A deformation thermometer measures temperature by using materials that change shape with temperature variations. As the temperature increases, the material expands or contracts, causing a mechanical deformation. This change in shape is then converted into a temperature reading based on the known relationship between the material's deformation and temperature.

elastic deformation

elastic deformation, ductile deformation and fracture

Elastic deformation is recoverable deformation. As such, when the load that caused the deformation is removed the material will return to it's original shape.

ductile deformation