Defect Detectability in CFRP Composites Using Long Pulse Thermography: A Comparative Study of Five Defect Types

This study investigates the applicability of Long Pulse Thermography (LPT) for detecting artificial defects in carbon-fiber-reinforced polymer (CFRP) composites. The work aimed to determine which types of defects can be detected in a CFRP panel using active infrared thermography and to assess the influence of defect type, size, and depth on their detectability. The test specimen was a CFRP laminate manufactured from unidirectional prepreg using the autoclave process. The panel had a variable thickness, and artificial defects were introduced at different locations to simulate typical manufacturing- and service-related damage. The investigated defects included: prepreg foil, simulating interlayer insertion caused by errors during prepreg lay-up; flat-bottom holes, simulating delamination caused, for example, by low-energy impacts; paper inserts, simulating interlayer insertion caused by the inclusion of a foreign body during manufacturing; flat-bottom holes filled with epoxy resin, simulating resin-rich areas; and graphite foil, simulating interlayer insertion defects. Thermographic measurements were performed using long-pulse thermal excitation, and the thermal response was analyzed during the cooling phase. The results showed that defect detectability depended strongly on defect type and increased with increasing defect diameter. Flat-bottom holes and resin-rich defects produced the clearest thermal indications and the highest detection repeatability, whereas prepreg foil and paper inserts yielded less distinct signals, particularly for smaller diameters. Under the applied test conditions, graphite foil inserts were not detected. The results confirm that LPT is a promising non-destructive testing method for the rapid inspection of CFRP structures; however, its effectiveness is limited for defects generating low thermal contrast.