Novel optico-acoustic nondestructive testing for wire break detection in cables

SUMMARY A novel optico-acoustic sensing system based on digital image correlation (DIC), guided ultrasonic waves (GUW), and acoustic emission (AE) and its application in detecting breaks on seven-wire steel strands is presented. The implementation of the emerging optical nondestructive testing (NDT) method of DIC in parallel with established acoustic NDT techniques enables the cross-correlation/validation of in situ recorded information related to progressive damage accumulation, which is the focal point of this research. To the authors' best knowledge, it is the first time that full field strain accumulations have been obtained on the surface of the seven-wire strands. Furthermore, acoustic waveforms and their extracted features are found complementary to full field strain measurements and prove capable to detect damage initiation in critical structural sites ('hot spots'). To demonstrate the potential of the novel NDT system, pristine and prenotched strands were loaded using a mechanical testing frame while simultaneously recording DIC, GUW, and AE data. The DIC and GUW were acquired and excited at specific load intervals to avoid overlapping of GUW with AE activity. The reported DIC results directly reveal strain accumulations at the notched areas prior to breaking, whereas AE waveforms and related features show sudden changes at time instances that correspond to wire breaks. In addition, the GUW signals show a decrease in their amplitude upon progressive load of the strands and wave speed variations. Detailed post-processing of the acoustic results was performed to cross-correlate recorded information from novel optico-acoustic sensing system and create methodologies for effective data filtering, alignment, synchronization, and fusion (through unsupervised pattern recognition techniques) that could lead to robust damage identification in structural health monitoring applications. The results demonstrate for the first time that the use of full field strains in correlation with acoustic techniques visually validates the damage source location and, in turn, enhances the damage predictive capabilities of each NDT method for prestressed and post-tensioned cables found in stay cable bridges.