On structural integrity with emphasis on rate effects of fracture toughness

Sammanfattning: This thesis is concerned with structural integrity from a fracture mechanics point of view. The emphasis in this work is on rate effects of fracture toughness but all investigations deal with phenomena or problems within the frame work of non-linear fracture mechanics. In Paper A, three-point bend and compact tension specimens, taken from beam sections of modern and older ordinary C-Mn structural steels, were tested at intermediate loading rates at room temperature and -30 °C. The experimental work, except the loading rates used, was performed according to ASTM E-813. In order to investigate transferability of data, full-scale beam sections were also tested at intermediate loading rates. The fracture toughness of C-Mn structural steels depends strongly on the loading rate, and decreases rapidly with increasing loading rate at and just above the maximum prescribed in ASTM-E813. Fracture toughness data for structures exposed to intermediate loading rates indicate the requirement for testing at appropriate loading rates. In Paper B and C, a similar investigation of both base and heat affected material (HAZ) of a typical high strength low alloy (HSLA) steel is presented. The results show that the HAZ fracture toughness in HSLA depends strongly on the loading rate and temperature. In Paper D, a finite element analysis of rate dependent crack growth initiation under Mode I plane strain and small scale yielding conditions is presented. The combined effects of a process region, in form of a line shaped cohesive zone model with rate-independent constitutive parameters, embedded in an elastic-viscoplastic isotropically hardening material, were studied. The results show that both process region and material parameters have a significant influence on the rate dependence of the fracture toughness. Parameter sets yielding results that agree qualitatively with experimental observations for an ordinary structural steel tested at low temperatures and intermediate loading rates, were identified. In Paper E, a path independent integral expression for the crack extension force of a two- dimensional circular arc crack is presented. The integral expression, which consists of a contour and an area integral, was derived from the principle of virtual work. It was implemented into a FEM post processing program and the crack extension force was calculated for a circular arc crack in a linear elastic material. Comparison with exact solutions for the effective elastic stress intensity factor shows acceptable accuracy for the numerical procedure used. In Paper F, stable growth of short cracks in an elastic- plastic isotropically hardening material was studied. The investigation was carried out by means of the finite element method. The numerical results indicate that a small variation of the governing constitutive parameters is sufficient to influence the fracture toughness and the extension of the process region at the onset of crack growth, during stable crack growth and at steady-state.