Progressive failure analysis for impact damage and compressive strength of composite laminates

It is well known that the compressive strength of composite laminates decreases after impact load, even if the impact damage is barely visible in appearance. This compression after impact (CAI) strength is one of the most important design criteria for composite structures. Currently, in order to reduce time-consuming model tests and simultaneously ensure structural reliability, analytical or numerical methods are required which are capable of reproducing the actual failure behavior of composite structures (failure mode and load). In this study, impact damage and CAI strength were continuously evaluated by means of numerical progressive failure analysis using dynamic explicit FEA. Since impact and subsequent CAI are dynamic and a static loading processes, respectively, analyses for removing the dynamic effect in the CAI process were performed in the following three steps: 1) impact, 2) relaxation of vibration and 3) CAI. In these analyses, the LaRC failure criteria which take into account fiber-kinking failure mode were employed as stress-based damage initiation criteria, and damage evolution for each lamina was simulated through energy-based damage mechanics. The damage initiation criteria and evolution law were implemented in analyses using the user-subroutine VUMAT of Abaqus/Explicit. In addition, delamination was represented by cohesive elements in stress-based damage initiation criteria and energy-based damage mechanics. As a result of comparison with tests, both of the projected delamination areas caused by impact loading and CAI strength were satisfactorily predicted within an accuracy of ± 15 %. In the CAI simulation, fiber-kinking damage propagated in the direction of width at the maximum applied load, but delamination did not start to propagate. The fiber kinking failure mode was caused by bending due to local buckling at impact area where delamination existed. Accordingly, both of the in-plane fiber kinking damage (which is critical failure mode in CAI) and the delamination (which strongly affects local buckling and subsequent in-plane fiber kinking) are quite important for the accurate prediction of CAI strength.