Damage detection in carbon fiber reinforced polymeric composites and honeycomb sandwich panels by active thermography

Infrared thermography is one of the effective non-destructive testing methods for damage characterization and identification in structural materials. Infrared thermography induces a temperature variation on the specimen and monitors the surface temperature to detect defects deep inside the structure. It offers advantages such as being a non-contact inspect method, the capability to scan large surface area and recording in real-time. Although various techniques have been developed for infrared thermography, lock-in thermography (LT) and pulse thermography (PT) are the most preferred ones due to their rapid detection and in-service applicability. LT method uses sinusoidal heat waves in different frequencies, whereas PT, employs an instantaneous heat pulse to increase the specimen’s temperature and monitor its evolution to identify the defects due to manufacturing and service conditions. This thesis focuses on both lock-in and pulse active thermography methods are used to detect different types of defects to assess the feasibility of these methods for non-destructive testing. To achieve this goal, delamination was first created in carbon fiber reinforced polymeric composites (CFRPC) during manufacturing, and then examined and compared by two methods. Secondly, delamination, liquid ingress and debonding were created artificially during/after manufacturing in a glass/phenolic prepreg with NomexTM honeycomb core sandwich composites which is a widely used material in the aviation industry. The results were presented comparatively on the basis of advantages and disadvantages. Moreover, the results were examined with acoustic emission (AE) method to have a better understanding of the delaminated sandwich structure. Finally, it is concluded that the most appropriate thermography method is LT method as compared to another method, i.e. PT, used in the study in terms of depth and characterization of the defect. Using the results obtained in this thesis, barely visible damage modes in CFRPC and phenolic-based sandwich structures used in the aviation industry were detected by active thermography. In addition, the damage modes that occurred during the bending tests were correlated with AE. This makes a great contribution to the improvement of structural health monitoring of these materials in the field of the aerospace industry