Failure mode of laser welds in lap‐shear specimens of high strength low alloy (HSLA) steel sheets

Abstract

In this paper, the failure mode of laser welds in lap‐shear specimens of non‐galvanized SAE J2340 300Y high strength low alloy steel sheets under quasi‐static loading conditions is examined based on experimental observations and finite element analyses. Laser welded lap‐shear specimens with reduced cross sections were made. Optical micrographs of the cross sections of the welds in the specimens before and after tests are examined to understand the microstructure and failure mode of the welds. Micro‐hardness tests were also conducted to provide an assessment of the mechanical properties in the base metal, heat‐affected and fusion zones. The micrographs indicate that the weld failure appears to be initiated from the base metal near the boundary of the base metal and the heat‐affected zone at a distance away from the pre‐existing crack tip, and the specimens fail due to the necking/shear of the lower left load carrying sheets. Finite element analyses based on non‐homogenous multi‐zone material models were conducted to model the ductile necking/shear failure and to obtain the J integral solutions for the pre‐existing cracks. The results of the finite element analyses are used to explain the ductile failure initiation sites and the necking/shear of the lower left load carrying sheets. The J integral solutions obtained from the finite element analyses based on the 3‐zone finite element model indicate that the J integral for the pre‐existing cracks at the failure loads are low compared to the fracture toughness and the specimens should fail in a plastic collapse or necking/shear mode. The effects of the sheet thickness on the failure mode were then investigated for laser welds with a fixed ratio of the weld width to the thickness. For the given non‐homogenous material model, the J integral solutions appear to be scaled by the sheet thickness. With consideration of the plastic collapse failure mode and fracture initiation failure mode, a critical thickness can be obtained for the transition of the plastic collapse or necking/shear failure mode to the fracture initiation failure mode. Finally, the failure load is expressed as a function of the sheet thickness according to the governing equations based on the two failure modes. The results demonstrate that the failure mode of welds of thin sheets depends on the sheet thickness, ductility of the base metal and fracture toughness of the heat‐affected zone. Therefore, failure criteria based on either the plastic collapse failure mode or the fracture initiation failure mode should be used cautiously for welds of thin sheets.

Blast resistance of high-strength structural steel welds

“…Both fracture images show dimples, but the dimples at the border area are elongated in shape. This is characteristic for ductile fracture resulting from shear loading [22], which was also determined at air blast experiments by Geffroy et al [23].…”

Tensile–Shear Fracture Behavior Prediction of High-Strength Steel Laser Overlap Welds

“…Terasiki et al [28] proposed the numerical model to predict the static fracture strength of a laser lap-welded specimen. Also, by using finite element analyses, the ductile fracture initiation and necking phenomena were simulated during the tensile-shear test of high strength low alloy steel sheets [29,30]. Ha et al [31] developed a failure criterion under combined normal and shear loading conditions for laser welds by experimental approaches and numerical analyses.…”

Stress intensity factor solutions for welds in lap-shear specimens of dissimilar sheet materials with and without kinked cracks

“…Note that the upper and lower sheets have nearly the same thickness. Specimens with similar shapes were adopted by a number of researchers for the study of laser welded joints, for example, see Lee et al [20], Asim et al [21],…”

“…, [27] Superscripts A stress intensity factors from Abaqus which are expressed in the form of Equation 35E stress intensity factors of Erdogan [25] which are expressed in the form of Equation (21) Subscripts E stress intensity factors of Erdogan [25] which are transformed into the form of Equation (1) u , l upper and lower sheets of weld…”