The Added Value of Infrared Thermography to Impact Damaging Assessment of Carbon Fibre Reinforced Composites

Abstract

Introduction. The weakness of composites to impact load is a well known problem. In particular, their impact damaging happens through complex mechanisms which are still not completely understood also because of the multitude of materials that can be created by changing: matrix, reinforcement and/or stacking sequence. It is enough to change the direction of a layer of fibres to have a new composite material. Once a new material is created, it is important to assess its performance under impact, or its impact resistance. The scope of this work is to show how advantageous may be to use infrared thermography to monitor impact tests, which are performed to ascertain the resistance-to-impact of a new composite material. 
 Methods. Infrared thermography (IRT) and phased array ultrasonic testing (PAUT) are used. IRT is used with a twofold function: 
 • on-line monitoring the impact event to visualize thermal signatures which bear the witness for the existence of any occurred damage;
 • non-destructive evaluation with lock-in thermography (LT) of the impacted specimens. 
 Results. Some results are reported in terms of:
 • thermal images taken during impact tests on carbon/epoxy specimens;
 • phase images taken on the impacted specimens with lock-in thermography;
 • C-scan images taken with PAUT on the impacted surface of one specimen are compared with phase images obtained with lock-in thermography and also with thermal images taken during the impact.
 Discussion. Both LT and PAUT are effective in detecting the impact damage; LT is fast and more effective to map large surfaces, conversely PAUT is better to get information along the thickness especially in case of thick parts. Then, an integrated use of both techniques would be advantageous. However, they are characterized by some uncertainty in discriminating very thin delaminations when they are compared with the thermal signatures visualized during monitoring of the impact event, especially in presence of composites with complex stacking sequence. 
 Conclusion. The obtained results show that, if the aim is to assess the performance under impact of composite materials for design purposes, monitoring the impact with an infrared imaging device appears to be the fastest and better solution.