RESIDUAL-STRESSES AS PARAMETERS OF X-RAY CRITERION OF FRACTURE AND OF FATIGUE LIFE PREDICTION

Fatigue testing is notorious for considerable scatter in specimen life even if special measures are taken to precisely control parameters of stress cycling.The reason for that must be ascribed to variation in material microstructure existing prior to fatigue loading. Structural condition of material may be determined by the following function: [GRAPHICS] Extensive fatigue tests on alpha-Ti and (alpha + beta)-Ti alloy at temperatures 77 K and 295 K showed that failure did not occur until function V(N)hkl reached its critical value at a plane characteristic of each lattice type (failure plane). The condition V(Cr)hkl = K(Pr)hkl may be therefore regarded an X-Ray criterion and, once determined numerically, a sort of material constant.The life N(B) of fatigue tested specimen was found to be clearly depended on structural condition of virgin material exhibited natural variation of structural characteristics. The finite fatigue life was found to conform with the relationship N(B) = a . (V(N0)hkl)b The expression may be used for predicting life N(B) if Prior to fatigue loading the material condition function V(N0)hkl value is made known for a characteristic lattice plane.