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Why does a bent metal coat hanger remain partly bent after the force is removed?

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Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).
mechanics of solids Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).

A bent metal coat hanger remains deformed because the force applied to it exceeds its elastic limit.1

When you first bend the hanger, it undergoes elastic deformation, meaning it would return to its original shape if the force were removed. However, once the applied stress surpasses the material's yield point, the metal exhibits plasticity. This is a property of ductile materials that allows them to undergo permanent changes in shape without rupturing.213

At the microscopic level, this permanent deformation is explained by the movement of structural defects known as dislocations within the metal's crystal lattice. When the stress is high enough, these dislocations shift, causing the atomic layers to slide past one another and settle into a new configuration that persists even after the external force is removed.23