In high-temperature components, secondary stresses are often applied by surface thermal fluctuations that may be sufficient to cause crack initiation and propagation and they may be also accompanied by primary loads. Since the code endurance design lines are based on isothermal uniaxial fatigue data in air, a series of tests have been conducted to study the crack initiation and growth behavior of Type 316 stainless steel under conditions of thermal fatigue in air from a bulk temperature of 650°C.

In general the observed crack initiation correlated with isothermal fatigue data relevant to temperatures towards the lower end of the surface thermal cycle temperature range indicating that the tensile plastic strain was the most important component of the initiation process. For tests without a primary load the crack growth rates in the inner fully elastic regions of the specimens correlated with calculated stress-intensity factors but again relevant to isothermal data for lower temperatures than the specimen bulk temperature. The crack growth rates observed in the plastic surface regions gave a good correlation with total strain range. A test having a surface strain range of half the isothermal fatigue endurance limit showed that limited crack propagation was possible in defected material at such low strain ranges. The addition of primary end loads gave little effect on the crack initiation behavior but did increase the crack growth rates to give an offset in the correlation with total strain range, which increased with increasing primary load.