Fatigue crack propagation is one of the design drivers for modern damage tolerant airframe structure. Retardation effects from overloads in a loading spectrum are usually taken into account when performing this calculation, since the beneficial effect on crack life can be significant. If the spectrum contains cycles of compression, in some cases, acceleration can also occur.

The aluminum industry has recently developed several new high damage tolerance alloys. 2024A and 2027 are commercially available; IS237 is still under development. Compared to 2024, 2024A (lower wing skin) provides an increase in fracture toughness and crack growth resistance, especially under variable amplitude loading. 2027 (lower wing skins and stringers) provides increased strength as well as improved damage tolerance. IS237 will be an age formable variant of 2027, providing both increased strength and damage tolerance. The alloys mentioned above prove that it is possible to develop alloys specifically for improved spectrum fatigue.

The objective of the present work was to find a way to predict the improvements in variable amplitude crack propagation due to these new alloys, with limited testing, or during alloy development, when only limited experimental data is available. For 2024, 2024A, 2027 and IS237, predictions were compared with experimental data from variable amplitude tests for a large transport wing. For 2024 and 2024A, there was also data from a business jet spectrum test. NASGRO 3.0.18 and AFGROW 4.0005.12.10 with their various built-in retardation models were used for the simulations.

The results show that a reasonable estimate of an alloys spectrum propagation performance can be obtained from just constant amplitude testing and basic static properties. However, the current simulation methods also appear to have some serious shortcomings. The more elaborate models did not always perform best in this comparison.