Satellites and launch vehicles are subject to pyroshock events that come from the actuation of separation devices. The shocks are high frequency transients that decay quickly—within 5-20 ms—and can be damaging events for satellites and their components. The damage risk can be reduced by good design practice, taking advantage of the attenuating properties of structural features in the load path. NASA and MIL handbooks provide general 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. One of the challenges is adequately modeling the dissipative mechanisms in structural features to better understand the risk to shock sensitive components. Previously, we examined the modeling of pyroshock attenuation in a cylindrical structure and used peak acceleration to evaluate how much shocks are attenuated by distance and structural features in a cylindrical structure. In this work, we investigated different quantities to gain more insight into how and why pyroshocks get attenuated by a bulkhead. We found that the bulkhead affects the SRS peak more than the SRS ramp and that approximately 30% of the structural intensity of the pyroshock flows into the bulkhead regardless of the thickness.
A case study highlighting the computational steps to establish credibility of a solid mechanics model and to use the compiled evidence to support quantitative program decisions is presented. An integrated modeling and testing strategy at the commencement of the CompSim (Computational Simulation) activity establishes the intended use of the model and documents the modeling and test integration plan. A PIRT (Phenomena Identification and Ranking Table) is used to identify and prioritize physical phenomena and perform gap analysis in terms of necessary capabilities and production-level code feature implementations required to construct the model. At significant stages of the project a PCMM (Predictive Capability Maturity Model) assessment, which is a qualitative expert elicitation based process, is performed to establish the rigor of the CompSim modeling effort. These activities are necessary conditions for establishing model credibility, but they are not sufficient because they provide no quantifiable guidance or insight about how to use and interpret the modeling results for decision making. This case study describes a project to determine the critical impact velocity beyond which a device is no longer guaranteed to function. Acceleration, weld failure and deformation based system integrity metrics of an internal structure are defined as QoIs (Quantities of Interest). A particularly challenging aspect of the case study is that predictiveness of the model for different QoIs is expected to vary. A solid mechanics model is constructed observing program resource limitations and analysis governance principles. An inventory of aleatory, computational and model form uncertainties is assembled, and strategies for their characterization are established. Formal UQ (Uncertainty Quantification) over the aleatory random variables is performed. Validation metrics are used to evaluate discrepancies between model and test data. At this point, the customers and the CompSim team agree that the model is useful for qualitative decisions such as design trades but its utility for quantitative conclusions including demonstration of compliance with requirements is not established. Expert judgment from CompSim SMEs is elicited to bound the effects of known uncertainties not currently modeled, such as the effect of tolerances, as well as to anticipate unknown uncertainties. The SME judgement also considers the expected accuracy variation of the different QoIs as recorded by previous organizational history with similar hardware, gaps identified by the PIRT, and completeness of PCMM evidence. Elicitation of the integrated team consisting of system engineering and CompSim practitioners results in quantified requirements expressed as ranges on acceptance threshold levels of the QoIs. Evidence theory is applied to convolve quantitative and qualitative uncertainties (aleatory UQ, numerical, model form uncertainties and SME judgement) resulting in belief and plausibility cumulative distributions at several impact velocities. The process outlined in this work illustrates a structured, transparent, and quantitative approach to establishing model credibility and supporting decisions by an integrated multi-disciplinary project team.
Several programs at Sandia National Laboratories have adopted energy spectra as a metric to relate the severity of mechanical insults to structural capacity. The purpose being to gain insight into the system's capability, reliability, and to quantify the ultimate margin between the normal operating envelope and the likely system failure point -- a system margin assessment. The fundamental concern with the use of energy metrics was that the applicability domain and implementation details were not completely defined for many problems of interest. The goal of this WSEAT project was to examine that domain of applicability and work out the necessary implementation details. The goal of this project was to provide experimental validation for the energy spectra based methods in the context of margin assessment as they relate to shock environments. The extensive test results concluded that failure predictions using energy methods did not agree with failure predictions using S-N data. As a result, a modification to the energy methods was developed following the form of Basquin's equation to incorporate the power law exponent for fatigue damage. This update to the energy-based framework brings the energy based metrics into agreement with experimental data and historical S-N data.