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Tensile Tests ( Part – VI)

The stress at which yield occurs is dependent on both the rate of deformation (strain rate) and, more significantly, the temperature at which the deformation occurs. Early work by Alder and Philips in 1954 found that the relationship between yield stress and strain rate (at constant temperature) .

The latter generally increases with temperature, and materials where m reaches a value greater than ~0.5 tend to exhibit super plastic behaviour.

Yielded structures have a lower and less constant modulus of elasticity, so deflections increase and buckling strength decreases, and both become more difficult to predict. When load is removed, the structure will remain permanently bent, and may have residual pre-stress. If buckling is avoided, structures have a tendency to adapt a more efficient shape that will be better able to sustain (or avoid) the loads that bent it. Because of this, highly engineered structures rely on yielding as a graceful failure mode which allows fail-safe operation. In aerospace engineering, for example, no safety factor is needed when comparing limit loads (the highest loads expected during normal operation) to yield criteria. Safety factors are only required when comparing limit loads to ultimate failure criteria, (buckling or rupture.) In other words, a plane which undergoes extraordinary loading beyond its operational envelope may bend a wing slightly, but this is considered to be a fail-safe failure mode which will not prevent it from making an emergency landing.

Compiled by Sabita Das