Detection and localisation of pre-damage heat spots in CFRP panels using Lamb waves

Undetected defects arising from the effect of heat exposure within composite panels can develop into critical cracks or delamination, potentially leading to structural failure. This study describes a novel approach to the structural health monitoring (SHM) of carbon fibre-reinforced polymer composite panels to detect and localise pre-damage levels of heat-spot effects. An appropriate heat-spot identification technique was proposed using ultrasonic Lamb-wave propagation from a surface-mounted sensor network. Preliminary tests validated the possibility of detecting heat-exposed areas and assessed the influence of temperature and excitation frequency on signal characteristics and detectability. Multiple indices were compared to detect the signal deviation due to the heat effect, and a time-shift deviation index (DI) was proposed. Consequently, the sensor-path coverage was experimentally determined for sensor-network optimisation. Heat-spot detection and localisation experiments were then conducted for three locations across a temperature severity range and using multiple excitation frequencies. The heat spot was detected and localised at all locations, regardless of severity; however, at a temperature of 35°C, accurate localisation was not always attained. The experimental results supported a correlation between localisation accuracy, excitation frequency, temperature severity and the employed DI. Higher excitation frequencies were found to generally improve localisation accuracy among various DIs. A root mean squared deviation anomaly measure taking into account the first five cycles of the signal emerged as the most effective DI, demonstrating robustness across tested frequencies, temperature severities and locations. Experimentation determined that a Lamb-wave-based SHM solution is viable to detect and localise heat spots in carbon fibre-reinforced polymer components, even at a pre-damage level.