Biaxial fatigue mechanisms and life predictions for 1045 steel and 7075 aluminum alloy including anisotropy and notch effects

This thesis focuses on biaxial high cycle fatigue mechanisms and fatigue life predictions for metals under constant amplitude axial-torsional loadings. The effects of non-proportional loadings (out-of-phase loading and mean tensile stress), material anisotropy and stress concentration on the axial-torsional fatigue behaviors are studied. Different fatigue life prediction approaches are evaluated or proposed on the basis of detailed observation of fatigue cracking mechanisms.; Stress-controlled high cycle fatigue tests (105--10 7 cycles) were carried out on smooth thin-walled tubular specimens made of as-rolled 1045 steel under proportional loadings (axial, torsional and in-phase fatigue) and non-proportional loadings (90° out-of-phase fatigue and torsional fatigue with static tension). A replica technique was used for crack monitoring from initiation sites. External surfaces of failed specimens and fracture surfaces were also observed in detail under SEM.; The cracks initiate at PSBs oriented close to maximum shear planes in all loading conditions. Under non-proportional loadings, more cracks initiate on the maximum shear plane where the normal stress (amplitude or mean) is the highest.; The maximum normal stress on the critical plane is more appropriate than the maximum hydrostatic stress to account for both effects of normal mean stress and normal stress amplitude. Fatigue life in the special loading case (&phis; = 90°; lambda = 0.5) is often overestimated.; Material anisotropy effects on cracking mechanisms and fatigue life were investigated from tests carried out on smooth tubular specimens made of a 7075 aluminum alloy having elongated grains and many inclusions strings in the longitudinal direction.; No anisotropy effect is observed under axial fatigue. Under torsional fatigue, anisotropy effect is more important in the underaged material than in the overaged material. Mechanical fibering plays a role in both materials, but planar slip and (111) texture in the underaged material enhance the weakness of the longitudinal plane and result in a low torsional fatigue performance (crystallographic fracture). Transverse shear cracking appears only when the static tension is high enough.; Notch effects on axial and torsional fatigue behaviors were studied using two circumferentially V-notched cylindrical specimens made of as-rolled 1045 steel. The specimens have the same notch angle of 90 degree but have different notch depths. Detailed FEM stress analyses at notch tip for both mode I and mode III loadings have been performed using the Catia V5 code. (Abstract shortened by UMI.)...