Novel Double-Half Spot Weld Testing Technique For Damage Progress And Failure Analysis Using Digital Image Correlation Techniques

Abstract

In-situ spot weld failure analysis under complex loading conditions is significantly restricted by the enclosed space around the weld nugget. Failure can be only observed at the post-failure stage which provides only limited information of the manner in which damage progresses and leads to crack propagation. Novel testing and analysis techniques were developed to observe the through thickness evolution of cracks and failure in real-time. Two new double-half weld (DHW) specimens for normal and shear load testing, compatible with digital image correlation (DIC) strain measurement, were developed that can be used to directly observe and record damage processes and cracking within spot welded samples. The test samples were analyzed with a speckle pattern to create strain maps and with a macro-etched surface to observe in-situ crack propagation. This novel testing method was used to determine complex failure mechanisms/modes for spot welds in hot stamped, ultra-high strength Al-Si coated 22MnB5 steel. Interfacial and pull-out failure mechanisms in normal and shear loading were observed though the thickness of spot weld and were recorded for the first time. Interfacial failure is initiated by strain localization at the weld notch followed by corona debonding and crack propagation towards the weld nugget. In pull-out failure, the crack follows the shape of a transient softened zone at the fusion boundary, while, interfacial failure is dominated by ductile shearing at the faying surface of the sheets. The results for the DHW test geometries reveal complex failure mechanisms/modes in spot welds that can be quantified using DIC analysis.