Fatigue crack growth under combined thermal cycling and mechanical loading

Fatigue crack growth tests are commonly conducted under isothermal conditions. In this study two specimens were subjected to combined thermal cycling (CTC) and mechanical loading (ML). Using information from only these two specimens it was possible, with good accuracy, to derive the crack propagation behaviour for any temperature in the tested temperature range. The tests were conducted with a loading frequency of 1 Hz and the temperature cycling was made between 170 °C and 570 °C and at heating and cooling rates of 3 or 5 °C/min. Extraction of data for prediction of isothermal conditions was made using a temperature interval of 20 °C. The obtained data was used to create Paris law type equations for any temperature in the tested temperature range. The prediction obtained from combined thermal cycling and mechanical loading were shown to be in very good agreement with an isothermal crack propagation test performed at the same mechanical loading frequency. The CTC–ML procedure will not replace all isothermal crack propagation testing but it may be a very time and cost effective way to extract more data from crack propagation tests.