Application of dynamic fracture mechanics to the investigation of catastrophic failure in aircraft structures

A dynamic fracture mechanics approach to the estimation of the residual strength of aircraft structures is presented. The dependence of the dynamic crack initiation toughness of aluminum 2024-T3 on loading rate is first studied experimentally. Based on the experimental results and on established dynamic fracture mechanic concepts, a fracture mechanics based failure model is established and is used to estimate the residual strength of aircraft structures. A methodology to determine residual strength of dynamically loaded structures based on global structural analysis coupled with local finite element analysis is introduced. Local finite element calculations were performed for different loading rates to simulate the conditions encountered in an explosively loaded aircraft fuselage. The results from the analyses were then used in conjunction with the experimental results for the dynamic fracture toughness of a 2024-T3 aluminum alloy as a function of loading rate, $KdIC$ vs. K˙d(t), to determine the time to failure, t_f, for a given loading rate. A failure envelope, $sf$ vs. , based on the failure model and finite element analysis, is presented for the different cases and the implications for the residual strength of aircraft structures is discussed.; Mixed mode dynamic crack initiation in aluminum 2024-T3 alloy is investigated by combining experiments with numerical simulations. The optical technique of coherent gradient sensing (CGS) and a strain gage method are employed to study the evolution of the mixed mode stress intensity factors. The dynamic mixed mode failure envelope is obtained using the crack initiation data from the experiments at a nominal loading rate of 7 × 105 $MPam/s$. Numerical simulations of the experiments are conducted to both help in designing the experiments and to validate the results of the experiments. The numerical simulations show good correlation with the experimental results.