The constitutive behavior of laser welds in 304L stainless steel determined by digital image correlation
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RODUCTION A. Constitutive Behavior in Welds
THE mechanical performance of welded components is closely linked to the material properties in the fusion zone or heat-affected zone of the weldment. To assess failure, such as by finite-element methods, researchers and engineers require an accurate description of the constitutive stress-strain response in the weldment material. Yet, the properties in the fusion and heat-affected zone are not always easy to quantify. For large-scale welds, conventional,[1,2] subsize,[3] or miniature[4] tensile bars can be extracted directly from the weld metal for subsequent tensile testing. This provides the most direct method for measuring the mechanical properties of the weld material, but requires sufficiently large weldments, and assumes a homogeneous material property across the dimensions of the tensile bar. A notable exception is the recent use of digital image correlation (DIC) to track inhomogeneous strains in a miniature tensile bar taken from a spot weld.[4] Alternatively, welded coupons of various geometries are tested mechanically in one or more loading conditions to assess the mechanical performance. These tests sometimes provide an empirical, nonuniversal description of load-displacement and failure response without providing any fundamental constitutive understanding, especially in the case of cross-weld tensile tests that sample the base metal, heat-affected zone, and fusion zone simultaneously. In some cases, these complex tests are analyzed with finite element analysis in an attempt to extract intrinsic material properties from the resulting overall load-displacement response of the test structure.[5–7] However, these solutions still generally rely on assumptions to describe the range of B.L. BOYCE, Principal Member of the Technical Staff, P.L. REU, Senior Member of the Technical Staff, and C.V. ROBINO, Distinguished Member of the Technical Staff, are with Sandia National Laboratories, Albuquerque, NM 87185. Contact e-mail: [email protected] Manuscript submitted March 1, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
material variability and can often result in nonunique solutions. In spite of these limitations, several researchers have previously investigated the properties of austenitic stainless steel weldments. Brooks et al.[3] evaluated the tensile properties of gas-tungsten arc and electron beam welds in several compositions of free-machining AISI 303 stainless steel by extracting tensile samples from the fusion zone. They showed that yield and ultimate strengths were affected by ferrite number, whereas ductility, as measured by reduction in area, was dramatically reduced with increasing sulfur content. Compared to typical solutiontreated 303 alloy base metal properties,[8] these weld alloys showed 5 to 30 pct increases in strength and 0 to 50 pct reduction in ductility, depending on the composition. The alloys with sulfur content ,0.1 wt pct had mechanical properties that would meet the ASTM minimum ductility specification (50 pct RA) for solution-treated AISI 3
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