Fracture toughness of the molten zone of resistance spot welds
A methodology for measuring the fracture toughness at crack initiation and the crack extension resistance of the molten zone of resistance spot welds under Mode I loading has been developed. The cross tensile test of U-shaped specimens was modified by crack growth monitoring and stress intensity factor determination. The resulting values of fracture toughness at crack initiation are independent of the nugget diameter and of the base material mechanical properties. The crack extension resistance seems to depend on base material mechanical properties and nugget diameter. Mixed cleavage + ductile mode associated to medium values of fracture toughness (54–90 $$\text{ MPa }\,\text{ m }^{0.5}$$) suggested a ductile to brittle transition behaviour. The relatively low fracture toughness (55–59 $$\text{ MPa } \,\text{ m }^{0.5}$$) associated to full interfacial ductile failure was quantitatively related to the high number density of small particles in the molten zone. This study opens the possibility to apply the local approach to fracture under monotonic loading to interfacial failure of resistance spot welds.
Fracture toughness of martensitic stainless steel resistance spot welds
Abstract The paper is focused on the strength and fracture toughness of AISI420 martensitic stainless steel resistance spot welds under the tensile-shear loading. The failure behavior of AISI420 spot welds was featured by quasi-cleavage interfacial failure with low load bearing capacity and weak energy absorption capability which was a function of the weld fusion microstructure, predominately carbon and chromium rich martensite plus δ-ferrite. Fracture toughness of the fusion zone proved to be the most important factor controlling the peak load of the spot welds made on AISI420 failed in interfacial mode. A geometry-independent fracture toughness of the weld nugget (c.a. 23 MPam0.5) was determined using fracture mechanics concept. Through modeling it was found that there is a critical fracture toughness which beyond that the pullout failure mode can be obtained which is a function of sheet thickness and tensile strength of the base metal.
A FATIGUE DESIGN PARAMETER FOR SPOT WELDS
— Mode I and mode II stress intensity factors for two half-spaces connected by a circular patch were used to develop a mixed-mode stress intensity factor (termed the stress index Ki) which can correlate the fatigue life of all spot weld geometries, base metals, and specimen dimensions. Empirical corrections were applied to Broek's equivalent stress intensity factor (Klq) to account for the weldment geometry (sheet thickness, nugget diameter, specimen width) and the effect of mean stress. The final expression, (Ki), is a measure of the notch-root stress field in the location where crack initiation and early crack growth occur. The stress index (ki) should be a useful tool for spot-weld fatigue design.