Special Problems of Thermal Integrity

TLDR: In this paper, the authors discuss the effects of temperature variations on the properties of materials and constructions at elevated temperatures, including the effect of the material thermal extension on detail natural oscillation frequencies.

Abstract: The chapter contains information on the special problems of thermal integrity, which can be of interest for the readers aimed to gain deepened understanding about some peculiarities of materials’ and constructions’ behavior at elevated temperatures. For problems of non-isothermal dynamics (Sect. 12.1), it is shown that influence of the material thermal extension on detail natural oscillation frequencies with temperature increase can be different. Alteration of contact conditions between machine parts (for example, change negative clearances on tightness or on the contrary) and occurrence of the temperature stresses, especially in thin-walled structures, can change their rigidity and the frequency spectrum. A possibility of directional change of a body shape by serial variable heating of its separate body sections having various elastoplastic temperature-dependent mechanical properties is demonstrated in Sect. 12.2. This effect, used in a number of technological processes, is illustrated on simple examples. Though in the majority of practical thermal strength problems, the temperature state of a body is supposed known and not dependent on its stress–strain state, and for an explanation of some effects, it is necessary to address to the “coupled” theory of the thermoelasticity considering thermodynamic link of mechanical and thermal processes. The bases of this theory in rather simple treatment are stated in Sect. 12.3. The coupling effect is illustrated by calculations of energy dispersion during elastic vibrations and influence of convective heat exchange on material deformation at tensile trials. In Sect. 12.4, the fundamentals of the wave theory of thermal conductivity considering (unlike the classical theory) a finite speed of thermal stream and temperature propagation over a solid are briefly stated, and the influence of this factor on unsteady temperatures and stresses propagation at a heat shock on a semi-infinite body is shown.