Measurement of axial stress using mode-converted ultrasound

A new ultrasonic technique for stress measurement using mode conversion is proposed and applied to the estimation of axial stresses in high-tension bolts. The effect of axial stresses on acoustic wave velocities in axisymmetric cylindrical solids is analyzed, and a linear acousto-elastic equation is formulated. Theoretical ray analysis is performed to compute time of flight (TOF) of mode-converted waves in cylindrical solids. To evaluate the validity of the proposed method, mode-converted longitudinal and shear waves in carbon-steel bolts are generated and captured to measure the TOF of the waves. Simultaneous velocity measurement of two differently polarized acoustic waves are made with enough resolution to estimate axial stresses with values less than 10% of the yield stress of the bolt material. It is observed from experimental results that the TOF ratio of longitudinal and shear waves is linearly proportional to the axial stress in bolts within 5% error, as expected from theory.