Tight Index Research Articles

A bolt preload monitoring method based on the refocusing capability of virtual time reversal

Bolted joints are widely used in aerospace and civil structures. Nevertheless, bolts may easily lose preload because of inappropriate initial preloads or time-varying servicing loads. Hence, bolt loosening monitoring is important for ensuring the safety and reliability of the bolt-jointed structures. In this paper, a guided wave method based on virtual time reversal (VTR) and tightness index representing refocusing capability is developed for bolt preload monitoring and to mitigate the requirement of baseline. In VTR, a response signal acquired from the bolted structure in fully tightened condition is reversed and used as the excitation signal for the bolted structure in loosening states. On this basis, the ratio of the energy of the refocused wave packet in final received wave signal to the energy of the entire signal is defined as tightness index (TIE). In this way, the calculation of TIE does not need to compare with baseline acquired in fully tightened condition. The effects of roughness of contact surfaces on guided wave propagation and the refocusing capability are analyzed by a 2D FE model. The proposed method is also experimentally validated by bolt-jointed structures with single and multiple bolts. The experimental results show that TIE increased almost linearly with the decease of bolt preload and is effective at the early stage of bolt loosening. In addition, the effect of temperature on preload detection accuracy is weak. Furthermore, it is important to select a right frequency to ensure that the TIE is sensitive throughout the entire bolt torque range.

Real time bolt preload monitoring using piezoceramic transducers and time reversal technique—a numerical study with experimental verification

Bolted joints are ubiquitous structural elements, and form critical connections in mechanical and civil structures. As such, loosened bolted joints may lead to catastrophic failures of these structures, thus inspiring a growing interest in monitoring of bolted joints. A novel energy based wave method is proposed in this study to monitor the axial load of bolted joint connections. In this method, the time reversal technique was used to focus the energy of a piezoelectric (PZT)-generated ultrasound wave from one side of the interface to be measured as a signal peak by another PZT transducer on the other side of the interface. A tightness index (TI) was defined and used to correlate the peak amplitude to the bolt axial load. The TI bypasses the need for more complex signal processing required in other energy-based methods. A coupled, electro-mechanical analysis with elasto-plastic finite element method was used to simulate and analyze the PZT based ultrasonic wave propagation through the interface of two steel plates connected by a single nut and bolt connection. Numerical results, backed by experimental results from testing on a bolted connection between two steel plates, revealed that the peak amplitude of the focused signal increases as the bolt preload (torque level) increases due to the enlarging true contact area of the steel plates. The amplitude of the focused peak saturates and the TI reaches unity as the bolt axial load reaches a threshold value. These conditions are associated with the maximum possible true contact area between the surfaces of the bolted connection.