Application of Nonlinear-Modulation Technique for the Detection of Bolt Loosening in Frame Structure

Bolted joints are susceptible to loosening in the vibrational service environment, which would adversely affect the joint structure integrity. In this work, a nonlinear-modulation approach was explored as a potential method to effectively detect bolt loosening at its early stage, by analyzing the modulation spectrum that arises from the nonlinear vibrations caused by the loosening bolts. To reveal the mechanism of nonlinear modulation, a mathematical model was developed. An effective energy-based damage index was formulated based on the high-frequency intrinsic mode functions (IMF), which contains modulation components processed by empirical mode decomposition (EMD). Vibration tests on a frame structure were carried out to investigate the feasibility of the proposed method. The experimental results demonstrated that modulation occurred in the high-frequency region of response signals; it was found that the energy-based damage index can accurately detect nonlinear damage caused by bolt loosening with superb sensitivity.