The Experiment and Numerical Simulation of CompositeCountersunk-head Fasteners Pull-throughMechanical Behavior

Junwu Mu & Zhidong Guan & Tianya Bian &

Zengshan Li & Kailun Wang & Sui Liu

# Springer Science+Business Media Dordrecht 2014

Abstract Fasteners made of the anisotropic carbon/carbon (C/C) composite material have beendeveloped for joining C/C composite material components in the high-temperature environment.The fastener specimens are fabricated from the C/C composites which are made from laminatedcarbon cloths with Z-direction carbon fibers being punctured as perform. Densification processcycles such as the thermal gradient chemical vapor infiltration (CVI) technology were repeated toobtain high density C/C composites fastener. The fasteners were machined parallel to the carboncloths (X-Y direction). A method was proposed to test pull-through mechanical behavior of thecountersunk-head C/C composite material fasteners. The damage morphologies of the fastenerswere observed through the charge coupled device (CCD) and the scanning electron microscope(SEM). The internal micro-structure were observed through the high-resolution Mirco-CTsystems. Finally, an excellent simulation of the C/C composite countersunk-head fasteners wereperformed with the finite element method (FEM), in which the damage evolution model of thefastener was established based on continuum damage mechanics. The simulation is correspondwell with the test result . The damage evolution process and the relation between the countersunkdepth and the ultimate load was investigated.

Keywords Composite fasteners . Pull-through test . FEMmodel

1 Introduction

A new type of high temperature lightweight composite material fasteners is required for thefuture thermal structure components’ joints. Carbon/carbon (C/C) composite materials arespoken highly because of their excellent high temperature properties(up to 3,000 °C [1, 2]).

Appl Compos MaterDOI 10.1007/s10443-013-9379-7

J. Mu : Z. Guan (*) : T. Bian : Z. Li : K. Wang : S. LiuSchool of Aeronautic Science and Engineering, Beihang University, XueYuan Road No.37, HaiDianDistrict, Beijing 0086-100191, Chinae-mail: d5062010@163.com

Zhidong. Guane-mail: mujunwubuaa@126.com

More and more attention has been paid for the C/C joint technology which reachesthe appearance of complicated shape C/C composite materials structural components[3]. The multi-directional braided C/C composite material fasteners have been consid-ered as the most important bolts for high temperature join application. The joinmethod of complicated C/C components of rapidness, safety, simplicity and low-costare needed. C/C composite materials are not able to be bond and currently there aretwo ways to solve this problem, one of which is chemical connection and the other ismechanical connection. The latter is more common join form, transferring high loadand having high reliability. Ceramic matrix composite (CMC) material fasteners suchas high temperature alloy fasteners, carbon/carbon and carbon/silicon carbide(C/SiC)fasteners have been studied by researchers [4–7]. Dogigli et al. [8] tested the tensileand shear properties of the C/SiC bolts under 1,600 °C. Bohrket al [9] conductedresearches on the mechanical behavior of C/C-SiC composite fasteners by tighteningtorque under thermal cycle. More lately,Hui Mei et al. [10] studied on the tensilestrength of the 2 DC/SiC bolts under high temperature(1,300 °C, 1,600 °C and1,800 °C).

The countersunk-head composite fasteners are used to maintain the external morphology.However, these composite fasteners have complicated microstructures and a variety of pull-through failure modes. For these reasons, it is definitely worthwhile to investigating themechanical behavior of C/C composite material countersunk-head fasteners. In this study,pull-through tests of the countersunk-head C/C composite material fastener will be done atroom temperature (RT). The pull-through damage load will be obtained and mechanicalbehavior will be analyzed.

2 Experimental

2.1 Materials & Specimens

The 3-D C/C are fabricated by using X-Y laminated carbon cloths (woven in warp and weftdirections) with Z punctured carbon fibers as perform. The preforms are carbonised at above1,000 °C and then the primary matrix material begins to breakdown only leaving its carbonbackbone. And the high density C/C composites are prepared with chemical vaporinfiltration(CVI) method [11–14] at temperature about 950~1,100 °C which is called densi-fication cycle. These cycles are carried out to fill porosity in the material and obtain a stronglybonded fiber matrix interface. Finally, C/C composite fasteners are fabricated by machiningprocessing along the fiber direction which is parallel to the carbon cloths (X-Y directions), asshown in Fig. 1 [15–17].

The preparation process of C/C composite bolts and the sample measurement are shown inFigs. 2, 3, and 4.

2.2 Procedure

The test procedure and fixture are proposed in Fig. 5.Two flat, constant circular cross-section steel plates, each containing a centrally located

fastener hole, were placed in a multi-piece fixture that had been aligned to minimize loadingeccentricities. Each plate contained four additional holes on the periphery where the uprightposts were located to support the up/down bedding. The two steel plates were joined togetherby the fastener and nut, with one plate being rotated 45° with respect to the second plate.

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This procedure (Compressive-Load Fixture) is suitable for screening and fastener develop-ment purposes. It can be used to perform comparative evaluations of candidate fasteners/fastener system designs.

From Fig. 5, the summary of the test method was: The steel plates were pried apart by theapplication of compressive force transmitting through the fixture and producing a tensileloading through the fastener. Force was applied until the failure of the composite fastener,nut or both occurs. Quasi-static loading used the velocity of 0.5 mm/min and the tests wereperformed in Fig. 6.

2.3 Experimental results

The pull-through ultimate load is shown in Table 1 which is 5361.38 N.Usually C/C composites are brittle materials, but three damage stages of the countersunk-head

composite fastener are observed from the load–displacement curve (Fig. 7), such as (1) non-destructive stage, the load–displacement was the great liner-type;(2) initial damage stage, the curvewas nonlinear and the specimen showed the characteristic of progressive damage;(3)When ultimateloads came, the specimenwas damaged. The load drops as a parabola rather than a straight line. Thecurve illustrates the failure mode of countersunk-head composite fastener is pseudo-plastic fracture.

The loaded state of the countersunk-head fastener is analyzed as follows, in Fig. 8.When the fastener bears a downward load, a shear force will be generated in the areas between

the head and the screw (shear region) to balance the reaction force coming from the head boundaryconstraint. Figure 9 illustrates characteristic fracture modes and morphology of the rupture for thecountersunk-head composite fastener. According to Fig. 9, it is inferred that themode of failure is thefibers hollowing around the center of the fastener head which includes the (a) delamination slippagefailure (b) carbon cloths intralaminar shear failure (c) fiber tensile failure. The rupture morphology is

Fig. 1 Three dimensional schematic illustration of fiber cloths and fasteners machining direction

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approximately a circle and the diameter of the region is equal to 10 mm (the diameter of the screw).The fibers are tensile failure in the bottom of the countersunk-head [18].

Fig. 2 Preparation process of C/C composite fasteners

Fig. 3 Countersunk-head com-posite fastener sample

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In this paper the internal fracture of C/C countersunk-head was observed andanalyzed by GE-nanotomo Micro-CT technology. Basic parameters of Micro-CT arein Table 2 as follow:

It is found that the screw is pulled out of the countersunk-head and there are a lot ofdelamination (Fig. 10a), intralaminar (Fig. 10b) and fiber tensile fracture according to theobservation by micro-CT.

As the micro-CT shows, a significant difference of the failure modes between C/C andmetal fasteners can be observed. Modes of failure in metal countersunk-head fasteners arefound to be the head plastic bending deformation (FFS) or shear failure (FFS) in Fig. 11.However, the failure modes of the C/C countersunk-head fastener is screws’ being pulled out,because of the weak shear strength of C/C composite materials (Fig. 9).

Fig. 5 Test procedure compressive-loaded fixture

Fig. 4 Fastener sample measure-ment

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The surface fracture of C/C countersunk-head was examined through a Zeiss scanningelectron microscope (SEM). Figure 12 illustrats SEM of delamination slippage, intralaminarshear and fiber tensile fracture morphology. A smooth fracture morphology of delaminationslippage can be seen in Fig. 12a. Numerous fibers are pulled out due to fiber/matrix (F/M)interface bonding being weak. Figure 12b illustrates the morphology of intralaminar shear wasfiber fracture and crude. Figure 12c shows the failure of fiber tensile at the bottom ofcountersunk-head. It also demonstrates the different failure models for just one countersunk-head from the Fig. 12.

The obtained fracture indicates the presence of shearing stresses acting on the countersunk-head area (Figs. 9 and 10). Thus, it is appropriate to use share strength to indicate the pull-through mechanical characters of the C/C countersunk-head fastener. The shear strength isdetermined from formula given in Eq. (1).

τ ¼ Fdamge

D� π� hð1Þ

Table 1 Results of C/C composite fasteners pull-through test

Fastener head Dimensions(mm) Ultimate load(N)

Countersunk 10 5361.38

Fig. 6 Fastener pull-through test

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Where the Fdamge is the pull-through ultimate load, h is the depth of countersunk-head andD is the screw diameter. The shear stress is 21.38 MPa when D is 10 mm and the h is 7 mm(Fig. 4).

3 Numerical Simulation

3.1 3D Finite Element Method (FEM) Model

For the numerical simulation of the fastener mechanical properties under pull-through load, 3Dfinite element fastener model was established by using software ABAQUS. Three-dimensionalsolid elements were adopted by the model. The type of the elements was eight-nodehexahedral elements (C3D8). Through the analysis of the loaded state of the countersunk-head fastener, the eight-node 3D cohesive element (COH3D8) was adopted to simulate theshearing sections between head and screw of the fastener(red region). The red region was alsocalled shear region. The 3D model is shown in Fig. 13.

Fig. 7 Load–displacement ofcountersunk-head fastener curve

Fig. 8 Load-bearing of thecountersunk-head

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The ABAQUS/Standard solver was used in the study, and the progressive damage processwas implemented by coding USDFLD, the subroutine of ABAQUS. The validity of numericalmodel could be verified by experimental results.

3.2 Cohesive Zone Failure Criteria and Damage Evolution Law

The cohesive element takes a linearly elastic-linearly softening traction-separation law in theanalysis which is also called the bi-linear traction-separation law [19] (Fig. 14).

The constitutive relationship of the cohesive elements is defined as

tntstt

8<:

9=; ¼

Knn 0 00 Kss 00 0 Ktt

24

35 εn

εsεt

8<:

9=; ð2Þ

Where tn, ts and tt are the normal and the two shear tractions respectively, and Kii (i=n, s, t),εi (i=n, s, t) represent the stiffness component and strain in i direction respectively.

Damage is assumed to initiate when a quadratic interaction function involving the nominalstress ratios reaches a value of 1.0. This criterion can be represented as:

tnh iton

� �2

þ tstos

� �2

þ tttot

� �2

¼ 1 ð3Þ

where The Macaulay bracket < > implies that a compressive normal stress does not contribute

to the damage initiation, it is shown as tnh i ¼ tn tn > 00 tn < 0

�. The tn

o, tso, tt

o are the interfacial

Table 2 Basic parameters ofMicro-CT Parameters Value

Scanning resolution 3 μm

Rotation 360°

Voltage 120KV

Electric 100 μA

Exposure time 3,000 ms

Fig. 9 CCD images depicting thefailure mode in fractured C/C fas-tener specimens

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normal and shear strength respectively. The shear strengths in the two orthogonal directions areassumed to be equal. [20]

The damage propagation of the head and screw region is determined by Benzeggagh-Kenane (BK) law which is based on the energy release rate which is particularly useful when

Fig. 10 Images of the C/C composite countersunk-head fastener fromMicro.CT. aDelamination failure mode. bIntralaminar failure mode

Fig. 11 Common fastener pull-through failure modes

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Fig. 12 SEM of the failure model around fastener head

Fig. 13 3D finite element fastener model

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the critical fracture energies during deformation purely along the first and the second sheardirections are the same; i.e. Gs

C=GtC, . It is given by

GCn þ GC

s −GCn

� � Gs þ Gt

Gn þ Gs þ Gt

� �α

¼ GC ð4Þ

where GnC, Gs

C are the critical energy release rate for pure Mode I and Mode IIfracture. Like the shear strengths, the critical energy release rates for the Mode IIfracture in the two orthogonal directions are also assumed to be the same. Theseenergy release rate are defined before analysis. The Gn, Gs, Gt are the calculatedstrain energy release rate. In this study, the value of the power is α=2. A linearstiffness degradation law based on damage variable D is that:

K ¼1−Dð ÞK0

nn 0 00 1−Dð ÞK0

ss 00 0 1−Dð ÞK0

tt

24

35;D ¼

0 δ < δ0

δmax δ0−δ� �

δ δ0−δmax� � δ0≤δ≤δmax

1 δmax < δ

8>><>>: ð5Þ

where δ0 is the effective displacement at the initiation of damage, which can becalculated through initial stiffness and strength properties, δmax refers to the maximumvalue of the effective displacement attained during the whole loading history.

Fig. 14 The bi-linear traction-separation law

Table 3 Three-dimensional Hashin criterion

Failure mode Formulas

Matrix tensile failure (σ2>0) e2m ¼ τ12SS12

� �2þ τ31

SS31

� �2þ σ2þσ3

Y t

� �2þ τ223−σ2�σ3

SS223

� �Matrix compress failure (σ2<0)

e2m ¼ σ2 þ σ3ð ÞYc

� Yc

2SS23

� 2

−1

!þ σ2 þ σ3ð Þ24 SS23ð Þ2

þ τ223−σ2 � σ3� �SS23ð Þ2 þ τ12

SS12

� 2

þ τ31SS31

� 2

Fiber tensile failure (σ1>0) e f ¼ σ1X t

� �2þ τ12

SS12

� �2þ τ13

SS31

� �2Fiber compress failure (σ1<0) e2f ¼ σ1

Xt

� �2

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3.3 3.3 Composite materials failure criteria and damage evolution law

Three-dimensional Hashin criterion [21] is very useful for determining the compositematerial failure. Stress-based formulas are used to judge the damage initiation. Fiberfailure, matrix failure and delamination can be governed separately by differentformulas in the criterion, as follows in the Table 3. The subscripts 1, 2 and 3 denote

Fig. 15 Flowchart for the FEM analysis

Table 4 Properties degradation

Where FV1, FV2 representedmatrix failure and fiber failure.The 0 represented non-failure, 1represented failure

Failure mode Material stiffness degradation

FV1 FV2 E1 E2 E3 V12 V13 V23 G12 G13 G23

0 0 E1 E2 E3 V12 V13 V23 G12 G13 G23

1 0 0 E2 E3 0 0 V23 0 0 G23

0 1 E1 0 E3 0 V13 0 0 G13 0

1 1 0 0 E3 0 0 0 0 0 0

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three principle material directions, respectively. The em, ef are the values of thecriteria, assuming failure initiates in relevant damage mode once any of them greaterthan 1.0 during the FEM analysis.

Figure 15 illustrates the flowchart for the FEM analysis.The material stiffness properties degradation (Table 4) was implemented by coding the

subroutine USDFLD in ABAQUS [22].Where FV1, FV2 represented matrix failure and fiber failure. The 0 represented non-failure,

1 represented failure.

3.4 3.4 Results and discussion

The comparison of the test and the FEM numerical simulation is listed in Table 5. There is only6.4 % error between two ultimate loads. Therefore, a good agreement indicates that the finiteelement model can predict the ultimate load of the composite fasteners pull-through test correctly.

The stress and damage evolution are obtained (Figs. 16 and 17). Figure 16 showsthe damage morphologies of the head and also pointes out that there is a compoundstress concentration region in the countersunk-head because the static force balance iskept by the shear-force when the pull-through load transferred from the bottom up inthe countersunk-head. It agrees with the analysis of head’s loaded state in Figs. 8, 9,and 10.

The damage progression in the test was as follow: the initial damage was foundvertical z-direction (Fig. 17(1)–(2)). It was the delamination of the laminated carbon

Table 5 Comparison between test and simulation results

Fastener Test ultimate load/N Simulation ultimate load/N Error

Countersunk fasteners 5361.38 5,000 6.74 %

Fig. 16 FEM result for head of fastener

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cloth which was around the head center region. This delamination was appearedmore and more obviously with the load increasing. The intralaminar shear failure oflaminated carbon cloth occurred when the load increased to a certain extent. Andthen the countersunk-head and the screw of fastener were separated, the damage waspropagated downwards until the whole fastener were damaged. In Fig. 17, it couldbe seen that the results of FEM simulation for the head damage evolution were ingood agreement with the experimental observation results (Fig. 12). From the above,the damage evolution of the composite fastener could be simulated very well by thisfinite element model.

Figure 18 shows the investigation of the ultimate pull-through loads between differentcountersunk depth fasteners by using the finite element fastener model. It indicates that thedepth value of countersunk-head is relative for the ultimate pull-through load, which increasesby the depth value increasing.

Fig. 17 FEM for countersunk-head fastener head of fastener damage evolution

Fig. 18 Ultimate pull-through loads for different countersunk depth

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4 Conclusions

In this paper, the countersunk-head C/C composite materials fasteners pull-through tests had beendone. Based on the experimental results and analysis, the following conclusions are offered:

1. The ultimate load of the pull-through mechanical is only 5.4KN because of the low shearstrength of the C/C composite materials.

2. The failuremechanism and damage of C/C composite materials fasteners are elementarily putforward. Under the pull-through load, the failure model of countersunk-head fasteners isdelamination slippage failure appearing in the circle region with diameter 10 mm around thecenter of the head, shear failure and fibers’ tensile failure appearing in the end of the head.

3. Comparing the simulation results of the FEM damage evolution model with the experi-mental results are accordant. The ultimate pull-through load increasing by the depth valueincreasing was studied based on the model.

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