Crack growth behavior in dissimilar welded Ti based alloys under biaxial fatigue loading

Abstract

This study aims at describing fatigue crack growth in dissimilar welding of Ti based alloys under macroscopic multiaxial loading. The proposed methodology involves the experimental analysis of fatigue crack behavior under equibiaxial tension and macroscopic combination of mode I and II for Ti17, Ti6242 and laser welded specimen of both base metals. Based on these experiments, crack path, fatigue crack growth rate and crack interaction with microstructure have been addressed. The 3D finite element analysis of cracks shapes has enabled to derive stress intensity factor (SIF) investigated for opening, in-plane and out-of-plane shear modes based on linear elastic fracture mechanics assumptions. Finally, an equivalent SIF has been proposed to take into account the local mode mixity induced by both macroscopic shear and 3D crack shape. As a conclusion, the dissimilar welding of Ti based alloys increase the fatigue crack growth rate (FCGR) for any macroscopic loading -with or without shear. Moreover, the microstructure of Ti6242 alloy, is well known to inhibit FCGR by multiples local bifurcation of crack path induced by the coarse microstructure of this alloy. This point was confirmed during equibiaxial tension but anomalous and very high FCGR was observed for macroscopic mode I + II loading. For the welded material, the fatigue crack to microstructure interactions have shown that the FCGR was clearly limited by coarse ↵ needles inducing local bifurcation and conversely that in both fusion zone and heat-a↵ected zone, local refinement of ↵ needles could not slow down the crack propagation. Highlights• Biaxial fatigue crack growth tests are carried out for Ti17, Ti6242, and welded specimen • FCGR is the highest for welded specimen and the lowest for Ti6242 for equibiaxial tension • Fatigue crack is not inhibited by microstructure in Ti6242 alloy for macroscopic mode I + II• SIF in mode I, II and III is derived from 3D FEA of relevant crack shape• An equivalent SIF is proposed to account for mode mixity

Influence of a 3D realistic crack path in the driving forces for fatigue crack growth under mode I+II loading

“…The experimental results and methodologies correspond to a previous paper that should be read to access full details, see [3]. Only major points are detailed below.…”

“…Their respective chemical compositions correspond to Table 1. A full analysis of the microstructures of both alloys have been achieved in [3].…”

“…This effect could be observed for flat to slant transition [2] or could coincide with cases including complex interactions of crack path with microstructure. Recent studies for titanium alloys have shown that, for coarse microstructure heterogeneity, as compared to the crack length the crack becomes complex in shape [3]. Besides FCGR could reach maximum and minimum values during the crack growth, making the behavior hardly assessed by a Paris-like law.…”

“…Conform remeshing technique associated with G-θ method is a powerful tool for LEFM by using the optimized mesh in the crack vicinity to insure path independence for G-Integral evaluation [16,17]. Because users of such FEA model need guidelines for crack growth analysis accounting to these new possibilities, FCGR criterion should be revised accordingly to more accurate models [18,3].…”

“…The aim of this paper is to test the influence of different crack path models, from oversimplification currently used in the literature of a straight crack front orthogonal to the specimen sides to a realistic 3D crack path. The main experimental results from previous studies are recalled [3] to precise crack path and FCGR main features observed for Ti17 and Ti6242 alloys under multiaxial loading before addressing the analysis of the impact of realistic 3D crack path on SIF and FCGR assessment.…”