Second Harmonic Detection Generated from Fastened Bolt

The second harmonic components before/after the bolt was fastened were detected by using double-layered piezoelectric transducer (DLPT). The resonance frequency of DLPT changes to 1 MHz (f 0/2) when connected in parallel, but remains at 2 MHz (f 0) when connected in series. An effective fundamental transmission (1 MHz) is obtained when the DLPT is electrically connected in parallel while efficient second harmonic reception (2 MHz) is obtained when the DLPT is connected in series. In our system, the pulse inversion averaging (PIA) method was applied to enhance the second harmonic component. A hexagon head bolt (the diameter of a screw: 12 mm, the length: 100 mm) was used in experimental. The bolt was fastened by 40 N-m. The detected second harmonic component after the bolt was fastened was increased by approximately 10 dB compared with before the bolt was fastened.