Estimation of Design Thermal Integrity

Abstract

Working capacity of machine parts in various possible operation conditions must be maintained by introduction of strength and durability margins (safety factors). These margins are necessary in view of possible random industrial deviations, unforeseen adverse combinations of loads, temperatures, and operating time at some regimes, and many other reasons. General principles for safety factor formation require analyzing the most probable ways for deviation of stresses, temperatures, and work duration or any other specific parameter, over its design value—on separateness or in aggregate with other ones. The possibility of creating a design of “equal strength” at non-isothermal loading is illustrated by the optimization of a turbomachine blade model non-uniformly heated along its length. The benefits of introducing a “weak link” that reaches destruction under overload before the entire system are discussed. It is shown that “equivalent” trials replicating the lifetime of the system can significantly accelerate the verification of the most stressed machine parts. The trials ensure a machine functioning with the same safety factors as under the work conditions, but during smaller duration. The cyclic durability margins for non-isothermal cyclic fatigue, taking into account influence of exposure at the maximum cycle temperature and asymmetric loading, are considered. Along with the evaluation of local strength and durability margins for the most stressed elements of a structure, computation methods, on a bearing ability of the structure “in whole,” are stated. Use of the determined safety factors is shown to be principally necessary for the reliable probabilistic estimation of details’ low-cycle fatigue (LCF). For this purpose, rational methods for generation of probabilistic-statistical strength and durability margins of machine parts, using results of the limited scope of sampling tests, are proposed.