溶接継手の疲労き裂伝ぱのフラクトグラフィ

Abstract

Fatigue crack growth tests in butt-welded joints of HT80 and STS42 steels were carried out using compact type (CT), center-cracked tension (CCT) and single edge-cracked tension (SECT) specimens at room temperature and 173K, and their fracture surfaces were examined using a scanning and a transmission electron microscopes to evaluate the influence of residual stress on the formation of fracture morphologies.As the fatigue crack propagated through compressive residual stress fields, the fracture surface was covered with a thin oxide film which was produced by mutual contact of crack surfaces. At ΔK above 20MPa√m or the crack growth rate above 2×10-8m/cycle, the fracture morphologies in the base metal and welded joints were dominated by striations independent of the test temperature. At the low ΔK region or the crack growth rate below 1×10-9m/cycle, fracture morphologies showed that the fatigue crack propagated along the microstructures with favourite orientation to the crack growth. The intergranular like facets were formed in the weld metal of HT80 steel at ΔK ranging from 4 to 12MPa√m. The maximum area percentage of the intergranular like facets, however, was considerably lower than that of intergranular facets formed in the base metal. The striation spacings in the welded joints were correlated well with the effective stress intensity factor range, ΔKeff R, which was estimated by superposition of the respective stress intensity factors for the residual stress fields and for the applied stresses.