Fatigue Crack Propagation Under Program and Random Loads

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Abstract
The influence of maximum stress, stress range, and sequence of load application on the rate and mechanism of fatigue crack propagation in 2024-T3 aluminum alloy was studied by means of electron fractography. Variable-amplitude loading programs were designed to provide tests under the following conditions: (1) constant maximum stress with three different levels of stress range, (2) constant stress range with three and four levels of maximum stress, (3) pseudo-random load application achieved by random distribution of the load spectra defined in Items 1 and 2, and (4) uniform maximum stress with peak overloads and underloads. The macroscopic growth rates were determined on center-notched crack growth panels, and the fracture surfaces were examined by electron fractography. The analysis of the influence of program loads on the rate and mechanism of fatigue crack growth was accomplished by: 1. Comparing plots of crack length versus measured rates of crack propagation for the different programs. The measured rates were also compared with rates calculated by a computer program. 2. Relating the count and spacing of the fatigue striations observed on the fracture surfaces with the applied load program.

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