= Residual stress
<Residual stress> is <stress> present after the external loading that produced it has been removed. Different regions can acquire incompatible preferred shapes through nonuniform <plastic deformation> or <thermal expansion>. Because the regions must still fit together, elastic <deformation> balances those mismatches and leaves an internal <stress> field.
A free specimen can therefore have zero resultant external <force> while containing regions in <tension> and compression. For example, cooling different parts at different rates can leave different permanent strains, even after their <temperatures> become equal. Cutting the specimen releases some constraints, and the resulting shape change can reveal the stored <stress>. In a simple traction-free static body with no <body force>, the <residual stress> must satisfy internal <force> balance and zero surface <traction>. It affects later loading because the applied <stress> adds to a pre-existing field rather than starting from zero everywhere.
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