Study on T-Bolt and Pin-loaded Bearing Strengths and Damage Accumulation in E-glass / Epoxy Blade Applications

Alexander JE Ashworth Briggs1, Zhongyi Y Zhang2, Hom N Dhakal2

1Australian Maritime College, University of Tasmania, Australia2School of Engineering, University of Portsmouth, PO1 3DJ, UK

Abstract

In this paper, the ultimate bearing strengths of pin-loaded double shear and T-bolt loaded

connections were studied in thick composites, where the diameter of the pin equates to the

thickness of the laminate. These bearing strengths were obtained for E-glass / Epoxy

laminates of [(±45, 03)n ,±45], and a Vf of 54%. It is found that the values for ultimate bearing

failure and first non-linearity of pin-loaded connections should be reduced by 25% and 38%

respectively, when applied to T-Bolt connections. The failure modes prior to ultimate failure

were primarily dominated by fibre matrix shear out and delamination. As far as laminates

with specific reinforcement architecture and a large percentage of reinforcement orientated

to the load axis are concerned, the long term service life of T-Bolt connections may be

impacted due to the visible onset of damage at a similar level to that accepted by

Germanischer Lloyd for load introduction zones.

KeywordsLaminate, Bearing strength, T-bolt, Polymer matrix composites (PMCs), Delamination

Corresponding Author:Zhongyi ZhangUniversity of Portsmouth, Portsmouth, Hampshire, PO1 3DJ, UK. Email: zhongyi.zhang@port.ac.ukNomenclature

To whom correspondence should be addressed.

Ab Bearing areaAbTB Bearing area T-Boltd Diameter of tension boltD Diameter of pin or holeE Edge margin, (distance between pin centre and specimen edge below pin)W Specimen width at pin centre

Fbru Ultimate bearing strengthF tu Ultimate tensile strength

Sxx, Syy , Szz Strength in principle directionsXC , ZC Axial and through thickness compressive strengths

σ xx, σ yy, σ zz Stress in principle directionsσ t Net section tensile stressσ br Bearing stress

Vf Volume fibre fraction

1 Introduction

Tidal turbine blade root end design is a compromise between structural and fluid dynamic

considerations. Blade performance optimisation indicates a requirement for narrow foil

sections, while long term structural integrity can be more easily achieved through an

increase in blade root Pitch Circle Diameter (PCD).

Commonly used composite blade connectors are the bonded insert and the T-bolt

connectors as illustrated in Fig.1. Load is transferred from the blade to the hub through the

adhesive connecting the insert to the laminate, or through the bearing interface between

the barrel or cylindrical nut and the laminate surface. The use of high count insert systems

offers both cost and weight savings for larger wind blades (1). T-bolt connections have been

used in wind energy applications since the 1980’s and are sometimes favoured in tidal

turbine applications, due to ease of damage inspection and concerns regarding the long

term hygrothermal degradation of epoxy adhesives (2). Tidal turbines have been installed at

depths of 30 to 40m, where pressures are similar to those that have been shown to

accelerate hygrothermal ageing (3).

When T-bolts are utilised in a root connection, laminates that have been optimised for blade

stiffness are reinforced to accommodate the local bearing pressure of the connection. This is

achieved by increasing the percentage of off-axis material in the root section, resulting in a

reduced laminate modulus in the load bearing axis. Local increase in laminate thickness or

PCD is required to maintain blade deflections and stay within design guidelines set by

certification bodies such as Germanischer Lloyd (GL) and Det Norsk Veritas (DNV). GL, for

example, stipulates a stress threshold within a load introduction zone of 100 MPa. The

former of these solutions adds weight to the structure, and may result in laminate

thicknesses exceeding the capability of current manufacturing and materials combinations.

The latter may conflict with optimal blade geometry and must be smoothed into the blade

sections, as a sudden geometry change leads to increased normal forces and inter-laminar

shear stress, which can be detrimental to the fatigue life of the blade.

1.1 Laminate

The ultimate strength of a pin-loaded joint is affected by laminate isotropy, stacking

sequence (4), pin clearance (5-7), and lateral constraint [8-10]. Joint strength is optimised

when the pin diameter is close to the thickness dimension of the specimen (8).

Repositioning of 90o plies to the surface of a pin-loaded laminate significantly increases

bearing strength, by constraining the laminate surface and reducing laminate brooming (4,

9, 10).

The bearing strengths of (0, 90)s laminates exceeds those of laminates (0,-45, +45)s , with

isotropic laminates demonstrating a greater increase in bearing strength in response to

clamping pressure than orthotropic laminates (9, 11). Stacking sequence also affects the

delamination bearing strength (12), with downstream effect on long term service life of the

joint.

Clamping pressure applied to a laminate along the axis of the pin has been shown to

increase both the delamination and ultimate bearing strengths of pin loaded laminates,

different stacking sequences are required to optimise the joint performance in each

instance. Beyond a saturated clamping pressure no further increase in joint performance

was noted (13). When applying clamping pressure the experimental error of those

researchers using instrumented bolts (13, 14) is considerably reduced over those relying on

a torque value due to a variety of causes including thread damage and surface roughness

(15).

1.2 Geometry

The bolt contact problem has been described as highly non-linear, due to the changing

stress distribution as the contact surface is increased , with initial non-linearity increasing

with radial clearance (6).

The influence of geometry ratios E/D and W/D are well documented [13, 14]. Laminate

bearing strength has been shown to increase with E/D for ratios of 1 to 3; beyond this level

some reduction in net tension stress at failure has been noted (16). A greater E/D ratio may

be required in T-bolt applications due to the removal of material below the cylindrical nut,

in order to fit the tension bolt, thus reducing the laminate shear area (Fig. 2).

Thin lap bolted joints exhibit double shear like behaviour, with load path eccentricity

increasing with joint thickness. The efficiency of a thin joint, despite having a lower load

carrying capability than a thick joint is characterised by a higher bearing strength which can

also be considered a measure of mass efficiency (17).

Most experimental studies have been restricted to laminate thicknesses <3mm. Weibull

theory could be applied to describe strength reduction to some extent when thickness was

increased from 3.18mm to 12.70mm in pin-loaded joints (18). At the larger end, theory

predicts a greater size effect than has been confirmed through experimentation. Studies

have not addressed the influence of compound ply waviness related to ply thickness

variability in thick laminates, as found in blade root sections where laminate thickness often

exceeds 100mm.

1.3 T-Bolts

Published work on T-bolt connections has focused on ultimate strength and fatigue

characterisation in FRP, in-plane stress concentration, bolt pretension, and stress relaxation

(19-21) .

Martínez et al studied the ultimate bearing strengths of T-bolts on laminate thicknesses of

36 and 37mm (19), demonstrating a point stress failure criteria for net tension failure.

However, no bearing failure criterion was determined in relation to the material tensile

strengths, nor were comparisons with pin-loaded connections made. Three dimensional

models have been applied to bearing failure, due to the combination of stresses in a bearing

interface which indicates an elevated likelihood of delamination in compression, and also of

fibre matrix shear out considering Eq. (1) and (2) (22, 23). Where σ zz<0.

Equation (1) for delamination in compression

( σ zzZC )2

+( σ xzSxz )2

+( σ yzSyz )2

≥1 (1)

Equation (2) for fibre matrix shear out

T-bolt geometry dictates large if not full scale testing for materials characterisation.

Comparative study and determination of reduction factors for this type of joint

configuration would enable usage of the considerable database of lab scale data derived

from published work.

T-bolts and the delamination-resistant strength of zero dominated laminate, does not

appear to have been studied in any depth.

The intersection of an in-plane hole, with a through-plane hole, creates a complex internal

laminate geometry with an additional unconstrained laminate edge. It is important to

understand whether this geometry affects the ultimate bearing strength of a laminate.

Improved understanding of the onset of laminate damage for this type of connection would

enable the development of a strategy for the improvement of T-bolt connections, in

conjunction with increasing blade service life and reducing servicing and maintenance costs.

In this study, T-bolt connections were investigated at onset of damage and ultimate load

and compared with a pin-loaded double shear connection, to determine the effect of the

additional internal geometry presented by the T-bolt, and the zero domination of the

reinforcement on the bearing strengths. Two different sizes of T-bolt specimen were

investigated in order to address size effects.

2 Materials and experimental procedures

2.1 Materials

Sicomin SR 8100 Epoxy and SD 8731 hardener were used to manufacture the specimens for

this study. The lay-up configuration was [(±45, 03)n ,±45] and the fibre volume fraction was

54%. Where n is the number of laminate complexes, resulting in 7 and 13 complexes of

(±45, 03) for the 20mm and 36mm specimens respectively. Within each laminate complex

the 0o ply consisted of a multi-axial combination material manufactured from Advantex®

Glass by Owens Corning, with 1170 gsm 0 o fibre, 70 gsm 90 o fibre and a nominal 30 gsm

( σ xxXC )2

+( σ xySxy )2

+( σ xzSxz )2

≥1 (2)

chopped strand mat (CSM) (Fig 3) and the weight of each ±45 o ply was 450 gsm. The

resulting laminate consisted of 81.5% at 0o, 11.5% at ±45o, 4.9% at 90o and 2.1% CSM.

2.2 Specimen Processing

A 1m x 0.5m panel was processed using the resin infusion method, followed by a post-cure

according to the manufacturer`s recommendations. Specimens were cut by water-jet, and

in-plane holes were machined using water fed diamond core drills to reduce heat or

delamination damage (24, 25). Bearing holes were reamed to a clearance fit of 0.2 mm.

2.3 Specimen Geometry

Two sizes of T-bolt specimen and a single size of double shear specimen were tested.

Specimen geometry is listed in Table 1 and illustrated in Fig.4.

2.4 T-bolt Geometry

Fig.5 shows a T-bolt; typical geometry results in the tension bolt failing before laminate

ultimate bearing failure is induced (19). To ensure laminate failure, preliminary bearing

failure loads were estimated using published data (11) and the T-bolt geometry was altered

from a typical ratio for cylindrical nut to tension bolt of 2:1, to a minimum of 1.5:1 (Table 2)

so as to ensure laminate failure prior to the tension bolt yielding.

2.5 Monotonic Tests

Monotonic tests were carried out using an ESH 100KN and a Dennison Mayes 630KN

machine with crosshead speeds of 1mm per min. These crosshead speeds comply with BS

EN ISO 14126:1999 for compression testing of polymeric materials, and are comparable with

the range of speeds of 0.5 to 1mm/min used by researchers. Fig 6 shows the experimental

setup for T-bolt and double shear tests.

2.6 Damage Observation

Video, photography and microscopy were used to observe surface damage evolution during

experimentation. MicroCT analysis using a Metris XT H 255 255KVa CAT back projection

system was used to observe subsurface damage at the point of major loss of stiffness.

Audible Acoustic Emissions were recorded using AV equipment, to correlate with features of

the load vs. displacement plot.

3 Results and Discussion

3.1 Ultimate Bearing Strength

The average experimental failure loads presented in Table 3 were obtained from

incremental and monotonic tests. Bearing strengths were calculated using Eq. (3, 4 & 5)

(19).

Bearing area of a T-bolt connection is

AbTB=¿t .D−

π .d2

4 ¿ (3)

Bearing area of a pin-loaded connection is

Calculation of mean bearing strength

Fbru=PultAb (5)

The ultimate bearing strength Fbru is a function of the failure load Pult and bearing area Ab

which was calculated from Eq. (3) for T-bolts and Eq. (4) for pin loaded connections, where t

is the specimen thickness, D the diameter of the bolt or cylindrical nut, and d the diameter

of the tension bolt.

The pin-loaded ultimate strength of 340 MPa is comparable with the range of values for E-

glass / epoxy laminates of Vf 55% with stacking of (0,±45)s and (0,90)s , with similar E/D and

W/D published by Sayman et al (11). The ultimate strength of these connections might be

increased by altering the material system as laminates (0, 90)s have been shown to exceed

that of (0,-45, +45)s laminates by approximately 25% (11).

Comparing the ultimate strengths the results indicate that T-bolts fail at 25% lower strength

than pin loaded connections. The intersection created by the removal of laminate material

to enable the fitting of the T-bolt tension bolt creates an additional unconstrained laminate

edge, with associated failure mechanisms of fibre buckling, brooming and delamination, all

of which reduce the connection’s bearing strength.

Fig. 7 presents MicroCT images of a 20mm T-bolt specimen loaded to the point of major loss

of stiffness. Visual inspection of the specimens revealed a slight surface bulging and

Ab=¿ t . D¿ (4)

delamination of the laminate under the load pin, and a split in the bearing surface (Fig. 7(a)).

The MicroCT images reveal that the laminate is diagonally cracked though three of the four

0o axis unidirectional plies (Fig. 7(b)). The displacement of the plies is indicative of laminate

normal stresses.

The maximum loading (Table 3) of the experimental T-bolts with reduced D:d ratio exceeded

the theoretical ultimate strength (Table 2) of typical T-bolt metallic assemblies with D:d ratio

of 2, indicating that with a holistic approach the metallic components of this attachment

method might be further optimised.

3.2 Delamination Bearing Strength

The T-bolts specimens exhibited visible damage on the surface or immediately sub-surface

adjacent to the bearing interface at stress levels approximating to the GL threshold. Park

(13) demonstrated that a laminate optimised for ultimate bearing strength (903,+453,-

453,03)s did not achieve optimal delamination bearing strength, and that by constraining

the 0º plies directly with surface 90º plies in the manner (903,03,+453,-453)s the

delamination strength could be increased by as much as 30%.

3.3 Damage Evolution

Bearing stresses at both visible damage and the first non-linearity of the load vs.

displacement plot are presented in Table 3. Fig. 8 presents the characteristically different

load vs. displacement plots for T-bolt and pin-loaded specimens of 20mm thickness. Optical

microscopy (Fig. 9) shows the first visible damage of the 20mm T-bolt specimens. At 107

MPa a narrow horizontal opaque band appeared on the centreline and below the pin. The

development of this damage appears to be within the region of linear elastic behaviour. The

stress level at which damage is first visible, is similar to that at which the simulation of

Kensche and Schulte predicted individual ply failure at one third of the test load (20). The

low stress level was attributed to the failure model selected, and stress concentrations

within the contact area. An increasing rate of clicks per kN was noted at an average bearing

stress of 107MPa during acoustic emission monitoring of zero dominated laminates loaded

by T-bolt connection, in the same acoustic emission monitoring study the same features

were recorded at 150 MPa for quasi isotropic laminates with a lower Vf (19).

The low visible failure threshold observed may be a consequence of the reinforcement

architecture; failure appears to be by way of delamination between a 90o tow and 0o fibres.

The combination material used has irregularly spaced 90o fibre tows resulting in waviness of

the 0o fibres, causing loading normal to the laminate plane which is known to promote

delamination, especially when combined with forces due to the Poisson effect. Audible

Acoustic emissions (AEs) are indicated in Fig. 10, demonstrating the correlation between AEs

and significant features of the plot.

At a bearing stress of 165 MPa, short striations, indicative of the occurrence of 0o fibre shear

out, were observed to develop in a narrow band, propagating away from the bearing

interface in the 0o orientation (Fig. 11 (b)). The length and width of the band increased with

load, as illustrated in Fig. 11 (c)-(f). At approximately 175 MPa a non-linearity is noted on

the load vs. displacement plot for the T-bolt (Fig. 10).

Visible inspection of the pin-loaded specimens during testing was not possible, due to

obscurement by the test linkages.

The development of visible damage on the mould face prior to the bag face of each

specimen, may be indicative of a difference in modulus between the laminate faces,

resulting in an unequal loading, through the thickness of the specimens.

T-bolt specimens failed through in-plane cleavage along the centreline of the specimen,

combined with delamination propagating tangentially from the in-plane hole. Fibre crushing

was apparent at the bearing interface, with extensive delamination visible on the outer

faces of the specimen, in comparison to the perimeter of the in-plane hole.

3.4 Size Effects

The higher bearing strength of the 20mm double shear connection as compared to the

equivalent T-bolt connection is indicative of a higher joint efficiency (17), however in the

implementation of this type of joint configuration is impractical with application to

composite wind and tidal turbine blades.

The T-bolt 36mm specimens exhibited an increase in ultimate bearing strength and a

reduction in bearing strength at the point of delamination or point of first visible damage

when compared to the 20mm T-bolt specimens (Table 3). Minor variation in experimental

setup –difference in D:d ratio- between the two specimen sizes to ensure structural integrity

of the metal components makes it difficult to draw any direct conclusions, however the

indication is that either no size effect is apparent or that joint efficiency increases with size.

Initially these results appear to be in contrast to (17), however if we consider that the basis

behind the size effect was increasing eccentricity due to thickness, then the lack of

eccentricity in the load path of T-bolt connection, results in a behaviour similar to a double

shear connection regardless of laminate thickness providing that the barrel nut stiffness is

sufficient to maintain a uniform pressure distribution. Any reduction in bearing strength

between double shear and T-bolt connections being due to the complex internal geometry

and additional free edge of the laminate.

4 Conclusions

Based on the experimental results, bearing strength data derived from pin-loaded tests

must be factored down when applied to T-Bolt connections. These reductions may be

attributed to a reduction in laminate normal constraint caused by the intersecting holes and

an additional unsupported laminate edge of the internal geometry.

The ultimate bearing strength for T-bolts is reduced by 25% when compared to double

shear connections, with damage accumulation occurring at a 38% lower bearing stress.

Reducing T-bolt D:d ratio where D is equal to laminate thickness may be used to

increase the ultimate strength of the connection and should be considered alongside

the design of the composite component.

Further Work

The effects of laminate orthotropy, and internal connection geometry on the bearing

pressure distribution should be quantified.

The E/D ratio for T-bolts should be investigated, considering the effect of material

removal below the bearing interface where the in-plane hole reduces the net laminate

thickness.

Acknowledgements

The authors acknowledge the Faculty of Engineering and the Environment at the University

of Southampton, and would like to thank Professor Jean-Luc Beney of La Haute Ecole

d'Ingénierie et de Gestion du Canton de Vaud (HEIG-VD), Switzerland. Nick Barlow and

Designcraft Ltd, UK are thanked for their support of this project.

Funding

This research received no specific grant from any funding agency in the public, commercial,

or not-for-profit sectors.

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Figure captions:

Fig. 1. Common root connectors (a) T-Bolts (b) Bonded inserts.Fig. 2. Reduction in shear area caused by t-bolt tension bolt.Fig. 3. Stitched UD reinforcement Combination mat, OCV . (a) 0O fibres (b) Distorted unevenly distributed 90o and short chopped strand fibresFig.4. Specimen geometry (a) Plan view and transverse section AAFig. 5. T-Bolt assembly.Fig. 6. Experimental setup for (a) T-bolted specimens (b) Double shear specimensFig.7. MicroCT images for specimen 1.2 after first major delamination in bearing at 65KN(a) A surface crack is visible below the bearing surface. (b) Section through laminate along the load axis showing displacement of the surface ply and depth of the crack. (c) Section YZ directly below the bearing surface. A 17mm transverse crack on an interface between a ±45o and 0o plies, and a 4mm split starting mid 0o ply, passing through a ±45o ply and terminating on the surface. (d) A diagonal crack directly below the bearing interface. The crack passes though 3 plies of 0o reinforcement, displacing the outer ±45o ply away from the surface.

Fig.8. Bearing stress vs. Machine Stroke (a) Pin-loaded specimen 2.1 - 20mm and (b) T-Bolt specimen 1.3 - 20mmFig. 9. Microscopy slide of a 20mm T-bolt specimen loaded to 32kN, 107 MPa. Delamination along a 90o tow. The loading axis is vertical, and the curved edge is the bearing interface. Fig. 10. Typical damage development of a 36mm T-Bolt specimen, indicating audible emissions and features of the load vs. displacement plotFig. 11. Development of Oo deg fibre shear out below the bearing interface (a) 147 MPa (b) 165 MPa (c) 174 MPa (d) 192 MPa (e) 210 MPa (f) 219 MPa. The images have been inverted to highlight the damaged areas.

(a) (b)Fig. 1. Common root connectors (a) T-Bolts (b) Bonded inserts.

T-b o lt she a r a re a = (t-d ).L

Ld t

Pin jo in t she a r a re a = t.L

Lt

Fig. 2. Reduction in shear area caused by T-Bolt tension bolt.

(a) (b)Fig. 3. Stitched UD reinforcement combination mat, OCV .

(a) 0O fibres (b) Distorted unevenly distributed 90o and short chopped-strand fibres.

Fig.4. Specimen geometry. Plan view and transverse section AA

Tens ion bo lt o r s tu dD iam ete r - d

C y lin dr ica l o r b a rre l nu tD iam ete r - D

Fig.5. T-Bolt assembly

90o fibre tows

CS fibres

Tension bolt

Specimen

Cylindrical nut

Machine interfaceplate

Machine interfaceplate

Shear pin

Specimen

Linkage

(a) (b)

Fig.6. Experimental setup for (a) T-bolted specimens (b) Double shear specimens

(a) (b)

(c) (d)

Fig.7. MicroCT images of 20mm T-bolt specimen after first major delamination in bearing at 65KN(a) A surface crack is visible below the bearing surface. (b) Section through laminate along the load axis showing

displacement of the surface ply and depth of the crack. (c) Section YZ directly below the bearing surface. A 17mm transverse crack on an interface between a ±45o and 0o plies, and a 4mm split starting mid 0o ply, passing through a ±45o

ply and terminating on the surface. (d) A diagonal crack directly below the bearing interface. The crack passes though 3 plies of 0o reinforcement, displacing the outer ±45o ply away from the surface.

0

100

200

300

400

0 1 2 3 4 5 6 7 8

σ br

(MPa

)

Machine Stroke (mm)

0

100

200

300

0 1 2 3 4 5

σ br(M

Pa)

Machine Stroke (mm)(a) (b)

Fig.8. Bearing stress vs. Machine Stroke (a) Pin-loaded specimen 2.1 - 20mm and (b) T-Bolt specimen 1.3 - 20mm

Fig.9. Microscopy slide of a 20mm T-bolt specimen loaded to 32kN, 107 MPa. Delamination along a 90o tow. The loading

axis is vertical, and the curved edge is the bearing interface.

0

50

100

150

200

250

300

0 2 4 6 8 10 12

σ br

(MPa

)

Machine Stroke (mm)

Major AE

Change in gradient

First visible damage

AE (Delamination)

Start of visible 0o striations

Fibre breakage

Fibre/matrix shearout

Minor delaminations

Minor AEs

Fig. 10. Typical damage development of a 36mm T-Bolt specimen, indicating audible emissions and features of the load vs. displacement plot.

(a) (b)

(c) (d)

(e) (f)Fig. 11. Oo deg fibre shear out development below the bearing interface of a 20mm T-bolt specimen, (a) 147 MPa (b) 165

MPa (c) 174 MPa (d) 192 MPa (e) 210 MPa (f) 219 MPa. Photographic images of the surface have been inverted to highlight the damaged areas.

Table captions

Table 1. Specimen geometry. Dimensions in mm.

Table 2. Typical and experimental T-bolt dimensions in mm.

Table 3. Experimental failure loads and bearing strengths

Table 1. Specimen geometry. Dimensions in mm.Series W L e t D dT-bolt 20 49 210 59.5 20.5 20 12T-bolt 36 89 360 108 37.5 36 24Double Shear 20 49 210 59.5 20.5 20 n/a

Table 2. Typical and experimental T-bolt dimensions in mm.Series D d D/d Grade Pitch Yield

kNUltimate kN

T-bolt 20 20 10 2 10.9 1.5 52 58T-bolt 20exp 20 12 1.7 12.9 1.8 92 102T-bolt 36 36 18 2 10.9 2.0 173 192T-bolt 36exp 36 24 1.5 10.9 2.0 346 384exp – modified geometry experimental T-bolt

Table 3. Experimental failure loads and bearing strengths.Specimen Type Bearing

Area mm2Vis. Damage Load kN

Non-linearityLoad kN

FailureLoad kN

Vis. Damageσbr MPa

Non-linearityσbr MPa

Fbru

MPaT-bolt 20mm 297 32 52 75.5 107 175 254T-bolt 36mm 841 78 153 228.5 93 181 272Double shear 20mm 420 na 119 142.9 Na 283 340