Validation of a Deformation Plasticity Failure Assessment Diagram Approach to Flaw Evaluation

Abstract

This paper is based upon work done to establish the validity of a simple engineering approach to assess ductile fracture. The approach is based upon the ideas of the British Central Electricity Generating Board's (CEGB) R-6 Failure Assessment Diagram (FAD) and the deformation plasticity solutions developed by General Electric. The FAD approach is an easy way to visualize a complex elastic-plastic analysis of a specimen or structure. The FAD is a safety/failure plane defined by the stress-intensity factor/fracture toughness ratio, Kr, as the ordinate and the applied stress/plastic collapse ratio, Sr, as the abscissa. For a particular stress level and flaw size, the coordinates (Sr′, Kr′) are determined. If the assessment point (Sr′, Kr′) lies on or outside of the failure assessment curve (FAC), the structure will fail. Design loads or margins of safety are readily determined graphically due to the unique linear proportionality of the diagram's axes. The FAC is constructed by taking the fully plastic solution results of deformation plasticity developed by Shih and Kumar along with Shih's estimation scheme to derive a curve in terms of (Sr, Kr) coordinates of the FAD. The validation of the deformation plasticity FAD was based upon the use of a Babcock & Wilcox-modified version of the incremental plasticity finite-element computer program ADINA (Automatic Dynamic Incremental Nonlinear Analysis). Since the FAC's are based upon the deformation plasticity J-integral, the validation of these curves is also a validation of the J-integral. Compact, center-cracked plate, double-edge-cracked plate, and single-edge-cracked plates were among the fracture configurations which were evaluated using the ADINA computer program. The additional validation of the FAD approach from experimental data of five test programs is also a validation of the elastic-plastic J-integral/J-resistance curve approach. These test programs included six different materials and five different test configurations. Based upon the assessment results the deformation plasticity failure assessment approach and the elastic-plastic J-integral/J-resistance curve approach appear to provide an excellent predictive method for determining specimen load behavior.