A hysteretic model of crack opening stress variation forms the basis for spectrum load notch fatigue crack growth analysis. The model is based on the observation of a significant difference between crack opening and crack closure stress, particularly under high stress-strain cycling. The two stress levels are controlled by the largest cycle in the load sequence. The analysis uses rainflow cycle count, constant amplitude long crack growth rate constants and cyclic stress-strain curve as inputs apart from closure data. Growth rate calculations for 6 mm thick L168 (2014-T651) Al-Cu alloy coupons under FALSTAFF load spectrum compare well with experimental data obtained on a long crack and at notch root crack depth ranging from under 50 microns to 2 mm.
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