Fatigue threshold-stress determination in AFP laminates containing gaps using IR thermography

Abstract The existence of gaps during manufacturing using automated fiber placement (AFP) is unavoidable. It is of interest to determine the threshold stress value below which no effect of the gaps on fatigue life exists. This work aims to use infrared thermography and apply Risitano method on AFP laminates containing gaps to provide a quick method for obtaining the threshold value. This method has a great potential in saving time and material required for performing traditional fatigue tests to develop stress/life curves. Based on the thermal observations of surface temperature of gapped specimen under fatigue loading with different maximum stress levels, a relation between the thermal response and the applied stress was observed. From this relation, the threshold stress value can be evaluated using a small number of specimens and in a short time. To investigate the applicability of this technique to different stacking sequences, fatigue tests were conducted on unidirectional, four-angle and cross-ply laminates containing defects (gaps). In addition, stress/life curves were developed for reference and defected specimens in order to get the threshold value using the traditional (Wohler) method. The results of threshold stress using IR thermography are in very good agreement with the results obtained using the traditional way.