Acoustoelastic Measurement of Bolt Axial Load with Hypothetical Velocity Ratio Method.

A new ultrasonic evaluation method of axial load is proposed for short and highly stressed bolts used for automobiles. The velocity-ratio method need not require time-of-flight measurement under stress free state, however, a considerable error is caused by the variation of bolt length which is unknown in the practical measurement. The proposed method estimates the bolt length variation and reduces the error of the axial load due to this variation. Based on the measured relations of the axial load with the time-of-flight of longitudinal and transverse waves, the variation of the unknown bolt length is estimated. By this correction, the variation of the axial load can be reduced to a half of those measured.