This paper presents the failure analysis of a steel torsion bar of the automobile suspension system. The bar fractures in service and the fracture surface was 45°to the axis of the bar, with radial marks and shear lips, characteristic of the brittle fracture of a mechanical element under torsion. Scanning electron microscope was used to characterize the fracture surface that did not present evidence of extensive plastic deformation. The torsion bar fracture initiated at a weld point which had produced a fatigue crack and a martensite transformation in the heataffected zone. The instrumented indentation technique was used for the mechanical characterization (microhardness and elastic modulus) of the microscopic and macroscopic regions near failure. The heat-affected zone was approximately 400-600 lm in thickness and the weld was a ''point'' approximately 4 mm diameter. The results obtained allowed differentiating the tempered martensite from the martensite (without tempering) in the heataffected zone, the latter being 47% harder.
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