Experimental and numerical study of strength mismatch in cross-weld tensile testing

Abstract

The mechanical properties of welded boiler tubes used in power plants can be significantly altered as a result of the fabrication history, such as pre-straining and heat treatment. The primary aim of the study was to determine the effect of fabrication history on local tensile properties across the welds. This was achieved by testing cross-weld specimens machined from welded thin-walled tubes (with unstrained or pre-strained base metal) before and after heat treatment. Digital image correlation (DIC), which is a full-field strain measurement technique, was implemented in order to obtain the local stress-strain curves and to extract the corresponding local tensile properties such as offset proof stress. Evidence of strain hardening due to the constraint and thermo-mechanical cycles during the welding process was found in the heat-affected-zone (HAZ) and evidence of softening was observed in the pre-strained base metal. It was found that the heat treatment process removed the effect of pre-straining and welding on the proof stress and the strength along the specimen was nearly homogenized. However, mapping the local stress-strain curves in the as-welded crossweld specimens with pre-strained base metal has revealed abnormal strain-relaxation with increase in load in the weld-affected region. For a better understanding of this behaviour, a tensile test of a cross-weld specimen with a large strength mismatch between the weld metal and the base metal was simulated using the finite element method. It was found that the strength mismatch in the specimen causes the development of bi-axial stresses in the HAZ once local yielding begins and the use of global axial stress to construct the local stress-strain 2 curve results in an apparent "reduced-strain" anomaly. Nevertheless, for the strength mismatch ratios studied, this anomalous behaviour did not seem to significantly affect the determination of the local proof stress in the specimens.

Experimental investigation of mechanical and fracture properties of offshore wind monopile weldments: SLIC interlaboratory test results

“…This is because of the fact that because of 3 distinct material microstructures (ie, BM, WM, and HAZ) in the gauge region, which have different sizes, the full deformation at the gauge section cannot be attained by all material microstructures. This is because once local yielding occurs in the material microstructure with lower yield strength, the surrounding material with higher yield strength forms a constraint around the softer material, and as a result of this, biaxial stresses develop in the region . However, it has been shown and discussed in that the percentage error between real proof stress values and those calculated from global stress is less than 8%, which can be considered low enough to produce acceptable indicative values of yield stress for different material microstructures from DIC tests performed in this study.…”

Measurement of Creep Deformation across Welds in 316H Stainless Steel Using Digital Image Correlation

“…Using a surface strain measurement technique such as DIC to measure strain in a complex test specimen, such as these cross-weld samples, should be approached with caution. The local constraint caused by the locally varying (elastic, plastic and creep) properties will lead to a tri-axial stress state to some degree, and whether this has a significant effect on the eventual results should be assessed on a case by case basis [31]. For example, in Fig.…”

Radiation Tolerance of Controlled Fusion Welds in High Temperature Oxidation Resistant FeCrAl Alloys

“…During the experiment, the maximum strain value is limited by some degree because of the limitation of the experimental method used to retrieve the true curves. For instance, with the DIC approach as the main tool, the inaccuracy increases when necking begins or when a complex stress state forms [29]. Detailed analysis of the true curves usually includes some constitutive equations [30] or modern approaches based on the physics of plasticity [27], but this aspect was out of the present work scope.…”

“…The procedure used here was close to the approach offered and evaluated in Ref. [29,32]. To retrieve the LYS-curves, the approach evaluates the transition from purely an elastic behavior to an elastic-plastic behavior for each point [29,32] along the specimen gauge, using DIC images collected during the test [28].…”