Ultrasonic measurement of axial stress in short bolts with consideration of nonlinear deformation

A precise method for estimating the axial stress is proposed for the short and highly stressed bolts used for automobile. These bolts show non-linear extension, because of local plastic deformation as well as non-uniform stre ss distribution in bolt cross section. The proposed method consists of (a) precise measurement of the time-of-flight of longitudinal and transverse wa ves using a digital pulse-echo technique, (b) to establish the calibration curves which take into account of nonlinear extension due to non-uniform st ress distribution based on simultaneous measurement of the axial elongation , time-of-flight of longitudinal and transverse waves and (c) evaluation of the axial stress with the change in the time-of-flight and the assumed len gth of equivalent stressed length. The axial stress is estimated within 1% error for one type bolts and within 2% error for another type bolts.