We describe an approach to predict failure in a complex, additively-manufactured stainless steel part as defined by the third Sandia Fracture Challenge. A viscoplastic internal state variable constitutive model was calibrated to fit experimental tension curves in order to capture plasticity, necking, and damage evolution leading to failure. Defects such as gas porosity and lack of fusion voids were represented by overlaying a synthetic porosity distribution onto the finite element mesh and computing the elementwise ratio between pore volume and element volume to initialize the damage internal state variables. These void volume fraction values were then used in a damage formulation accounting for growth of these existing voids, while new voids were allowed to nucleate based on a nucleation rule. Blind predictions of failure are compared to experimental results. The comparisons indicate that crack initiation and propagation were correctly predicted, and that an initial porosity field superimposed as higher initial damage may provide a path forward for capturing material strength uncertainty. The latter conclusion was supported by predicted crack face tortuosity beyond the usual mesh sensitivity and variability in predicted strain to failure; however, it bears further inquiry and a more conclusive result is pending compressive testing of challenge-built coupons to de-convolute materials behavior from the geometric influence of significant porosity.
Satellites are subject to pyroshock events that come from the actuation of separation and can be damaging events for satellites. The damage risk is mitigated by the fact that shock intensity is attenuated by the spacecraft structure. NASA and MIL handbooks and standards, which were developed from extensive tests performed in the 1960’s, provide guidelines for estimating the attenuating effects of distance, joints, and other structural features in the load path between the shock source and the shock sensitive component. Anecdotal evidence suggests that these rules are not always conservative while sometimes they are grossly over-conservative. The first part of the paper summarizes and interprets the attenuation rules-of-thumb. The second part presents a case study in which attenuation factors developed for a satellite are compared to attenuation factors measured in a pyro-shock test of the satellite. The third part looks at the feasibility of using 21st century computational tools to predict shock attenuation through a simple jointed structure. Such tools have the potential to recreate satellite specific shock attenuation factors that could provide greater confidence in the predicted loads on shock sensitive components by reducing, and perhaps eliminating, the over-under conservatism issue; however they are surprisingly difficult to use.