In a previous study, we showed that crack initiation life at a notch tip can be modeled as the micro crack propagation life from an initial microstructural defect up to a crack size of about 500 μm. This model is based upon the assumption that short cracks at a notch propagate without any significant closure effects. In our present experimental study, the results of fatigue tests to verify this hypothesis are presented and discussed.
The fatigue tests were conducted in ambient air on single edge notched specimens with a theoretical stress concentration factor of 3.14 on thin sheets of 1.2 mm thick Aluminum alloy 2024 T351. The test conditions studied are: constant amplitude tests at stress ratios of 0.1, 0.3, and 0.5 and variable amplitude tests using an extract from the TWIST spectrum. The tests were carried out at a nominal frequency of 10 Hz. At selected intervals, micro crack advance (if any) was determined using replicas. The measurements were made at the maximum stress for constant amplitude tests and at a chosen positive stress for variable amplitude tests (avoiding any overload). The smallest crack size detected is about 10 μm.