Clamp looseness detection using modal strain estimated from FBG based operational modal analysis

Clamps are widely used to fix and support the pipe in engineering community. Clamps looseness may further lead to the fatigue damage of the pipe, therefore the early detection of clamps looseness is very crucial. The robustness and the accuracy of damages detection have already been the greatest challenges in traditional methods in Structural Health Monitoring (SHM). Due to the higher sensitivity of Strain Mode Shape (SMS) to local damages and more effective performance of fiber Bragg grating (FBG) sensing technology for industrial and mechanical devices, present works focus on a new method to detect clamp looseness based on Strain Mode Shape Differences (SMSD) and FBG sensing technology. In order to estimate the SMSs online, a novel diagram of Operational Strain Modal Analysis (OSMA) with FBG sensing technology is also presented, in which a rational fraction composed by Forsythe Complex Orthogonal Polynomials (FCOP) is employed to fit the ‘Positive’ Strain Power Spectral Density (PSPSD) of a single output. Traditionally, the fitting of the all the outputs need to be conducted to obtain the SMSs, which increases the calculation cost. To further address this issue, an improved OSMA integrated with strain response transmissibility (SRT) is also proposed to calculate the SMSs by just fitting a single output. Firstly, the identified modal parameters based on the presented method is validated with simulation results. And then, the ability of the SMSD index is further investigated analytically and experimentally for clamp looseness detection under different locations and degrees, the results indicate that the clamp looseness can be detected by the SMSD index.