This work deals with the effect of a new “bolt-head” pin profile on the friction stir welding performance of the aluminum alloy 6061-T6, compared to traditional pin profiles. Friction stir welding parameters such as the tool rotation speed and the welding speed were investigated together with the different pin profiles; the results show that the new “bolt-head” pin profile leads to better mechanical properties of welded specimens. The pin profiles used in this work were the straight square (SS), straight hexagon (SH), taper cylindrical (TC), and the straight hexagon “bolt-head” (SHBH). It was found that the last pin profile improves the material flow behavior and the uniform distribution of plastic deformation and reduces the formation of macroscopic defects on the welded zone. Mechanical tensile tests on welded specimens were performed to determine the tensile strength: the specimens welded with the SHBH pin profile have shown the highest mechanical properties. An approach is presented for material flow on this aluminum alloy using the SHBH pin profile, which is related to the improvement on the resulting mechanical properties.

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Crack initiation and propagation have been investigated on the polymeric material ABS (Acrylonitrile Butadiene Styrene), under ultrasonic fatigue testing. Three controlled actions were implemented in order to carry out fatigue tests at very high frequency on this material of low thermal conductivity, they are: a) The applying load was low to limit heat dissipation at the specimen neck section, b) The dimensions of testing specimen were small (but fitting the resonance condition), in order to restraint the temperature gradient at the specimen narrow section, c) Temperature at the specimen neck section was restrained by immersion in water or oil during ultrasonic fatigue testing. Experimental results are discussed on the basis of thermo-mechanical behaviour: the tail phenomenon at the initial stage of fatigue, initial shear yielding deformation, crazed development on the later stage, plastic strain on the fracture surface and the transition from low to high crack growth rate. In addition, a numerical analysis is developed to evaluate the J integral of energy dissipation and the stress intensity factor K, with the crack length

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