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This paper introduces a vibro-acoustic method for detecting bolt loosening by exciting a structure with single-tone and frequency-modulated (FM) signals near its natural frequency and analyzing the resulting acoustic and vibrational responses. Experiments were conducted on a bolted joint demonstrator with varying preload conditions, measuring acceleration and acoustic signals. The results show that loose bolts exhibit distinct high-frequency spectral peaks under single-tone excitation and are further distinguishable using FM excitation, with harmonic band power ratios providing a clear metric for differentiating loose states from preloaded states.
You can hear a loose bolt: vibro-acoustic excitation reveals distinct high-frequency spectral signatures that pinpoint preload loss in bolted joints.
Preload loss in bolted joints results in alterations of the stiffness, damping, and nonlinearity of the structure, but existing monitoring techniques for rail-vehicle systems are often not capable of combining controlled shaker tests and sensing of nonlinear features. This paper proposes a method for detecting bolt loosening using a vibro-acoustic technique, where the structure is subjected to controlled shaker tests to sense the nonlinear features. A triaxial accelerometer was attached to the demonstrator, a microphone was placed in close proximity, and one of the bolts was tested under 0%, 20%, 40%, and 80% preload conditions. Single-tone and frequency-modulated (FM) signals close to the main natural frequency of 130 Hz, which was identified using sine sweep and narrow-band excitation, were applied to the demonstrator. When the structure was subjected to 130 Hz single-tone excitation, the loose state of the bolt exhibited several additional high-frequency spectral peaks. FM excitation between 125 and 135 Hz further distinguished between the states. Harmonic band power ratios, normalized to the carrier, distinguished between the loose state and the 80% preload state, where the difference between the loose and 80% preload states was 17.5 dB for l = 2 and 36.5 dB for l = 6.