PSI - Issue 25

7

Dalbir Singh et al. / Procedia Structural Integrity 25 (2020) 159–171 Dalbir singh/ Structural Integrity Procedia 00 (2019) 000 – 000

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2.4. Morphological analysis

The surface morphology of the composite specimens was examined by Scanning Electron Microscope (SEM), as shown in Fig.8. The images were taken with the help of a scanning electron microscope (SEM) to further analyse the effects of the oil solutions and of the exposure duration on the composites. The fractured surfaces of the tested specimens were subjected to sputter-coating (platinum) and was analysed using a Coxem CX (COXEM South Korea) at 20KV accelerating voltage. The failure mechanism SEM images of the bi-directional control sample and samples immersed with different aviation grade fluids (Avgas100LL, ATF-K50 and OX-38) are shown in Fig.8. In order further analyse the effect of exposure duration on the resins and fibre, images were taken with the help of a scanning electron microscope (SEM) as seen in Fig. 8 after the Tensile, Flexural and Impact tests. They showed the typical failure modes such as matrix cracking, de-bonding with fibre fracture, fibre tear, delamination, fractured laminates, mixed failure, and multi-stepped fracture were obtained for the bi-directional sample. The SEM analysis showed the cracks, breakage of fibre, delamination and damage of the matrix material. It also showed the misalignment of fibre & void formation in between the matrix which resulted in fibre-matrix damage.

Fig. 8. Failure mechanisms (L to R) after 45 days: (a) Control samples; (b) Samples exposed to Avgas 100L (c) Samples exposed to ATF K50 (d) Samples exposed to OX 38 3. Results and Discussion 3.1. Tensile test The variation of tensile strength is represented in Fig.9. The tensile strength decreases gradually with increase in the exposure duration with different oils. It was observed that the mechanical properties of the carbon composites decreased linearly with an increase in the exposure duration. It was noted that the Avgas 100LL sample had a 0.305% reduction in strength after 15 days of immersion while those of ATF and Ox-38 displays 1.219% and 2.057% reductions respectively. For the 30-day immersion period, the reduction in strength is further reduced by 1.021%, 2.225% and 2.834% from the control sample for Avgas-100LL, ATF and OX-38 fluids respectively. At the end of the 45-day immersion period, the samples were observed to display immense reductions in tensile strength. 2.743%,4.541% and 9.052% were the percentage reductions for Avgas, ATF and Ox-38respectively.To summarise, at the end of 15 days, the reduction in strength was ~2%and it could be neglected . At the end of 30 days, we saw a drop in strength for the ATF and Ox-38 samples while the Avgas sample remained relatively unaffected. After 45 days, it appeared that all the samples were severely affected by the fuels/oils and displayed significant reductions in tensile strength. The SEM images in Fig.10 show the mechanisms of failure of the control sample and sample exposed to different aviation grade fluids for different durations. For control sample, the failure mechanism stared by initiation of crack at the surface and penetrate in the matrix, followed by fibre breakage and small delamination shown in Fig.10 (a). For sample immersed in aviation grade fluids (Avgas-100LL, ATF K50-and OX-38), the analysis had shown mixed mode of failure as presented in Fig.10(b),10(c) and 10(d). The tensile strength is given in table 1.