Smart washer-a piezoceramic-based transducer to monitor looseness of bolted connection

The safety of a bolted connection, as one of the most common ways of making two or more parts/components work together in engineering structures, is very important in order to ensure the health of the whole structure. However, bolt loosening or pre-load degradation may induce the failure of the bolt connection, threatening the normal operation of the system's structure. As a result, it would be beneficial if the health condition of the bolt connection could be monitored in real time. In this paper, a 'smart washer', fabricated by embedding a piezoceramic patch into two pre-machined flat metal rings, was invented and then introduced as a transducer to detect the looseness of a bolted connection. A simple specimen, which consists of two steel plates connected by a nut, a bolt and two smart washers, was fabricated as the test object to study the performance of the 'smart washers' (SWs). For the specimen, a smart washer was used as an actuator to generate a stress wave, and the other one was used as a sensor to detect the propagated wave that traveled through the interface of the bolted connection. A time reversal method was employed to quantify the energy of the stress wave propagating between the two washers, and thus it was possible to build a relationship between the extent of any pre-loaded degradation of the bolt connection and the response signal of the stress wave traveling between the two washers. In addition, a normalized bolt looseness index was proposed for evaluating the looseness of a bolt connection based on wavelet energy analysis.