Carbon Fiber Reinforced Polymers (CFRP) are widely applied in light weight applications e.g. in aerospace because of their high strength to weight ratio compared to e.g. metal structures. However, due to the polymer matrix, CFRP show lower thermal stability. Therefore a quantification of thermal damage is indispensable. A non-destructive evaluation method for moderate thermal damage is ATR-FTIR spectroscopy. The focus of this method is to characterize the surface damage. However, for a more detailed comprehension of thermal damage it is also necessary to gain a deep insight into the bulk material. For this purpose, chemical and mechanical properties were considered through the profile of the CFRP with respect to matrix degradation, loss of fiber-matrix-adhesion such as occurring delaminations and loss of interlaminar shear strength.
For this study a frequently used CFRP (HexPly® 8552/IM7) was analyzed. All samples were thermally irradiated at one side with a heat flux of 50 kW/m² over various time intervals (from 0 to 140 seconds) i.e. every sample suffered from a temperature profile through its depth. ATR-FTIR spectroscopy along a grounded incline plane through a CFRP specimen allows a chemical characterization of the thermal damage into the depth. The intensity ratio of characteristic IR bands of the thermally less stable epoxy resin (1510cm-1) compared to the thermoplastic polyethersulfone (1486cm-1) decreases with increasing thermal degradation. The mechanical behavior after thermal damage of CFRP is determined by Interlaminar Shear Strength tests (ILSS). These tests were performed with irradiated samples with varying thickness (2 and 8mm) and samples (2mm) cut out from different depths of an irradiated sample (8mm). Resulting delaminations due to thermal loading are observed with various microscopic methods.
The correlation of chemical and mechanical data shows the relationship between residual strength and degradation of the polymer matrix due to thermal loading. A measured temperature profile through the CFRP specimen explains the degree of degradation. The intensity ratio of epoxy resin to thermoplastic increases from the irradiated to the non-irradiated side. The ILSS tests also show a lower residual strength at the irradiated side and higher residual strength at the back side. Both ILSS and ATR-FTIR measurements are indicating a pronounced degradation close to the irradiated surface and show good correlation of mechanical and chemical data.