Robust estimation of limit loads for cracked components

Abstract

The problem of stress corrosion cracking in natural gas pipelines in Central and Western Canada has led to a need for quick, efficient methods of evaluating pipeline integrity based on component geometry and operating parameters. The use of high toughness materials in modern engineering practice requires the application of elastic-plastic fracture mechanics, or limit type analysis, to evaluate defects. This thesis demonstrates the application of two robust finite element techniques, the Gloss R-node method and the mα-method, to finding limit loads for cracked components. Each of the methods relies on linear elastic finite element solutions in conjunction with a modulus adjustment scheme to provide a simple, systematic means for determining failure loads. The techniques are initially applied to standard fracture specimens to gage their effectiveness in analyzing crack geometries. The analysis is then directed to pipe geometries containing longitudinal defects (internal and external) of varying depths and culminates in the analysis of multiple defects typical of stress corrosion crack colonies. The robust limit load results are compared to traditional nonlinear finite element analysis results and analytical solutions, where applicable. The robust techniques consistently provide conservative results which compare well to both nonlinear finite element analysis and analytical solutions.