Evaluation of Hygrothermal Ageing In Cfrp Composite Material: A Study Using Non-destructive Techniques

Abstract

Advanced composite materials are replacing conventional metals/alloys in aerospace industries due to their excellent strength and stiffness-to-density ratio as well as some other better physical and mechanical properties such as fatigue and corrosion resis- tance. Inight operations, aircraft and spacecraft undergo rigorous environmental conditions that affect the performance of their structural components. Extreme temperature and humid weather conditions along with other environmental factors degrade the properties of structural material. Because of these conditions, the degra- dation happening over a long period of time is referred to as ageing. Further, ageing particularly because of the absorption of moisture and heat is known as hygrothermal (HT) ageing, which is a major concern for the ber-reinforced polymer composites that are being extensively used in modern aircraft structures. In aviation industries, a periodic maintenance check of aircraft structural compo- nents is performed after certain ying hours or cycles that involves around 50 to 6000 man-hours. This consumes enormous time, money and efforts. Non-destructive Evaluation (NDE) techniques, online as well as offlne, can prove to be cost and time effective for inspections of these aircraft and spacecraft structural components. Lit- erature shows that so far the limited focus is dedicated for evaluation of degradation in mechanical properties due to HT ageing in polymer composites by NDE approach. Thus, the present investigation focuses on studying the effectiveness of different NDE techniques for evaluation of HT ageing in Carbon Fiber Reinforced Polymer (CFRP) composite. Different NDE techniques viz., X-ray radiography, ultrasound scanning, thermoelastic wave propagation and infrared thermography were utilized in this re- search endeavour. To induce HT ageing, CFRP composite specimens were immersed in a hot water bath for varied durations up to more than 1000 hours. The in u- ence of HT ageing on X-ray linear attenuation coeffcient, ultrasound attenuation coeffcient, thermoelastic wave amplitude and velocity, thermal cooling constant and nally on mechanical properties was investigated. The trend of water absorption in a composite material has been found to follow Fickian behaviour, reaching satura- tion beyond a certain time period. The attenuation coeffcients obtained from X-ray radiography and ultrasound technique have been found to be signi cantly affected by HT ageing of CFRP. The cooling constant obtained through thermographic study has shown consistent variation with the extent of HT ageing. In addition to conventional NDE tools mentioned above, a novel, non-contact and unique NDE technique has been tried to study the effects of HT degradation in CFRP in this study. That is a thorough generation and propagation of thermoelastic waves in the polymer composite materials using a high power focused LASER energy. These thermoelastic wave propagation studies have shown considerable differences in the wave propagation characteristics proving the effectiveness of this innovative and unique acoustic technique. Further, in evaluating the in uence of HT ageing on mechanical properties, stan- dard three-point bend exural tests were carried out on sets of un-aged and aged specimens. Results obtained from these tests have shown a drastic reduction (26%) in exural strength and signi cant drop (7%) in exural modulus due to HT ageing. Through this work, a correlation between measurable NDE parameters and degra- dation in mechanical properties owing to HT ageing in CFRP composite material has been demonstrated. Further refi nement and establishment of NDE techniques towards the characterization of ageing in materials can be an important direction for the monitoring of the health of composite structures.