Stress measurement method and its apparatus

A stress measurement method comprising:

• a first process for measuring time of flight of ultrasonic wave to of a structural material under no stress or the least stress applied and time of flight of ultrasonic wave tn that responds to change of applied stresses for a distance from one face to the opposite one at a determined place;
• a second process for calculating stress dependent coefficients α using the theoretically conducted formula
• α32 f(Δtn, σn)
• which is made up based on tn and to obtained by the first process, and 2nd and 3rd order Elastic Modulus characterized by the materials to be measured when Δtn(=tn−to) shows the difference between tn and to which are obtained through the first process;
• a third process for measuring time of flight t of the material equipped for an instrument or a construction under unknown load applied for the distance from one face to the opposite one at the determined place at the almost same time of the first and the second processes;
• and
• a fourth process for, using the information obtained by the first, the second, the third processes and the previously conducted formula
• σ=f(Δt, α)
• when Δt=t−to, obtaining stress σ in the object to be measured, which is in situ measured value of the object under practically applied load in the instrument or the construction.