Patent ID: 7299138

Claim:
A stress measurement method comprising: providing a test piece comprising a same material as an object to be measured, and using an ultrasonic wave to measure a time of flight t o of sound propagating within the test piece when the test piece is under no stress and time of flight t n of sound propagating within the test piece when the test piece is under an applied stress σ n ; calculating a difference Δt n (Δt n =t n −t o ) between the time of flight t n and the time of flight t o based on plural information which corresponds to the time of flight t n and the time of flight t o , calculating a stress dependent coefficient α which is a characteristic of the material of the object to be measured, wherein calculation of the stress dependent coefficient α is based on plural information including at least an intrinsic elastic constant and is defined by the formula α=f (Δt n , σ n ); measuring a time of flight t of sound propagating within the object to be measured, the object to be measured being under an unknown applied stress; determining a practical internal stress σ of the object to be measured as an applied stress per unit time while the object to be measured is installed, wherein the practical internal stress is defined by the formula σ=f (Δt,α), where Δt=t−t o .