Thermoelastic phenomenon

Most, if not all, microwave biological effects and potential medical appHcations are beheved to be the result of heating, ie, thermal effects. The phenomenon of microwave hearing, ie, the hearing of clicking sounds when exposed to an intense radar-like pulse, is generally beheved to be a thermoelastic effect (161). Excellent reviews of the field of microwave bioeffects are available (162,163). 

Thermoelastic Effect A mechanical phenomenon that involves the thermal expansion coefficient is the thermoelastic effect, in which a material is heated or cooled due to mechanical deformation. The thermoelastic effect is represented by the following relation ... 

The explosive character of the photoinduced solid-state chlorination reaction of MCH was first described in ref. 31, the phenomenon being interpreted on the assumption of a decrease in the chain-growth activation energy due to the thermoelastic stresses induced in the sample. A possible role of brittle fracture was not considered in that case. However, it would be of interest also to take account of that effect under the conditions used in ref. 31, the more so in that the evaluated values of stresses required to reduce the activation energy markedly are far above the thresholds of brittle fracture of the corresponding matrices (for details, see Section XII). 

The stress-temperature behavior of natural rubber at various extension ratios has been measured by Shen et al. (1967) and shown in Figure 14.12. Compare trends between data at varying extension ratios with those shown in Figure 14.3 and provide an explanahon for the changes at low elongations (a phenomenon that is termed thermoelastic inversion). 

The titanium-nickel alloys show unusual properties, that is, after it is deformed the material can snap back to its previous shape following heating of the material. This phenomenon is called shape memory effect (SME). The SME of TiNi alloy was first observed by Buehler and Wiley at the U.S. Naval Ordnance Laboratory [Buehler et al, 1963]. The equiatomic TiNi or NiTi alloy (Nitinol) exhibits an exceptional SME near room temperature if it is plasticaUy deformed below the transformation temperature, it reverts back to its original shape as the temperature is raised. The SME can be generally related to a diffusionless martensitic phase transformation which is also thermoelastic in nature, the thermoelasticity being attributed to the ordering in the parent and martensitic phases [Wayman and Shimizu, 1972]. Another unusual... 

The exposure of a strueture to a sudden change of temperature may cause its ruin either because locally, most often on the surface, the tension stress reaches the critical rapture value, or because a flaw of the structure propagates and finally reaches a critical size, causing a catastrophic failure. In the former case, a breaking criterion can easily be obtained from the equations which govern the thermoelastic behavior (see section 8.3.1). In the latter case, the fracture occurs only after several cycles, or at the end of a certain period of time, depending on the difference of temperature and the initial microstracture this is the phenomenon of thermal fatigue, which is more difficult to model. 

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