Detecting bolt looseness of flanged joint based on multiple-mode conversion

This article detects the bolt looseness of a flanged joint based on the energy conversion between multiple wave modes. The mode conversion introduced by the flange carries abundant information referring to the bolt looseness, but it requires precise sensing technology to capture each mode. The mode transducer for cylindrical shells was first proposed based on the orthogonality of mode shapes. The distribution scheme and electrode function enable the piezo transducer array to selectively control multiple modes. Both the simulated and experimental validations were subsequently conducted to check the design scheme. We examined the shapes and mode signals of the excited wave field to ensure that the energy was concentrated in the target mode. Finally, two-stage detection was conducted for eight bolts: the first stage detected the initial looseness of a single bolt using two indicators; the second stage trained the machine learning classifiers to recognize the location of the loose bolts. The results exhibited a high sensitivity of mode conversion in detecting initial looseness, with an accuracy of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>97</mml:mn> <mml:mo>.</mml:mo> <mml:mn>9</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> in recognizing the loose location.