There is a tendency for the endurance limits of steels to increase at low temperatures. A more important design consideration, however, is that many materials experience a significant reduction in fracture toughness at low temperatures.
At high temperatures the endurance limit for steels disappears due to the mobilizing of dislocations. At temperatures beyond approximately one-half of the melting point of the material, creep becomes important. In this range the stress-life approach is no longer applicable. It is also important to note that high temperatures can cause annealing, which may remove beneficial residual compressive stresses.
When operating temperatures are below room temperature, brittle fracture is a strong possibility and should be investigated first. When the operating temperatures are higher than room temperature, yielding should be investigated first because the yield strength drops off so rapidly with temperature. Any stress will induce creep in a material operating at high temperatures; so this factor must be considered too. Finally, it may be true that there is no fatigue limit for materials operating at high temperatures. Because of the reduced fatigue resistance, the failure process is, to some extent, dependent on time.
The limited amount of data available show that the endurance limit for steels increases slightly as the temperature rises and then begins to fall off in the 400 to 700°F range. For this reason it is probably true that the endurance limit is related to tensile strength at elevated temperatures in the same manner as at room temperature. It seems quite logical, therefore, to employ the same relations to predict endurance limit at elevated temperatures as are used at room temperature, at least until more comprehensive data become available. At the very least, this practice will provide a useful standard against which the performance of various materials can be compared.
Two types of problems arise when temperature is a consideration. If the rotating beam endurance limit is known at room temperature, then use: