Residual strength prediction of impacted composite sandwich structures

Abstract

The paper presents two methods for predicting the in-plane compression strength of impact damaged sandwich structures. For developing these methods uniaxial in-plane compression tests of impact damaged unsymmetrical sandwich coupons are conducted. The specimens were made of two CFRP face sheets of different thickness and an embedded honeycomb core. Low velocity impacts from 1 J up to 15 J caused varying amounts of core and face sheet damage and a dent in the impacted face sheet. While increasing the in-plane compression in the residual strength tests, the dent grew in depth and transverse to the loading direction until final failure of the impacted face sheet occurred. This phenomenological observation is reproduced in computational simulations. The simulations use either an efficient finite element model or a semi-analytical Ritz approach including non-linear displacement-strain relations and progressive damage growth in the core. For the latter stress-based failure criteria and stiffness degradation models are applied. Residual strength criteria are developed depending on the numerical solver. The comparison of simulation and test results demonstrates that the presented methods are able to predict the damage tolerance of unsymmetrical sandwich structures. For damages due a high-energy impact the simulations show that the face sheet damage has to be included in the model. Including the face sheet damage into the semi-analytical model increases the computational time tremendously. Therefore, the semi-analytical model should be used only for impact damages with negligible face sheet damage.