Biaxial fatigue crack growth and crack closure under constant amplitude and periodic compressive overload histories in 1045 steel.

摘要

This thesis examines the fatigue crack growth and crack closure mechanisms of SAE 1045 Steel under various biaxial loading conditions. The influence of periodic compressive overloads on crack growth, crack closure, and fracture surface asperity are investigated.; A new measurement technique, confocal scanning laser microscopy, is used to measure crack length and depth, crack shape, and crack opening loads by optical tomography of small fatigue cracks.; Fatigue crack growth and fatigue life tests, under constant amplitude loading and load histories containing periodic compressive overload cycles, were performed four biaxial principal strain ratios (hoop strain/axial strain) of {dollar}lambda = -1{dollar} (pure shear loading), {dollar}rmlambda = -v{dollar} (uniaxial loading), biaxial strain ratio of {dollar}lambda = -0.625,{dollar} and {dollar}lambda = +1{dollar} (equibiaxial loading).; For biaxial strain ratios of {dollar}lambda = -1{dollar} and {dollar}lambda = -0.625,{dollar} surface cracks initially nucleated on slip bands at {dollar}45spcirc{dollar} to the axis of the specimen which coincides with the plane of maximum strain. Failure then occurred by a rapid linking of microcracks.; In uniaxial loading {dollar}(rmlambda = -v),{dollar} cracks initiated along the maximum shear plane at {dollar}45spcirc{dollar} to surface of the specimen (Stage I growth) and failure then took place by Stage II growth perpendicular to the axis of the specimen.; In equibiaxial fatigue loading {dollar}(lambda = +1),{dollar} cracks nucleated on the two maximum shear planes parallel and perpendicular to the specimen axis and propagated into the specimen on planes at {dollar}45spcirc{dollar} to the specimen surface.; Quantitative experimental measurements of fracture surface asperity were made to investigate interference shielding at the crack tip. Asperity heights and shapes were correlated with crack growth and crack closure for both constant amplitude and periodic compressive overload biaxial fatigue tests with various strain ratios using measurements made with a Confocal Scanning Laser Microscopy (CSLM) image processing technique.; The results of effective fatigue life predictions obtained from the various {dollar}rmDelta Ksb{lcub}eff{rcub}{dollar}-da/dN curves showed that the strain based critical shear plane approaches correlated the predicted and experimental effective life data within a factor of {dollar}pm{dollar}2 for the low cycle fatigue regime, {dollar}rm10sp3 {rcub}10sp5.{dollar}