This paper presents an overview of a newly developed code, NESTEM that analyzes structural components subjected to varying thermal and mechanical loads. This program is an enhanced version of NESSUS and has all the capabilities of NESSUS. In addition, it allows one to perform heat transfer analysis. The basic heat transfer variables can be included as random variables along with the mechanical random variables to quantify risk using probabilistic methods and to perform sensitivity analysis. The analysis capabilities of NESTEM have been demonstrated by analyzing a cylindrical combustor liner. This analysis includes evaluating stresses and their variations at critical points on the liner using material properties, pressure loading and basic heat transfer variables as the random variables. The heat transfer variables are convection temperatures, film coefficients, radiation temperatures, emissivity, absorptivity and conductivity. Cumulative distribution functions and sensitivity factors, for stress responses, for mechanical and thermal random variables are calculated. These results can be used to quickly identify the most critical design variables, in order to optimize the design, to make it cost effective.

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The primary structure of the ARES I-X Upper Stage Simulator (USS) launch vehicle is constructed of welded mild steel plates. There is some concern over the possibility of structural failure due to welding flaws. It was considered critical to quantify the impact of uncertainties in residual stress, material porosity, applied loads, and material and crack growth properties on the reliability of the welds during its pre-flight and flight. A criterion — an existing maximum size crack at the weld toe must be smaller than the maximum allowable flaw size — was established to estimate the reliability of the welds. A spectrum of maximum allowable flaw sizes was developed for different possible combinations of all of the above listed variables by performing probabilistic crack growth analyses using the ANSYS finite element analysis code in conjunction with the NASGRO crack growth code.

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