The bolts used for automobiles are relatively small compared with those used in power plants, steel bridges and frames. The precise control of the fastening force is required for important parts. The force measurement by torque wrenches is, however, unreliable because it does measure the frictional torque caused by the axial force not but axial force itself. Therefore direct measurement of the axial force in those bolts is required. The conventional ultrasonic bolt gages are not applicable for these short bolts because the bolt length uniformly stressed is small compared with the total length. An ultrasonic resonance method [1] is useful for axial stress measurement of these bolts, however, it requires long measurement time because of frequency sweep and both measurements in stressed and unstressed states.
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