Mechanism-based analysis of crater cracks in constrained lap joints: A stress-based indicator with quasi-Monte-Carlo estimation of propagation direction

Crater cracking at weld termination is a recurring manufacturing-induced failure in laser-welded lap joints under strong restraint, often leading to manual rework and quality risks in compact assemblies. This study investigates crater cracks in constrained lap joints of a smartphone inner frame by quantifying crack-length variations with process parameters and estimating the most probable in-plane propagation direction under cooling-induced stresses. A designed experiment combined with response surface methodology (RSM) maps the main and interaction effects on the measured crack length. Thermo-mechanical finite element analyses are then performed to extract the transient stress evolution around the termination crater, and a mixed-mode fracture-mechanics indicator converts the stress fields into a mechanics-ready metric for propagation tendency. To infer the propagation direction without microstructure calibration, a quasi-Monte-Carlo search is conducted on the unit sphere using spherical Fibonacci points, and the direction maximizing the driving metric is selected. The results support a prevention-oriented view in which solidification-related susceptibility enables crack initiation, whereas cooling-stage tensile restraint governs subsequent crack extension, final crack size, and propagation direction. The proposed framework provides actionable guidance for mitigating crater cracking through parameter-window selection and restraint/gap management in constrained lap-joint manufacturing.