Fatigue behaviour and damage evolution of CNT-modified recycled carbon fiber composites

In the present study, a comparative fatigue investigation was performed on virgin CFRP (VCFRP), recycled CFRP (RCFRP) and CNT grown recycled CFRP (CNT_RCFRP) composites. The CFRP waste was recycled using a microwave-assisted chemical recycling technique to recover recycled carbon fibers (RCF). Subsequently, CNTs were grown on RCF surfaces using a stepped microwave irradiation process for effective surface modification. Both experimental and computational approaches were employed to analyze fatigue-driven strength degradation and failure mechanisms in the developed composites. Fatigue testing was performed under tension–tension loading at a stress ratio of 0.1 and a frequency of 10 Hz. A stress level corresponding to 80% of RCFRP ultimate tensile strength (≈315 MPa) was selected for comparison. The results show that RCFRP exhibited an approximately 84% reduction in fatigue life compared to VCFRP. In contrast, CNT_RCFRP demonstrated improved performance, showing only about a 20% reduction in fatigue life relative to VCFRP. Fractographic analysis revealed that RCFRP experienced premature failure due to surface defects induced during the recycling process. These defects promoted early matrix cracking and accelerated microcrack coalescence under cyclic loading conditions. Conversely, CNTs in CNT_RCFRP acted as nano-bridges, effectively deflecting crack propagation paths and delaying microcrack coalescence. This resulted in improved fatigue resistance compared to untreated recycled composites. Furthermore, a three-dimensional Hashin failure criterion coupled with a damage accumulation model was used to predict stiffness degradation. The numerical predictions showed good agreement with experimental observations.