Effect of low temperature on mode I and mode II interlaminar fracture toughness of CFRP-steel hybrid laminates

Delamination is the dominant failure type in fiber metal laminates (FML), particularly when combining carbon fiber reinforced plastics (CFRP) with steel. Their interface behavior is frequently studied using the double cantilever beam (DCB) and end-notched flexure (ENF) setup for mode I and mode II delamination, respectively. By nature, analysis of hybrid interfaces requires asymmetric laminate layups. Thus, thermal residual stresses (TRS) are acting on the interfaces. A framework for the correction of the apparent fracture toughness from experimental testing and for accurate numerical modeling is provided. The approach is validated using DCB and ENF test results at -55 °C and 23 °C. The results demonstrate the necessity of incorporating TRS in the analysis of asymmetric CFRP-steel FMLs, i.e. including a temperature step and using the true fracture toughness value as simulation input. Otherwise, delamination onset is mispredicted by up to 29 %.