Wavelet analysis of piezoelectric transducer signals to detect rib fractures during impact tests

Abstract

Understanding when rib fractures occurred during an impact would provide valuable data to develop improved thoracic injury risk functions and ultimately safer restraint systems. The strain-gage based method commonly used has limited sensitivity and requires up to six gauges per rib to obtain a good spatial resolution. An approach based on time-scale analysis (TSA) of signals obtained from piezoelectric transducers (PZT) was evaluated with the view to overcoming these limitations. A white-tailed deer ribcage was instrumented with fifteen PZT affixed to the left and right ribs and vertebrae (consecutive rib levels 2–5). Six non-injurious impact tests and one fracture test were performed. The continuous wavelet transform (CWT) – a specific type of TSA – was used to track the impact and fracture signals in the ribcage. It was shown that the impacts could be detected by all the PZT within a few milliseconds, and that the peak value of the CWT of the PZT signals greatly decreased for the PZT contralateral to the impact location. As for the fracture, the PZT affixed to the neighbouring ribs could detect the time of fracture within milliseconds. Further analysis of the PZT signals allowed to characterise the transmission of the impact/fracture signals through the ribcage structure (rib, sternum, vertebrae). The next step in the development of this method is to include the vibratory response of the ribcage to refine the algorithm used to process the PZT signals.