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An Experimental Method for Dynamic Delamination Analysis of Composite Materials by Impact Bending a

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J. Wiegand , A. Hornig , R. Gerlach , C. Neale , N. Petrinic & W. Hufenbach a

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Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ

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Institute of Lightweight Engineering and Polymer Technology (ILK), Technische Universität Dresden, Holbeinstraße 3, 01307 Dresden, Germany Accepted author version posted online: 24 Apr 2014.

To cite this article: J. Wiegand, A. Hornig, R. Gerlach, C. Neale, N. Petrinic & W. Hufenbach (2014): An Experimental Method for Dynamic Delamination Analysis of Composite Materials by Impact Bending, Mechanics of Advanced Materials and Structures, DOI: 10.1080/15376494.2012.736066 To link to this article: http://dx.doi.org/10.1080/15376494.2012.736066

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ACCEPTED MANUSCRIPT An Experimental Method for Dynamic Delamination Analysis of Composite Materials by Impact Bending J. Wiegand1*, A. Hornig2, R. Gerlach1, C. Neale1, N. Petrinic1 and W. Hufenbach2

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(format: first name as initials, family name in full)

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Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ

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Institute of Lightweight Engineering and Polymer Technology (ILK), Technische Universität Dresden Holbeinstraße 3, 01307 Dresden, Germany *

Corresponding author; e-mail [email protected]

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Abstract An improved experimental method for characterising dynamic delamination growth in composite structures has been developed and verified using high speed photography and explicit finite Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

element simulation. The method is based on a 3-point bending device. End notch flexure carbon fibre composite beam specimen were subjected to both quasi-static and impact rates of Mode II loading. The experimental results showed no significant strain rate dependency of the delamination fracture toughness. This important result complements the scarce and conflicting data available in the literature, and serves as a reference for calibration of numerical modelling strategies. Keywords 1. composites 2. delamination 3. impact 4. fracture toughness 5. bending 6. wave analysis

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INTRODUCTION Composite laminates are increasingly used in aerospace applications which require high resistance to impact loading, such as aircraft engine fan blades, the aircraft fuselage and wing Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

leading edges. However, while the fibres in such composite laminates reinforce the in plane direction, resulting in high specific strength and stiffness, the out of plane direction of composite laminate remains free of fibre reinforcement. Furthermore, the stacking of several plies in a laminate leads to an additional weak point at the interface between the plies. Impact damage (e.g. bird strike) can lead to debonding failure of these interfaces, called delamination. Delamination results in a complete loss of the composite’s ability to bear out of plane loads and significantly reduces the bending stiffness of the laminate, thus greatly influencing the structures ability to withstand the loads for which it was designed. Understanding delamination initiation and the ability to accurately predict the extend of delamination damage due to impact loading still remains a key challenge in the design of composite structures. An accurate understanding of the delamination fracture toughness (DFT) is of particular importance, mainly due to the dominant influence of the DFT on delamination propagation. Especially the increasingly more frequent application of cohesive zone models for delamination prediction in general [1-5] and under impact loading in particular [6-8] requires accurate DFT values for a wide range of strain rates. However, it is still not well understood whether the well documented strain rate dependency of polymeric matrix materials [9,10], composite materials based on polymeric matrix materials [11,

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ACCEPTED MANUSCRIPT 12, 13,14] and the interface between matrix accumulations and fibre bundles [15] also results in a strain rate dependency of the DFT. A number of experimental studies examining the quasi static DFT of composite laminates have been performed [16,17,18], and a review listing available experimental methods can be found in [19,20]. However, very little data for dynamic Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

DFT characterisation are available in the open literature [21-25]. Most work on carbon/epoxy or graphite/PEEK composites has shown decreasing fracture toughness with increasing loading rate [21, 22, 23, 24]. However, some studies also observed an increase of the fracture toughness [25]. These contradicting observations could so far not be explained through crack propagation mechanics, and the possibility of an influence of the experimental methods used has to be considered. In the following, a brief overview about existing experimental methods for measuring the DFT is presented. Bending experiments are already widely used for the characterisation of the delamination properties of composite materials [16,19,26]. The experimental setups are usually straightforward as they comprise simple and well defined boundary conditions thus enabling the verification and validation of numerical models aimed at predictive modelling of material response to mechanical loading. The three-point-beam-bending is probably the most commonly used type of bending experiment. In the case of composite materials, different failure modes can be investigated by changing only a few experimental parameters, such as span and beam thickness. At higher loading rates the experimental methods are more complex and different approaches have been proposed. For example, impact bending experiments with drop weight testing devices are widely used [27]. In these experiments an impactor of known weight is instrumented with an

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ACCEPTED MANUSCRIPT accelerometer and dropped from a defined height. During the impact, the deceleration of the drop weight is measured using accelerometers which enable calculation of the force-time and displacement-time histories. The drop weight testing has already been standardised [28] despite the fact that the obtained force history data is usually polluted by numerous stress wave Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

reflections. These reflections result from the complex geometry and boundary conditions of the commonly used bulky drop weights. The noise may be of a similar order of magnitude as the response generated from the interaction with the specimen itself. Consequently, extensive data filtering operations are required to estimate the force transmitted in the actual specimen [29]. An alternative force measurement approach incorporates measurement on the non-moving boundaries of the specimen. This method relies on a complicated design of the specimen support to minimise wave reflections [30, 31]. Additional measurement techniques, such as those relying upon high speed photography or miniature strain gauges fitted directly onto the specimens are necessary if measurement of deformation during loading in drop weight experiments is required. An alternative approach to dynamic three-point-beam-bending extends from the well-established Split-Hopkinson-bar technique commonly used in uniaxial tensile and compressive testing at high rates of strain [32]. A bending apparatus utilising long thin rods has been proposed by Hallet [33]. Hallet modified the Split-Hopkinson pressure bar (SHPB) technique in order to monitor longitudinal stress waves in an impactor rod resulting from impact loads applied at the bar's end. The waves are measured some distance away from the bar ends and so onedimensional analysis of the measured stress waves may be used in the calculation of force-time and displacement-time histories of the bar end in contact with the target specimen [34]. This

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ACCEPTED MANUSCRIPT makes additional displacement measurement systems redundant. The use of long thin bars also yields considerably more accurate measurements than drop weight towers. These advantages led to the decision to develop an experimental methodology based on the Hopkinson bar, with particular focus on accurate force and beam deflection measurement. The Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

end notched three point bending test setup [35] is chosen for the characterisation of the Mode II DFT. The impact bending experimental method is developed and verified, and a detailed investigation of the dynamic Mode II DFT growth under shear loading is undertaken. The results are compared to quasi-static experiments, and conclusions about the dependency of the DFT on loading rate are drawn.

EXPERIMENTAL METHODOLOGY The baseline of this study is an impact bending apparatus described in [33]. A thin rod (the projectile) is accelerated by compressed air and collides with an instrumented rod (the impactor) of exactly the same material and geometric properties. In theory, the kinetic energy is entirely transferred from the projectile to the impactor, which travels unstressed towards the targeted specimen. When the impactor bar collides with the specimen, the history of longitudinal stress in the impactor bar generated during the interaction with the specimen is recorded by a set of strain gauges mounted on the surface of the impactor, approximately mid length of the bar. The geometry of the impactor allows for the use of one-dimensional wave analysis to calculate the force-time (and subsequently stress –time) and velocity-time histories in any chosen cross section [37]. The displacement u of the projectile tip is obtained by integration the velocity of the impactor tip v with respect to time

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ACCEPTED MANUSCRIPT u   vdt .

(1)

The beam deflection is given by the impactor displacement up from the time of impact. It is important to note that this methodology relies on the assumption of a stress free impactor


prior to impact. A sketch of the original experimental setup used as the inspiration for this study is given in Figure 1. In this setup, the impactor bar is slightly shorter than the projectile. The impactor tip is shaped as a blunt symmetric chisel in order to apply a line load across the width of the beam specimen. Figure 1 The impact bending apparatus In order to verify and validate the experimental setup, a number of steel beams were impacted with an initial velocity of approx. 7 m/s. The signals recorded on the impactor bar showed an intense ringing before the impactor bar collides with the specimen. This indicates trapped strain energy from the collision of the projectile and the impactor bar which does not dissipate prior to the impact with the specimen and negatively affects the measurement accuracy during the interaction between the impactor and the beam specimen (see Figure 2). The ringing does not carry any information about the interaction of impactor and specimen and can be considered as noise. In order to improve the understanding of the system, some experiments were recorded using a PHANTOM digital high speed camera. Images with a resolution of 800x256 pixels were obtained at a frame rate of 15,000 frames/s. The displacement of the impactor tip was monitored by means of fast Fourier transformation (FFT) based cross correlation image analysis with subpixel accuracy [38]. The impactor tip displacement from the image analysis can be directly

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ACCEPTED MANUSCRIPT compared with the results of the wave analysis and enables a verification of the analysis procedure. The results of the comparison are displayed in Figure 3. Figure 2 Example strain gauge reading of the original setup Figure 3 Measured strain gauge signal and comparison of deflection history as obtained from Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

the wave analysis and the image analysis From Figure 3 it becomes clear that the deflection calculation by wave analysis can yield a significant error. Initially, both wave analysis and image analysis correspond very well. However, as the specimen continues to bend, the wave analysis over predicts the beam deflection significantly, which is a direct consequence of the residual stress oscillations in the impactor. It has been suggested in [33] that a simple low pass filter, such as a running average, could be used to remove the oscillations from the data. This would appear reasonable as it has been demonstrated analytically in [27] that the high frequency vibrations in the loading bar could not be transmitted into the specimen. However, the experimental investigation supported by highspeed-photography showed a greater error in deflections calculated using filtering (see Figure 3). Even though the high frequent oscillations are not transmitted into the specimen, they still contribute to the calculation of the displacement of the impactor itself and subsequently the predicted force and deflection histories. In addition, any filtering would also affect the measurement of stress waves initiated by the collision with the target beam specimen. Therefore, the accuracy of the experimental procedure has to be improved in order to provide useful data for constitutive model verification. This requires the source of the trapped strain energy in the strain gauge signal recording to be traced and removed.

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ACCEPTED MANUSCRIPT The initial impact of projectile and impact bar causes a compressive wave to travel along the impact bar. In an ideal configuration, the initiated stress wave should be close to a square pulse which is cancelled out by the tensile reflection at the impactor tip (see Figure 4 a). However, a second stress wave is detected shortly after the first stress wave passes the strain gauge (Figure Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

5). This second wave is not entirely cancelled out by the first tensile reflection and is therefore trapped as both bars separate. The time difference between first and second wave enabled to identify the second stress wave as being caused by an impedance jump in the projectile due to nylon runners attached for low friction sliding during the acceleration (Figure 4 b). Consequently, the experimental setup is modified to minimise any impedance jumps along the path of the stress waves. This was achieved by changing the setup of the gas gun loading device such that the impactor bar is supported by nylon tubes attached to the gas gun, not to the impactor. The stress waves initiated by the collision of projectile and impact bar are measured and compared to the original experimental setup in Figure 5. The comparison clearly demonstrates that no significant stress waves are trapped in the impactor bar. Figure 4 Lagrange charts for projectile-impactor interaction Figure 5 Stress waves of the collision of projectile and impactor bar

In order to verify the deflection calculation, steel beam specimens were subjected to impact bending. The deflection of a beam specimen obtained by wave analysis and image analysis is compared in Figure 6, and demonstrates the accurate prediction of the beam deflection well beyond the maximum deflection of the beam. Figure 6 Deflection verification

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ACCEPTED MANUSCRIPT The challenges in dynamic load measurement do not allow for a direct experimental measurement of the contact force. Therefore, an accurate numerical model of the impact bending experiment is used to compare the calculated contact force from wave analysis to the force obtained numerically. Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

This virtual test is performed using the explicit FE solver LS-DYNA. The impactor bar and the specimen are modelled elastically in full size using hexahedral elements. The boundary conditions of the experiment are closely reproduced by giving the impactor bar an initial velocity. The beam bearings are modelled as rigid bodies and frictionless contact algorithms are used to simulate the interaction between the parts. The cross section force is extracted from the FE model at the same position where the strain gauge is located in the real experiment. This cross section force is then used as input for a wave analysis to calculate beam deflection and contact force. These are then compared with the deflection and contact force as computed by the FE analysis (see Figure 7). The deflection compares very well, with only a small difference in the maximum deflection. This is probably caused by the non-perfect geometric representation of the impactor tip in the FE model. The contact force initially compares very well. The force obtained by wave analysis becomes noisier during the impact. This is caused by the simplifications of the wave analysis which neglects wave dispersion and Poisson’s effects. The force up to the maximum force, however, is fairly accurate. Also, initial oscillations of the force typical for impact experiments are captured very well. This demonstrates that the presented method enables an accurate measurement of the beam deflection and a good measurement of the contact force during impact bending experiments. Figure 7 Verification by means of virtual testing

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END NOTCH FLEXURE EXPERIMENTS FOR DFT


CHARACTERISATION UNDER VARIABLE LOADING VELOCITIES In order to assess whether the DFT of carbon fibre composites depends on the loading rate, three point end notch flexure (3ENF) test were performed at quasi-static and impact loading rates. The experimental device described above (see Figure 1) was used for the impact experiments, whilst a standard screw driven testing machine (loading rate: 0.1mm/min) was used for the quasi-static tests. Parent Material and Specimen Preparation Beam specimens were cut from a cross ply laminate [0/90/0]2s of HTS40 fibres and toughened PR520 epoxy resin. Two specimen geometries were used. The first geometry (indicated as DIN) followed as close as possible the geometry suggested in the DIN standard (withdrawn) [35], but could only be used within the quasi-static-experiments. Therefore, a modified non-standard specimen (indicated as IMP) is proposed. The IMP specimen geometry is influenced by the geometry of the impact apparatus. Both geometries are presented in Table 1. Table 1 ENF specimen geometry (all in mm) The recognised method of pre-cracking for DFT characterisation involves manufacturing panels with a lubricated PTFE film inserted between the appropriate plies, where a pre-crack can be propagated by Mode I loading. In this study, the notch was introduced using an alternative method, as proposed by Tanzawa [36]. A precision cutting machine with 0.35 mm diamond cut off wheel was used to machine a 10 mm long notch into the 0/0 plies in the beam centre. A mid-

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ACCEPTED MANUSCRIPT plane crack was then encouraged by using razor blades to make a small cut into the 0/0 plane. Pre-cracks were propagated to their desired lengths inserting a sharp 10 degree wedge into the machined notch. The specimens were painted white to enable tracking of the crack during the pre-cracking. The position of pre-crack was marked on the specimen. An example of a typical Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

pre-crack is given in Figure 8. Figure 8 Pre-cracked notch Data Analysis A digital high speed camera (Phantom, 21,000 frames/s) is used to monitor the crack tip on the specimen during the impact experiments, while a high resolution digital camera was used during the quasi-static experiments. The images recorded in both quasi-static and impact experiments were processed using MATLAB. The global coordinates of a specimen's crack tip in each frame were measured from the images, and the random error associated with the measurement of the crack tip position data was effectively filtered by linearly interpolating between key points on the crack position-time plot. This enabled the clear identification of the times when the delamination progressed. See Figure 9 for an example of this process for an impact experiment. Figure 9 Crack propagation obtained from the high speed footage

It should be noted that the impact bending experiment does not yield a constant strain rate throughout the experiment. This is because the loading velocity reduces as the beam deflects. The actual local strain rates could be observed by the use of digital speckle photography and subsequent full field strain measurement. Alternatively, numerical models could be used to

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ACCEPTED MANUSCRIPT obtain strain rate estimates. The lack of evidence for strong rate dependent material behaviour in this study, however, did not justify the significant effort such measurements would require. The fracture toughness was calculated using equation 2 taken from the [35] where GIIC is the critical fracture toughness (Jm-2), d is the crosshead displacement at crack initiation (mm), P is Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

the critical load at crack initiation (N), a is the crack length within the supports at initiation (mm), w is the width of specimen (mm) and L is the span length (mm). It is derived using simple beam theory assuming homogeneous specimen stiffness and is known as the direct beam method [Error! Reference source not found.].

GIIC 

9000 Pa 2 d . 2w 0.25L3  3a 3





(2)

Equation 2 is used to generate the fracture resistance curves (R-curves) for the quasi-static and impact experiments respectively. Mean R-curves and impact velocities were calculated by interpolating the individual R-curves over an array of crack lengths and averaging each point.

RESULTS AND DISCUSSION The first set of experiments was performed quasi-statically using the DIN geometry, followed by quasi-static experiments using the IMP geometry. As the indenter geometry for DIN and IMP geometries is different, these comparisons were solely used to establish the general validity of the impact specimen geometry. The investigation of the effect of the loading rate on DFT, which is the main focus of this study, was conducted by performing impact experiments using the IMP geometry only, at impact velocities of 4 m/s and 6 m/s respectively.

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ACCEPTED MANUSCRIPT The measured force-deflection diagrams are displayed in Figure 10. Figure 10 b) directly compares the force measured during the quasi-static and impact experiments. For clarity the impact force is displayed after being filtered using a low-pass frequency filter. This enables a better direct comparison to the quasi-static trends. The comparison of the quasi-static R-curves Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

for the DIN and IMP geometry is displayed in Figure 11. Figure 12 shows the comparison of the quasi-static and impact R-curves for the IMP geometry. An example of experimentally observed delamination growth is displayed in Figure 13. Figure 10 Force deflection curves from the ENF experiments Figure 11 Comparison of the quasi-static R curves Figure 12 Comparison of quasi-static and impact R-curves Figure 13 Delamination propagation during an impact ENF experiment The quasi static data in Figure 10 shows the difference in beam stiffness due to the different specimen thickness and width. Additionally, a comparison of quasi-static and an impact bending force-deflection data is given. Although the impact data shows significant oscillations, which is expected, the general trend is very similar to the quasi-static experiments. The quasi-static R-curves in Figure 11 show very similar results for the DIN and IMP geometries. This indicates that the results from both geometries are comparable. However, the experimental scatter is relatively large for both specimen geometries. The DFT increases dramatically when the crack crosses the specimen centre where the load is applied. This is due to through thickness compression and the resulting interlaminar friction. A similar behaviour was observed in the impact experiments. The data beyond the specimen centre cannot be used.

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ACCEPTED MANUSCRIPT Figure 12 shows the comparison of the 3ENF tests for impact and quasi-static loading. Both data sets show very similar behaviour. The DFT values are of similar magnitude and no obvious strain rate effects were observed. Also, the increase of the impact velocity from 4 m/s to 6 m/s did not show any effect on the DFT. Downloaded by [the Bodleian Libraries of the University of Oxford] at 05:30 17 July 2014

The experimental scatter observed for the impact experiments was of a similar order as observed in the quasi-static experiments. As a result, no reliable absolute numbers for the DFT of the material tested could be derived. The range of values obtained, however, indicates that the DFT of the tested material does not significantly depend on the loading rate. An overview of the measured fracture toughness values is given in Table 2. Table 2 Experimentally determined fracture toughness values

CONCLUSIONS An improved method for experimentally characterising the DFT of composite beams subjected to impact loading has been developed, and verified via numerical analysis. The method allows for the accurate measurement of force-time and deflection-time histories, in turn allowing for the measurement of the DFT of composite laminates. Using the developed methodology, carbon fibre composite beam specimen using the end-notchflexure geometry were subjected to both quasi-static and impact rates of loading. No significant strain rate dependency of the DFT could be observed. This is a very important result, especially with regards to modelling strategies, as it suggests that no rate dependency needs to be included in numerical fracture toughness simulations.

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ACCEPTED MANUSCRIPT ACKNOWLEDGMENTS This research has been partially sponsored by the EU (VITAL, FP6-012271) whose support is


kindly acknowledged.

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A. Jumpasut, N. Petrinic, B.C.F. Elliott, C.R. Siviour, and M.R. Arthington, An Error Analysis into the Use of Regular Targets and Target Detection in Image Analysis for Impact Engineering, Applied Mechanics and Materials, 13(14), pp. 203-210, 2008. 39. J.R Reeder, K. Demarco, and K.S. Whitley, The use of doubler reinforcement in


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Figure 1 The impact bending apparatus

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Figure 2 Example strain gauge reading of the original setup

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Figure 3 Measured strain gauge signal and comparison of deflection history as obtained from the wave analysis and the image analysis

Figure 4 Lagrange charts for projectile-impactor interaction

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Figure 5 Stress waves of the collision of projectile and impactor bar

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Figure 6 Deflection verification

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Figure 7 Verification by means of virtual testing

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Figure 8 Pre-cracked notch

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Figure 9 Crack propagation obtained from the high speed footage

Figure 10 Force deflection curves from the ENF experiments

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Figure 11 Comparison of the quasi-static R curves

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Figure 12 Comparison of quasi-static and impact R-curves

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Figure 13 Delamination propagation during an impact ENF experiment

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Table 1 ENF specimen geometry (all in mm)

Table 2 Experimentally determined fracture toughness values

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