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Mechanical stress: physical condition which exists within any material because of strain or deformation by external forces or by non-uniform thermal expansion.
All metals deform (stretch or compress) when they are stressed, to a greater or lesser degree. This deformation is a visible sign of metal strain. In metallurgy, strain can be defined as the deformation of a metal due to stress. In other words, it's a measure of how much a metal has been stretched or compressed when compared to its original length. If there is an increase in the length of a piece of metal due to stress, this is referred to as tensile strain. But if there's a reduction in length, this is compressive strain. Metal Strain in Ductile (Bendable) Materials