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. This is the first time direct submicron resolution of the interior of cracks has been obtained.

Fatigue crack growth and fatigue life tests, under constant amplitude loading and block load histories containing periodic compressive overload cycles, were performed under four biaxial principal strain ratios (hoop strain/axial strain) of λ=-1 (pure shear loading), λ=-ν (uniaxial loading), biaxial strain ratio of λ=-0.625, and λ=+1 (equibiaxial loading).

For biaxial strain ratios of λ=-1 and λ=-0.625, surface cracks initially nucleated on slip bands at 45° to the axis of the specimen which coincides with the plane of maximum shear strain. For these ratios, growth on shear planes (microcracks) into the specimen occupied up to 90% of the fatigue life during which time the surface length of the microcracks remained nearly constant It was found that at about 90% of the fatigue life the aspect ratio a/c reached unity (semi-circular), and the shear cracks started growing in the length direction as well as into the material. Failure then occurred by a rapid linking of microcracks.

In uniaxial loading (λ=-ν), cracks initiated along the maximum shear plane at 45° to the 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 (λ=+1), cracks nucleated on the two maximum shear planes parallel and perpendicular to the specimen axis and propagated into the specimen on planes at 45° to the specimen surface. In equibiaxial fatigue loading, once a crack initiated, it grew in the length and depth directions until failure took place.

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 effective strain intensity factor range was modeled based on parameters in the maximum shear strain plane in which fatigue cracks initiated. The components of the strain intensity factor range model consisting of the maximum shear strain range and the normal strain range acting on the critical plane were used as driving forces to describe the crack growth rates and predict the fatigue life.

The results of effective fatigue life predictions obtained from the various ΔK_eff-da/dN curves showed that the strain based critical shear plane approaches correlated the predicted and experimental effective life data within a factor of ±2 for the low cycle fatigue regime, 10³<N≤10⁵, and a factor of ±3 for high cycle fatigue regime, N>10⁵.