Identification of Impact Damage in Composite Laminates through Integrated Pulsed Phase Thermography and Embedded Thermal Sensors

This dissertation develops a methodology to identify impact damage in aerospace composite laminates using integrated pulsed phase thermography and fiber Bragg grating (FBG) sensors. Initially, a two-dimensional woven, carbon fiber epoxy laminate is used to calibrate the defect depth with blind frequency for the particular material system using pulsed phase thermography (PPT). The calibration specimen contains simulated defects in the form of polymer foam inclusions. The calibrated depth vs. blind frequency relation is then applied to specimens with barely visible impact damage due to low velocity impacts. The results demonstrate that the use of the polymer insert simulated defects, in contrast to drilled holes or inserts with higher thermal contrast, provides thermal phase shifts similar to that observed in the impacted specimens. Despite the differences between the simulated and impact damage (e.g. the irregular boundaries and thin nature of the delaminations), the minimum depth of delamination from the impacted surface and the extent of damage on the rear surface of the specimen calculated from the PPT images are shown to correspond well with those of visual observations.;The next group of laminated composite specimens are fabricated with embedded FBG sensors to test the ability of the combined inspection method using pulsed phase thermography and FBG sensors to identify impact damage severity. Initially three sets of specimens containing a single FBG sensor at the mid-plane, along with data from previous studies, are used to optimize the distance of low velocity impact damage from the FBG sensor and also to optimize the FBG interrogator data acquisition rate. The results from these specimens show a wide scatter in the FBG sensor temperature measurements during cooling. Also, due to its low conductivity, specimen took long time to cool, increasing the inspection time. Therefore for the final specimen the FBG sensor data acquisition is performed in the heating transient. The final specimen contains three FBG sensors, embedded at different depths and is used to quantify the initiation and through the thickness progression of impact damage of varying severities. Pulsed phase thermography is used for the global assessment of the impact damage, meaning to quantify the absolute damage size and also to quantify the damage severity in terms of the phase contrast between damaged and pristine specimen areas. The results demonstrate the higher sensitivity of the internal FBG temperature sensors than that of the PPT during damage initiation. The PPT images were able to capture the later during damage progression. Visual inspection, microscopic images and PPT results are used to compare the damage modes and severity at different depths detected by the FBG sensors. A damage assessment model is then formulated based on the results of combined inspection method.