Influence of Test-Procedure and Frequency on Cyclic Crack Propagation and Fatigue Behaviour of 7xxx-Alloys for Aircraft Applications

Cyclic crack propagation and fatigue behaviour of aluminium alloy-temper-combinations are an important part of the material approval process for aircraft applications. Even after approval, both properties have to be measured periodically to verify quality uniformity of established and standardised production processes. Test duration and expense are driven by test frequencies. On the one hand the influence of test-frequency on crack propagation is not stated in the relevant ASTM standard and modern test machines are capable of frequencies up to 100 Hz. Otherwise some aircraft manufactures require test-frequencies below 20 Hz.

The influence of test-frequency on cyclic crack propagation behaviour with different test configurations was investigated for 7075-T7351. Tests at lower frequencies were carried out by using a servohydraulic test system with an automated wide field image sensor for optical crack length measurements. A resonant machine was used to obtain crack growth data at higher frequencies.

The value of the test-frequency in the range of 10 to 100 Hz, at stress ratios of 0.1 to 0.5, did not have any significant impact on stage II of the fatigue crack growth curve, the most important for material qualification.

For 7050-T7451 frequencies of up to 80 Hz were used for smooth bar fatigue testing. A round robin test with different labs in sample preparation and testing was carried out. Residual stress measurements showed that sample preparation within the requirements of the specification has an important impact on the results. However, an increase of test frequency did not influence the life time of the samples.

In both cases an increase in test frequency can help in saving testing time and cost, especially for smooth bar fatigue testing, a required release test above a certain plate thickness.