Progressive Damage of GFRP Composite Plate Under Ballistic Impact: Experimental and Numerical Study Progressive Damage of GFRP Composite Plate Under Ballistic Impact: Experimental and Numerical Study Md Muslim Ansari * and Anupam Chakrabarti Civil Engineering Department, Indian Institute of Technology Roorkee, Roorkee, India Summary In this paper, the behaviour of laminated GFRP composite plate under ballistic impact has been studied with experimental as well as numerical model. Variation of residual velocity, energy absorption, damaged area and modes of damages have been studied. A simplified three dimensional FE model for composite plate and impactor with gap interaction and Lagrangian mesh has been presented. Material characterization of GFRP composite for progressive damage analysis based on material stress/strain failure criteria has been carried out. Shock effect with nonlinear volumetric response of laminated composite during impact is also considered. Pressure wave propagation in composite plate due to impact is also studied. Numerical results from present FE model are validated with those of experimental results showing close agreements in terms of damaged length, energy absorption and variation of residual velocities. Keywords: Ballistic impact, Damage modes, FE model, GFRP laminate 1. Introduction FRP composites are versatile material for the structural application due to their light weight, high stiffness, high strength and ease of erection in any environment. Due to orthotropic nature of laminated FRP composite, behaviour of these materials are complicated mainly in terms of damage and its propagation under ballistic impact. Numerous researchers have studied the impact behaviour of composite and their works can be classified broadly in three categories such as experimental, numerical and analytical. Impact behaviour of composite plate with drop weight machine was performed by some researchers in terms of energy absorption, deflection, indentation in plate and contact force etc. 1-4. Some experimental works on high velocity impact on composite plate were reported in literature 5-6. Sevkat 7 conducted experimental and numerical study to estimate the ballistic limit *Corresponding authors E-mail: muslimdecivil@gmail.com Smithers Information Ltd., 2016 velocity of graphite fiber/toughened SC-79 resin composite beam. Jordan and Naito 8 performed an experimental investigation to study the ballistic limit, residual velocity for different target thickness. Some analytical as well as numerical investigations on impact behaviour of composite plate in terms of residual velocity and ballistic limit were reported in past 9-11. From literature review, it indicates that there is a lack in ballistic impact analysis on FRP composite plates on damage evolution and their propagation. Progressive damage and their modes in composite plate under ballistic load can be studied more efficiently by a numerical model with lesser expenditure and efforts compare to experimental and analytical works. However, there is no numerical study available in the literature highlighting the three dimensional progressive damage and modes of failure of laminated composite plates. Aim of this paper is to study the ballistic impact behaviour of GFRP composite plate and to develop a three dimensional finite element model for progressive damage analysis of GFRP composite in addition to validation of experimental results. Material characterization is carried out for those required in AUTODYN hydro code which is discussed in detail in the following section. 2. Material, Specimens and Methods 2.1 Material Characterization GFRP laminate was made by hand layup method and samples were cut according to required dimension for material characterization following the guidelines as in ASTM D3039/ D3039M and related literature 12. Nominal pressure was applied by soft roller on each layer during casting of GFRP laminate to reduce air voids. A total pressure of 250N was applied on casted samples and left for 24 hours at room temperature for complete setting. Finally, samples were cured in hot air oven at 80 0 C for three hours Polymers & Polymer Composites, Vol. 24, No. 7, 2016 579
Md Muslim Ansari and Anupam Chakrabarti for homogeneous solidification. Test samples of size 300 mm x 25 mm x 10 mm were procured from solid GFRP laminate for material characterization. Figure 1 shows stepwise process of making test specimen and testing of sample in UTM under tension. Strain gauges are applied along longitudinal and transverse directions to measure the strain in respective directions as shown in Figure 1(c). Stress-strain curve for tensile tests of GFRP is plotted in Figure 2(a). In plan Poison s ratio of GFRP is calculated from lateral strain vs. longitudinal strain curve as shown in Figure 2(b). Material properties of GFRP composite as obtained from tensile test and material properties of steel (4340) from literature 12 are listed in Table 1. 2.2 Numerical Modeling and Methods In the present study, numerical simulation has been carried out using ANSYS/AUTODYN v14.5, a commercial hydro code. The GFRP composite plates of dimension of 140 mm x 140 mm x 3.12 mm and cylindrical bullet with ogival nose shape have been modeled with Lagrangian process and hexahedron brick element. Interaction between plate and impactor is defined by gap interaction method with gap size of 0.05 mm and frictionless contact. Due to symmetric nature of plate, quarter plate is considered with symmetric boundary condition on X=0 and Y=0 to reduce the computational domain as shown in Figure 3. Mesh convergence study has been carried out and it is found that mesh division of 70 x70 along x and y direction (in plane) shows good convergence in results. In the coordinate axis system, direction-11 is taken along z-direction or through the thickness direction of composite plate, direction-22 and 33 are along x and y direction or in plane axis of plate, direction convention are same Figure 1. Material processing and testing of GFRP; (a) air removal by soft roller, (b) hot air oven, (c) tensile test in UTM Figure 2. (a) Stress strain curve for tensile test, (b) lateral strain vs. longitudinal strain curve for in plan Poison s ratio 580 Polymers & Polymer Composites, Vol. 24, No. 7, 2016
Progressive Damage of GFRP Composite Plate Under Ballistic Impact: Experimental and Numerical Study Table 1. Material properties of GFRP and steel 4340 14 GFRP composite Equation of state: Orthotropic Tensile failure Stress 22 (kpa) 4.318e+005 Sub-Equation of State: Polynomial Maximum Shear Stress 23 (kpa) 8.0e+004 Reference density (gm/cm 3 ) 1.800 Tensile Failure Strain 11 0.009 Young s modulus 11 (kpa) 6.000e+006 Tensile Failure Strain 22 0.02 Young s modulus 22 (kpa) 1.971e+007 Tensile Failure Strain 33 0.02 Young s modulus 33 (kpa) 1.971e+007 Post Failure Response: Orthotropic Poisons ratio 12 0.150 Fail 11 and 11 Only Poisons ratio 23 0.130 Fail 22 and 22 Only Poisons ratio 31 0.492 Fail 33 and 33 Only Strength: Elastic Fail 12 and 12 and 11 Only Shear modulus (kpa) 1.790e+006 Fail 23 and 23 and 11 Only Failure: Material Stress/Strain Fail 31 and 31 and 11 Only Residual shear Stiff. Frac. 0.20 Steel (4340) Equation of States: Linear Strain rate constant 0.014 Reference density (gm/cm 3 ) 7.83 Thermal softening exponent 1.04 Bulk modulus (kpa) 1.59E+07 Melting temperature (K) 1793 Reference temperature (K) 300 Failure model: Johnson-Cook Specific heat capacity (J/kgK) 477 Damage constant, D1: 0.05 Strength:Johnson-Cook Damage constant, D2: 3.344 Shear modulus (kpa) 7.7E+07 Damage constant, D3: -2.12 Yield Stress (kpa) 7.92E+05 Damage constant, D4: 0.002 Hardening constant (kpa) 5.10E+05 Damage constant, D5: 0.61 Hardening exponent 0.26 as taken in AUTODYN coordinate system convention 14. Figure 3. FE model of composite plate and impactor in AUTODYN Experimental impact test on GFRP composite plate of size 140 mm x 140 mm x 3.12 mm were carried out using pneumatic gun. All samples are impacted by ogival nose shaped steel bullet of diameter 19 mm and mass 52.0 gm under fully clamped boundary condition. 2.2.1 Damage Initiation Criteria Target plate and bullet both have been taken as a deformable body. Failure initiation criteria and growth of damage in GFRP composite plate is based on the combination of material stress and strain. Hashin failure criteria is used extensively for the modeling and to study the damage in composite due to impact. However, this criterion for matrix and fiber failure is considered only plain stresses s 22, s 33 and s 23. Modified version of these failure criteria along with the criteria for delamination has been implemented in AUTODYN. In the fiber failure and matrix cracking, out of plan shear stresses are also considered with original criteria as below; Failure along 11 plane, (1) Polymers & Polymer Composites, Vol. 24, No. 7, 2016 581
Md Muslim Ansari and Anupam Chakrabarti Failure along 22 plane, Failure along 33 plane, (2) (3) Where s ij, e ij = stress and strain along i and j direction respectively, s ijf, e ijf = failure stress and strain along i and j direction respectively Subsequent to failure initiation, stiffness and strength properties for failed element are changed according to the modes of failure. 3. RESULTS and DISCUSSIONS Ballistic impact behaviour of laminated GFRP composite plate has been investigated with experimental as well as numerical model. In experimental impact tests, GFRP composite plate was impacted by ogival shaped cylindrical steel bullet at incidence velocities ranging between 10 m/s- 500 m/s. Numerical results of residual velocities, energy absorption and damaged length in composite plate are compared with those results obtained from the present FE model. Damage pattern in GFRP composite plates due to ballistic impact are also compared for both experimental as well as FE based numerical model. linearly at higher incidence velocity. It is also observed that the residual velocities of bullet as obtained from FE model is more than the results obtained from experimental results for every incidence velocities. This difference is due to frictionless contact as defined between bullet and composite plate in the present FE model. Ballistic limit for GFRP composite plate of size 140 mm x 140 mm x 3.12 mm is observed to be 33 m/s. Time histories of contact force are also studied for different incidence velocities with FE model as shown in Figure 5. At incidence velocity of 500 m/s, peak value of contact force is 6.57 kn at 0.0122 ms whereas this peak force is decreased to 0.923 kn at 0.0544 ms for incidence velocity 100 m/s. It means that the time of occurrence of peak force increases as the incidence velocity of bullet decreases. Transfer of impact energy from bullet to composite plate is faster at high incidence velocity. It is also observe that the contact force-time history gets narrower at high incidence velocity. All the kinetic energy lost by impactor during impact is supposed to be absorbed in composite plate as below; Figure 4. Variation of residual velocity with incidence velocity where E abs = absorbed energy in composite plate, m i = mass of impactor, V i and V r = incidence and residual velocity respectively This absorbed energy causes damage in composite plate. Energy absorption in composite plate due to impact is plotted with results obtained from the present FE model and also from the available experimental results. Energy absorption in composite plate decreases with incidence velocities from both experimental and the present FE model as shown in Figure 6. It is also observed that the numerical results of energy absorption from the present FE model are less as compared to those results obtained from experimental tests and this may be due to the frictionless contact as defined in the present FE model and as discussed earlier. Variation of acceleration/retardation of impactor during the penetration process of laminated GFRP composite plate is also studied which indirectly represents the penetration resistance offered by composite plate. Retardation of ogival impactor with time at four different incidence velocities between 100-500 m/s are presented in Figure 7., Figure 4 shows the variation of residual velocity with incidence velocity obtained from both experimental impact tests and FE model. Results from FE model have close agreements with experimental results as observed from Figure 4. Nonlinear variation in residual velocity is observed near ballistic limit which is behaving almost 582 Polymers & Polymer Composites, Vol. 24, No. 7, 2016
Progressive Damage of GFRP Composite Plate Under Ballistic Impact: Experimental and Numerical Study It is observed that the retardation of impactor increases up to maximum value and then decreases to zero for all incidence velocities as considered. Zero value of retardation shows the impactor move with constant velocity i.e. complete perforation of composite plate occurs. For lower incidence velocity (100 m/s), acceleration-time history of impactor shows so many up and downs after reaching maximum retardation which may be due to more number of constituting lamina. Acceleration-time histories of impactor get narrower at high incidence velocity. Damage pattern and modes of damages in the GFRP composite plate due to ballistic impact are studied. Bullet is fired at incidence velocity of 274.5 m/s to the fully clamped composite plate and damage on the back face with side view are presented in Figure 8. Due to irregular shape of damage, it is measured in terms of damage length (L x and L y ) along x and y directions. Length of damages in plate from experimental as well as FE model is listed in Table 2. Numerical values of damaged lengths (i.e. damaged area) in composite plate obtained from the present FE model have good agreements with those obtained from experimental impact test. Figure 5. Time-histories of contact force for different incidence velocities Figure 6. Variation of absorbed energy in composite plate at different incidence velocities Table 2. Damage lengths on back face of GFRP composite plate of size 140 mm 140 mm 3.12 mm Damage length L x (mm) Damage length L y (mm) FE model 63.85 65 Experimental 58.94 61.4 Figure 7. Variation of acceleration/retardation of impactor at different incidence velocity Considering the modes of failure in laminated composite plate due to ballistic impact, it is observed that the most of damaged area in composite plate occurs mainly due to delamination as shown from the results of both experimental test and present FE model. Failure pattern of composite plate is symmetric along x and y directions which is just because of symmetric woven glass fiber. Polymers & Polymer Composites, Vol. 24, No. 7, 2016 583
Md Muslim Ansari and Anupam Chakrabarti Figure 8. Damage pattern in FRP composite plate; FE model, a-back face, b- side view; Experimental, c-back face, d-side view It is observed that the main cause of delamination is matrix failure in tension except some delaminated part that occurs due to in plane failure (Failed 23) as indicated from material status bar of the present FE model. Fiber breakage occurs just below the bullet nose as shown in Figure 8(b, d). As the bullet hit on the composite plate, a pressure is applied on the impact point, which pressure propagates in form of pressure wave. This pressure wave causes the generation of various stresses in composite plate. Variation of pressure wave in laminated GFRP composite plate due to impact by ogival nose shaped impactor is studied. Effect of incidence velocity on pressure wave propagation is also studied. For which two different incidence velocities namely 30 m/s and 100 m/s have been chosen, one is less than ballistic limit and other is far more than ballistic limit. From Figure 9, it is observed that the pressure variation in terms of magnitude and nature is more in case of ballistic impact (V i = 100 m/s) on Figure 9. Pressure wave contour on the front face of composite plate due to impact at different incidence velocity 584 Polymers & Polymer Composites, Vol. 24, No. 7, 2016
Progressive Damage of GFRP Composite Plate Under Ballistic Impact: Experimental and Numerical Study composite plate. Concentration of pressure variation is more near the impact point at high velocity impact. 4. Conclusions Behaviour of GFRP composite plate under ballistic impact by a cylindrical steel bullet of ogival nose shape has been studied. Experimental impact tests were carried out with pneumatic gun. A 3D FE model is developed in AUTODYN hydro code to validate the experimental results and also studied the modes of damages in composite plate due to ballistic impact. For progressive damage study of GFRP composite plate, material characterization is also carried out in light of ASTM D3039/D3039M and related literature 14 and the same is implemented in AUTODYN. Shock effect is also considered in material modeling to study the exact damage behaviour of composite plate as appeared from experimental impact tests. Some important observations from the present ballistic impact analysis are discussed below; 1. Damage in laminated GFRP composite plate due to ballistic impact occurs mainly due to delamination. 2. Matrix failure in tension is the predominant cause of delamination as observed from experimental as well as from the present FE model. 3. Some part of delamination also occurs due to in plane failure as observed from the results obtained of the present FE model. 4. Fiber breakage occurs just below the bullet nose and some parts of materials are also flown off with bullet due to high incidence velocity. 5. Energy absorption in composite plate decreases as the incidence velocity of bullet decreases. 6. Pressure wave concentration on the surface of laminated composite plate is more in case of impact at high incidence velocity than the lower incidence velocity. REFERENCES 1. Tiberkak R., Bachene M., Rechak S. and Necib B., Compos. Struct. 83, (2008), 73-82. 2. Zhang D., Sun Y., Chen L. and Pan N., Mater. and Des.50, (2013), 750-756. 3. Evci C. and Gülgeç M., Int. J. of Impact Eng., 43, (2012), 40-51. 4. Hossainzadeh R., Shokrieh M.M. and Lessard L., Compos. Sci. and Tech., 66(1), (2006),61-68. 5. Cantwell W.J. and Morton J.A., Compos., 20(6),(1989), 545-551. 6. Cantwell W.J. and Morton J.A., Compos. Sci. and Tech., 38, (1990), 119-141. 7. Sevkat E., Int. J. of Impact Eng., 45, (2012), 16-27. 8. Jordan J.B. and Naito C.J., Int. J. of Impact Eng, 63, (2014), 63-71. 9. Landa B.P. and Olivers F.H., Int. J. of Impact Eng, 16, (1995), 455-466. 10. Wen H.M, Compos. Struct., 49, (2000), 321-329. 11. Wen H.M., Compos. Sci. and Tech., 61, (2001), 1163-1172. 12. Hayhurst C.J., Livingstone I.H.J and Clegg R.A. et al., Int. J. of Impact Eng, 26, (2001), 309-320. 13. Johnson G.R. and Cook W.H., Eng. Frac. Mech., 21, (1985), 31-48. 14. ANSYS/AUTODYN 14.5, User s manual, ANSYS Inc. South Pointe. (2012). Polymers & Polymer Composites, Vol. 24, No. 7, 2016 585
Md Muslim Ansari and Anupam Chakrabarti 586 Polymers & Polymer Composites, Vol. 24, No. 7, 2016