| As a common form of connection,bolted joints are widely used in various engineering fields.The reliability of bolted joints will influence the safety of the entire structure.Affected by the internal and external factors for a long-term,the performance of bolt will be degraded,such as the bolt looseness and the degraded of the pre-tightening force,which may lead to the overall destruction of the structure.Therefore,it's valuable in theory and engineering practical to carry out damage detection and position,estimation of damage degree and forecast of residual life of bolted joints.As a kind of stress wave that propagates in the waveguide,ultrasonic guided wave was widely used in the field of structural health monitoring due to the advantages of being sensitive to damage and small energy attenuation.In this paper,the bolted joints used in steel structure were taken as the research object,and the ultrasonic wave guided wave was used as the detection method to characterize the bolt pre-tightening force and locate the looseness bolts.Based on the theory of elastic mechanics,the propagation equations of Lamb waves in free plates with boundary and unconstrained were deduced.The characteristic of dispersion and multimodality were analyzed based on dispersion curves plotted by numerical solutions.Furthermore,experimental excitation frequency can be selected based on dispersion curves.In order to understand the impact of windowing on the emission signal,we analyzed the influence of the number of cycles to the time and frequency domain based on the signal processing technology.In this paper,we combined the Hertz contact theory with the average energy density of waves to explain the effect of changes in bolt preload to the energy of the guided wave signal at the theoretical level.The signal's energy in time domain and frequency domain,the amplitude of the first wave packet,the signal's energy of each order obtained from wavelet decomposition and wavelet packet decomposition,the power spectrum calculated from the short-time Fourier transform and the peak value of the average period spectrum were all used to characterize the preload of the single bolt.The energy of signal in time and frequency domain increases gradually with the increase of bolt preload can be found from the experiment.The peak amplitude of the first wave and average period spectrum can represent the bolt preload due to they are characterization of signal energy in a certain sense.By performing wavelet decomposition and wavelet packet decomposition processing on the signal,it is found that the low-order components of signals can represent the bolt pre-load well.Compact bolted joints and sparse bolted joints as two structural types were researched to locate the looseness bolt in this paper.The degree of bolted joints damage was estimated based on the signal similarity and the mean square error for the intensive bolted joints.The position of single looseness bolt was located by the row and column positioning method.The location of signal and multiple looseness bolt were studied based on the arrival time and peak amplitude of the first wave for the sparse bolted joints.In order to overcome the limitations of the baseline positioning method,the non-baseline positioning technique based on time reversal and the instantaneous baseline positioning technique based on average period spectrum were studied in this paper.The experiment found that the similarity index were not sensitive to damage caused by bolt looseness and can't achieve the purpose of locating the damage bolt for non-baseline positioning technology.For instantaneous baseline technology,the looseness bolt can be located by comparing the amplitude of the first wave obtained from the equal-length path.This paper proposed an iterative calculation method to determine the looseness bolt based on stability of damage index of each path when the damage threshold cannot be accurately determined and was confirmed to be feasible. |
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