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.
The microstructural evolution and elemental distribution of a 3rd generation 1 GPa advanced high strength steel during double pulse resistance spot welding
This paper presents the effects of double pulse resistance spot welding (RSW) on the microstructural evolution, elemental distribution and mechanical properties of a 3rd generation 1 GPa advanced high strength steel (AHSS). In order to investigate the effect of double pulsing, the steel was exposed to single and various double pulse RSW schedules. The first current pulse was applied to create the weld nugget, while the second current pulse generated a secondary weld nugget and annealed or (partial) re-melted the primary weld nugget, depending on the magnitude of the current. The effect of the second current pulse on the weld nugget and heat-affected zone characteristics was investigated using optical microscopy and electron probe microanalysis (EPMA). Optical and electron microscopy revealed that the secondary weld nugget is fully martensitic, showing a typical solidification microstructure, while the annealed zone reveals an equi-axed martensitic structure. EPMA results showed that elemental segregation has been considerably reduced in the annealed zone. Mechanical properties of the welds show that the AHSS studied is prone to weld metal failure for single pulse RSW. However, the double pulse RSW method can lead to significantly improved mechanical performance and favourable failure modes.
Surface roughness monitoring by singular spectrum analysis of vibration signals
Abstract This study assessed two methods for enhanced surface roughness (Ra) monitoring based on the application of singular spectrum analysis (SSA) to vibrations signals generated in workpiece-cutting tool interaction in CNC finish turning operations i.e., the individual analysis of principal components (I-SSA), and the grouping analysis of correlated principal components (G-SSA). Singular spectrum analysis is a non-parametric technique of time series analysis that decomposes a signal into a set of independent additive time series referred to as principal components. A number of experiments with different cutting conditions were performed to assess surface roughness monitoring using both of these methods. The results show that singular spectrum analysis of vibration signal processing discriminated the frequency ranges effective for predicting surface roughness. Grouping analysis of correlated principal components (G-SSA) proved to be the most efficient method for monitoring surface roughness, with optimum prediction and reliability results at a lower analytical-computational cost. Finally, the results show that singular spectrum analysis is an ideal method for analyzing vibration signals applied to the on-line monitoring of surface roughness.