Strain energy density based failure criterion for GFRP coupons under tension and bending

Strain Energy Density based Failure Criterion for GFRP Coupons under Tension and Bending Accurate prediction of failure strength is one of the major concerns in the design of fabric reinforced polymer composites. Though there are numerous failure criteria available, each criterion suits particular test data, and also lack of critical experimental results makes it difficult to assess the accuracy of these criteria. Hence, much research needs to be done to understand the mechanical characterization of fabric based polymer composites and develop a generalized theory considering the overall properties, so that the proposed theory is valid for a wide range of experimental data. In this study, five different E-glass/vinyl ester symmetric laminates (unidirectional, bi-directional, tri-directional with Continuous Strand Mat (CSM), quadridirectional with and without CSM), manufactured using a compression molding process, and were tested under tension and bending in both the longitudinal and transverse directions. Bi-linear stress-strain response up to 90% of ultimate stress was observed for all composites with different fiber architectures, except for the tri-directional fabric composite with CSM tested in transverse direction, which showed a tri-linear stress-strain response. Strain energy densities, locations where a change of slope occurs in a bi-linear or tri-linear curve with respect to strains, ratios of longitudinal moduli, were evaluated for all test specimens at different strain levels. Consistency of test data was observed regarding the points where change of slope occurred, ratios of longitudinal moduli, etc. Also, it was found that the sum of the product of stiffness and strain energy density up to ultimate strain is constant. For all composite materials with different fiber architectures except tri-directional fabric based composites with CSM tested in transverse direction, the first and second points where change of slope occurred were found to be (K1) 0.34, and (K2) 0.87, under tension and (K1) 0.28, and (K2) 0.88, under bending, respectively. The ratios of the moduli were found to be (KE1) 1.2, and (KE1) 1.27 under tension and bending, respectively. In tri-directional composites with CSM tested in transverse direction three points where change of slope occurred were (K1) 0.25, (K2) 0.68 and (K3) 0.87 for tension, and (K1) 0.15, (K2) 0.45 and (K3) 0.88 for bending, respectively, and the ratios of the modulus were found to be (KE1) 1.41, (KE2) 1.47 for tension and (KE1)1.56, (KE2) 1.51 for bending. Based on the consistent experimental data, distress/damage mechanisms were identified and considered progressive starting from matrix micro-cracking, matrix microcracking leading to delamination, to fiber pull-out or fiber breakage (after crack interaction). The above distress/damage progressions have been used herein as a basis to the development of the proposed theory. A strain energy density based failure criterion is proposed, and the model was used to predict strength, stresses and strains at various stages for different fiber architectures considered in this study. The experimental and predicted strains, stresses, and strengths at various damage stages were found to be conservative, i.e., within 10% error for all different fiber architectures.