Joining of metal and fiber composites

Joining of metal and fiber composites

Edwin Legdin

Stockholm, Sweden 2017

Master thesis report

Joining of metal and fiber composites

Edwin Legdin

Box 7047, 164 40 Kista, Sweden + 46 8 440 48 00, www.swereakimab.se

Abstract

There is an increasing demand from the

vehicle industry to create strong joints

between fiber reinforced plastics and metals.

The aim of this report is to explore different

possible existing joining methods through a

literature study and also evaluate and

compare three methods with physical

experiments. The three different methods that

were chosen were friction element welding

(FEW), RIVTAC and blind riveting. The

joint performance was evaluated through

testing of cross tension and shear tensile

strength. It was found that FEW generally

performed better but is restricted to some

material combinations. RIVTAC in

combination with adhesives reached higher

joint strength than without adhesive.

Blind rivets had the weakest joints. If they

were modified, they reached close to the

same joint strengths as the other methods.

1

Table of contents

1 Introduction ........................................................................................... 3

1.1 Scope ...................................................................................... 3 1.2 Automotive relevant materials ................................................ 3

2 Literature study: Joining methods for metal to fiber composites ......... 6

2.1 Methods based on mechanical locking ................................... 6

2.1.1 RIVTAC® (High-speed impact nailing) ................................ 7

2.1.2 Friction element welding (FEW) ............................................ 9

2.1.3 Resistance element welding (REW) ..................................... 11 2.1.4 Blind rivet ............................................................................. 12 2.1.4.1 Spin-blind riveting (SBR) ..................................................... 13 2.1.4.2 Rotation friction pressing riveting (RFPR) .......................... 13 2.1.5 Self-Piercing Riveting (SPR) ................................................ 14

2.1.5.1 Friction Self-Piercing Riveting (F-SPR) .............................. 16 2.1.6 Solid Self Piercing Riveting (SSPR) .................................... 17

2.1.7 Friction riveting .................................................................... 18 2.1.8 Flow drill screws (FDS) ....................................................... 19

2.1.9 Clinching .............................................................................. 21 2.1.9.1 Ultrasonic assisted clinching ................................................ 22

2.1.10 Ultrasonic joining with pins (U-joining) .............................. 23 2.2 Methods based on adhesion .................................................. 24

2.2.1 Added adhesive agent ........................................................... 25 2.2.2 Friction stir welding (FSW) .................................................. 25 2.2.3 Refill Friction Spot Joining .................................................. 27

2.2.4 Laser beam welding .............................................................. 28 2.3 Hybrid methods .................................................................... 29

2.3.1 ComeldTM

.............................................................................. 29

3 Experimental setup ............................................................................. 31

3.1 Cross sections ....................................................................... 34

4 Results and discussion ........................................................................ 35

4.1 Lap-Shear tests ..................................................................... 36 4.2 Cross-Tension tests ............................................................... 39 4.3 CFR polyamide 6.6 to DP800 steel ...................................... 42

4.3.1 RIVTAC ............................................................................... 42 4.3.2 FEW ...................................................................................... 44 4.4 CFR Epoxy to DP800 steel ................................................... 46 4.4.1 RIVTAC ............................................................................... 46 4.4.2 FEW ...................................................................................... 48

4.5 CFR polyamide 6.6 to 22MnB5 steel ................................... 49 4.5.1 RIVTAC ............................................................................... 49

4.5.2 FEW ...................................................................................... 52 4.6 CFR Epoxy to 22MnB5 steel ................................................ 54 4.6.1 RIVTAC ............................................................................... 54

2

4.7 CFR polyamide 6.6 to Usibor steel ...................................... 57

4.7.1 RIVTAC ............................................................................... 57 4.7.2 Blind Rivet made of aluminum ............................................ 60 4.7.3 Blind Rivet made of Steel ..................................................... 61 4.7.4 Blind Rivet made of Steel with the mandrel kept ................. 62 4.8 CFR Epoxy to Usibor steel ................................................... 62

5 Conclusions & Summary .................................................................... 66

6 Future Work ........................................................................................ 67

7 Acknowledgements ............................................................................. 67

8 References ........................................................................................... 67

9 Appendix A ......................................................................................... 71

9.1 CFR polyamide 6.6 to DP800 steel ...................................... 71 9.1.1 RIVTAC ............................................................................... 71

9.1.2 FEW ...................................................................................... 72 9.2 CFR Epoxy to DP800 steel ................................................... 73

9.2.1 RIVTAC ............................................................................... 73 9.2.2 FEW ...................................................................................... 75 9.3 CFR polyamide 6.6 to 22MnB5 steel ................................... 75

9.3.1 RIVTAC ............................................................................... 75

9.3.2 FEW ...................................................................................... 77 9.4 CFR Epoxy to 22MnB5 steel ................................................ 78 9.4.1 RIVTAC ............................................................................... 78

9.5 CFR polyamide 6.6 to Usibor steel ...................................... 79 9.5.1 RIVTAC ............................................................................... 79

9.5.2 Blind Rivet made of aluminum ............................................ 81 9.5.3 Blind Rivet made of Steel ..................................................... 81 9.5.4 Blind Rivet made of Steel with the mandrel kept ................. 82 9.6 CFR Epoxy to Usibor steel ................................................... 82

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1 Introduction The awareness of climate change is increasing worldwide which puts pressure on the vehicle

industry to minimize their emissions. The vehicle manufacturers are constantly looking for

ways to decrease the emissions from their products. One way of doing this is by reducing the

weight of the vehicles. Although, this has to be done at the same time as acceptable

mechanical and comfort properties are maintained.

Manufacturers today are trying to solve this by using lightweight materials such as high

strength steels to create vehicles with increased crash-safety and decreased weight. Others are

looking at fiber composites, aluminum or magnesium, which also are materials with a high

strength to weight ratio [1].

However to join these dissimilar materials can prove to be a challenge. Different material

combinations can be difficult to join because of either their individual chemical compositions

or large differences in physical properties. Considering high strength steel and fiber

composites with their distinct chemical composition difference and the fact that there are also

many different variations of these materials, the creation of a sound joint can be an issue [2].

1.1 Scope

This report aims to explore the different possible joining methods that exist for the joining of

metals and fiber composites and also evaluate and compare three of these methods with

physical experiments.

1.2 Automotive relevant materials

Common metals used in the automotive industry are listed below. With the exception of the

aluminum all other materials were investigated in this study.

DP800

Docol 800 DP (hereafter referred to as DP800) is a so called dual phase (DP) steel with a

tensile strength of 800 MPa. The DP steels have undergone a heat treatment in order to obtain

a two-phase microstructure. The structure is comprised of soft ferrite with a hard martensitic

phase dispersed inside. The ferrite gives the steel good formability and ductility properties

while the martensite accounts for the high strength. DP steels generally have very good

weldability due to the low alloy content relative to the high strength[3].

Usibor®

Usibor® (hereafter referred to as Usibor) is a hot rolled ultra-high strength coated boron steel,

more accurately 22MnB5 steel with an Al-Si coating (90% aluminum and 10% Silicon).

Usibor was developed for use in the automotive industry and is hardened during a hot

stamping process. Prior to the hot stamping it has a tensile strength in the range of 500 – 700

MPa. During the forming the material is heated to a temperature of 900°C for austenitization

and then quenched, which leads to a martensitic transformation of the steel. This results in a

yield strength of 1500 MPa, making it one of the strongest steels used in the automotive

manufacturing today. The Al-Si coating prevents the parts from oxidizing during the hot

stamping process and also gives the finished part good corrosion resistance [4].

Aluminum (AA6016/AA5182)

Aluminum is a metal with a good strength to weight ratio. The density of aluminum is

approximately 2.7 g/cm3

, which is about one third of the density of steel. Although, comparing

the tensile strength of aluminum to steel it can be considered a weak metal. Pure aluminum

has a tensile strength of approximately 90 MPa. However, it can be improved all the way up

4

to 570 MPa by cold working, thermal treatment or by alloying. Aluminum is an excellent heat

and electricity conductor, with a low melting point and high ductility. Another attribute of

aluminum is that it is highly resistant to corrosion hence it rapidly reacts with air and naturally

produces a thin oxide coating which protects the metal [5].

Figure 1 shows how the corrosion resistance (green), fatigue strength (purple), ductility (red)

and tensile strength (blue) changes in aluminum depending on the alloy type.

Figure 1: Change in corrosion resistance (green), fatigue strength (purple), ductility (red) and tensile

strength (blue) depending on alloy type [6]

Aluminum alloy AA6016 is alloyed with Manganese and Silicon, has a tensile strength of 280

MPa and generally has a good formability and weldability. AA5182 is alloyed with

manganese only and has a lower tensile strength than AA6016 (see Figure 1). It is also not a

heat treatable alloy like AA6016.

Welding aluminum to steel is problematic. Steel and aluminum have different metallurgical

properties making them incompatible. If a heat based joining method is used there is a great

risk of intermetallic phases occurring which are brittle and due to the unpredictable load paths

dangerous regarding failure [7]. Also joining aluminum to other materials can be difficult due

to the high thermal expansion. It is approximately twice as big as the thermal expansion of

steel, which easily leads to large deformations during welding or can introduce stresses in the

materials if exposed to temperature cycles. The oxide layer that is formed on aluminum is

strong and does not easily melt during welding. The melting temperature of aluminum is

approximately 660°C while the aluminum oxide melts at approximately 2050°C [6].

Fiber composites

In general, a composite material is composed of at least two materials, a matrix and

reinforcement. The name suggested that the reinforcement is improving the matrix material in

for instance stiffness and strength. Reinforcements can be different types of particles, fibers

etc. However, here reinforcement is restricted to fiber reinforcements.

The reinforcement is embedded in the matrix material which in this report is restricted to

polymer materials. The matrix binds the fibers together, introduces external loads to the

reinforcement, protects the reinforcements from the surrounding environment etc. While the

reinforcement carries most of the structural load the matrix gives the composite its shape and

durability [8].

As mentioned above, the reinforcing fiber gives the composite material stiffness and strength.

However, the reinforcements are fibers and therefore the orientation of the fibers in the

composite material has a significant influence on the mechanical properties. From a weight

specific point of few, the best reinforcement is continuous and aligned in loading direction

which gives the fibers the chance to carry as much load as possible and is resulting in an

anisotropic material (see figure below). If the reinforcement is discontinuous and aligned, the

5

load carrying capacity is reduced. In some application the discontinuous and randomly

orientated fibers or a fabric / weave is used.

Figure 2: Different arrangements for the fiber reinforcement in fiber composites [9]

The most common fiber reinforcements are carbon, glass or aramid fibers. Carbon fibers have

high weight specific stiffness and strength and are therefore often used in weight critical

applications such as aircrafts or vehicles. Glass fibers are less expensive and have a higher

strain to failure than carbon fibers. The weight specific stiffness and strength properties of

glass fibers are lower than carbon fibers but still high and are therefore often used in weight

and cost critical applications such as interior parts of vehicles, boats etc. Aramid fibers are

also known as Kevlar fibers and are often used in protection equipment such as bullet proof

vests or other safety equipment.

Since carbon fiber has the highest strength and stiffness properties it is therefore the favored

reinforcement type in many applications and will be the reinforcement used in this study.

Although carbon fibers have excellent mechanical properties it has some disadvantages which

might make the utilization of carbon fibers less appealing. The main drawback of carbon

fibers is their low strain to failure and high price. Another drawback is its electrical

conductivity, which may short out electrical equipment if there is carbon particles freely

suspended in the air and also cause galvanic corrosion when it comes into contact with metals.

The second part of the composite, the matrix, was as mentioned above restricted to polymer

materials. The polymer materials can be divided into two main categories, thermosets and

thermoplastics. Thermosets have irreversible three dimensional polymer chain bonds resulting

in a polymer which cannot be melted. In contrast to this, in thermoplastics the polymer chains

associates with internal forces which depend on the temperature and therefore thermoplastics

can be melted. This leads to the fact that the two polymer categories solidify in different

ways. Thermoplastics can be melted and therefore solidify through cooling. Thermosets

instead solidify by a chemical reaction, which normally is initiated when two or more

compounds are mixed together, in order to start the curing process. In general, thermosets

have better mechanical properties and can obviously be used at higher temperature than

thermoplastics. The most commonly used thermoset are vinyl-ester and epoxy. The most

common plastics are polyamide (PA), polyethylene (PE) etc.

In this study fiber composites made out of two different matrixes were used, one type from

each category. One made out of a thermoset (epoxy) which have great temperature tolerance

and mechanical properties. The other made out of the common thermoplastic polyamide (PA),

which is best-known as nylons [8].

6

2 Literature study: Joining methods for metal to fiber composites

Figure 3 shows an overview map displaying different joining methods relevant for joining

metals to fiber composites. The methods can, very simplified, be divided into two categories

depending on their main bonding mechanism. They are either adhesion based or based on a

mechanical locking mechanism. The following sections will explain how each method works.

Figure 3: Overview map over methods for joining metals to fiber composites

2.1 Methods based on mechanical locking

This section presents the joining methods that mainly rely on residual tensile stresses, created

from frictional forces or some sort of mechanical lock, to create a successful bonding. These

are often based on a point attachment that holds the components in compression. Depending

on the area of use of the object that is to be joined, there will be several different loads that

these joints are exposed to. To simplify, these loads have been broken down into two

theoretical load cases: transverse- and axial loading. These are illustrated in Figure 4, where

the transverse loading forces are forces that act perpendicular to the fastener and the axial

(tensile/compressive) forces act in the direction of the fastener. Compressive forces are

normally not the issue and will therefore not be examined in this study.

7

Figure 4: Illustrates the direction of transverse and axial loading on a fastener [10].

Also, since that it almost is a necessity for the creation of a mechanical joint that a hole is

made, it will result in local stress concentrations in these areas. Generally these local peak

stresses determine the load capability of the entire structure. In Figure 5 an illustration of how

the stresses increase in the proximity of the hole is shown.

Figure 5: Illustration of local peak stresses in a tensile loaded mechanical overlap joint [11]

2.1.1 RIVTAC® (High-speed impact nailing)

The RIVTAC® (hereafter referred to as RIVTAC) method is based on a nail with grooves

being inserted with high speed (20 to 40 m/s [12]) to penetrate the materials. The process is

shown in Figure 6. Firstly, the nail is positioned. Then it is accelerated to high speed so that it

penetrates the materials. Since improved flowability in the metal is obtained due to a short

heat increase, the material is allowed to first be displaced and then contract into the grooves

of the rivet body. This mechanism creates a form and force bond from the material pressing

against the nail body which locks it in place. The head of the rivet locks the composite to the

metal and also creates another form lock if material is displaced into the groove inside the

head [13].

Figure 6: The RIVTAC process: Positioning – Entering – Penetration - Bracing [13]

One advantage with RIVTAC is that it only needs one sided access unlike many other

methods. Another advantage is the extremely short process time, meaning that robotized

RIVTAC can reach a very high productivity. The drawback with RIVTAC is the loud noise

the process produces which affects the working environment.

The different materials that can be joined with RIVTAC according to the supplier

BÖLLHOFF [14] is listed in Table 1. Figure 7 shows cross-sections of different material

8

combinations. The general doctrine is that the combinations should be joined from thin into

thick or soft into hard. The top layer can have a maximum strength of 1000 MPa and the

bottom 1600 MPa [15].

Table 1: List of materials RIVTAC can be used on

List of materials RIVTAC can be used on according to literature [14]

Aluminum (pressure cast, extruded, sheet)

Steels with Rm up to 1,400 N/mm

Plastics and fiber-reinforced plastics (e.g. fiber glass or carbon)

Material combinations with magnesium, copper, films, metal mesh, wood, sandwich

materials

Joining of mixed joints, multiple-layer joints and hybrid joints of these materials

Materials with adhesive, sealant or other intermediate layer

Figure 7: Cross sections of material combinations that can be joined with RIVTAC [16]

Mercedes-Benz was the first car manufacturer worldwide to use RIVTAC in series production

in their Mercedes-Benz SL class [17], although not for the joining of fiber composites to

metal but for joining different metals together.

Table 2 summarizes advantages and disadvantages with the RIVTAC method.

Table 2: List of Advantages and disadvantages with RIVTAC

Advantages Disadvantages

High productivity/short process times Risk for galvanic corrosion

Does not require pre-drilled holes Stress concentrations

No surface preparation needed Rivet adds weight

Temperature sensitive materials can be joined

(No change in material properties due to heat)

Restrictions regarding maximum penetration

(material strength and thickness)

One sided access (depends on structural integrity) Risk of damage from hole generation

No dangerous smoke or fume emissions Not user friendly (Loud noise)

No Smooth surface finish

Equipment Suppliers:

BÖLLHOFF

9

2.1.2 Friction element welding (FEW)

The theory behind FEW is that a customized rivet is friction welded onto the lower sheet

which will, with the rivet head, lock the upper sheet in place. Figure 8 describes the FEW

process. First a blank holder applies pressure to the two parts to prevent gaps, and the friction

element is accelerated to a high rotational speed. The spinning element is then brought into

contact with the surface of the upper sheet and the resulting frictional heat cause a

plasticization in the material. This in turn allows the friction element to penetrate the sheet

when further pressure is applied.

When the friction element comes into contact with the underlying sheet, the frictional heat

increases even further and plasticizes the rivet and activates the surfaces. Welding beads are

formed around the rivet, which is now decreasing in length because of the axial force. After a

certain shortening, the rotation is stopped. Additional force is now applied which results in a

pressure welding effect that shortens the rivet even further and produces a strong metal bond

between rivet and lower sheet. The displaced material from the cover sheet is collected in a

groove under the head of the friction element, resulting in a form fit while the decreasing

temperature of the rivet results in an axial shrinkage and a force-lock between upper sheet and

the rivet [18]. Figure 9 shows a cross-section of a FEW joint.

Figure 8: The FEW process [19]

Figure 9: Cross section of FEW joint [19]

FEW is beneficial when brittle Ultra-high strength (UHS) steels are to be joined. Since there

is no punching or forming of the steel sheet the low ductility does not cause any problems.

Also in this case, just like for RIVTAC, the doctrine is that the combinations should

10

preferably be joined from soft into hard. FEW is also suitable for temperature-sensitive metals

since it is a solid-state joining method.

In previous studies where tensile shear tests have been performed for different rivet and bolt

joining techniques it appears that FEW-joints can transfer high tensile shear loads [12]. Table

3 lists the materials that can be joined by FEW and Table 4 mentions the advantages and

disadvantages.

Table 3: List of materials FEW can be used on

List of materials FEW can be used on according to literature

Aluminum (although not as bottom sheet due to its high thermal conductivity)

Ultra-High strength Steels with up to 1500 MPa in tensile strength [20]


Materials with adhesive, sealant or other intermediate layer

Table 4: List of Advantages and disadvantages with FEW


Does not require pre-drilled holes and no exact

hole alignment

Emissions when joining CFR thermoset plastics

(CF particles)

No surface preparation needed Element adds weight

Small heat affected zone/no material melting Risk of damage from hole generation in

composite

Can be used on brittle UHS steel Risk for galvanic corrosion

Two-sided access (depending on structural

integrity)


Stress concentrations


EJOT (EJOWELD®)

Kerb-Konus (LWF at the University of Paderborn)

EJOWELD® is utilized in the series production of Audi Q7 in Bratislava [21] for joining

aluminum to steel, see Figure 10.

Figure 10: parts of Audi Q7 where EJOWELD® is utilized

11

2.1.3 Resistance element welding (REW)

Resistance element welding is a joining method developed by Volkswagen and utilized in

their Passat GTE model [22]. REW is a form of resistance spot welding developed for the

joining of dissimilar material combinations by using a so called weld rivet. The technique

requires that a hole is either pre-drilled in the cover sheet alternatively punched with the rivet

before the welding process. Figure 11 shows the REW process. When the perforation is done

and the weld rivet is in place, the electrodes are positioned both on top of the rivet and under

the lower sheet, similar to a conventional resistance spot welding process. In the next stage

when pressure and electric current is applied, heat is generated due to electrical resistance and

a weld nugget is formed between the base sheet and the rivet. The electrode force is then

increased and results in the rivet compressing and creates a force connection between cover

sheet and rivet head [18]. The advantages and disadvantages are listed in Table 6, and the

different materials that can be joined with this method in Table 5.

Figure 11: The process of REW [18]

Table 5: List of Materials that REW can be used on

List of materials REW can be used on according to literature

Aluminum

Up to Ultra-High strength steels


materials with adhesive, sealant or other intermediate layer

Table 6: Advantages and disadvantages of REW


Can be used on brittle UHS steel and other

materials that are difficult to weld Stress concentrations

Fast method when cover sheet is prepared Requires a pre-drilled hole in the upper sheet

Can join thick sheets, weld rivet size is the

restricting factor Two-sided accessibility needed

Conventional RSW equipment can be used heat affected zone


Rivet adds weight

Risk for galvanic corrosion

12

2.1.4 Blind rivet

Blind riveting is a common mechanical joining method. Even though there are many different

variations of the blind rivet, both in rivet shape and the rivet material, the principle is the

same. Blind riveting is based on applying a pulling force on the mandrel (see Figure 12)

which will result in rivet deformation and creation of a form lock on the two sheets [23].

Figure 13 shows a cross-section of a blind rivet made out of aluminum which holds a carbon

fiber reinforced plastic (CFRP) to Usibor.

Figure 12: Cross section of a blind rivet and illustration of the process [24][25]

Figure 13: Cross section of a blind rivet made out of aluminum bonding CFRP to Usibor

BMW is using blind rivets in their 2016 7-series model to combine CRFP with aluminum and

high strength steel [26], see Figure 14.

Figure 14: CFRP parts joined to metal in the BMW 2016 7-series [26]

13

Table 7: Advantages and disadvantages of blind riveting



(No change in material properties due to heat) Stress concentrations

One sided access Risk for galvanic corrosion

User and environmentally friendly (no fume,

smoke or other emissions) Requires pre-drilled holes in both sheets

Rivet adds weight

2.1.4.1 Spin-blind riveting (SBR)

Hybrid methods with blind riveting that are aiming to eliminate the need of pre-drilled holes

are being researched. One is SBR and the process can be seen in Figure 15. A customized

blind rivet is accelerated to a high rotational speed and pressed against the metal sheet in

order to generate frictional heat. Plasticization of the sheet occurs locally and the rivet begins

to penetrate the upper sheet. The displaced material creates a “sleeve” with high temperature

which in turn heats the lower sheet. When the rivet has pierced through both sheets it is

brought to a halt and the mandrel head is pulled back into the rivet so that the rivet is

deformed [24] similar to conventional blind riveting. A cross section of an SBR joint can be

seen in Figure 16.

Figure 15: The process of spin-blind riveting [24]

Figure 16: Cross section of a spin-blind rivet joint with magnesium and plastic [24]

2.1.4.2 Rotation friction pressing riveting (RFPR)

This method is based on the same principle as SBR but with the difference that it uses a two-

piece rivet with a plug and shank (seen in Figure 18) and that a punch pushes the plug

upwards into the rivet shank. See Figure 17.

14

A study made on magnesium alloy sheets show that the RFPR joints have improved shear

strength and fatigue properties when compared to other rivet deformation based methods [27].

Figure 17: Rotation friction pressing riveting process [27]

Figure 18: Rivet used for rotation friction pressing riveting [27]

Table 8 lists the materials which are possible to join with these hybrid methods.

Table 8: List of materials SBR and RFPR can be used on

List of materials SPR and RFPR can be used on according to literature [24][27]

Aluminum

Magnesium


These hybrid methods have the same advantages and disadvantages as conventional blind

riveting but with some differences. Table 9 mentions the advantages and disadvantages with

SBR and RFPR compared to conventional blind riveting.

Table 9: Advantages and disadvantages with SBR and RFPR


Can transmit higher shear loads than

conventional blind rivets due to the sleeve[24] Only “soft” materials can be joined

Does not require pre-drilled holes A small heat affected zone

If the fiber composite has a thermoplastic matrix

there is a possibility for less damage being done

to the fibers (since they are movable and can be

displaced instead of destroyed) [24]

2.1.5 Self-Piercing Riveting (SPR)

SPR is based on controlled deformation of a hollow rivet in order to create a sound joint. The

process is shown in Figure 19 and it starts with the tool clamping the sheets together between

the blank holder and the anvil. Hereafter the punch forces the rivet into the materials. The

rivet first pierces the top sheet and continues through until the shape of the anvil upsets the

rivet and causes it to flare within the bottom layer and forms a mechanical bond between the

15

sheets [28]. This shape also causes a small bulge to form underneath the stack-up, which

ideally the rivet should not pierce. A cross-section of a SPR joint can be seen in Figure 19.

Figure 19: Procedure of SPR and cross section of a SPR joint [29][30]

The hardness of both the rivet and the sheet material is highly important properties, since the

rivet has to be able to penetrate the upper sheet and also be ductile enough to flare afterwards

in the lower sheet. If the hardness of the rivet is too low it might collapse and if it is too high

it may pierce or cause thinning in the bottom sheet. The rivets are usually made from medium

carbon or boron treated steel using a cold forming process. However, for special

circumstances, copper, aluminum or stainless steel is also used. It is also important that the

sheet materials have sufficient ductility to deform into the anvil. The total sheet thickness

which is possible to join ranges from 0.5 mm to 12 mm depending on materials and the joint

property requirements[20][31].

Self-piercing riveting is a common method in the automotive industry. It provides the

possibility to join materials which are difficult to join with resistance spot welding (which

also is a common joining method in the automotive industry) and it can also endure fatigue

load better than some element based mechanical bonding methods such as RIVTAC joints

[32]. Some examples of car models using SPR are [33]:

Jaguar XJ: 3185 Self piercing rivets

Jaguar XK: 2620 Self piercing rivets

Audi A8 (D4): 1847 Self piercing rivets

Lamborghini Gallardo: 1300 Self piercing rivets

Rolls Royce Phantom: 725 Self piercing rivets

Mercedes-Benz SLS AMG: 975 Self piercing rivets

Mercedes-Benz SL (R231): 1235 Self piercing rivets

Aston Martin Vanquish: 176 Self piercing rivets

Chevrolet Corvette Z06: 236 Self piercing rivets

BMW 5 and 6 series: 598 Self piercing rivets

Audi TT: 1606 Self piercing rivets

Table 10 lists the materials SPR can be used on and Table 11 lists the advantages and

disadvantages with SPR.

16

Table 10: List of materials SPR can be used on

List of materials SPR can be used on according to literature [31][33]

low carbon and micro-alloyed steels

zinc-coated, organic-coated or pre-painted steels

ductile aluminum alloys

dissimilar material combinations such as steel to aluminum alloys, aluminum to magnesium

alloys

ductile plastics and composites to metal


Table 11: List of Advantages and disadvantages of SPR


Does not require pre-drilled holes Stress concentrations

No surface preparation needed Risk of damage from deformation of sheets


(No change in material properties due to heat) Risk for galvanic corrosion

Little or no damage to pre-coating Cannot be used on brittle materials

High strength and good fatigue resistance

compared to other element based mechanical

joining methods such as RIVTAC [34]

Access to both sides is required

User and environmentally friendly (no fume,

smoke or other emissions)

No Smooth surface finish on bottom sheet as it

produces a bulge

Rivet adds weight


BÖLLHOFF (Rivset)

Avdel Fastriv

RIVTEC

TUCKER

2.1.5.1 Friction Self-Piercing Riveting (F-SPR)

As mentioned above, self-piercing riveting is a preferred technique when it comes to joining

of dissimilar materials. However, when riveting low ductility materials there is a great risk of

cracks occurring in the worksheets because of the large deformations. The risk of cracking

can be decreased by preheating and increasing the ductility of the material, which lead to the

development of the friction self-piercing riveting method (see Figure 20). This method is

basically SPR but adding a high rotational speed to the rivet, which will locally soften the

material due to the friction heat [35]. F-SPR produces a joint that is combining the mechanical

lock from SPR and the bonding forces of the plasticized material that have been mixed and

solidified. Figure 21 show cross-sections made on AA6061-T6 to AZ31B with SPR and F-

SPR [36]. No cracks can be seen with the F-SPR method.

17

Figure 20: Friction self-piercing riveting method [35]

Figure 21: Comparison of cross-sections with AA6061-T6 to AZ31B[35]:

(a) SPR and (b) F-SPR

2.1.6 Solid Self Piercing Riveting (SSPR)

The solid self-piercing riveting process is a SPR process, except that the rivet is solid and

does not deform inside the worksheets. Instead the rivet penetrates both worksheets and

replaces the removed piece of material, as seen in Figure 22. This way the drawback that SPR

has concerning the joining of brittle materials is overcome and it is possible to join hard

materials with low ductility and also a plane surface without a bulge can be achieved.

Different materials and rivet geometries can be used depending on the application

[12][18][37]. Figure 23 shows cross-sections of two different SSPR-joints. The rivet is

embossed with a grove/groves which desirably is supposed to be filled with worksheet

material if the process force is large enough to initiate plastic deformation

The solid self-piercing riveting method is applied in the Audi TT (8J) where a total of 96 solid

rivets are used, 48 which are made of coated stainless steel and 48 made from aluminum [33].

Figure 22: Description of the SSPR process [18]

18

Figure 23: Cross sections of two different Solid self-piercing rivets [20]


Kerb-Konus

TOX PRESSOTECHNIK

2.1.7 Friction riveting

The process of friction riveting is shown in Figure 24. It is based on mechanical fastening

with a rod-like metallic rivet and the principles of friction welding. The method is mainly

developed for joining of polymeric material such as composites. The rivet is accelerated and

when the required rotational speed is achieved, the rivet is brought into contact with the

workpiece. The local temperature increase causes the material of the workpiece in the

surrounding area (that is in contact with the rivet) to melt. Since polymers have low thermal

conductivity the heat input locally will eventually (after a certain depth) become larger than

the heat transferred to the surroundings, leading to local temperature increase until it reaches

the plasticizing temperature of the metallic rivet. Forging pressure is now applied while the

rotation is stopped and the plasticized tip of the rivet is deformed into a parabolic shape inside

the worksheet.

Figure 24: The friction riveting process [11]

This bonding method can be used in various ways to join composites and metals. One way is

to drill a hole in the metal sheet and then assembly the two sheets by an already fastened rivet

(in the composite) with a nut. Other possibilities are shown in Figure 25. (A) is by inserting a

rivet with a threaded hole so that screws can be used to bond the sheets together, (B) shows a

sandwich construction, consisting of two metal plates and a plastic core, that have been joined

with a friction rivet.

No published/presented work could be found where this method is used in the industry.

19

Figure 25: Possible configuration of friction riveted joints [11]

(A) point-on-plate insert-joint. (B) sandwich type joint

Table 12: Advantages and disadvantages with friction riveting


Single side accessibility Rivet adds weight

Low cost and simple machinery No Smooth surface finish

No pre-drilled holes or surface preparation of

polymeric worksheet needed Stress concentrations

Risk for galvanic corrosion

2.1.8 Flow drill screws (FDS)

A high strength steel screw is used to locally heat the top sheet with high rotational speed and

axial load. When reaching a certain temperature the material starts to plasticize and the screw

penetrates the sheet materials (see Figure 26, 1 and 2). The softened material flow towards the

bottom of the worksheets and forms an extrusion around the screw. This extrusion is, while

the material hardens due to the temperature decreasing, threaded by the grooves on the screw

(3. and 4.). This result in the threads being shrink-fitted to the screw since they are made at

elevated temperatures. Some material also flows upwards and is collected in a cavity under

the head of the screw and results in a form fit. During this stage in the process the rotation is

decreased to prevent material chip as much as possible. When full thread engagement is

reached the screw is tightened [38]. Figure 27 shows a cross-section of a FDS joint.

Figure 26: The FDS process [39]

20

Figure 27: Cross-section of FDS joint [40]

The cycle time for this process can vary from 1.5 – 4 seconds depending on the used tool and

the materials being joined. Also for FDS the general stack-up recommendations are from thin-

thick or soft-hard [41]. The different materials that the FDS method can join can be seen in

Table 13. The maximum total sheet thicknesses that can be joined without pre-punched holes

are roughly [42]:

Aluminum - 6 mm

Magnesium - 4.5 mm

Steel - 2 mm

Stainless steel - 1.5 mm

FDS has an advantage over other mechanical joining technologies: it is easy to disassemble

the workpieces afterwards for either service or inspection. Furthermore, like RIVTAC, it only

requires one sided access. Table 13 lists the material FDS can be used on and Table 14

mentions some of the other advantages and disadvantages with FDS.

There are several car manufacturers using FDS in their models [33][39][42]:

Audi R8: 310 flow drill screws

Audi A8 (D4): 632 flow drill screws

Audi TT: 229 Flow drill screws

Lamborghini Gallardo: 200 Flow drill screws

Mercedes-Benz SLS AMG: 581 Flow drill screws

Mercedes-Benz SL (R231): 152 Flow drill screws

Aston Martin Vanquish: 76 Flow drill screws

Porsche 911/Boxster: 190 Flow drill screws

GM Opel in the speedster

Jaguar in the XK and X150

Volkswagen in the Cross Touran

Lotus Evora

Table 13: List of materials FDS can be used at

List of materials FDS can be used on according to literature

Steel (DP600 is the general recommended limit, higher strength levels might be possible in

lower thicknesses or with pre-drilled hole [41])

aluminum

magnesium

stainless steel

Plastics and fiber-reinforced plastics


21

Table 14: Advantages and disadvantages with FDS


Single side accessibility screw adds weight

Easy disassemble No Smooth surface finish

No pre-drilled holes and no exact hole

alignment Stress concentrations

Small heat affected zone/no material melting Risk for galvanic corrosion

No surface preparation needed Risk of damage from hole generation


EJOT

2.1.9 Clinching

Clinching is a method which is used to create a mechanical interlock by producing an

indentation in the worksheets. The procedure starts with the sheets being clamped together

between a blankholder and a die. After that a punch is brought down applying pressure. The

punch forces material into the die and locally deforms the sheets creating a button on the

underside, which holds the sheets together with a mechanical shape lock [43]. Figure 28

illustrates the process. Process time is typically 1-2 seconds.

Figure 28: clinching process[43]

Similar to SPR the hardness of the sheet material is highly important since the joint is

achieved by local plastic deformation. If the worksheets are not ductile enough it might result

in cracking, making clinching less suitable for brittle materials. However, manufacturers are

attempting to overcome this problem by optimization of the process and the die design [44].

This leads to many different variations of the clinching method and hence variations of the

tool used. Some of them are shown in Figure 29, but the principle of the joint is the same.

The general stack-up recommendation for clinching from the suppliers to achieve strong

joints, is to join from thick to thin [45]. This was approved to be true for round joints in high-

strength structural steel [46]. Other recommendations from suppliers are that the difference of

sheet thickness should be below factor two [47].

22

Figure 29: clinching tools for different clinch joints [48]

Examples of car models where clinching is used [33]:

Rolls Royce Phantom: 30 clinch points

Mercedes-Benz SL (R231): 213 clinch points

Audi TT: 172 clinch points

Table 15: List of materials clinching can be used on

List of materials clinching can be used on according to literature [44]

low carbon and micro-alloyed steels

zinc-coated, organic coated and pre-painted steels

stainless steels

lightweight materials, such as ductile aluminum alloys

fiber reinforced plastic (FRP) [48]

Table 16: Advantages and disadvantages with clinching


No added weight from bolts and rivets Two-sided access required

Does not require pre-drilled holes nor surface

preparation

Risk of damage from deformation and for

galvanic corrosion


(No change in material properties due to heat) brittle materials cannot be joined

stress concentrations

2.1.9.1 Ultrasonic assisted clinching

To create stronger joints between metals and fiber reinforced plastics and also an attempt at

overcoming the drawbacks with clinching on brittle materials, a method that combines

clinching and ultrasonic joining is being developed. The process begins with a circular shaped

sonotrode clamping down on the workpieces and subjects them to ultrasonic vibrations. The

vibrations melt the polymer of the composite sheet in the subjected area and an adhesive bond

is obtained. Then a punch is brought down and produces a clinch bond inside the “sonotrode

circle”, cross-section and joint is shown in Figure 30 [49].

23

Figure 30: joint and cross-section of an ultrasonic assisted clinch joint [49]

2.1.10 Ultrasonic joining with pins (U-joining)

Ultrasonic joining with pins uses a metal sheet with pins that later on is joined together with a

thermoplastic composite with the use of ultrasonic vibrations. There are several ways of

placing the pins onto the metal sheet. After that, the pins are brought into contact with the

composite surface and a sonotrode is lowered onto the workpieces. Figure 31 shows an

illustration of the ultrasonic joining procedure. The sonotrode applies pressure and the metal

sheet is subjected to ultrasonic vibrations (usually at a frequency of 20 – 40 kHz [50]) which

will lead to a temperature increase where the pins are in contact with the composite due to

friction (2 and 3). The increased heat leads to local softening of the polymer matrix and

allowing the pins to penetrate into the composite from the pressure of the sonotrode (4), and

thereby form a mechanical interlock between the worksheets. The vibrations are then stopped

and the molten polymer solidifies, which also results in an adhesive bond being created

between the composite and metal sheet.

So far, ultrasonic joining with pins for fiber composites to metal is in an early stage of

development and to the authors knowledge not used in the industry today. However tests

made on U-joined titanium and GFRP hybrid joints showed increased lap-shear strength and

ductility compared to reference joints [51].

Figure 31: The procedure of ultrasonic joining [51]

24

Table 17: Advantages and disadvantages with ultrasonic joining


Fast process (weld times typically below

one second)

Need for advanced preparation of pins on metal

workpiece

Smooth surface finish Access to both sides is required

Energy efficient

2.2 Methods based on adhesion

Just like for the mechanical joining methods the application decides the loads that the joints

will undergo but for the sake of simplicity they have been broken down into the same two

theoretical load cases as for mechanical joining, transverse- and axial loading. However here,

the joints instead rely on adhesion forces to create a bond.

Generally adhesively bonded joints are stronger in transverse loading and weak against axial

loads, or as they are also called, “peel loads”. However similarly like for mechanical joints,

where there were stress concentrations close to the holes, there will be an uneven shear stress

distribution in adhesively bonded overlap joints. How the shear stress changes along the

overlap is shown in Figure 32. There will be local peak stresses at the edges of the bond and

they decrease as you move towards the middle of the bond [11].

Figure 32: Shear stress distribution of an adhesively bonded overlap joint loaded in tension

There are mainly two different ways to create an adhesive bond with fiber composites, either

by using an additional adhesive agent or by utilizing the thermoplastic matrix of the

composite. The second is done by using heat to melt the thermoplastic and then letting it

resolidify in order to adhesively attach to the metal (meaning that composites with a

thermoset matrix cannot be joined this way). There are many methods used for welding of

metals that can be used as a heat source to melt the polymeric matrix, but in this report only a

few will be mentioned.

25

2.2.1 Added adhesive agent

A bond between the fiber composite and metal sheet is created with the use of a substance

which sticks to the surface of the two materials and fastens the two objects together by

adhesive and cohesive forces. The substances used are adhesives, various plastic agents or

epoxies that bond by curing either with heat, pressure, time or by the use of a solvent.

Adhesive bonding is widely used in the automotive industry today. Up to around 100 m of

adhesives are used in several car models. Some examples are listed below [33].

Jaguar XJ: length of the adhesive bonds is a total of 116 m.

Jaguar XK: a total of 99 m

BMW 5 and 6 series: a total of 15.8 m

Audi TT: a total of 97.2 m

Mercedes-Benz SL (R231): a total of 76.2 m

Mercedes-Benz SLS AMG

Lotus Evora

Table 18: Advantages and disadvantages with adhesive bonding


Improves stress distribution which improves

fatigue properties Stress concentrations at the edges of the joint

No galvanic corrosion Sensitive to peel stress


(No change in material properties due to heat)

Sensitive to mismatches in CTE (coefficient of

thermal expansion), will create residual stresses

No thickness restrictions Surface preparation often needed

Smooth surface finish Cannot easily be dissembled for service or

inspection

No added weight from bolts and rivets

2.2.2 Friction stir welding (FSW)

A cylindrical-shouldered tool with a probe (see Figure 33) is accelerated to a high rotational

speed and brought to the contact point between the composite and metal. Frictional heat is

generated between the materials and the tool and leads to the creation of a soft region under

the shoulder. The tool is moved forward and stirs the now softened materials together and

forces them to the back of the tool, where they consolidate when the temperature decreases.

Although a requirement for this to work is that the composite is made out of a thermoplastic

resin.

26

Figure 33: The FSW process [52]

The FSW method enables the creation of butt welds between metal and fiber composites[53],

unlike the other methods which require that the worksheets overlap for the bonding to be

successful. The downside is that there is a great risk of fiber pull-out if the reinforcement in

the composite consists of woven fiber mats. If they are instead discontinuous (“short”) fibers

then the risk is less significant.

One way to avoid fiber pull-out is by shifting the FSW tool placement so that it is placed

closer to, or entirely on the metal side. This way the thermoplastic resin (that have been

melted by the heat transferred from the metal and then solidified again) will create an

adhesive bond without the FSW tool coming in contact with the fiber reinforcement [54].

FSW is not used in the industry for joining fiber composites to metal today, however it is for

example used in the Audi R8 and Mercedes-Benz SL (R231) for welding different metals

together [33].

The different materials that can be joined with FSW are listed in Table 19 and the advantages

and disadvantages in Table 20.

Table 19: List of materials FSW can be used on

List of materials FSW can be used on according to literature [55]

Lead

Zinc

Magnesium

Aluminum

Copper

Titanium

Steel

Stainless steel

Nickel alloys

Fiber reinforced thermoplastic polymers

27

Table 20: Advantages and disadvantages of FSW


Smooth surface finish Large Clamping forces needed

No added weight from bolts and rivets Fiber pull-out

No pre-punched holes or surface preparation

needed Exit hole left after withdrawing the tool

Small heat affected zone Only works for thermoplastics

Overlap or butt joint possible

2.2.3 Refill Friction Spot Joining

The friction spot joining method uses a tool that consists of three parts: pin, sleeve and

clamping ring, which can all move independently from each other (see Figure 34). The

process starts with the clamping ring pressing down the worksheets in order to hold them in

place. Thereafter the pin and sleeve starts rotating and are then lowered onto the top sheet and

the temperature rises locally due to the heat from the friction between these parts. The top

sheet experiences a local softening and the sleeve starts plunging into the material, meanwhile

the pin retracts upwards at the same time. The continuous feeding of the sleeve causes the

plasticized material to flow into this annular space which is created between sleeve and pin.

When the sleeve has reached a pre-set depth it will start to retract back to the surface of the

top sheet while the pin starts moving downwards instead and forces the previously collected

plasticized material back, in order to refill the void left by the sleeve. Then the tool is either

lifted of or kept in place (if pressure during consolidation is desired), but the rotation is

stopped to let the plasticized material cool down and solidify to create the joint.

When joining of metal to fiber composites is done by this method the metal sheet is placed on

top, and the pre-set depth of the sleeve is set to the thickness of the metallic workpiece to

avoid damage to the fibers in the FRP sheet.

There are two interlocking mechanisms that are bonding the sheets together. One is adhesion

forces from the polymer that have been melted from the frictional heat transferred from the

metal to the FRP and then afterwards consolidated. The second mechanism is a mechanical

lock from a metallic nub on the lower side of the metal sheet that has been created from the

plasticized metal.

Advantages and disadvantages with this method can be seen in Table 21.

Figure 34: The friction spot joining method [56]

28

Table 21: Advantages and disadvantages with friction spot joining


Smooth surface finish Two-sided access

No added weight from bolts and rivets Stress concentrations


hole alignment Only works for thermoplastics

No exit hole left after withdrawing the tool

unlike FSW

Small heat affected zone

Little or no damage to the fibers in the FRP

2.2.4 Laser beam welding

Laser is an acronym for “light amplification by stimulated emission of radiation". Meaning

that it is light emitted through a process of optical amplification based on the stimulated

emission of electromagnetic radiation [57].

Like the previous methods the principle in this method is the same, the metal and composite is

brought into contact with each other, the metal is heated by the laser in order to melt the

thermoplastic resin which attaches adhesively to the metal when it resolidifies.

Laser for CFRP to metal joining seems to still be in the research stage since no

published/presented work that showed laser being used for the joining of CFRP to metals in

the industry could be found. [58] did trials with laser joining where 3 different aluminum

alloys were successfully joined to CFRP. Figure 35 shows the aluminum and CFRP test

specimens that were bonded with a laser.

Figure 35: AA5182 and CFRP joined by a diode laser.[58]

https://en.wikipedia.org/wiki/Optical_amplification

https://en.wikipedia.org/wiki/Stimulated_emission

https://en.wikipedia.org/wiki/Stimulated_emission

https://en.wikipedia.org/wiki/Electromagnetic_radiation

29

Table 22: Advantages and disadvantages with laser


Smooth surface finish Only works for thermoplastics

No added weight from bolts and rivets So far only relatively low strengths achieved


hole alignment

Single-sided accessibility

Small heat affected zone

Fast process

2.3 Hybrid methods

A number of methods can be combined into hybrid joints. Most common are adhesives

combined with mechanical joining (screw-bonding, riv-bonding etc.) however there are also

some hybrid methods that create joints in other ways.

2.3.1 ComeldTM

In the first step in this method, the surface of the metal sheet is treated with a power beam

technology called Surfi-Sculpt. This treatment creates metal protrusions, so called proggles,

on the surface of the metal sheet (see Figure 36). It does so by constantly melting a small

amount of material on the surface and then displacing it using the power beam by utilizing the

surface tension and vapor pressure effects[59].

Figure 36: Surfi-sculpt process [60]

Then the bond between the fiber composite and metal is created by letting this, now

hedgehog-like, surface pierce the fiber weaves and laminating them with a polymer directly

onto it. The polymer will act as both matrix of the fiber composite and as an adhesive joint.

Figure 37 shows a cross-section of a joint without the proggles and a ComeldTM

joint.

30

Figure 37: cross-section of a control joint and a ComeldTM

joint [61]

It is assumed that ComeldTM

is not a used in the industry today since no published/presented

work of this could be found.

Table 23: Advantages and disadvantages with ComeldTM


Smooth surface finish Surface preparation needed

No added weight from rivets and bolts Many process steps, low productivity (curing of

polymer)

Improved stress distribution Risk for galvanic corrosion

Cannot be easily disassembled, inspection can

be difficult

31

3 Experimental setup Three different joining methods were evaluated in this project. The evaluated methods were

FEW, RIVTAC and blind riveting. The specimens for FEW and RIVTAC were tested with

and without an adhesive in the joining area. The used adhesive was Henkel Terokal 5055

which is a 2-component epoxy based adhesive which was allowed to cure in room

temperature for more than 2 days. It had been applied manually with a handheld gun with a

target value on 0.2 mm for the adhesive layer thickness.

This resulted in a total of 5 different joining configurations. The tensile properties and cross-

tension properties of the joint were investigated. The specimen geometries for all of the

tensile and cross-tension specimens were the same and are presented in Figure 38. The plate

length and width of the plates was 125 mm and 38 mm, respectively. For the tensile test

specimen the overlap was 38mm. All testing was performed at room temperature of around

20°C.

Figure 38: Dimensions of the test specimens and testing forces

Five different plate materials are used in this study and they are summarized in Table 24.

32

Table 24: The different plate materials used in this study and the abbreviations used

Epoxy Carbon Fiber Reinforced epoxy polymer 2,5 mm

PA Carbon Fiber Reinforced polyamide 6.6 plastic 3,0 mm

DP800 Dual-phased high strength steel 1,5 mm

22MnB5 ultra-high strength boron steel 1,5 mm

USIBOR ultra-high strength boron steel (22MnB3) with Al-Si coating 1,5 mm

The specimens joined with RIVTAC and FEW were produced and provided by the

Laboratorium für Werkstoff- und Fügetechnik (LWF) of the University of Paderborn in

Germany. Lab technicians at University of Paderborn selected the process parameters for the

RIVTAC and FEW joining methods through trials, these parameters are presented in Table

25. Specimens with blind rivets were manufactured at Swerea KIMAB.

Table 25: Process parameters for the RIVTAC and FEW test specimens

RIVTAC material combinations process parameters

join pressure [bar] blank holder [bar]

Epoxy DP800 4 1,5 Epoxy 22MnB5 4,5 1,5 Epoxy USIBOR 4,5 1,5

PA DP800 3,85 1,5 PA 22MnB5 4,5 1,5 PA USIBOR 4,5 1,5

FEW rotation [rpm] max.force [kN]

Epoxy DP800 18000 7,16 PA DP800 18000 7,16 PA 22MnB5 18000 7,7

Table 26 summarizes the different material and joining combinations tested in this work and

presents the quantity of tested specimens as well.

33

Table 26: The test specimens provided by Paderborn

Lap-Shear Cross-tension RIVTAC RIVTAC

No adhesive Mtrl 1 Mtrl 2 quantity No adhesive Mtrl 1 Mtrl 2 quantity 2 Epoxy DP800 5 21 Epoxy DP800 5 3 Epoxy 22MnB5 6 22 Epoxy 22MnB5 4 4 Epoxy USIBOR 5 23 Epoxy USIBOR 4 6 PA DP800 5 24 PA DP800 5 7 PA 22MnB5 6 25 PA 22MnB5 4 8 PA USIBOR 6 26 PA USIBOR 4

With adhesive With adhesive 9 Epoxy DP800 5 27 Epoxy DP800 5

10 Epoxy 22MnB5 5 28 Epoxy 22MnB5 4 11 Epoxy USIBOR 4 29 Epoxy USIBOR 5 12 PA DP800 5 30 PA DP800 4 13 PA 22MnB5 5 31 PA 22MnB5 4 14 PA USIBOR 3 32 PA USIBOR 5

FEW FEW No adhesive Mtrl 1 Mtrl 2 quantity No adhesive Mtrl 1 Mtrl 2 quantity

15 Epoxy DP800 5 Epoxy 22MnB5 Not created (CF dusts) - - Epoxy USIBOR Not possible

16 PA DP800 5 33 PA DP800 5 17 PA 22MnB5 5 34 PA 22MnB5 4

PA USIBOR Not possible With adhesive With adhesive

18 Epoxy DP800 5 - - 19 PA DP800 5 35 PA DP800 4 20 PA 22MnB5 3 36 PA 22MnB5 5

Blind rivet Blind rivet

No adhesive Mtrl 1 Mtrl 2 quantity No adhesive Mtrl 1 Mtrl 2 quantity POP al PA USIBOR 5 POP al PA USIBOR 4 POP st PA USIBOR 4

POP st mandrel kept PA USIBOR 4

The tests were performed on a Zwick/Roell Zmart Pro instron 4505 tensile testing machine

seen in Figure 39. The specimens were pulled until rupture with a constant crosshead test

speed of 10 mm/min. During testing, a load cell calibrated for 100 KN was used to measure

the force and extensometers were used to measure displacement. Both, force and

displacement were logged.

For the lap-shear testing, specimens were clamped directly at the crosshead clamps which can

be seen in Figure 39b. For the tensile testing shim plates (shown by the red arrows), of the

same thickness as the testing coupons, were used to eliminate the moment forces.

34

Figure 39: a) Zwick/Roell Zmart Pro instron 4505 tensile testing machine

b) Close-up of the crossheads clamping a lap-shear test specimen

The cross-tension specimens were first mounted into the cross-tension tool seen in Figure 40

and then clamped in the crossheads of the testing machine. Since there was no way to use the

extensometers in this configuration the crosshead displacement was logged.

Figure 40: a) The cross-tension tool b) Cross tension tool clamped by the crossheads

3.1 Cross sections

To evaluate the cross section of the joints, the joints were molded in a transparent epoxy resin

and grinded down to the center of the joint with paper grit P4000. The epoxy resin cured over

night at room temperature and was therefore not influencing temperature sensitive materials.

Pictures of the cross sections were taken with an AXIO Zoom.V16 and Zeiss 5 megapixel

AxioCam ICc color camera. This was done for every material / joining method combination.

35

4 Results and discussion In Figure 41 the average maximum force of each method / material combination is

summarized, including both shear and cross tension tests. The abbreviations NA and WA

stand for no adhesive and with adhesive, respectively. In general, the joints with adhesive

bond perform better. A significant increase can be seen for the lap-shear testing where adding

adhesive could result in almost 5 times higher lap shear strength. For the cross-tension tests,

the adhesive was increasing the max. force with only about 20%.

Figure 41: Bar graphs showing the average values with the scatter between minimum and maximum

value for the different methods

It was not possible to use FEW on Usibor since the coating with its high melting point. The

coating disabled proper surface activation of the steel sheet with the friction heat generated

(see Figure 42). This is the explanation why no test specimens were made with FEW on

Usibor.

36

Figure 42: FEW joint between CFRP and Usibor, the coating has disabled proper surface activation

When using FEW to join a CFR epoxy, small particles and hazardous dusts of carbon fiber

and epoxy was created. This is due to the thermoset epoxy not having a melting point and

therefore dusts are created. Such dust and particles are most likely not acceptable within the

automotive industry. However, specimens were manufactured for one material combination

with epoxy composite and only tested in lap-shear, to investigate the joint strength.

The highest lap-shear joint strengths were obtained when CFR epoxy was bonded with

adhesive and various mechanical joining methods. The same joint strength could not be

reached with polyamide polymer due to the low surface energy of the material. This low

energy makes is difficult for the adhesive to bond. In addition to this, the used epoxy

composite had a much coarser surface which allowed the adhesive to create a better

attachment and fail cohesively instead of adhesively, like it did with the polyamide

composite, resulting in greater joint strengths.

When comparing FEW and RIVTAC it can be seen that FEW reached a higher average joint

strength in both lap-shear and cross tension for almost all the material combinations.

However for the epoxy + DP800 joints with adhesive, the FEW joints were weaker than the

RIVTAC joints. A theory for why it was so is that during the FEW joining process heat is

generated which could degrade / destroy the adhesive. Since that is the main reason for the

high joint strength in the CFR epoxy material combination, this can result in a lower strength.

The purple bars in Figure 41 show the average joint strength for blind rivets. The first set of

blind rivet joints were made out of aluminum rivets which resulted in low lap-shear and cross

tension joint strength. After this, lap-shear tests with blind rivets made out of steel were

performed showing higher performance but still lower than other methods. However, a

significant increase of lap-shear force could be achieved with replacing aluminum rivets with

steel ones. In an ordinary blind rivet, the mandrel is removed resulting in the typical hole in a

blind rivet. The shear strength of the blind rivet could be increased if the mandrel instead was

kept intact inside the rivet. So another set of steel blind rivet specimens were produced and

instead joined so that the steel mandrel of the rivet was kept intact inside the rivet. These

specimens reached a lap-shear load close to the ones manufactured with RIVTAC.

In the following sections the load curves for each method are compared by taking one

representative curve from each test series. Thereafter cross sections and the failure modes for

every specimen are presented. All load curves can be found in Appendix A.

4.1 Lap-Shear tests

In Figure 43 the representative curves for CFR PA 6.6 and DP800 steel manufactures with the

FEW and RIVTAC is shown. When comparing the two methods it can be seen that the FEW

joint can carry higher loads, but fails at a lower displacement in both the hybrid and non-

hybrid configuration.

In the hybrid methods the load curves reaches a distinct peak load followed by a load drop

and this symbolizes the failure of the adhesive. This load drop is relatively small and shows

37

that the bonding of the adhesive on the thermoplastic PA6.6 is poor. However, it still accounts

for the maximum load of the RIVTAC hybrid method.

Figure 43: representative lap-shear curves for the methods used to join CFR polyamide to DP800

steel

With CFR epoxy and DP800 steel it is the same that the RIVTAC joints have a more ductile

break than FEW. However, the adhesive in combination with the epoxy results in the much

higher peak load for both hybrid methods. When comparing the epoxy results with the PA

6.6, it can be seen that the bonding of the adhesive is much better on the thermoset.

Figure 44: representative lap-shear curves for the methods used to join CFR epoxy to DP800 steel

When comparing RIVTAC with FEW for the higher strength steel 22MnB5 (Figure 45 and

Figure 46) it can be seen that the RIVTAC fails at much higher displacement, but FEW

reaches higher peaks (in combination with CFR polyamide since FEW was not tested with

22MnB5+epoxy). However comparing the two RIVTAC joints it shows that the adhesive

allowed a much more ductile break for the RIVTAC hybrid method. The opposite was seen

with CFR epoxy.

0

2

4

6

8

0 2 4 6 8 10

Load

[kN

]

Displacement [mm]

PA 6.6 + DP800

Rivtac - No Adhesive FEW - No AdhesiveRivtac - With Adhesive FEW - With Adhesive

0

10

20

30

0 2 4 6 8 10

Load

[kN

]

Displacement [mm]

Epoxy + DP800


38

Figure 45: representative lap-shear curves for the methods used to join CFR polyamide to 22MnB5

steel

Figure 46: representative lap-shear curves for the methods used to join CFR epoxy to 22MnB5 steel

In Figure 47 the blind rivet tests were presented together with RIVTAC but no FEW results

were presented since as mentioned earlier the FEW could not be used on Usibor. The blind

rivet performed worst. If the steel mandrel was kept intact instead, the load carrying capacity

almost reached the same level as specimens joined with RIVTAC. The RIVTAC hybrid had a

much more ductile failure than the non-hybrid RIVTAC.

0

2

4

6

8

10

0 2 4 6 8 10 12 14 16

Load

[kN

]

Displacement [mm]

PA 6.6 + 22MnB5


0

10

20

30

0 2 4 6 8 10 12 14 16

Load

[kN

]

Displacement [mm]

Epoxy + 22MnB5

Rivtac - No Adhesive Rivtac - With Adhesive

39

Figure 47: representative lap-shear curves for the methods used to join CFR polyamide to Usibor

steel

Figure 48: representative lap-shear curves for the methods used to join CFR epoxy to Usibor steel

4.2 Cross-Tension tests

In this section representative cross tension curves for each method and material combination

are presented.

In general it can be summarized that the adhesive did not improve the joint significantly. The

biggest effect can be seen for the material combinations with CFR epoxy where it resulted in

small spikes in the beginning of the load curves.

Other than FEW being a little bit stronger there was no other significant difference between

the methods.

0

2

4

6

8

0 2 4 6 8 10 12 14 16 18

Load

[kN

]

Displacement [mm]

PA 6.6 - Usibor

Rivtac - No Adhesive Blind riv Alu - No AdhesiveRivtac - With Adhesive Blind riv steel - No AdhesiveBlind riv steel mandrel left - No Adhesive

0

10

20

30

0 2 4 6 8 10 12 14 16 18

Load

[kN

]

Displacement [mm]

Epoxy + Usibor


40

Figure 49: representative cross-tension curves for the methods used to join CFR polyamide to DP800

steel

Figure 50: representative cross-tension curves for the methods used to join CFR epoxy to DP800 steel

Figure 51: representative cross-tension curves for the methods used to join CFR polyamide to

22MnB5 steel

0

2

4

6

0 5 10 15 20 25

Load

[kN

]

Displacement [mm]

Cross - PA 6.6 + DP800


0

2

4

6

0 5 10 15 20 25

Load

[kN

]

Displacement [mm]

Cross - Epoxy + DP800


0

2

4

6

0 5 10 15 20 25

Load

[kN

]

Displacement [mm]

Cross - PA 6.6 + 22MnB5

Rivtac - No Adhesive FEW - No Adhesive

41

Figure 52: representative cross-tension curves for the methods used to join CFR epoxy to 22MnB5

steel

Figure 53: representative cross-tension curves for the methods used to join CFR polyamide to Usibor

steel

Figure 54: representative cross-tension curves for the methods used to join CFR epoxy to Usibor steel

joint

0

2

4

6

0 5 10 15 20 25

Load

[kN

]

Displacement [mm]

Cross - Epoxy + 22MnB5


0

2

4

6

0 5 10 15 20 25

Load

[kN

]

Displacement [mm]

Cross - PA 6.6 + Usibor

Rivtac - No Adhesive Blind riv Alu - No Adhesive

0

2

4

6

0 5 10 15 20 25

Load

[kN

]

Displacement [mm]

Cross - Epoxy + Usibor


42

4.3 CFR polyamide 6.6 to DP800 steel

4.3.1 RIVTAC

Figure 55 shows the cross sections of CFR polyamide to DP800 steel RIVTAC joints, with

and without adhesives. It can be seen that the deformation of the steel sheet in the setup

without adhesives leads to large hollow areas between the CFRP and the steel sheet. For the

adhesive joints these hollow areas are filled with adhesives. Delamination and cracked fibers

can be seen in the joining area for both configurations. Some deformed material is collected

under the head of the joining element.

Figure 55: Cross section of a CFR polyamide plastic to DP800 steel RIVTAC joint.

Left – No adhesive Right – With adhesive

Lap-shear

All of the specimens fail in the same way as shown in Figure 56, where the RIVTAC nail gets

tilted, enlarging the hole and being pulled out of the lower steel sheet (Figure 56). The DP800

sheet is too thin and ductile too withstand the point load that the RIVTAC nail exerts on the

metal. It is the same for the hybrid method with adhesives. However, the adhesive layer fails

abruptly first (Figure 57) resulting in a higher joint strength initially. See Figure 107 and

Figure 108 in Appendix A for respective load curves.

Figure 56: Failure modes in lap-shear testing of CFR polyamide plastic to DP800 steel RIVTAC joint.

43

Figure 57: Failure modes in lap-shear testing of CFR polyamide plastic to DP800 steel RIVTAC joint

with adhesive.

Cross tension

The general failure mode in these test series was nail pull-out of the lower metal sheet which

can be seen in Figure 58 and Figure 59. However, for one specimen without adhesive slippage

of the clamped material in the rig occurred, which means the specimen started being pulled

out of the rig and the test was cancelled so it did not fail in the interface. For the series with

adhesive, two specimens experienced nail pull-out while one failed in the composite. The nail

remained in the metal sheet and instead got pulled through the composite. Figure 109 and

Figure 110 in Appendix A shows the load curves for these test specimens.

Figure 58: Failure modes in cross tension testing of CFR polyamide plastic to DP800 steel RIVTAC

joint.

Figure 59: Failure modes in cross tension testing of CFR polyamide plastic to DP800 steel RIVTAC

joint with adhesive.

44

4.3.2 FEW

Three different polished cross sections are presented in Figure 60. The top left figure was

made by LWF at the University of Paderborn whereas the other two pictures were taken of

cross sections made at Swerea KIMAB. All specimens show sufficient friction welding zones

but a significantly different shaped “torus”, the part of the FEW element that is plasticized

and pressed outwards into the CFRP. When looking at cross sections made at Swerea

KIMAB, it can be seen that the fibers are almost in contact with the cylindrical part of the

joining element. In contrast to this, the picture made at LWF at the University of Paderborn

show a distance between the fibers and joining element. Some fibers are broken or deflected

by the toruses but there are no fibers deflected into the head cavity in any of the cross

sections. There is little or no deformation of the steel sheet or delaminations in the CFRP

sheet. When comparing the torus of the two specimens made at Swerea KIMAB with each

other, it can be seen a slightly difference in shape. The hybrid method with adhesives seems

to have a “colder” torus which is deflected downwards by the CFRP. This difference could be

a due to the adhesive.

Figure 60: Cross section of a CFR polyamide plastic to DP800 steel FEW joint.


Lap-shear

Two failure modes with FEW joints of CFRP to DP800 appeared during the lap-shear testing

(see Figure 61 and Figure 62). During the welding zone failure the joining element failed,

which occurred in specimen 3-5 for the non-hybrid method and 1-2 for the hybrid. The second

failure mode was plug failure which happened to the remaining specimens. Both of these

45

failure modes occurs in the heat affected zone (HAZ). This area has experienced a heat

treatment which was unfavorable for the joining element and the steel sheet material. An

unwanted annealing due to the heat input results in a change in microstructure and weaker

material. The adhesive in the hybrid method failed before the load curve reached its

maximum, unlike for the RIVTAC hybrid where the adhesive was the reason for the

maximum value.

There is a larger scatter in the load curves for FEW (see Figure 111 and Figure 112) than in

the load curves for RIVTAC (see Figure 107 and Figure 108) and the deformation to failure

also varies more for FEW. However, all of the lap-shear RIVTAC joints failed because the

steel sheet was deformed. This would imply a high joint strength since generally joints are the

weakest points. Even so, the FEW joints reached a higher average joint strength. The reason is

believed to be because the RIVTAC nail body functions as a lever exerting a force on the

underside of the steel sheet. Also since the RIVTAC nail body has a smaller cross section area

than the FEW the force that it exerts is gathered in a smaller area creating a larger pressure

leading to sheet material deformation already at lower loads.

Figure 61: Failure modes in lap-shear testing of CFR polyamide plastic to DP800 steel FEW joint.

Figure 62: Failure modes in lap-shear testing of CFR polyamide plastic to DP800 steel FEW joint

with adhesive

Cross tension

Figure 63 and Figure 64 shows the failure modes for the cross tension testing. For one

specimen in both test series, slippage in the rig occurred. All remaining specimens failed in

the composite material. The composite failed due to bending. If the plate thickness would be

increased, it is likely that the joint would fail instead.

46

Figure 63: Failure modes in cross tension testing of CFR polyamide plastic to DP800 steel FEW joint.

Figure 64: Failure modes in cross tension testing of CFR polyamide plastic to DP800 steel FEW joint

with adhesives.

4.4 CFR Epoxy to DP800 steel

4.4.1 RIVTAC

Figure 65 presents cross sections of the CFR Epoxy polymer joined with RIVTAC to DP800

steel. The steel plate is deformed resulting in a gap between the composite sheet and the steel

plate. This gap is empty for the specimens without adhesives and filled up with adhesives for

the hybrid specimens. This effect is the same as in Figure 55 where RIVTAC was used to join

CFR PA 6.6 to DP 800. When comparing the thermoplastic with the thermoset it can be seen

that there is not as much material in the head groove for the thermoset. The thermoset has a

higher stiffness and strength than the thermoplastic. In addition to this, the thermoset is not

that sensitive to temperature changes. This could result in less material deformation and

therefore less material collected in the head groove.

Figure 65: Cross section of a CFR epoxy polymer to DP800 steel RIVTAC joint.


47

Lap-shear

In Figure 66 and Figure 67 the lap shear tested RIVTAC specimens for the non-hybrid

material and hybrid material, respectively are presented. Similarly as the already presented

RIVTAC specimens, the nail gets tilted and enlarges the hole in both materials. As it can be

seen from the figures, the enlarging of the hole was much more distinct in the composite

material. As loading is progressed, the nail gets pulled out of the steel sheet.

When comparing the hybrid specimens with the non-hybrid ones, the hybrid specimens had a

more abrupt failure. This abrupt failure is a result from the adhesive failure. However, the

adhesive gives the specimen higher joint strength (see Figure 115 and Figure 116 in Appendix

A). It should be mentioned that one specimen had a different failure mode where the nail

crushed the polymer and was pulled through the composite material. This could be a result of

varying composite material in the joining area. However, this was not investigated in detail.

Figure 66: Failure modes in lap-shear testing of CFR epoxy polymer to DP800 steel RIVTAC joint.

Figure 67: Failure modes in lap-shear testing of CFR epoxy polymer to DP800 steel RIVTAC joint

with adhesive.

Cross tension

Figure 68 and Figure 69 present the failed cross tension specimens for the non-hybrid and

hybrid specimens, respectively. Nail pull-out of the metal sheet was the most common failure

mode of the non-hybrid RIVTAC. Specimen number 2 also failed by nail pull-out although

the plates are not completely separated. For the hybrid material, specimen number 1 was not

completely pulled apart. However, for two of the hybrid specimens the nail was pulled

through the composite material. The load – displacement curves for the RIVTAC specimens

were presented in Figure 50. When comparing the hybrid with the non-hybrid response, the

difference was very small. During a cross tension test of a hybrid specimen, the steel plate

was bended resulting in peel forces in the adhesive. It is well known that adhesives cannot

carry peel forces. In addition to this, it looks like the adhesion of the adhesive on the steel was

poor. In other words, there is almost no adhesive on the steel part. The combination of the

above mentioned reasons could result in a relatively small difference between the hybrid and

48

the non-hybrid configuration. However, when comparing the hybrid with the non- hybrid

joints, the added adhesive resulted in a different failure mode.

Figure 68: Failure modes in cross tension testing of CFR epoxy polymer to DP800 steel RIVTAC joint

Figure 69: Failure modes in cross tension testing of CFR epoxy polymer to DP800 steel RIVTAC joint

with adhesive.

4.4.2 FEW

Figure 70 shows a cross section of a FEW CFR epoxy polymer and DP800 steel joint without

adhesive made at LWF at the University of Paderborn. The cross section shows a relatively

large welding zone and no deformations of the steel sheet. As mentioned above, FEW on CFR

epoxy creates hazardous dusts of epoxy and carbon fiber. The dusts have been removed

before pictures were taken but some particles can be seen around the head of the element.

Figure 70: Cross section of a CFR epoxy polymer to DP800 steel FEW joint.

49

Lap-shear

Figure 71 and Figure 72 presents the failed specimens with and without adhesive. The most

common failure mode was welding zone failure which occurred in all tests except for three

specimens (one without adhesive and two with adhesive). These three specimens experienced

plug failure in the steel base material. The welding zone failures look different for the two

methods since the toruses probably look different, just like for previous FEW hybrid in the

material combination DP800 and CFR polyamide. The torus seems to be colder and pressed

downwards more than in the non-hybrid method.

The hybrid method gave higher joint strength compared to the non-hybrid method (see Figure

119 and Figure 120). In addition to this, the FEW methods showed a larger scatter when

compared to RIVTAC method (see Figure 115 and Figure 116).

Figure 71: Failure modes in lap-shear testing of CFR epoxy polymer to DP800 steel FEW joint.

Figure 72: Failure modes in lap-shear testing of CFR epoxy polymer to DP800 steel FEW joint with

adhesive.

4.5 CFR polyamide 6.6 to 22MnB5 steel

4.5.1 RIVTAC

For this material combination, the RIVTAC method has been improved by LWF at the

University of Paderborn for the joining of composites and boron steel. To get a kind of

punching effect, the nail tip has been modified to a blunt shape leading to a more stable

joining process causing less damage to the fiber composite.

50

Figure 73 is presenting the cross sections for the hybrid and non-hybrid configuration. It can

be seen that there are no large cavities between steel and CFRP. Recall the RIVTAC joints

with CFRP and DP800 (see Figure 55 and Figure 65) where a gap between the CRFP and

DP800 can be seen. The DP800 has the same young’s modulus and thickness as the 22MnB5

steel resulting in the same bending stiffness. The only difference is the shape of the nail which

results in punching for the 22MnB5 material.

There is a lot of material collected in the head groove and it is a good looking rim hole

without any cracks. However, it seems that the profile peaks of the grooves on the nail body

get a little deformed from the process when punched through the high strength steel (see

Figure 75). This deformation will most likely reduce the nail pullout force.

Figure 73: Cross section of a CFR polyamide plastic to 22MnB5 steel RIVTAC joint.


Figure 74: shows the deformation of the profile peaks of the grooves

Lap-shear

In the RIVTAC without adhesives test series, the failure modes was either nail pull-out of the

steel sheet or the nail itself failing (see Figure 75). For the hybrid method only 1 out of 4 nails

failed and the remaining 3 specimens failed in the fiber composite after the abrupt failure of

the adhesives (see Figure 76). The adhesive seems to improve the joining between the nail

and metal sheet since no nail pull-out occurred. The adhesion on the steel parts seems to be

better than the adhesion on the CFR PA. This is not surprising since the thermoplastic PA has

a low surface energy which makes wetting difficult and results in a bad adhesion on the PA.

However, the adhesion is improving the joint and results in a different failure mode (see load

51

curve in Figure 122). The abrupt failure of the adhesive leads to an impulse force resulting in

that the nail hitting the polyamide plastic and damaging it. Then the nail continues to plough

through the composite as the crack propagates. However, the average joint strength is still

larger for the hybrid method.

Figure 75: Failure modes in lap-shear testing of CFR polyamide plastic to 22MnB5 steel RIVTAC

joint.

Figure 76: Failure modes in lap-shear testing of CFR polyamide plastic to 22MnB5 steel RIVTAC

joint with adhesives.

Cross tension

Two different failure modes occurred for the RIVTAC specimens without adhesive (see

Figure 77). One single specimen failed because the nail was pulled out of the steel. The three

remaining specimens failed in the composite material near the nail head. Two specimens with

composite material failure show a bending failure of the plate as well. This bending failure

could be a post failure. In other words, first the specimen could fail around the nail followed

by a bending failure.

For the RIVTAC hybrid specimens, three different failure modes occurred (see Figure 78).

One specimen failed because the nail was pulled-out through the composite material and

another experienced bending failure of the composite plate. The remaining two specimens

failed by nail pull-out of the steel plate or composite material failure.

52

Figure 77: Failure modes in cross tension testing of CFR polyamide plastic to 22MnB5 steel RIVTAC

joint.

Figure 78: Failure modes in cross tension testing of CFR polyamide plastic to 22MnB5 steel RIVTAC


4.5.2 FEW

Figure 79 shows the cross section for the FEW - CFR PA - 22MnB5 hybrid and non-hybrid

configuration. Both cross section show a sufficiently large welding zone which should lead to

good mechanical properties of the joint. However, the torus seems to have been colder than in

the previous material combinations with the lower strength steels. Due to this it is deflected

by the CFRP inwards towards the element body or for the hybrid case downwards. When

looking at the composite material, only a small amount of broken fibers can be seen. Some

composite material was collected under the element head.

Figure 79: Cross section of a CFR polyamide plastic to 22MnB5 steel FEW joint.


53

Lap-shear

Like with the lower strength steels two failure modes were observed for FEW of CFR PA and

22MnB5: welding zone failure and plug failure (see Figure 80 and Figure 81). Both failures

occurred in the HAZ. The plug failure in the steel sheet material occurred only in the non-

hybrid joints.

In the load curves (Figure 125 and Figure 126) it can be seen that the non-hybrid FEW could

withstand larger loads than the hybrid method. This indicates that the adhesive has a negative

impact during the joining process resulting in a weaker joint strength for this material

combination in FEW. Another hypothesis could be that when the impulse force from the

abrupt adhesive failure creates a crack which will then require less force to pull until rupture.

Figure 80: Failure modes in lap-shear testing of CFR polyamide plastic to 22MnB5 steel FEW joint.

Figure 81: Failure modes in lap-shear testing of CFR polyamide plastic to 22MnB5 steel FEW joint

with adhesives.

54

Cross tension

For the cross tension testing, it can be seen that almost all specimens failed in the fiber

composite (see Figure 82 and Figure 83). There is only one exception for the hybrid material

which failed in the FEW element.

Figure 82: Failure modes in cross tension testing of CFR polyamide plastic to 22MnB5 steel FEW

joint.

Figure 83: Failure modes in cross tension testing of CFR polyamide plastic to 22MnB5 steel FEW


4.6 CFR Epoxy to 22MnB5 steel

4.6.1 RIVTAC

For this configuration, the modified RIVTAC nail tip with the flat tip was used. The polished

cross sections for both the hybrid and non-hybrid method are presented in Figure 84. The

RIVTAC with the modified nail is punching the material and therefore the bottom sheet does

not deform. This results in no gap between the two materials. Some composite material is

deformed into the head groove but it is less than in the polyamide and 22MnB5 combination.

The profile peaks of the grooves on the nail body get a little deformed from the process (see

Figure 85).

55

Figure 84: Cross section of a CFR epoxy polymer to 22MnB5 steel RIVTAC joint.


Figure 85: shows the deformation of the profile peaks of the grooves

Lap-shear

In Figure 86 the lap-shear tested specimens for RIVTAC are presented where it can be seen

that specimen 1, 3 and 4 fails in the fiber composite while for specimen 2 and 5 the RIVTAC

nail failed. For specimen 2 the nail failed by rupture whereas in specimen 5 the nail deformed

the composite material before it failed.

The hybrid version for this material combination was the first hybrid specimens which was

lap-shear tested. At that time the tensile testing machine was set with a stop criteria which got

active at a force drop of 80 % of Fmax. Since this criterion is reached directly after the abrupt

failure of the adhesive, the first test was stopped and the specimen was not pulled until

rupture. Thereafter the settings were changed resulting in RIVTAC nail failure after the

abrupt adhesive failure for the remaining specimens. Also here the epoxy + adhesives

combination results in high joint strengths (see Figure 130).

However, these results are reversed when compared to the results for the RIVTAC methods

for the CFR polyamide + 22MnB5 steel combination from the previous section. There the

hybrid method failed in the composite by the nail ploughing through the material (see Figure

76) and the non-hybrid RIVTAC experienced nail failure and nail pull-out instead (see Figure

75). For this combination the theory (from the CFR polyamide + 22MnB5 material

56

combination) of the matrix cracking from the impulse force due to abrupt adhesive failure

does not seem to match, since the main failure mode is nail failure. However the impulse

force here, which is larger due to the CFR epoxy and adhesive combination, could be large

enough to instead create a crack in the nail and lead to the rupture of the nail instead of the

composite.

Figure 86: Failure modes in lap-shear testing of CFR epoxy polymer to 22MnB5 steel RIVTAC joint.

Figure 87: Failure modes in lap-shear testing of CFR epoxy polymer to 22MnB5 steel RIVTAC joint

with adhesives.

Cross tension

For both, the RIVTAC with adhesives and without, all of the specimens experienced

composite failure where the nail head has been pulled through the fiber composite during the

cross-tension testing (see Figure 88 and Figure 89). Comparing these results to the cross

tension results in CFR polyamide + 22MnB5 steel combination above, where a total of 3 out

of 8 specimens failed because of nail pull-out. It implies that either the connection between

the nail and metal sheet is improved when the material combination contains epoxy matrix

instead of polyamide or that the RIVTAC process damages the epoxy matrix more compared

to polyamide. However, the epoxy matrix does have a higher stiffness and should result in a

more brittle failure but it also resulted in higher maximum joints strengths for these test

specimens (see Figure 41).

57

Figure 88: Failure modes in cross tension testing of CFR epoxy polymer to 22MnB5 steel RIVTAC

joint.

Figure 89: Failure modes in cross tension testing of CFR epoxy polymer to 22MnB5 steel RIVTAC


4.7 CFR polyamide 6.6 to Usibor steel

4.7.1 RIVTAC

In Figure 90 it can be seen that RIVTAC without adhesives had a similar cross section as the

CRF PA - 22MnB5 combination. It shows no large cavities and the head groove is filled with

material. In contrast to this, RIVTAC with adhesives showed larger deformations which

created some pores in the adhesive and also less material collected in the head groove when

compared to the 22MnB5 and CFR polyamide combination (Figure 73).

58

Figure 90: Cross section of a CFR polyamide plastic to Usibor steel RIVTAC joint.


Similar as for the other material combinations with high strength steel, the profile peaks of the

grooves on the nail body get a little deformed from the process. On the bottom of the

specimen, some areal damage of the Al-Si coating around the rim hole area could be detected

(seen in Figure 91). This damage was caused by the deformation of the steel sheet where the

brittle Al-Si coating got damaged.

Figure 91: Shows the deformation of the profile peaks of the grooves and damage of the Al-Si coating

of Usibor caused by the RIVTAC process.

Lap-shear

All RIVTAC specimens without adhesives showed the same failure mode, which was failure

of the RIVTAC nail (see Figure 92). For the hybrid method, two specimens failed in the

composite after the abrupt adhesive failure and only 1 test specimen experienced RIVTAC

nail failure, although after it had started to tear through the fiber composite (see Figure 93).

The respective load curves can be found in Figure 133 and Figure 134.

These results are similar to the ones obtained for the CFR polyamide + 22MnB5 combination

in the previous section (4.5.1) where the results imply that the adhesive improves the

connection between nail and metal sheet. However, since the only failure in the non-hybrid

method was nail failure, it is believed that the load transmission properties are altered in a

more favorable manner for the RIVTAC nail. It undergoes less critical loads to the nail

strength and experiences nail failure less frequently compared to the non-hybrid RIVTAC

(when in combination with CFR polyamide).

59

Figure 92: Failure modes in lap-shear testing of CFR polyamide plastic to Usibor steel RIVTAC joint.

Figure 93: Failure modes in lap-shear testing of CFR polyamide plastic to Usibor steel RIVTAC joint

with adhesives.

Cross tension

For non-hybrid RIVTAC all of the test specimens failed in the fiber composite, two where the

nail head was pulled through and two where the composite material failed (see Figure 94).

In contrast to this, the RIVTAC hybrid showed two failure modes (see Figure 95). One where

the composite failed and the nailed head was pulled through the composite (1 and 3), the other

where the nail was pulled out of the metal (2 and 4)

60

Figure 94: Failure modes in cross tension testing of CFR polyamide plastic to Usibor steel RIVTAC

joint.

Figure 95: Failure modes in cross tension testing of CFR polyamide plastic to Usibor steel RIVTAC


4.7.2 Blind Rivet made of aluminum

Figure 96 shows a cross section of the aluminum blind rivet joint that was tested. Here a hole

has been pre-drilled in both the CFRP and metal sheet before joining. The brighter metal is

the aluminum part of the rivet while the darker “knob” is the steel mandrel. From a corrosion

point of view, using an aluminum rivet in combination with carbon fiber is probably not the

best combination. However, corrosion is not part of this work.

Figure 96: Cross section of a CFR polyamide plastic to Usibor steel blind rivet joint.

61

Lap-shear

The lap-shear testing results are shown in Figure 97 and it can be seen that the rivet failed and

that there is no particular damage or deformation to either the CFRP or the Usibor steel. This

was the weakest joint which was tested in lap-shear during this study.

Figure 97: Failure modes in lap-shear testing of CFR polyamide plastic to Usibor steel aluminum

blind rivet joint.

Cross tension

Also in the cross-tension tests this rivet had the weakest joint and all of the specimens

experienced rivet failure without any noticeable damage to the materials that were joined.

Figure 98: Failure modes in cross tension testing of CFR polyamide plastic to Usibor steel aluminum

blind rivet joint.

4.7.3 Blind Rivet made of Steel

Since the specimen with the aluminum blind rivet showed poor load bearing capacity,

specimens with rivets made out of steel was tested.

Lap-shear

Rivet failure was the one and only failure mode obtained in the lap-shear testing of the steel

blind rivet (see Figure 99). Although a twice as large joint strength compared to aluminum

blind rivets was registered and there is some noticeable damage done to the CFRP.

62

Figure 99: Failure modes in lap-shear testing of CFR polyamide plastic to Usibor steel blind rivet

made of steel joint.

4.7.4 Blind Rivet made of Steel with the mandrel kept

Since the blind rivets still had lower joint strength than the other two methods the steel

mandrel was kept intact and left inside the blind rivet in another set of test specimens.

Lap-shear

The failure mode obtained was the same as before; rivet failure and can be seen in Figure 100.

However, the joint strength was increased with about 35% when the steel mandrel was kept

intact and reaches similar levels as specimens joint with the RIVTAC and FEW methods.

Figure 100: Failure modes in lap-shear testing of CFR polyamide plastic to Usibor steel blind rivet

made of steel with mandrel kept joint.

4.8 CFR Epoxy to Usibor steel

Comparing to the combination with CFR polyamide (see Figure 90), here it can be seen that

there are less delaminations and that the steel sheet did not deform as much for the hybrid

method (Figure 101). This leads to less hollow areas between the sheets. Although there is a

smaller amount of material deformed into the head groove in both of the RIVTAC methods.

Figure 102 shows the damage done to the Al-Si coating by the RIVTAC process.

63

Figure 101: Cross section of a CFR epoxy polymer to Usibor steel RIVTAC joint.


Figure 102: Damage of the Al-Si coating of Usibor caused by the RIVTAC process.

Lap-shear

For specimen 1, 2 and 4 for RIVTAC without adhesive, first the nail started to plow through

the CRFP and thereafter the RIVTAC nail failed (see Figure 103). For specimen 3, the nail

was pulled through the composite material and did not fail.

It was the same for the RIVTAC hybrid method. After the adhesive failed abruptly, the nail

started plowing through the fiber composite until it failed. There is one exception, specimen

4 where the nail was intact but the composite material lost its load bearing capacity (see

Figure 104). Higher joint strength was obtained again with the epoxy + adhesives

combination.

64

Figure 103: Failure modes in lap-shear testing of CFR Epoxy polymer to Usibor steel RIVTAC joint.

Figure 104: Failure modes in lap-shear testing of CFR Epoxy polymer to Usibor steel RIVTAC joint

with adhesives.

Cross tension

For cross tension tests, the unanimous failure mode for both RIVTAC hybrid and non-hybrid

was composite failure where the nail head was pulled through the CFRP (see Figure 105 and

Figure 106).

65

Figure 105: Failure modes in cross-tension testing of CFR Epoxy polymer to Usibor steel RIVTAC

joint.

Figure 106: Failure modes in cross-tension testing of CFR Epoxy polymer to Usibor steel RIVTAC


66

5 Conclusions & Summary Highest joint strength was achieved when combining CFR epoxy with adhesives. The

thermoset epoxy had a higher surface energy which made wetting easier. Therefore the

failure was cohesively and not adhesively at the interface.

LWFs (at the University of Paderborn) FEW method could not create joints with

Usibor. This because the Al-Si coating was difficult to melt with the generated friction

heat and therefore a proper surface activation was disabled.

Using FEW on CFR epoxy did create hazardous dusts of carbon fiber and epoxy due

to non-existing melting point of the thermoset.

In the lap-shear CFR epoxy + DP800 combination the RIVTAC with adhesives

showed a higher average joint strength than FEW with adhesives. It could be that the

adhesive experiences degradation from the heat generated by the FEW method

resulting in a weaker adhesive bond and lower joint strength, since the reason for high

joint strength in lap-shear combinations with CFR epoxy was the adhesive bond.

For the CFR polyamide + 22MnB5 material combination, the FEW hybrid showed a

lower average joint strength than the FEW alone. The adhesive might have a negative

impact during the joining process resulting in a weaker joint for this material

combination. In the cross sections the FEW torus in the hybrid joints generally seemed

to be colder compared to the FEW torus in the joints without adhesives, which could

be a reason for the weaker joint. This might be due to the adhesive decreasing the

friction, resulting in less frictional heat generated. Changing joining process

parameters for hybrid joining should be considered. In addition to this when the

adhesive fails abruptly it might result in an impulse force hitting the FEW element

which could create a small crack which will require less force to pull until rupture.

RIVTAC in combination with adhesive do reach higher joint strength than without

adhesive, for these material combinations.

A theory when joining CFR polyamide with RIVTAC was developed. When in

combination with adhesive, the load transmission properties are altered in a more

favorable manner for the RIVTAC nail. It undergoes less critical loads to the nail

strength and experiences nail failure less frequently compared to the non-hybrid

RIVTAC. This theory does not apply for CFR epoxy where the results imply the

opposite, however these joints do experience greater loads due to the epoxy matrix +

adhesive combination. Should be researched further.

Two failure modes happened with FEW joints in the lap-shear testing. One is plug

failure and the other is failure in welding zone. Both of these failure modes happen in

the area that is the heat affected zone (HAZ). This area has probably experienced a

heat treatment which was unfavorable for the joining element and the steel sheet

material. An unwanted annealing due to the heat input results in a change in

microstructure and weaker material.

For the blind riveting method, the best result was achieved with steel rivets where the

mandrel body was kept left inside the rived. Joint strength similar as for other methods

could be achieved with a manufacturing method which is less cost intensive.

In general FEW joints achieved a higher average joint strength in both lap-shear and

cross tension.

67

6 Future Work EJOTs friction element welding method (EJOWELD) can today weld USIBOR combinations,

and it would be interesting to investigate and compare the joint strengths of the CFR

polyamide and USIBOR combination not investigated for FEW in this report.

The impact of the adhesive on to the failure mode for RIVTAC should be investigated further.

7 Acknowledgements First I would like to thank my supervisors at Swerea KIMAB, Karl Fahlström and Christof

Schneider, for their scientific guidance and support throughout this work. I would like to

thank Aldin Delic for teaching me the testing equipment and his help during my physical

experiments. I would also like to thank Paul Janiak for fruitful discussions and sharing of

information and Miroslava Sedlakova for guidance and help during the cross section

specimen preparation. Lastly I would also like to thank David Löveborn for good discussions

and general help.

8 References [1] “Reducing car weight and cutting down greenhouse gas emissions - Transport -

Research & Innovation - European Commission,” 2016. [Online]. Available:

http://ec.europa.eu/research/transport/projects/items/reducing_car_weight_and_cutting

_down_greenhouse_gas_emissions_en.htm.

[2] F. C. Campbell, Joining: Understanding the Basics. ASM International, 2011.

[3] SSAB, “Docol DP / DL Cold reduced dual phase steels Docol DP / DL Cold reduced

dual phase steels,” pp. 2–5, 2016.

[4] A. Worldwide, “Steels for hot stamping -Usibor® Steels for hot stamping -Usibor ®

Ultra high strength steels,” 2016.

[5] “Aluminium - Properties That Make Aluminium An Attractive Engineering Material

Properties of Aluminium That Make It An Attractive,” pp. 1–5, 2004.

[6] SWEREA KIMAB, “Svetsning av Aluminium, aluminiumlegeringar,” pp. 1–26, 2013.

[7] K. Fahlström and N. Malmberg, “Challenges using mixed materials , Al-Fe,” 2012.

[8] B. T. (B. T. Aström, Manufacturing of polymer composites. Chapman & Hall, 1997.

[9] “Contemporary Materials,” 2016. [Online]. Available:

http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0/s16-09-

contemporary-materials.html.

[10] “10 Tricks Engineers Need to Know About Fasteners – EngineerDog,” 2016. [Online].

Available: https://engineerdog.com/2015/01/11/10-tricks-engineers-need-to-know-

about-fasteners/.

[11] S. T. Amancio Filho, “Friction Riveting development and analysis of a new joining

technique for polymer-metal multi-material structures,” Riv. Ital. della Saldatura, vol.

63, no. 2, pp. 197–208, 2011.

[12] G. Meschut, V. Janzen, and T. Olfermann, “Innovative and highly productive joining

technologies for multi-material lightweight car body structures,” J. Mater. Eng.

Perform., vol. 23, no. 5, pp. 1515–1523, 2014.

[13] “RIVTAC ® Automation P High-speed joining Innovative and flexible.”

[14] “RIVTAC ® - The innovative high-speed joining process Reliable !”

68

[15] “Joining challenges in automotive lightweight applications,” pp. 1–46, 2014.

[16] I. Gotlib, “An Analysis of High-Speed Impact Nailing for Lightweight Automotive

Structures,” 2014.

[17] H. Schubert and J. Möhring, “Flexible material joining technology for tomorrow ` s

innovative lightweight design Today ´ s challenges in automobil production,” 2012.

[18] G. Meschut, O. Hahn, V. Janzen, and T. Olfermann, “Innovative joining technologies

for multi-material structures,” Weld. World, vol. 58, no. 1, pp. 65–75, 2014.

[19] K. Fahlström, “Forskningstrender inom mekanisk fogning vid ” Centre for Joining and

Structures ” R & D.”

[20] Henkel, “EAA Aluminium Automotive Manual – Joining,” Eur. Alum. Assoc., pp. 1–

31, 2015.

[21] S. Schrodt, “Herzlich willkommen zum lieferanten-innovationstag 2015 18.,” 2015, no.

November.

[22] S. Huland, M. Möller, M. Franke, and T. Vakalopoulos, “THE ELECTRIFICATION

OF THE PASSAT The GTE,” ViaVision, no. July, 2015.

[23] “Bolt Depot - About Blind Rivets,” 2016. [Online]. Available:

https://www.boltdepot.com/fastener-information/rivets/About-Blind-Rivets.aspx.

[24] F. Podlesak, A. Hälsig, K. Höfer, R. Kaboli, and P. Mayr, “Spin-blind-riveting: secure

joining of plastic with metal,” Weld. World, vol. 59, no. 6, pp. 927–932, 2015.

[25] “Marshall Sales, Inc., Your full line fastener source for brands you know and trust. -

Products: Rivets.” [Online]. Available: http://www.marshallsales.com/pr_rivets.php.

[26] “Carbon core: 2016 BMW 7-series gets carbon-fiber implants | Autoweek.” [Online].

Available: http://autoweek.com/article/car-news/carbon-core-2016-bmw-7-series-gets-

carbon-fiber-implants.

[27] H. Duan, G. Han, M. Wang, X. Zhang, Z. Liu, and Z. Liu, “Rotation friction pressing

riveting of AZ31 magnesium alloy sheet,” Mater. Des., vol. 54, pp. 414–424, 2014.

[28] A. Chrysanthou, “Self-Piercing Riveting Properties, Processes and Applications, 1.

Introduction,” in Self-Piercing Riveting, 2014, pp. 1–7.

[29] S. A. Westgate, R. Doo, F. Liebrecht, S. Braeunling, and T. Mattsson, “The

development of lightweight self-piercing riveting equipment (March 2001),” 2001.

[Online]. Available: http://www.twi-global.com/technical-knowledge/published-

papers/the-development-of-lightweight-self-piercing-riveting-equipment-march-2001/.

[30] “RIVTEC - Self Piercing Rivets.” [Online]. Available: http://www.rivtec.com.au/157-

self-piercing-rivets.

[31] “TWI - FAQ: What is self-piercing riveting used for?,” 2016. [Online]. Available:

http://www.twi-global.com/technical-knowledge/faqs/process-faqs/faq-what-is-self-

piercing-riveting-used-for/.

[32] P. Janiak and T. Vuoristo, “Fatigue testing of mechanical joints,” 2016.

[33] European Aluminium Association, “Car body – Body structures,” Alum. Automot.

Man., vol. 6, no. 2011, pp. 1–84, 2013.

[34] “Fatigue testing of mechanical joints - Svetskommissionen,” 2016. [Online]. Available:

http://www.svets.se/fou/rapporterfrancjs/fatiguetestingofmechanicaljoints.4.74c144581

4b329feaf91467.html.

[35] Y. Ma, Y. Li, W. Hu, M. Lou, and Z. Lin, “Modeling of Friction Self-piercing Riveting

(F-SPR) of Aluminum to Magnesium,” J. Manuf. Sci. Eng., vol. 138, no. June, pp. 1–9,

69

2015.

[36] Y. Li, Z. Wei, Z. Wang, and Y. Li, “Friction Self-Piercing Riveting of Aluminum

Alloy AA6061-T6 to Magnesium Alloy AZ31B,” J. Manuf. Sci. Eng., vol. 135, no. 6,

p. 61007, Nov. 2013.

[37] J. Mucha, “The numerical analysis of the effect of the joining process parameters on

self-piercing riveting using the solid rivet,” Arch. Civ. Mech. Eng., vol. 14, no. 3, pp.

444–454, 2014.

[38] “Flow-Drilling Screws Help Carmakers Shed Weight | 2016-02-01 | Assembly

Magazine.” [Online]. Available: http://www.assemblymag.com/articles/93220-flow-

drilling-screws-help-carmakers-shed-weight.

[39] S. Engineer and G. Nygren, “EJOT FDS ® – EJOWELD ® – Flytborrande

skruvsystem Friktionssvetssystem för höghållfasta material,” 2010.

[40] “EJOT FDS® - Flow drilling screw | Ejot & Avdel.” [Online]. Available:

http://www.ejot-avdel.se/sv/product/show/262.

[41] P. Janiak, “Questionnaire EJOT - FDS - med svar.”

[42] J. Camillo, “Fastening: Going With the Flow | 2011-03-31 | Assembly Magazine,”

2011. [Online]. Available: http://www.assemblymag.com/articles/88634-fastening-

going-with-the-flow.

[43] “TWI - FAQ: What is clinching and how does it work?,” 2016. [Online]. Available:

http://www.twi-global.com/technical-knowledge/faqs/process-faqs/faq-what-is-

clinching-and-how-does-it-work/.

[44] “FAQ: What is clinching used for?,” 2016. [Online]. Available: http://www.twi-

global.com/technical-knowledge/faqs/process-faqs/faq-what-is-clinching-used-for/.

[45] “Clinching for Sheet Metal Assembly | 2016-05-05 | Assembly Magazine.” [Online].

Available: http://www.assemblymag.com/articles/93388-clinching-for-sheet-metal-

assembly.

[46] J. P. Varis, “The suitability of clinching as a joining method for high-strength structural

steel,” J. Mater. Process. Technol., vol. 132, no. 1–3, pp. 242–249, 2003.

[47] “Fasten sheet metal efficiently with clinching technology | Norlok.” [Online].

Available: http://norlok.com/what-is-clinching/.

[48] F. Lambiase, M. Durante, and A. Di Ilio, “Fast joining of aluminum sheets with Glass

Fiber Reinforced Polymer (GFRP) by mechanical clinching,” J. Mater. Process.

Technol., vol. 236, pp. 241–251, 2016.

[49] P. Schweißtechnik, U.-P. D. Peter Mayr, D.-I. Wolf Georgi, U.-P. D. Michael Gehde,

and D.-I. Sven Friedrich, “Technizche Universität Chemnitz, Hybridfügen, Clinchen

und Schmelzkleben mittels torsionalem Ultraschall.”

[50] “TWI - FAQ: What is ultrasonic welding of thermoplastics?,” 2016. [Online].

Available: http://www.twi-global.com/technical-knowledge/faqs/process-faqs/faq-

what-is-ultrasonic-welding-of-thermoplastics/.

[51] E. E. Feistauer, R. P. M. Guimarães, T. Ebel, J. F. Dos Santos, and S. T. Amancio-

Filho, “Ultrasonic joining: A novel direct-assembly technique for metal-composite

hybrid structures,” Mater. Lett., vol. 170, pp. 1–4, 2016.

[52] “ESAB - What is friction stir welding of aluminum?,” 2016. [Online]. Available:

http://www.esabna.com/us/en/education/blog/what-is-friction-stir-welding-of-

aluminum.cfm.

70

[53] F. Khodabakhshi et al., “Microstructure-property characterization of a friction-stir

welded joint between AA5059 aluminum alloy and high density polyethylene,” Mater.

Charact., vol. 98, pp. 73–82, 2014.

[54] T. Czigány and Z. Kiss, “Friction stir welding of fiber reinforced polymer composites,”

18th Int. Conf. Compos. Mater., 2011.

[55] “TWI - FAQ: What materials can I join with friction stir welding?,” 2016. [Online].

Available: http://www.twi-global.com/technical-knowledge/faqs/process-faqs/faq-

what-materials-can-i-join-with-friction-stir-welding/.

[56] “Friction spot welding | www.bil-ibs.be,” 2016. [Online]. Available: http://www.bil-

ibs.be/en/friction-spot-welding.

[57] S. D. Dipl, “1 . Basics of LASER Physics Outline : Biomedical Optics,” pp. 1–33,

2015.

[58] J. Hägg, “Laser direct joining of CFRP and aluminum,” KTH, 2012.

[59] “FAQ: What is Surfi-Sculpt?,” 2016. [Online]. Available: http://www.twi-

global.com/technical-knowledge/faqs/process-faqs/faq-what-is-surfi-sculpt/.

[60] “Industrial laser solutions - A new method of laser surface modification - Industrial

Laser Solutions,” 2016. [Online]. Available: http://www.industrial-

lasers.com/articles/print/volume-27/issue-03/features/a-new-method-of-laser-surface-

modification.html.

[61] “TWI - FAQ: What is Comeld TM?,” 2016. [Online]. Available: http://www.twi-

global.com/technical-knowledge/faqs/process-faqs/faq-what-is-comeld-tm/.

71

9 Appendix A

9.1 CFR polyamide 6.6 to DP800 steel

9.1.1 RIVTAC

Lap-shear

Figure 107: Load curve for a CFR polyamide plastic to DP800 steel RIVTAC joint

Figure 108: Load curve for a CFR polyamide plastic to DP800 steel RIVTAC joint with adhesive

Cross tension

Figure 109: Load curve for a CFR polyamide plastic to DP800 steel RIVTAC joint

0

2

4

6

8

10

12

0 2 4 6 8 10

Load

[kN

]

Displacement [mm]

1

2

3

4

5

0

2

4

6

8

10

12

0 2 4 6 8 10

Load

[kN

]

Displacement [mm]

1

2

3

4

5

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0 5 10 15 20 25 30

Load

[kN

]

Displacement [mm]

1

2

3

4

72

Figure 110: Load curve for a CFR polyamide plastic to DP800 steel RIVTAC joint with adhesive

9.1.2 FEW

Lap-shear

Figure 111: Load curve for a CFR polyamide plastic to DP800 steel FEW joint

Figure 112: Load curve for a CFR polyamide plastic to DP800 steel FEW joint with adhesive

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0 5 10 15 20 25 30

Load

[kN

]

Displacement [mm]

1

2

3

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14 16

Load

[kN

]

Displacement [mm]

1

2

3

4

5

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14 16

Load

[kN

]

Displacement [mm]

1

2

3

4

5

73

Cross tension

Figure 113: Load curve for a CFR polyamide plastic to DP800 steel FEW joint

Figure 114: Load curve for a CFR polyamide plastic to DP800 steel FEW joint with adhesive

9.2 CFR Epoxy to DP800 steel

9.2.1 RIVTAC

Lap-shear

Figure 115: Load curve for a CFR epoxy polymer to DP800 steel RIVTAC joint

0

1

2

3

4

5

6

0 5 10 15 20 25 30

Load

[kN

]

Displacement [mm]

1

2

3

0

1

2

3

4

5

6

0 5 10 15 20 25 30

Load

[kN

]

Displacement [mm]

1

2

3

0

5

10

15

20

25

30

0 2 4 6 8 10 12

Load

[kN

]

Displacement [mm]

1

2

3

4

5

74

Figure 116: Load curve for a CFR epoxy polymer to DP800 steel RIVTAC joint with adhesive

Cross tension

Figure 117: Load curve for a CFR epoxy polymer to DP800 steel RIVTAC joint

Figure 118: Load curve for a CFR epoxy polymer to DP800 steel RIVTAC joint with adhesive

0

5

10

15

20

25

30

0 2 4 6 8 10 12

Load

[kN

]

Displacement [mm]

1

2

3

4

5

0

1

2

3

4

5

0 5 10 15 20 25 30 35

Load

[kN

]

Displacement [mm]

1

2

3

4

0

1

2

3

4

5

0 5 10 15 20 25 30 35

Load

[kN

]

Displacement [mm]

1

2

3

4

75

9.2.2 FEW

Lap-shear

Figure 119: Load curve for a CFR epoxy polymer to DP800 steel FEW joint

Figure 120: Load curve for a CFR epoxy polymer to DP800 steel FEW joint with adhesive

9.3 CFR polyamide 6.6 to 22MnB5 steel

9.3.1 RIVTAC

Lap-shear

Figure 121: Load curve for a CFR polyamide plastic to 22MnB5 steel RIVTAC joint

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14

Load

[kN

]

Displacement [mm]

1

2

3

4

5

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14

Load

[kN

]

Displacement [mm]

1

2

3

4

5

0

1

2

3

4

5

6

7

8

0 5 10 15 20 25

Load

[kN

]

Displacement [mm]

1

2

3

4

5

76

Figure 122: Load curve for a CFR polyamide plastic to 22MnB5 steel RIVTAC joint with adhesives

Cross tension

Figure 123: Load curve for a CFR polyamide plastic to 22MnB5 steel RIVTAC joint

Figure 124: Load curve for a CFR polyamide plastic to 22MnB5 steel RIVTAC joint with adhesives

0

1

2

3

4

5

6

7

8

0 5 10 15 20 25

Load

[kN

]

Displacement [mm]

1

2

3

4

0

1

2

3

4

5

0 2 4 6 8 10 12 14

Load

[kN

]

Displacement [mm]

1

2

3

4

0

1

2

3

4

5

0 2 4 6 8 10 12 14

Load

[kN

]

Displacement [mm]

1

2

3

4

77

9.3.2 FEW

Lap-shear

Figure 125: Load curve for a CFR polyamide plastic to 22MnB5 steel FEW joint.

Figure 126: Load curve for a CFR polyamide plastic to 22MnB5 steel FEW joint with adhesives.

Cross tension

Figure 127: Load curve for a CFR polyamide plastic to 22MnB5 steel FEW joint.

0

2

4

6

8

10

12

0 2 4 6 8 10

Load

[kN

]

Displacement [mm]

1

2

3

4

0

2

4

6

8

10

12

0 2 4 6 8 10

Load

[kN

]

Displacement [mm]

1

2

3

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14 16

Load

[kN

]

Displacement [mm]

1

2

3

4

78

Figure 128: Load curve for a CFR polyamide plastic to 22MnB5 steel FEW joint with adhesives.

9.4 CFR Epoxy to 22MnB5 steel

9.4.1 RIVTAC

Lap-shear

Figure 129: Load curve for a CFR epoxy polymer to 22MnB5 steel RIVTAC joint.

Figure 130: Load curve for a CFR epoxy polymer to 22MnB5 steel RIVTAC joint with adhesives

Cross tension

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14 16

Load

[kN

]

Displacement [mm]

1

2

3

4

0

5

10

15

20

25

30

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

4

5

0

5

10

15

20

25

30

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

4

79

Figure 131: Load curve for a CFR epoxy polymer to 22MnB5 steel RIVTAC joint.

Figure 132: Load curve for a CFR epoxy polymer to 22MnB5 steel RIVTAC joint with adhesives.

9.5 CFR polyamide 6.6 to Usibor steel

9.5.1 RIVTAC

Lap-shear

Figure 133: Load curve for a CFR polyamide plastic to Usibor steel RIVTAC joint.

0

1

2

3

4

5

6

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

4

0

1

2

3

4

5

6

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

4

0

1

2

3

4

5

6

7

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

4

5

80

Figure 134: Load curve for a CFR polyamide plastic to Usibor steel RIVTAC joint with adhesives.

Cross tension

Figure 135: Load curve for a CFR polyamide plastic to Usibor steel RIVTAC joint.

Figure 136: Load curve for a CFR polyamide plastic to Usibor steel RIVTAC joint with adhesives.

0

1

2

3

4

5

6

7

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

0 2 4 6 8 10 12 14

Load

[kN

]

Displacement [mm]

1

2

3

4

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

0 2 4 6 8 10 12 14

Load

[kN

]

Displacement [mm]

1

2

3

4

81

9.5.2 Blind Rivet made of aluminum

Lap-shear

Figure 137: Load curve for a CFR polyamide plastic to Usibor steel aluminum blind rivet joint.

Cross tension

Figure 138: Load curve for a CFR polyamide plastic to Usibor steel aluminum blind rivet joint.

9.5.3 Blind Rivet made of Steel

Lap-shear

Figure 139: Load curve for a CFR polyamide plastic to Usibor steel blind rivet made of steel joint.

0

0,5

1

1,5

2

0 0,5 1 1,5 2

Load

[kN

]

Displacement [mm]

1

2

3

4

5

0

0,5

1

1,5

2

2,5

0 1 2 3 4 5

Load

[kN

]

Displacement [mm]

1

2

3

4

0

1

2

3

4

0 0,5 1 1,5 2 2,5 3 3,5

Load

[kN

]

Displacement [mm]

1

2

3

4

82

9.5.4 Blind Rivet made of Steel with the mandrel kept

Lap-shear

Figure 140: Load curve for a CFR polyamide plastic to Usibor steel blind rivet made of steel with

mandrel kept joint.

9.6 CFR Epoxy to Usibor steel

Lap-shear

Figure 141: Load curve for a CFR epoxy polymer to Usibor steel RIVTAC joint.

Figure 142: Load curve for a CFR epoxy polymer to Usibor steel RIVTAC joint with adhesives.

0

1

2

3

4

5

6

0 1 2 3 4 5

Load

[kN

]

Displacement [mm]

1

2

3

4

0

5

10

15

20

25

30

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

4

0

5

10

15

20

25

30

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

4

Box 7047, 164 07 Kista, Sweden

Visiting Isafjordsgatan 28 A, 164 40 Kista, Sweden

+46 8 440 48 00, [email protected], www.swereakimab.se

Cross tension

Figure 143: Load curve for a CFR epoxy polymer to Usibor steel RIVTAC joint.

Figure 144: Load curve for a CFR epoxy polymer to Usibor steel RIVTAC joint with adhesives.

0

1

2

3

4

5

6

0 5 10 15 20

Load

[kN

]

Displacement [mm]

1

2

3

4

0

1

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Displacement [mm]

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Documents

Joining of metal and fiber composites