Nondestructive testing of composite T-Joints by TNDT and other methods

Detecting delaminations in the stringer foot areas of T-joints made of carbon fiber reinforced polymer (CFRP) composite is a challenging task for standard nondestructive testing (NDT) techniques. In this study, several methods of thermal NDT (TNDT) have been used to inspect eight CFRP T-Joint specimens with polyolefin film implants which represent subsurface defects. Both one- and two-sided TNDT procedures were used. Thermal modeling was performed to predict the results of TNDT tests. The potential of laser vibrometry was also investigated, and UT (ultrasonic) phased array C-scan was used for verification of test results. Two-sided TNDT was able to detect simulated defects throughout the entire thickness of the test material. Thermal images of defect-free T-Joints clearly establish baseline thermal patterns of good stringers, and subsurface defects may create an identifiable distortion to the baseline patterns. The ability of one-sided TNDT to detect defects depends strongly on their depth and size. Ultrasonic (sonic) infrared thermography was not successful in detecting the implants, due to the formation of standing waves and complicated thermal patterns observed on the stringers. And laser vibrometry has proven to be ineffective in the detection of the implants. Phased array ultrasonic C-scan testing has also been successful in detecting polymeric implants located outside the ultrasonic dead zone but the best sensitivity of UT is achieved in immersion techniques, with water between the ultrasonic transducer and a part to be tested. In all cases, the use of advanced data processing techniques has been indispensable to provide reasonable test results.

Automated summary

This study utilized multiple thermal NDT methods to inspect CFRP T-joints, with the results showing that two-sided TNDT was able to detect simulated defects throughout the entire material thickness, while the detection capability of one-sided TNDT depended on the depth and size of the defects. Furthermore, ultrasonic infrared thermography and laser vibrometry were not effective in detecting the implants.