Terahertz pulsed imaging of low velocity impact damage in woven fiber composite laminates

Recently, terahertz pulse imaging has been used extensively to characterize internal defects in various electrically insulating materials. In this work, terahertz pulse imaging is used to identify the damage induced by low-velocity impact on woven glass-fiber reinforced polyamide laminates. Several impact energies are considered to study the damage initiation and propagation. The permanent indentation, known to be a relevant damage indicator, is extracted from the terahertz results and validated through comparison with profilometry. Further, the criticality of the damage, in terms of the number of composite plies in which the cracks propagate, can be clearly determined from terahertz imaging. These observations are validated through X-ray tomographic observations and analysis. Finally, a strong connection is established between the evolution of the permanent indentation and the appearance, and criticality, of the low-velocity impact-induced damage. These observations suggest that terahertz imaging is a reliable technique for the nondestructive assessment of impact damage in glass fiber composites.