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This paper presents a novel second-order output spectrum (SOOS) based method with a local tuning approach (LTA) for the precise localization of multiple bolt-loosening faults in complex structures with a simple sensor chain. The development of this new method is based on a recently developed virtual beam-like structure (VBLS) concept and the nth-order output spectrum estimation (nth-OSE) algorithm using only properly measured data. In this new method, a more general multi-degree-of-freedom (MDOF) model with general nonlinear restoring forces, caused by not only faults but also inherently existing material or boundary nonlinearities in structures, is considered, and a special local tuning mechanism is intentionally proposed to derive the novel SOOS based damage indicator. Results of numerical and experimental studies demonstrate that this novel SOOS based local tuning method can give more accurate, sensitive and reliable information about fault positions, and then can be used effectively and reliably for the precise localization of multiple bolt-loosening faults in complex structures even with inherent material or boundary nonlinearities. The results of this study would present a totally new insight into initial structural fault detection by employing sensitive nonlinear features from a systematic frequency domain approach.

In order to diagnose potential serious and initial damages (loose bolts or fatigue cracks) in industrial structures more precisely, a systematic second-order output spectrum (SOOS) based method with a local tuning approach (LTA) is proposed in this article. In the novel method, structures with faults, material nonlinearities and nonlinear boundary conditions are represented by the Volterra series, and then a general ring type multi-degree-of-freedom (MDOF) model simulating nonlinearities as nonlinear restoring forces is built to describe the structures’ behavior. With defined nonlinear fault features, two local damage indicators based on the SOOS and LTA are derived for diagnosing serious and initial faults in sequence. Results of experiments on a satellite-like structure with loose bolts show that the systematic SOOS based method can identify serious and initial bolt loosening faults more accurately. Thus, it can be more applicable for fault diagnosis in complex structures in practice.