A methodology for evaluating the reliability and risk of structures under complex service environments

Abstract

SUMMARYAerospace components are often subjected to complex service environments such as random excitations and random temperature conditions. In addition, small variations in the material properties or geometry may significantly affect the structural performance and durability of aerospace components. Therefore, a methodology was developed to determine structural reliability and to assess the associated risk due to various uncertainties in the design variables. The methodology consists of a probabilistic structural analysis by a special computer code NESSUS (Numerical Evaluation of Stochastic Structures Under Stress), a generic probabilistic material property model (multifactor, interaction equation), and a probabilistic fatigue analysis. The methodology is versatile and is equally applicable to structures operating at hightemperature and/or cryogenic environments. The relationship between the probability of crack initiation and fatigue cycles is developed. The probabilistic crack propagation at a given risk level is studied. The most probable fracture path at a given cycle is determined. Those results serve as a guideline for component certification and for setting inspection criteria. The methodology is demonstrated to be capable of formally evaluating structural reliability and risk -including uncertainties in material properties, structural parameters, and loading conditions. It is described in detail with an application to the space shuttle main engine (SSME) blade.

Probabilistic Progressive Buckling of Conventional and Adaptive Trusses

“…It is important to note that all of the truss members were assumed to be initially perfectly straight and that when any member buckled it would yield [5]. Therefore, the maximum eccentricity at which the yielding in the member (first-bay front diagonal) will take place due to the combined effects of axial and in-plane bending moments was calculated.…”

Probabilistic progressive buckling of trusses

“…Representative examples include a probabilistic assessment of a mistuned bladed disk assembly 4 and an evaluation of the reliability and risk of a turbine blade under complex service environments. 5 Furthermore, NESSUS has also been used to computationally simulate and probabilistically evaluate a cantilever truss typical for space-type structures 6 and to quantify the uncertainties in the structural responses (displacements, member axial forces, and vibration frequencies). The objective of this paper is to develop a methodology and to perform probabilistic progressive buckling assessment of space-type trusses using the NESSUS computer code.…”