Energetic and Fractographic Investigation of Crack Growth Under Variable Amplitude Loading

This study investigates fatigue crack propagation in aluminum alloy 2024-T351 under constantamplitude loading (CAL) and variable-amplitude loading (VAL), with a focus on energy parameters. The hysteretic energy dissipated at each load level (or per block) exhibits trends comparable to those observed under CAL conditions. At high propagation rates, both the dissipated hysteretic energy and the crack growth rate vary linearly, consistent with a propagation mechanism characterized by the formation of striations in each cycle. At lower propagation rates, the relationship between crack growth rate and dissipated energy per block follows a power-law form. The analysis is extended through quantitative microfractographic observations using scanning electron microscopy, which reveal the spatial distribution of key fractographic features and the mechanisms governing crack propagation. Based on these findings, an energy-based model is proposed that allows the replacement of load spectra with an equivalent constant-amplitude loading.