Pulsed phase thermography imaging of fatigue-loaded composite adhesively bonded joints

• Pulsed phase thermography was used to size damage in adhesively bonded lap joints.
• Specimens with various manufacturing defects were loaded in low-cycle fatigue.
• Imminent adhesive failure could be identified prior to joint failure.
• Fiber-failure could not be detected prior to failure, due to its brittle nature.
• The extent of damage was accurately estimated through a threshold-sizing algorithm.

We applied pulsed phase thermography to image and size damage in adhesively bonded joints. Specifically, the initiation and propagation of fatigue-induced damage in single lap joints with carbon fiber epoxy adherends was investigated. Lap joint specimens with various levels of manufacturing defects were fabricated and loaded in low-cycle fatigue. A calibration specimen with artificial defects was used to design a threshold algorithm for sizing of the damaged regions. The dominant failure mode in specimens without manufacturing defects was fiber-failure, whereas joints failing prematurely demonstrated adhesive failure. Imaging of the lap joints after regular number of fatigue cycles revealed that manufacturing defects could be detected and the resulting, imminent adhesive failure could be identified prior to joint failure. Additionally, the extent of this damage could be accurately estimated through the sizing algorithm. Due to the brittle nature of fiber-failure, it could not be detected prior to failure of the joint, however this was not critical, as the goal was to identify premature failure of the adhesively bonded joint.