PSI - Issue 4

David Simunek et al. / Procedia Structural Integrity 4 (2017) 27–34 D. Simunek/ Structural Integrity Procedia 00 (2017) 000 – 000

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Fig. 4 Crack growth under constant amplitude loading

2.2. Overload (OL) tests

A declaration of the characteristic parameters at overload tests is depicted by Richard and Sander (Richard et al. (2012) and (Sander et al. (2006)). Overload tests are performed to investigate retardation effects after overloads. To this purpose, ten single overload revolutions with a overload factor of R OL = 2.5 are applied during constant amplitude tests at a specific stress intensity factor range ΔK. Afterwards , the load is reduced back to the previous stress amplitude. The test results are shown in Figure 5. All tests show an acceleration of the crack growth directly after the overloads, followed by retardation. This phenomenon is observed and described by Bichler et al. 2007. The number of load-cycles with acceleration N A is approximately 6 to 9 % of the delay cycles N D.

Fig. 5 Results of overload tests

The amount of crack growth retardation is pictured in Figure 5 in terms of the crack propagation rate vs. stress intensity factor. At EA4T 6 and 9 a comparable reduction of crack propagation rate is observed, whereas EA4T 5 indicates a more distinct retardation effect. The quantification of the retardation effect is based on the constant amplitude tests EA4T 1 and EA4T 4, which represent the slowest and highest crack propagation rate, see Fig.4. Whereas the ratio of delay cycles N D to comparable cycles at constant amplitude loading N CA is approximately N D /N CA ~ 1.6 to 2.0 for EA4T 6 and EA4T 9, at EA4T 5 the ratio is N D /N CA ~ 2.6 to 3.3.