Predicting Delaminations and Residual Strength of Carbon Fibre Reinforced Polymers (CFRP) After One-Sided Thermal Loading by Means of Infrared Spectroscopy

Abstract

This study investigates the change of chemical, structural, and mechanical properties of carbon fibre reinforced polymers (CFRP) after one-sided thermal loading. Therefore, CFRP samples (HexPly® 8552/IM7) with varying thickness (4 and 8 mm) are irradiated at different heat fluxes (15, 30, and 50 kW/m2). For a depth-resolved view on matrix degradation inside the CFRP, infrared spectroscopy (ATR-FTIR) is applied along a ground incline plane. A change of structural properties in the form of developing delaminations is investigated with microfocused computed X-ray tomography (µCT). The loss of residual strength is determined by means of interlaminar shear strength testing (ILSS). The evaluation of the data shows that delaminations occur predominantly dependent on temperature and only beyond a certain level of matrix degradation traceable by IR spectroscopy. It is also shown that delaminations are mainly responsible for the loss of strength. Furthermore, linear discriminant analysis (LDA) is performed to predict the presence of delaminations. This information provides a basis for a reliable prediction of the residual strength by IR spectroscopy after one-sided thermal loading.

Using infrared spectroscopy in combination with multivariate data analysis to predict residual strength of carbon fiber reinforced polymers after one‐sided thermal loading

“…As the development of delaminations is inter alia dependent on temperature, structural damage occurs faster for thinner specimens. [18] Furthermore, Figure 2 shows the ILSS for 4 and 8 mm thick specimens irradiated at 50 kW/m 2 . Also, residual strength for thinner CFRP decreases faster than for thick CFRP.…”

“…For these lower heat fluxes, however, a longer irradiation time was necessary to induce delaminations and cause a loss in residual strength (see Table 1). [18]…”

“…These lines cross the gradients of matrix degradation at a value of $0.71 for I 1510 cm -1/I 1486 cm -1 for 120 and 160 s. This value corresponds to the calculated threshold value of 0.74 ± 0.05, below which the formation of delaminations in 8552/IM7 occurs. [18] The threshold value is indicated by the horizontal line in Figure 4. Interestingly, the horizontal and vertical lines cross where the drop of the I 1510 cm -1/ I 1486 cm -1 curves start when approaching the front side of the sample.…”

“…Damage investigations undertaken by Vetter et al showed the coherence between decreasing nondelaminated thickness and decreasing residual ILSS of CFRP after one-sided thermal loading. [18] A reliable estimation of residual strength is only possible if the CFRP is penetrated with delaminations less than 50% of its sample thickness. Without an exact localization of the delamination depth within the CFRP, no statement about residual strength and a further usage of a structure is possible.…”

“…This often leads to a complete replacement, which can be expensive and time intensive. [18] Therefore, an ability to reliably predict the residual strength after thermal loading is preferable, especially directly on the damaged aircraft.…”