Effect of Stress on the Expansion Coefficient Available to Purchase

The expansion coefficients of specimens under tensile stress and after the stress has been removed have been measured for three steels (0.20, 0.40, 0.80% carbon) and for two types of Invar. If the applied stress is below the elastic limit the change in expansion coefficient α, with stress σ is shown to be given by $(∂α/∂σ)T=−1/E2(∂E/∂T)σ$⁠, where E is Young's modulus and T the temperature; if the stress is removed the expansivity returns to its original value. When the applied stress exceeds the elastic limit, there is a discontinuous change in expansion coefficient, and the expansivity on unloading differs from the original value. The lattice distortion associated with the onset of even a slight plastic strain (2×10⁻⁶) appears sufficient to change the expansion coefficient both under load and after the load is removed.