A cohesive-based method to bridge the strain rate effect and defects of RTM-6 epoxy resin under tensile loading

The objective of the present work is to investigate the relationship between the strain rate effect of RTM-6 epoxy resin and the presence of defects under tensile loading by means of a numerical modelling approach. High-strain-rate tensile tests were conducted using a split Hopkinson tension bar (SHTB) test facility. Axial strains were locally measured within the gauge section of the sample using a high-speed stereo digital image correlation technique (high-speed 3D DIC). Additionally, quasi-static tensile tests were conducted to study the tensile behaviour over a wide range of strain rates. The dynamic experimental results showed an increase in strength and modulus, but also a noticeable reduction in the failure strain, compared to the quasi-static tests. Latter observation may be attributed to the effect of defects present in brittle polymeric materials. Defects lead to the generation of microcracks before the failure of samples, as confirmed by experimental observations. Two different cohesive models were therefore created to replicate the constitutive model of the material with and without defects. Through an inverse method fitting, the failure mechanism of cohesive elements was calibrated and the tensile behaviour at various strain rates was replicated. The results showed that the strain rate effect can be accurately simulated by implementing cohesive elements that mimic the presence of defects. The number of simulated defects that allows an accurate reproduction of the behaviour depends on the strain rate level and the material appears more sensitive to defects at high strain rates. Therefore, the present work validates the assumption of the relationship between strain rate effect and defects for brittle polymeric materials.