Thermo-elastic inversion effect

The data in Fig. 5.18 were obtained at a fairly high extension (350 percent). When the experiment is repeated at lower extensions the slope of the curve decreases and below about 10% extension it becomes negative. This is caused by a reduction in stress due to thermal expansion of the material as the temperature is increased and it is known as the thermo-elastic inversion effect. If the effective change in extension due to thermal expansion is allowed for then the thermo-elastic inversion effect disappears and the stress increases proportionately with temperature at low extensions as well as high extensions. 

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