Fracture toughness and fatigue crack growth rates of butt-welded joints of copper and steel: Mode I cracks oriented parallel to the weld

Abstract

Fracture toughness and fatigue crack growth resistance (FCGRs) of butt-welded joints of copper and steel were measured. For this purpose, compact tension C (T) specimens were machined out from plain (unwelded) base materials and welded joints. The different zones of the welded joints viz., Ni-weld, Cu (HAZ), Steel (HAZ), interface between Cu and Ni-weld (Int. A) and interface between Ni-weld and steel (Int. B) were subjected to testing. Mode I crack, parallel to the weld was introduced in each zone. In both tests, the welded assembly showed inferior performance against crack compared to the unwelded base materials. In fracture toughness tests, load vs. load line displacement curves were obtained. A stable ductile crack growth was observed for copper and Ni-weld. While brittle-fracture was observed for steel. Cu (HAZ) exhibited highest load and displacement. Steel (HAZ) had shown highest load but the least displacement. The interface A and Ni-weld have shown loads and displacements closer to that of Cu (HAZ). The interface B had shown the least load but displacement comparable to that of Cu (HAZ) and Ni-Weld. In elastic regime, HAZ of En31 displayed highest toughness and the Int. B, the lowest. On the other hand, in elastic-plastic regime, HAZ of Cu had shown the highest toughness and the Int. B, the lowest. The number of fatigue cycles undergone by the Ni-weld was highest and by the steel (HAZ), the lowest. Hence, the Ni-weld showed the highest resistance to the growth of the crack under fatigue. In both the tests, if the crack had deflected it was towards the low strength-material and not necessarily towards the low toughness-material. The different behaviours of the zones are attributed to their different microstructures, yield strengths, thermal expansion coefficients and elastic moduli.