Investigation of Stress-Strain History Modeling at Stress Risers,Phase II.

摘要

An analytical and experimental study of the stress and strain history at stress risers was conducted to evaluate the effects of time- and cycle-dependent changes on the fatigue life of aluminum alloy structures. This report covers Phase II of a two-phase program . Both creep and stress relaxation were modeled and measured. An elastic-plastic finite element code simulation was utilized to model the nonlinear stress-strain field around the stress riser and to model creep sustained load hold periods. A four-part experimental program was conducted to generate constitutive data necessary for the formulation of a hysteresis analysis model. The experimental program included simple coupon specimens, a unique simplified stress concentration specimen, center circularly notched super-scale specimens, and notched fatigue specimens. Significant creep and stress relaxation was measured during the experimental program. These data were used in the development of creep/stress relaxation module for the hysteresis analysis. The automated hysteresis analysis developed during this program includes a material hardening/softening module, a creep/stress relaxation module, locus and branch curve definition for the stable material response, and a damage accumulation module. Correlation studies have been conducted using this analysis as well as a linear damage analysis method to compare predicted versus actual specimen life.