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

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. 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. A drop of up to 40% in the value of dynamic initiation toughness, [...], is discovered for loading rates in the range of [...]. This range of loading rate corresponds to the typical rates found in an aircraft fuselage experiencing explosive loading conditions. A dramatic increase in the value of dynamic crack initiation toughness is also found for loading rates above [...]. Based on these 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, [...], ranging from [...] to [...], to simulate the conditions encountered in an explosively loaded aircraft fuselage. Simulations were conducted at a number of loading rates for the following cases of relevance to aircraft fuselage: (i) center cracked panels, (ii) rivet holes with wing cracks, (iii) biaxially loaded panels and (iv) panels prestressed to simulate pressurization. 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, [...], to determine the time to failure, [...], for a given loading rate. A failure envelope, [...], 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. Pre-fatigued single edge notched specimens and three point bend specimens are subjected to dynamic symmetric and asymmetric loading to generate a range of mode mixity at the cracktip. 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 [...] and is compared with the static counterpart for 2024-T3 aluminum alloy. The fracture surfaces near the crack initiation site are investigated using a scanning electron microscope and reveal ductile void growth and coalescence. 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