FATIGUE DAMAGE EVOLUTION OF TORSIONALLY LOADED ALUMINUM-ALLOY SPECIMENS - INFLUENCE OF SHOT PEENING

The evolution of fatigue damage due to torsion loading has been invest igated in aluminium alloy specimens as functions of cycle number and d epth below the specimen surface, respectively. The experimental techni ques used were scanning electron microscopy, optical microscopy of pla stic replicas, and fluorescent penetrant inspection. A number of shear -type deformation mechanisms were found: axial and circumferential cra cks on unpeened specimens, axial cracks and circumferential slip bands , later evolving into cracks on shot peened specimens. In both cases, cracks were delineated by fretting debris formed by the rubbing action of the fracture facets. The deformation mechanisms found have been us ed in an attempt to rationalize the influence of shot peening on the t orsion fatigue properties which have been reported earlier by 2 of the authors.