A critical investigation of the use of infrared thermography in determining the condition of composite materials

• Main Author: Addepalli, Sri Naga Pavan
• Published: University of Wales Trinity Saint David 2017
• Subjects: 620.1; TS Manufactures
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.720420

Since the introduction of synthetic composite materials as primary airframe structures in aircrafts, especially the Carbon Fibre Reinforced Polymers (CFRP) in the late 1990’s, there has been increased use of these advanced materials that have completely replaced key metallic parts of the aircraft contributing to overall reduction in the weight of the aircraft. These innovative materials are increasingly preferred due to their material properties and better strength-to-weight ratio offering not just weight savings but increased resistance to issues such as abrasion and corrosion. As these composite materials are non-metallic in nature, their behaviour especially in the presence of defects and damage is less understood as they do not follow properties exhibited by metals and their alloy systems. This study thus focusses on establishing methods that could detect these defects and damage in a non-destructive manner such that the inspection systems do not cause further damage to the component. This study is primarily experimental in nature and has been presented in two parts. The first section looks at establishing pulsed thermography as a key technique capable of detecting sub-surface defects and its applicability to detect them. This has been presented by inspecting field representative samples and by introducing commonly occurring materials as inserts during the layup stage of the CFRP at controlled depths to determine the detection capability of the system. The second part of the work presented a parametric low-energy impact study where laminates were subject to modified Charpy and ballistic testing to create barely visible impact damage (BVID). The damaged parts were then subjected to inspection using techniques such as pulsed thermography, thermoelastic stress analysis (TSA), immersion ultrasonic testing, microscopy and laser doppler vibrometer (LDV). The aim was to establish TSA method as an alternative tool to detect surface breaking damage. It was found that pulsed thermography, though capable of detecting subsurface damage, was less sensitive to near surface damage. Further, it was noticed that the TSA method showed a positive response when it came to detecting surface breaking damage created during ballistic impact, thus establishing the technique as an in-situ technique.