A hybrid approach for quantitative evaluation of residual torque of loose bolts in bolted joints using passive and active acousto-ultrasonics : theory, simulation and experimental validation

Abstract

Bolted joints are one of the most common elements to assemble structural components for engineering assets. To sustain structural integrity, cut down maintenance cost, and extend service lives of bolted structures, it is highly desirable to characterize bolt loosening in engineering structures as accurately as possible when the bolt loosening is still at its embryo stage. To serve the task of detection of bolt loosening, a diversity of nondestructive evaluation (NDE) and structural health monitoring (SHM) approaches have been well developed, based on the use of linear and nonlinear features of acousto-ultrasonics. In this Ph.D. study, theoretical analyses are firstly conducted to correlate the linear/nonlinear features of acoustic waves to the residual torque of a loose joint, which facilitates understanding of the detection philosophy of bolt loosening. Subsequently, bolt loosening-related linear/nonlinear indices are established using the linear and nonlinear features of acousto-ultrasonics. Specifically, for the linear passive acoustic method using acoustic emission (AE), microcontact theory is used to derive a qualitative relationship between extracted AE parameters and contact conditions at the interface of a joint. In the active linear acoustic method using vibration modal parameters (VMP), resonant frequency and damping ratio are theoretically linked to the tightening status of a joint. In the active acoustic method using wave energy dissipation (WED) of GUWs, a linear index is defined based on WED to identify the tightening condition of multi-type (e.g., single-lap, cross-lap and hybrid lap) bolted joints. Given the necessity of identifying early bolt loosening in the multi-type joints, three nonlinear indices are constructed theoretically using the magnitudes of SOH, TOH and modulated sidebands. To validate the theoretical development, the proposed indices using linear and nonlinear acoustic features, are comparatively adopted to detect bolt loosening in both aluminum-aluminum (Al-Al) and composite-composite (C-C) bolted joints. Two detection strategies-the active and passive linear feature-based methods are firstly adopted to evaluate the tightening status of the joints. In the passive method, intrinsic mode functions (IMFs) of acoustic emission (AE) signals, are used to evaluate the tightening condition of the joints quantitatively. Experimental results reveal that vibration loosening of composite joints results in an increase in the energy ratios of high-frequency IMFs, on which basis the detectability of the AE-based evaluation is further improved, making it possible to evaluate the tightening condition of a bolted joint under vibration fatigue. In the active method regarding bolt loosening identification using vibration modal parameters, damping ratio is found more sensitive to bolt loosening compared to resonant frequency. However, these two parameters are not capable of evaluating the residual torque of a loose joint at the early stage. In the acoustic method using linear propagating features of GUWs, the defined WED-based linear index is comparatively employed to evaluate the residual torque in three types of Al-Al joints. As the applied torque increases, such a linear index is found to increase in a single-lap joint while decrease in a cross-lap joint. The defined WED-based linear index fails to predict the residual torque of hybrid-lap and composite bolted joints. The detectability of this linear acoustic method in bolt loosening is improved by minimizing the effect of boundary reflection, but still in a limited range. To circumvent the deficiency of the use of linear signal features of GUWs, nonlinear acoustic features, including SOH, TOH and sidebands, are recalled. The independence of the CAN-based nonlinear indices regarding the excitation intensities and their efficiencies in bolt loosening detection including the early stage are investigated through experiments in both Al-Al and C-C bolted joints. To observe that the sensitivity of the nonlinear index constructed from magnitudes of modulated sidebands persists throughout the whole torque range from fully fastened to fully loose. Moreover, the detectability of such a nonlinear index is found to be independent of joint configurations and joining materials, showing enhanced applicability regarding bolt loosening evaluation compared to the linear index. Nevertheless, nonlinear indices constructed from magnitudes of SOH and TOH, are found only capable of evaluating the residual torque of joints when the applied torque exceeds certain values. Last, for facilitating an enhanced comprehending of generation mechanisms for modulated sidebands and of quantitative dependence of sideband magnitudes on the residual torque of the bolt, numerical simulation of the VAM-based method regarding detecting bolt loosening in both Al-Al and C-C single-lap bolted joints is comparatively conducted. Numerical results obtained from the modified contact model is found highly consistent with those of experimental data. Based on this, the occurrence and increase in response magnitudes of modulated sidebands due to deterioration of bolt loosening are theoretically attributed to a decrease in the linear contact stiffness along with an increase in the nonlinear contact stiffness. Conclusively, through theoretical modeling, numerical simulation, and experimental investigations, a series of SHM techniques for identifying bolt loosening in multi-type bolted joints are developed in this thesis based on linear and nonlinear attributes of acousto-ultrasonics. The presented study provides a reliable theoretical foundation, relying on which numerical analysis can be accurately conducted to facilitate the tasks of bolt loosening characterization in a variety of bolted structures.