Abstract
The object of this paper is to inspect loosening of bolt by projecting an ultrasonic wave on the screw hole side. When an ultrasonic pulse is projected on the screw hole side, the pulse is reflected intricately. So, it is difficult for the conventional ultrasonic inspection. In this paper, the authors focus their attention to frequency modulation of the ultrasonic wave, and attempt to inspect loosening of bolt by using it. In the proposed method, the frequency modulation is observed as a variation of instantaneous frequency of the ultrasonic wave, and the instantaneous frequency is defined as the derivative of a time-dependent phase angle of the ultrasonic wave. When the tightening torque of a bolt becomes small, the authors find that the peak value of the frequency of the ultrasonic wave reflected from a screw hole becomes low, and the time of a peak becomes early. Therefore, the loosening of bolt can be inspected by the variation of the instantaneous frequency. Moreover, it found out by considering screw sectional form geometrically that the number of the reflective courses of an ultrasonic pulse is four. Then, reflection of the ultrasonic wave in a screw hole was modeled, and the cause of variation of the instantaneous frequency was detected.
