New Z Pinning Method

8/10/2019 New Z Pinning Method

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American Institute of Aeronautics and Astronautics

1

New Manufacturing Method of Z-pinned Composite

Laminates

Ik Hyeon Choi1

Korea Aerospace Research Institute, 45 Eoeun-dong, Yuseong-gu, Daejeon, 305-333, Republic of Korea

In Gul Kim2Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, Republic of Korea

and

Seok Min Ahn3, Chan Hong Yeom4, In Hee Hwang5and Dae Sung Lee6Korea Aerospace Research Institute, 45 Eoeun-dong, Yuseong-gu, Daejeon, 305-333, Republic of Korea

Z-pinning technique is one of the methods to enhance inter-laminar strength of

laminated composites. In this paper conventional z-pinning technology will be introduced

and new concept recently proposed by authors will be introduced. The performance inimpact resistance of some trial specimens manufactured using the new concept will be

investigated.

I. Introduction

any techniques including 3D weaving, stitching and braiding have been developed to enhance inter-laminarstrength of laminated composites. However, z-pinning is the only technique which can be applied to prepreg

laminated composite structures. If the other techniques are used to prepreg which is half cured composite materials,probably it might result in excessive fiber damage that degrades in-plane mechanical properties. This is a seriouslimitation because presently many highly loaded composite components, including many aircraft structures, aremade using prepreg laminates [1, 2].

In 1989, the first z-pinning concept as shown in Fig. 1 was registered as a USA patent [3]. In this concept they

use a kind of foammaterial called pre-form which

contains z-pins to beinserted inlaminates. The pre-form placed on thelaminated prepregsis easily collapsed

by curing pressureinside autoclave, sothe z-pins areinserted to the

laminated prepregsduring autoclave

1Principal Researcher, Aerodynamics and Structures Dept.,[email protected], Aerospace Engineering Dept.,[email protected], Aerodynamics and Structures Dept.,[email protected], Aeronautics Technology Division,[email protected], Rotorcraft Program Office,[email protected] Director, Aeronautics Research & Development Head Office,[email protected],Senior Member AIAA.

M

Figure 1. First z-pinning concept.

51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference
18th2 - 15 April 2010, Orlando, Florida

AIAA 2010-313

Copyright 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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curing process.After the insertingof the z-pins, thecompacted pre-form by autoclave

pressure and theremained part ofthe z-pins over theupper surface ofthe z-pinnedlaminates areremoved by cutter.However thisconcept has not

been known to beapplied in realstructure yet.

In 1996,another conceptusing ultrasonic

tool as shown inFig. 2 was

patented in USA[4] and this concept was actually applied to real aircraft structural joints. In this concept, they use also the similarpre-form but they do inserting process of z-pins outside of autoclave just after laminating of prepregs and beforeautoclave curing process. In this case the pre-form is composed of two types of material, i.e. the upper part is madefrom easily collapsible foam material and the lower part from relatively hard foam material in order that the lower

part has a role to prevent inclining of the z-pin during inserting process. Usually prepreg before curing is very stickyin room temperature, so in order to insert the pins into prepreg without damage of reinforced fibers in prepreg, aspecial tool is needed like ultrasonic horn with very high frequency vibration, about 20 kHz. It is known thatinserting process needs generally 3 to 4 sec, typically producing a square inch of pinned area. With fully automatedequipment, point-to-point moves can be made in around 10 sec [5]. After the inserting process and removing processof remained parts of the pre-form and the z-pin, the z-pinned laminated prepregs is cured by conventional autoclave

curing procedure.Usually diameter of z-pin is 0.2~1.0mm, and volume ratio of z-pin is 0.5~4.0%. It is known that only a relatively

small volume fraction of z-pin is needed for considerable enhancement of the through-thickness properties anddamage tolerance performance. However the only application of z-pining technology in aerospace field is in F/A-18E/F Super Hornet, which is used to replace titanium fasteners in assembling air inlet duct and engine bay door.This provides a good cost saving (US$83,000) and modest weight reduction (17 kg) per aircraft [1].

II. New Z-pinning Concept

Recently authors invented a new z-pinning concept as shown in Fig. 3 [6]. In this concept, instead of using thepre-form including z-pins, a couple of fixtures are used, which can be used repeatedly and permanently. One of thefixtures is upper fixture, in which many guide pins are installed rigidly in vertical direction. The other is lowerfixture, in which many holes are machined and z-pins to be inserted to the laminates are placed inside the holes. Inautoclave processing, curing pressure pushes the upper fixture toward lower fixture and the guide pins of upper

fixture pushes the z-pins placed inside the holes of the lower fixture to the laminated prepregs. Then the z-pins areinserted into laminated prepregs and finally excessive resin is flow out from the laminated prepregs since the lengthof the z-pins is just fit for the thickness of normally cured laminate without z-pinning.

This new concept has some advantages in comparison to the conventional concepts, i.e. this concept does notneed any disposable material like pre-form because the fixtures can do the role of the pre-form and can be usedrepeatedly and permanently. Only z-pins to be inserted are needed to be replaced in the hole of lower fixture aftercuring of z-pinned composite structure. So, there is no waste of disposable material at all. As well as this advantage,this concept has another advantage that it can be applied to traditionally established autoclave curing procedurewithout any change of it. Only an additional simple work is needed to put a couple of fixtures on the laminated

Figure 2. Typical pre-form and z-pinning concept using ultrasonic horn.


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prepregs like acurl plate which isusually used when

both surfaces ofcured laminate areneeded to besmoothed. The

procedureinserting z-pin can

be doneautomaticallyduring curing

procedure inautoclave. So thisconcept is veryuseful in mass

production of z-pinned compositestructures.

III. First Trial Fixture and Specimen

Fig. 4 shows first trial fixture manufactured by authors and a typical specimen preliminarily cured using this newconcept. Left fixture is upper fixture in which the guide pins are installed to push the z-pins in holes of lower fixture.Right fixture is lower fixture in which many holes are machined.

In order to compare damage tolerance performance of the z-pinned specimen, low-velocity impact test wasperformed. We can see central rectangular area in the specimen which is the z-pinned region and local damageinduced by the low-velocity impact test at the center of it.

Fig. 5 and 6 show the contact force histories measured from low-velocity impact test on the preliminarily curedz-pinned specimens and normal specimens without z-pinning. Left figures of Fig. 5 and 6 show contact force historyof normal specimens and right figures of Fig. 5 and 6 show contact force history of z-pinned specimens. Fig. 5 is for3J impact energy and Fig. 6 is for 4J impact energy. From the two figures it can be seen that contact force historycurves of normal specimens show more serious sharp fluctuations. Usually it means more damage might happen inthe specimen during contact process by impact. Relatively contact force history curves of z-pinned specimens show

less fluctuation. So it can be guessed that less damage might happen in the z-pinned specimen.Fig. 7 and 8 show the C-scanned damage areas after low-velocity impact test. Left figures of Fig. 7 and 8 show

C-scanned damage areas of normal specimens and right figures of Fig. 7 and 8 show C-scanned damage areas of z-pinned specimens.Fig. 7 is for 3Jimpact energy andFig. 8 is for 4Jimpact energy.From these twofigures it can besurely seen thatless damagehappened in the z-

pinned specimensas expectedthrough comparingof fluctuation oftheir contact forcehistory curves inFig. 5 and 6.

Figure 3. New concept of z-pinning.

Figure 4. The first trial z-pinning fixture and a typical z-pinned specimen using the


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IV. Second Trial Fixture and Specimen

After success of manufacturing some preliminary specimens with the first trial fixture, the second trial fixtureand specimens are manufactured in order to check enhancement of CAI (compression after impact) strength of z-

pinned composite laminates. The specimens were manufactured by following SACMA (Suppliers of AdvancedComposite Materials Association) specifications on CAI test. Using the fixture it produces two z-pinned specimensand two normal specimens for each curing procedure. Fig. 9 shows the second trial z-pinning fixture and typical

carbon/epoxy z-pins which diameter is 0.28mm. We manufactured two types of specimens which stackingsequences are [45/0/-45/90]3Sand [45/0/-45/90]4S.

Table 1 shows CAI performance enhancement of z-pinned laminates. In case of [45/0/-45/90]3S shows onlyabout 6% enhancement and [45/0/-45/90]4Sshows about 28% enhancement of CAI performance. From this result itcan be seen that thicker z-pinned laminates take more enhancement of CAI performance. Probably this differencehappens because thin specimen may collapse with a kind of buckling mode, so z-pinned effect can not be appearedfully. On the other hand, this enhancement looks like not so big in comparison with the value reported in reference[1]. It is why the present density of the z-pin is relatively lower than it from the reference and the detailed curing

procedures on the new z-pinning concept are notyet optimally established.So we need to check itagain on the other z-

pinned specimens withhigher density of z-pinmanufactured throughwell established the newz-pinning curingtechniques. Probably itwill be expected to beshow more enhancementthan the present.

0

500

1000

1500

2000

2500

0. 00 7 0. 008 0. 009 0. 01 0. 01 1 0. 012 0. 013

Time (sec)

ContactForce

(N)

No load(3J)

0

500

1000

1500

2000

2500

0 .007 0. 00 8 0. 00 9 0 .01 0 .011 0. 012 0. 01 3

Time (sec)

ContactForce(N

Z-pinning-1 (3J)

Figure 5. The contact force histories of normal

(left) specimen and z-pinned (right)

specimen under 3J impact energy test.

0

500

1000

1500

2000

2500

0. 006 0. 007 0.008 0. 009 0. 01 0.011 0.012

Time(sec)

ContactForce

(N

No Load-2(4J)

0

500

1000

1500

2000

2500

0. 006 0.007 0. 008 0. 009 0. 01 0. 011 0. 012

Time(sec)

ContactForce

(N

Z-pinnin -2(4J)

Figure 6. The contact force histories of normal

(left) specimen and z-pinned (right)

specimen under 4J impact energy test.

Figure 7. Typical C-scanned impact damage area

of normal (left) specimen and z-pinned

(right) specimen under 3J impact energy

test.

Figure 8. Typical C-scanned impact damage area

of normal (left) specimen and z-pinned

(right) specimen under 4J impact energy

test.

Figure 9. The second trial z-pinning fixture and typical carbon/epoxy z-pins.


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V. Lower Fixture Made From Composite Material

Manufacturing of the z-pinning fixture needs considerably more expansive machining process since machiningmany minute holes of very small diameter needs some elaborate process and special tool. So we tried manufacturingof lower fixture using composite material by composite curing procedure instead of mechanical machining process.

Fig. 10 shows trial lower fixture made from composite materials and conventional upper fixture. In order tomake composite lower fixture, at first upper fixture needs to be manufactured from a metal material like steel usingconventional machining process. Because composite material usually used to shrink after curing, the diameter of theguide pin to make a hole at lower fixture should be slightly bigger than target diameter of the hole of lower fixture.The guide pin should have a sharpened end shape in order to be easily driven into laminated prepregs which will belower fixture after curing. After curing lower fixture which has many holes, the guide pins of upper fixture should bereplaced with new guide pins which have slightly smaller diameter since the new guide pins should be smoothlyinserted and extracted into the holes of the composite lower fixture. In this study after manufacturing of the triallower fixturewe succeededin curing somesingle lap jointz-pinnedspecimensusing it.

Consequently, it can beconcluded thatthe machiningcost formanufacturinglower fixture

will be savedconsiderably byusing this method.

VI. Conclusion

Authors invented a new z-pinning concept. The new concept does not need any disposable materials but onlyneeds repeatedly usable fixture system. It can be applied to the traditional procedure of autoclave curing procedure

Table 1. Enhancement of CAI performance of z-pinned laminates.

[45/0/-45/90]4S at z-pin density 0.54%

[45/0/-45/90]3S at z-pin density 0.54%

27.75

93.2

73.0

#6

6.55

71.9

67.5

#6

27.55

94.6

74.2

Average

6.08

68.0

64.1

Average

#5#4#3#2#1Specimen No.

10.465.690.71-0.3914.34Enhancement ratio (%)

77.672.572.273.676.0Compressive load of normal

specimen (kN)

91.295.099.099.090.1Compressive load of z-pinned specimen (kN)

17.4731.0637.0934.5618.60Enhancement ratio (%)

69.868.763.164.669.9

Compressive load of z-

pinned specimen (kN)


specimen (kN)


[45/0/-45/90]4S at z-pin density 0.54%

[45/0/-45/90]3S at z-pin density 0.54%

27.75

93.2

73.0

#6

6.55

71.9

67.5

#6

27.55

94.6

74.2

Average

6.08

68.0

64.1

Average


10.465.690.71-0.3914.34Enhancement ratio (%)


specimen (kN)

91.295.099.099.090.1Compressive load of z-pinned specimen (kN)

17.4731.0637.0934.5618.60Enhancement ratio (%)

69.868.763.164.669.9

Compressive load of z-

pinned specimen (kN)


specimen (kN)


Figure 10. The lower fixture made from composite material (left) and conventionally

manufactured upper fixture made from steel (right).


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without any change except for putting the fixture system on the laminated prepregs like a curl plate. So this newconcept is very useful in applying to mass production of z-pinned laminate composite structures.

Two trial preliminary fixture systems, in which the new z-pinning concept was applied, were manufactured andsome trial specimens were cured. Low-velocity impact test and CAI (compression after impact) test were performedon the specimens. CAI strengths and damage areas as well as contact force histories were measured. The contactforce histories from the z-pinned specimen had less fluctuation than normal specimen, which means less damagehappened in z-pinned specimens. From C-scanning results it was surely visualized that z-pinned laminates has better

performance in damage tolerance than normal laminates without z-pinning. The value of CAI strength of the presentz-pinned specimens was some enhanced even though it was not reached the reported value in reference because ofthe present low density of z-pin and not yet fully established the new z-pinning curing techniques.

In order to save machining cost of lower fixture it was tried to make it from composite materials using compositecuring method. Using the composite lower fixture, it was succeeded to manufacturing z-pinned single lap jointspecimens. Consequently, it can be concluded that the machining cost for manufacturing lower fixture will be savedconsiderably using this method.

References1Mouritz, A. P., Review of Z-pinned Composite Laminates, Composite: Part A, Vol. 38, 2007, pp. 2383-2397.2 Dickinson, L. C., Farley G. L., and Hinders M. K., Translaminar Reinforced Composites: A Review, Journal of

Composites Technology and Research, Vol. 21, Issue 1, 1999, pp. 3-15.3Boyce, J. S., Wallis, R. R. and Bullock, D. E., Foster-Miller Inc, Waltham, MA, U.S. Patent Application for a Composite

Structure Reinforcement, PatentNo. 4,808,461, filed 28 Feb. 1989.4Fusco, T. M., Magee, C. and Freitas, G., Foster-Miller Inc, Waltham, MA, U.S. Patent Application for a Method and

System for Inserting Reinforcing Elements in a Composite Structure, U.S. PatentNo. 5,589,015, filed 31 Dec. 1996.5Partridge, I. K., Cartie, D. D. R. and Bonnington, T., Manufacture and Performance of Z-Pinned Composites, Advanced

Polymeric Materials, edited by G. O. Shonaike and S. G. Advani, CRC Press, 2003, pp. 103-138.6Choi, I. H., Hwang, I. H., Ahn, S. M., Kim, E. T., Yeom, C. H. and Lee, D. S., Korea Aerospace Research Institute, Daejeon,

Republic of Korea, R.O.K. Patent Application for a A Method and an Apparatus for Making Composite Laminated StructureReinforced by Inserting Pins, and a Method for Making the Apparatus,Patent No. 10-0932302, filed 8 Dec. 2009.

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New Z Pinning Method