Biaxial fatigue tests were performed on thin-walled tubular 1045 steel specimens in a test fixture that applied internal and external pressure and axial load. There were two test series, one in which constant amplitude fully reversed strains were applied and another in which large periodic compressive strain cycles causing strains normal to the crack plane were inserted in a constant amplitude history of smaller strain cycles. Ratios of hoop strain to axial strain of λ = - 1 (pure shear), λ = -0.625, λ = - v (uniaxial straining), and λ = +1 (equibiaxial straining) were used in each test series. A confocal scanning laser microscopy (CSLM) image processing technique that gave a resolution of 0.25 μm was developed and used to obtain three-dimensional images of small cracks as they were opened under load. The information in the images was used to construct cross sections of cracks from which crack depth, shape and crack opening stress were obtained.
For pure shear straining tests (λ = -1) and for tests at a strain ratio of λ - 0.625 surface cracks rapidly initiated on planes of maximum shear in both constant amplitude and overstrain tests but did not increase significantly in length until near the end of the tests. Instead they grew into the specimen until they were semi-circular in shape. This phase occupied approximately 90% of the fatigue life. Then they began to grow in length as well as depth and linked up to cause failure.
In the uniaxial and equibiaxial straining tests, crack growth initiated on planes of maximum shear at 45° to the surface of the specimens (Stage I crack growth) before changing to Stage II growth perpendicular to the applied stress. The cracks once initiated grew continuously in both the length and depth direction until failure took place.
The magnitude and frequency of application of the periodic compressive overstrain cycles in the second test series was chosen to reduce the crack opening stress level below the minimum stress level of the constant amplitude cycles so that they experienced closure-free crack growth. The compressive overstrains significantly increased crack growth rates and decreased the threshold strain intensity for the smaller constant amplitude cycles. They also caused a large decrease in the small cycle fatigue resistance as measured by their equivalent strain-life curves.