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Experimental and Numerical Investigations of Kenaf Natural Fiber Reinforced Composite Subjected to Impact Loading

Sareh Aiman Hilmi Abu Seman ,1 Roslan Ahmad,1 Hazizan Md Akil2 1 School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang 14300, Malaysia 2

School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang 14300, Malaysia

The aim of this work is to develop a Finite Element Model based on continuum damage mechanics to predict the structural response of Kenaf composite when subjected to high-velocity impact. The stress–strain response of the Kenaf composite through the fiber direction was simulated to examine prediction accuracy of this simulation approach. A combined elasticdamage material model, incorporating the 3D Hashin’s failure criteria, was implemented into the ABAQUS/ Explicit finite element code by user-defined VUMAT material subroutines. The results from the model predictions were compared with experimental data obtained by SHPB compression testing. The developed Finite Element Model of the full-scale test presented an accurate prediction of the strain waves in the bars and successfully replicated the structural impact response of Kenaf composite over the range of impact loadings studied. The model also provides a reasonably good agreement between numerical and experimental results on the effect of strain rate on the failure stress and failure C 2018 Society of strain. POLYM. COMPOS., 00:000–000, 2018. V Plastics Engineers

INTRODUCTION Over the past few decades, industry has tried to minimize the use of fibers manufactured from petroleum as they are non-degradable and non-sustainable [1]. Natural fiber has many advantages including; being cost effective, environmentally friendly, highly available and sustainable with high performance characteristics. Natural fibers have piqued the interests of scientists and engineers as a viable alternative for industrial applications [2–4]. In service, natural fiber reinforced composites may experience static Correspondence to: H. Md Akil; e-mail: [email protected] Contract grant sponsor: Universiti Sains Malaysia (USM). DOI 10.1002/pc.24758 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2018 Society of Plastics Engineers V

POLYMER COMPOSITES—2018

and dynamic loading conditions. It has been demonstrated that most polymer composite materials behave differently under static and dynamic conditions [5]. Currently, understanding of the dynamic mechanical properties of natural fiber reinforced composite is very limited. The majority of the data is quasi-statically collected using Universal Testing Machines with a limitation of strain rate at 0.1 s21 [6]. Lack of understanding of differences in material strength, with regards to the impact rate, will cause materials to be inefficiently and ineffectively implemented in designs. Therefore, there is a need to distinguish and document the behavior of Kenaf composite materials under high rate loading to enable the effective use of these composites in a variety of high performance applications. In dynamic conditions that is, crash situations, the strain rates occurring can range from 100 to 1,000 s21, therefore a dynamic and robust test device must be employed. Several dynamic devices have been used to study the behavior of materials under high strain rates; these include a Split-Hopkinson pressure bar (SHPB), Kolsky bar [6], a flying wedge [7] or air gun system [8]. Among these, SHPB seems to be the most popular and widely used method of measuring dynamic mechanical properties of materials. This is attributable to the simple arrangement and ease of operation that this system offers in comparison to the other techniques. Experimental tests involving a broad range of strain rates are necessary to characterize the behavior of the material. In this paper, a test program involves a Universal Testing Machine imposing strain rates up to approximately 0.01 s21 and a compression SHPB has been employed for obtaining the rates occurring between 1,500 and 2,500s21. The majority of dynamic tests are both destructive and costly in nature. However, by applying a numerical technique through the powerful finite element method (FEM),

virtual testing can be conducted which appropriately describes the impact situation and can forecast the damage of the components involved. This is done through the implementation of different failure criteria in a relatively short time. A complete numerical build-up of a splitHopkinson bar is required to accurately predict the experimental measurement data. Satisfactory results, that is, simulating the fragmentation and detailed stress distributions of the specimen, can be obtained by applying an appropriate number of elements. In recent years, several numerical models of the SHPB were developed to predict the impact behavior of many composite materials. Meng and Li [9] modeled their compression Split-Hopkinson bar set-up using Abaqus. Strain time histories obtained from experiments and numerical simulations were compared and found to satisfactorily agree. Wang and Li [10] developed a full-scale compression Split-Hopkinson bar in Abaqus to predict the stress distribution and damage evolution in alumina. The agreement between observations obtained through high speed photography during the experiment and the finite element (FE) predicted damage process, supports the developed FE model. Several researchers have been involved in exploring the failure criteria for fiber reinforced composites [11–16]. Several methods and resources have been addressed to establish the failure criteria for fiber reinforced polymer composites. Damage modeling in composites can be studied either by a stress or strain-based failure criteria approach; or following damage mechanics concepts. The earliest failure criteria established was based on Von Mises Criterion which was developed for isotropic materials, such as metals. These polynomial failure criteria, such as the Tsai-Wu [11] or Tsai-Hill [16] are an extension of Von Mises Criterion, developed to fulfill the orthotropic nature of composite material. Based on the equivalent stress or strain, these criteria are usually employed to describe the failure envelope of any given multidirectional laminate subjected to multi-axial loading. However, the damage mechanisms of different modes cannot be clearly characterized using the polynomial failure criteria, unless they are applied at the ply level. Hence a ply-by-ply method has been widely used to model the progressive failure in composites. As the previous criteria is unable to influence the properties of Fiber Reinforced Polymers (FRP) by degrading the stiffness and strength, Hashin has proposed a failure criterion that consists of four distinct failure modes: fiber damage in tension and compression and matrix tensile and compressive failure [12, 17]. Different from the polynomial failure criteria, the progressive damage model is considered in Hashin-based criterion in which the stiffness is degraded after initial structural failure. Hashin-based criterion has been successfully adopted by many researchers in the past to study and predict the behavior of many orthotropic or anisotropic materials; especially polymer composite laminate with either glass [18] or carbon fiber [19, 20] reinforced. Despite being 2 POLYMER COMPOSITES—2018

intensively used to simulate the strength of structures, Hashin-based damage criterions have rarely been used for transversely isotropic natural fiber reinforced composites. To the authors’ knowledge, only the work done by Rubio-Lopez et al. [21], involves dealing with Hashinbased damage criterion to predict the behavior of all cellulose composite under low-velocity impacts. During this work, validation of the model was obtained through comparison with the experimental results. Regardless of that, the modeling of Kenaf composite is an almost unexplored field. Motivated by this background, the objective of this paper is twofold. First, to characterize the Kenaf composite under quasi-static and high strain rates using the SHPB technique. Quasi-static test data was used to determine the undamaged elastic material parameter in the material model, while the results from SHPB tests were served for validation. Second, to develop the first numerical model to predict the high-velocity impact behavior of Kenaf composite involving a SHPB device using the FEM. The results obtained by the FE analyses are then illustrated and compared with experimental data obtained by the SHPB impact tests to assess the predictive capabilities of the model. EXPERIMENTAL Materials The materials tested were the Pultruded Kenaf Fiber Reinforced Composites samples which were prepared using a thermoset pultrusion machine at the School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Penang, Malaysia. Tex size of Kenaf fiber yarns used is 1,400 with average diameter of 1.27 mm and the resin used is unsaturated polyester resin (Crystic P9901). The Kenaf composites that demonstrated high thermal stability Azwa and Yousif [22] were cured at 1208C temperature to meet the ideal characteristics required. The composite then shaped into a continuous rod with an average diameter of 12 mm. The pultruded natural fiber composites were fabricated with approximately 70 wt% of the fiber and typically 30 wt% of the matrix. Cylindrical Kenaf composite specimens of the diameter, d 5 12 mm were used for uniaxial compression experiments at dynamic and quasi-static strain rates. For the SHPB test, an optimum slenderness ratio of 0.5 has been used as suggested by Davies and Hunter [23] meanwhile for static tests, a specimen with a slenderness ratio of 1.5 was selected to meet the ASTM Designation of E9–89. Compressive Test A Universal Testing Machine was employed to conduct the static compression test. This test was performed to determine a baseline response of the material to DOI 10.1002/pc

FIG. 1. The schematic of the SHPB system.

understand differences induced by different loading rates. A constant crosshead speed of 0.1 mm/min which corresponds to the strain rate of 1.0 3 1022 s21 was applied to the specimens at room temperature. Figure 1 shows the schematic diagram of a SHPB system used to conduct the dynamic uniaxial compression test. The striker, input and output bars of 12mm in diameter were 150 mm, 1500 mm, and 1500 mm in length, respectively. The bars were made of YAG300 maraging steel with Young’s modulus of 184 GPa, Poisson’s ratio of 0.30 and yield strength of 2,000 MPa. A valid Hopkinson bar experiment can be realized by carrying out a calibration method first before the actual tests are conducted to ensure obtainability and reliability of the accurate values of specimen strains. Based on Fig. 2, it can be seen that the stress states on both bars were coincident and nearly identical which shows that no dispersion is present. Therefore, it validates that the employed SHPB for this study was practically aligned and frictionless. CONSTITUTIVE MODEL Research done by Hashin and Rotem and Hashin [12, 17] have led to the establishment of the Hashin Damage Criterion. Instead of predicting damage initiation through a single Equation as proposed in the polynomial criteria such as the Tsai-Hill and Tsai-Wu criteria [11, 16], four failure modes with four corresponding failure indices related to fiber and matrix failures have been considered in the Hashin damage initiation criterion [6]. Degradation of a material point or damage initiation starts when the corresponding failure of fiber/matrix in tension/compression is satisfied. The Hashin failure criteria can be expressed as follows: Tensile fiber mode: r11 > 0 Compressive fiber mode: r11 < 0 2 2 2 2 r13 r11 r12 r11 1 1 51 d 51 if, If, ft X1T S12 F13 X1C 51 dfc 51 Tensile matrix modes: r22 1r33 > 0 If,

ðr22 1r33 Þ2 22 r33 1 r23 2r 2 S223 X2t

1

r212 1r213 S212

Where, dft, dfc, dmt, and dmc denote the damage variables associated with the corresponding failure modes in the fiber and matrix. X1t , X1c , X2t X2c denotes tensile and compressive failure strength in fiber Directions 1 and 2 while S12 S13 and S23 denotes failure shear strength in 1–2, 1–3, and 2–3 planes. Failure criteria for laminated composites are available in ABAQUS/Explicit; however they can only be applied for continuum shell elements. The continuum shell elements, with the existing failure criteria, are not capable of taking large through-the-thickness deformations into account. Therefore, it is necessary to apply a constitutive model and failure criteria suitable for simulating the composite using 3D solid elements (C3D8R). For this purpose, the Hashin failure criterion was extended to 3D to simulate the Kenaf composite under dynamic loading. The failure criteria, with the related constitutive model, are implemented into ABAQUS/Explicit using a VUMAT subroutine provided by ABAQUS [24]. DEVELOPMENT OF FINITE ELEMENT MODELLING A 3D FE model of the SHPB compression on Kenaf composite was developed in the ABAQUS/Explicit software (Dassault Syste`mes Simulia Corp., RI) to simulate the stress–strain behavior of the Kenaf composite. The full-scale geometries of the SHPB testing setup and Kenaf composite were modeled and meshed with hexahedral elements, C3D8R. The deletion of FE elements was applied to the corresponding part in Kenaf composite where the damage occurred. The mesh density of FE elements was selected for the computational efficiency/accuracy and energy dissipation during the damage process. Different FE mesh densities were applied to the bars and Kenaf composite (Fig. 3). In order to obtain the appropriate FEs size for the Kenaf composite, the dynamic response of specimen in the SHPB test was simulated as a function of the FE mesh density. Referring to Fig. 4, it was found that the increasing of mesh density leads to a slight overestimation of failure stress value (with approximation percentage difference < 5%) and too large elements might lead through an underestimation of failure stress value.

51 dmt 51

Compressive matrix modes: r22 1r33 < 0 h i ðr2 2r r Þ ðr22 1r33 Þ 1r33 Þ2 X2c 1 ðr224S 1 23 S2 22 33 1 If, 2 2S23 21 X2c

r212 1r213 S212

23

51 dmc 51

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23

FIG. 2. Stresses on incident and transmitter bars during calibration.

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FIG. 3. Finite element mesh of SHPB and specimen.

A good convergence reflecting the global response of specimen under impact loading was achieved (with approximation percentage difference < 1.5%) when specimen was meshed with at least 978 FE elements. Even though higher mesh density leads to a slight overestimation, 5% percentage difference is acceptable considering complex dynamic structural analysis. Besides, a higher mesh density is much better when simulating the fragmentation of the specimen and provides more accurate stress distributions during the SHPB test. In this study, coarse mesh has been chosen to provide satisfactory results on all simulations since the global stiffness is the focus in this investigation. Consequently, less time-consuming analyses have been achieved. The entire incident and transmission bars of the SHPB were also modeled which a total of 25,200 elements were employed to both bars. The initial impact velocity of the striker bar (16–18 ms21) in the FE model was determined from the SHPB experiments, corresponding to the average dynamic strain rate of 1,500–2,500 s21. A frictional coefficient of 0.06 was applied to all the interfacial contacts. Considering the boundary conditions, the striker, incident and transmitted bars were free to move in the longitudinal direction but were not allowed to have any displacements in the transverse directions during the wave propagation phase of the simulation. The specimen however was free to move in all three directions, facilitating an accurate representation of radial movements. A condition of general contact interaction was defined between the surface pairs of either bar–bar or bar–specimen. The contact interaction properties for interaction between the surface pairs were also defined. Specimen made of Kenaf composite used in this investigation has been considered a transversely isotropic unidirectional composite. The x coordinate is defined as the direction aligned with the fibers in the bundle, and the yz plane is perpendicular to this direction. Table 1 summarized the main properties adopted in the analyses. The undamaged elastic properties used in the FE model were obtained from a number of Quasi-static experimental tests and some were gathered from the literature [25, 26]. For the striker, input and output bars, the elastic properties were specified with E 5 210,000 MPa and m 5 0.3 because they deformed elastically during the SHPB experiments.

room temperature, and the strain rate ranged from 1022 to 2.5 3 103 s21. The quasi-static tests at a strain rate of 1022 s21 were carried out using a Universal Testing Machine. To ensure that there is no effect of changing the slenderness ratio when going from low to high strain rates, specimens with different slenderness ratio were tested under static tests to evaluate if there was any variation on mechanical properties obtained. As shown in Fig. 5, the compressive strength was slightly affected by varying the slenderness ratio. Table 1 lists the mechanical properties for the examined Kenaf composite. Based on this experimental investigation value, there is discrepancy with the theoretical value. It is believed that the misalignment of fibers, presence of voids and low fiber-matrix interfacial strength which are typically the result of an imperfection during the fabrication process of the material were the culprits that causes degradation to the overall mechanical properties of the composite. This was also reported by Liebig et al. [27] and Arib et al. [28] The dynamic compressive tests involving strain rates in the range of 1,500–2,500 s21 were performed in the SHPB. The high strain rate stress–strain behavior of Kenaf composites under compressive loading is presented in Fig. 6 at three different strain rate levels. A representative quasi-static stress–strain curve is plotted in the same figure. The quasi-static behavior is almost linear up to the ultimate stress level for this loading. Like quasi-static tests, all the high strain rate specimens showed some linear behavior at lower loads. As the propagating stress pulses increasingly load the specimens, they show progressive non-linear behavior in the stress–strain response. During the high strain rate test, the ultimate stress observed is higher than its quasi-static value. This increase is from 83 MPa at 3.3% strain to 115 MPa at 2.9% strain. To explain about this phenomenon, Abrate [29] have come out with a theory. Under transient strainrate deformation, damage propagation time is decreased. Therefore, as the strain rate increases from low to high, the cumulative damage is decreased. With insignificant amount of damage at high strain rates, the material is able to withstand much higher load and deform much longer before it fails. The elastic responses show an increasing trend from static to dynamic loadings. However, the

RESULTS AND DISCUSSION Experimental Results Uniaxial compressive tests were carried out in the fiber direction of the composites. All tests were performed at 4 POLYMER COMPOSITES—2018

FIG. 4. The relative percentage difference between predicted and measured compressive strength as a function of the mesh density.

DOI 10.1002/pc

TABLE 1. Elastic constants and strengths of Kenaf fiber reinforced composite. E1 (GPa)

E2 (GPa)

E3 (GPa)

V12

V13

V23

G12 (GPa)

G13(GPa)

G23 (GPa)

q (kg/m3)

1.14

1.14

0.132

0.043

0.043

0.16787

0.05087

0.06056

890

X1c (MPa)

X2t (MPa)

X2c(MPa)

S12(MPa)

S13(MPa)

S23 (MPa)

86

57

47

83

55

3.86 X1t (MPa) 128

elastic phase of the stress—strain curves were shifting downwards with the increase of strain rates, indicating lower elastic modulus. The maximum stress of the Kenaf composite specimens is presented in Fig. 7 as a function of average strain-rate. The maximum stresses in the range of 1,500– 2,500 s21 appear to be almost constant. The average value of this strain-rate insensitive ultimate stress is found to be 115 MPa. Different result have been observed by Omar, et al. [30] on Kenaf composite between a strain rate range of 1,021–1,340 s21 where the ultimate stress was affected by strain rate. Contradiction in results can be explained in terms of molecular-level motions based on Ree–Eyring theory [31]. According to Ree and Eyring [31], when a particular degree of freedom of the polymer chain suddenly becomes restricted at high strain rates, the corresponding process begins to contribute to the overall material deformation resistance. This theory is true between strain rate ranges of 1,021–1,340 s21. However, when the strain rate enters the range of 1,500–2,500 s21 or the transition threshold, it is believed that the heat has been generated in the material. Witnessed by Moss and Pond [32], some part of the work done on the specimens is being converted to heat and it is all contributing to increased molecular mobility within the material. The polymer chains will only be stretching until reach a limiting stretch ratio which will cause fracture if surpassed [33]. The ultimate strength of glass/epoxy and glass/vinyl ester were also found to be strain rate insensitive as reported by Hayes and Adams [34] and Gama et al. [35] The specimens’ strains at the maximum stress are presented in Fig. 8 as a function of strain-rate. Unlike the data points of maximum stress, the strain at the maximum

FIG. 5.

Effect of slenderness ratio on compressive strength.

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57.3

stress of the Kenaf composite specimens’ increases with test strain-rate, and the trend is represented by a solid line. Increase in the failure strain can be described by the movement of molecular polymer chains within the material. As the strain rate increased, much higher heat will be generated within the material which mean increased in molecular mobility. Consequently, this makes the material less stiffer and prone to stretch until reaching a limiting stretch ratio [33]. Validation of FE Model The strain waves ðEi ; Er ; Et Þ in the middle of the input and output bars were predicted in the FE model of the full-scale SHPB system. These predicted waves were compared with the waves that were measured during the SHPB tests. Note that the time in both the FE model and the experiment was synchronized for comparison purpose. The FE predictions for the strain waves in the bars (Fig. 9) of Kenaf composite match those measured in the SHPB experiments. This reveals that the Abaqus model indeed is capable of capturing the physics in an SHPB test. The applied velocity of the striker bar in the simulation ensured that the force and the strain rate was approximately the same as in the test which is between 1,500 and 2,500s21. According to the figure, the strain in gauge increases from zero to approximately 0.00163 at t 5 0.285 ms. This is associated with the incoming stress wave in the incident bar. The stress wave propagates from incident bar towards the free end, where it is partly transmitted into the specimen, and partly reflected. The reflected stress wave is picked up by gauge at t 5 0.486 ms, and simultaneously, a strain increase is registered by gauge in the transmission bar.

FIG. 6. Stress–strain curves at different levels of strain rate.


FIG. 7.

Effect of strain rate on failure stress.

In all the SHPB tests, a notable occurrence in the reflected wave was observed. The reflected wave would increase to a peak value and then start to decrease. This phenomenon occurred due to the unidirectional fibrous structure of Kenaf composite. By means of a high value of the incident wave, the initial deforming rate is high shown by the reflected wave value. However, when the transmitted wave is increased, the reflected wave starts to decrease. The cause of this is due to a wave propagating through a non-homogeneous material. The unidirectional fibrous structure of Kenaf composite has disturbed the propagating waves entering it which triggers internal reflections and has led to wave interactions that disrupt the shape of reflected wave. As shown in Fig. 9, the developed FE model could provide a correct prediction on this phenomenon. Some oscillations are observed in the incoming strains in simulations, see Fig. 9. Contrary to the experiments, where the stress wave has a finite rise time of about 50 ms, the striker was released abruptly in the numerical model. This causes oscillations of Pochhammer-Cree variation in the simulations. Same phenomenon has also been documented by Wang and Li [10] and Dass Goel, et al. [36] in their numerical simulations of SHPB. There are ways to reduce these numerical vibrations; either by introducing damping in to the model, or to sample the simulation response with the same logging frequency as in the test, that is, 1 MHz. However, those methods are not applied here. Looking at Fig. 9, the numerically predicted

FIG. 8. Effect of strain rate on failure strain.

6 POLYMER COMPOSITES—2018

FIG. 9. Comparison between experimentally measured and FE predicted strain waves in the middle of the input and output bars in the SHPB system.

strains in the input bar seem to oscillate around a mean value agreeing well with the experimentally observed strain. Considering the output bar, the oscillations in the transmitted strains at gauge are much less serious than they are for the incoming strains. Besides, the average value of the transmitted strain is well predicted in almost all simulations. The engineering stress–strain curves of Kenaf composite, at three different strain rates obtained from simulations and experiments, are compared in Fig. 10 to verify the effectiveness of the proposed macro-scale homogeneous model. All curves are determined in the same way, by which the strain history presented from the experiments and simulations are calculated from the strain registrations at the same positions on the incident and transmitted bars. As indicated by Fig. 10, the stress–strain behavior of the Kenaf composite at these three different strain rates is well described. Each stress–strain curve depicts three stages of deformation and damage that the specimen undergoes. These initially consist of approximate linear deformation or elastic stage, plastic stage and followed by post-peak stage. A reasonable agreement in predicting the initial slope, the maximum stress and the maximum failure strain can be found from the developed composite model. This shows that the developed models were able to replicate the vital features on the response of

FIG. 10. Comparison of the strain-time curves from simulations and experiments.

DOI 10.1002/pc

the composite under compressive load conditions. The behavioral predictions towards strain-rate of the Kenaf composites: rate insensitivity of failure stress and rate sensitivity of failure strain, are adequately supported by the test results. This consistency validates the developed FE model of the SHPB system and the use of the Hashin failure criterion for Kenaf composite. CONCLUDING REMARKS In this article, an experimental study on the quasistatic and high strain-rate uniaxial compression tests of a Kenaf composite was presented. The SHPB and a Universal Testing Machine were employed to characterize the compressive properties of a Kenaf composite under various strain-rate conditions. Specimens were compressed both statically and dynamically in the extrusion direction. Data from the Quasi-static tests were used to identify the undamaged elastic material parameter in the material model, while the SHPB tests were served for validation. The results from SHPB tests indicated that the maximum stress is strain-rate insensitive, while failure strain was observed to increase with the increase in strain rates. A Finite Element numerical model was carried out in the present investigation to predict the observed behavior of Kenaf composite under dynamic impacts. The model was validated through comparison with experimental results. Kenaf composite was analyzed with three different striker velocities leading to three different strain rates obtained from experiment, that is, 1,500, 2,000, and 2,500 s21. Using explicit solvers of the Abaqus, the incident and transmitted strain obtained from the simulations were in good agreement with the experimental data. The full-scale stress–strain curves at different strain rates of Kenaf composite are well predicted by the developed FE model of the SHPB system with the use of the applied material model incorporating the Hashin damage model. The material model also provides sufficiently accurate simulations towards the effect of strain rates on the failure strain. As a general conclusion, the developed numerical model is deemed both suitably and effectively competent at simulating the effect of strain rates on natural fiber materials such as Kenaf composite. REFERENCES 1. M.P.M. Dicker, P.F. Duckworth, A.B. Baker, G. Francois, M.K. Hazzard, and P.M. Weaver, Compos. A Appl. Sci. Manufact., 56, 280 (2014). 2. D.N. Saheb, and J.P. Jog, Adv. Polym. Technol., 18, 351 (1999). 3. J. Holbery, and D. Houston, J. Miner. Metal and Mater. Soc., 58, 80 (2006). 4. Y. Xue, D.R. Veazie, C. Glinsey, M.F. Horstemeyer, and R.M. Rowell, Compos. B Eng., 38, 152 (2007).

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