Using the material specimen of a constant velocity universal drive shaft processed with strengthening technology as a test object,the form and type of torsional fracture,the hardness characteristic of fracture and the relationships among fracture hardness,residual static strength and residual static stiffness were investigated.The results show that the static fracture morphology is of smooth transverse shear whether the strengthening by light load or the damaging by heavy load has been carried out on the specimen under the action of fatigue load spectrum.All the morphologyies of fatigue fracture are of rugged transverse shear or tensile fracture,and they bear no obvious internal relations to the strengthening process,damage process and fatigue life.During the process of specimen fatigue frature,the residual static strength,residual static stiffness and average hardness of fracture area retain the same chamging trend and rule.