Weibull multiscale interlaminar fracture analysis of low-weight percentage CNT composites

Abstract

Delamination due to interlaminar fracture has limited the application of composite materials in highly integrity-sensitive structural systems. The novelty of this experimental and statistical investigation is twofold: (i) the effects of the size of multi-wall carbon nanotube (MWCNT) bundles and their interphase with the surrounding matrix on the multimodal fracture toughness and crack propagation regime; (ii) the effectiveness of single lamina vs. multiple laminas on fracture toughness in carbon fiber nanocomposites. The Atomic Force Microscopy Peak Force Quantitative Nanomechanics Mapping (AFM PFQNM) technique was utilized to characterize the CNT bundle size and interphase. Weibull analysis of a myriad of 480 CNT bundle sizes and interphases confirmed a high scatter in the data with more occurrences at the larger size. Two different nanocomposite laminates were considered: (i) laminates with 1wt% CNT in all laminas (FCNT); (ii) laminates with 1wt% CNT in the middle layer in front of the pre-crack (CNT). The average size of the CNT bundles and the interphase thickness were 549.7 nm and 22.9 nm in the CNT laminates and 480.1 nm and 22.2 nm in the FCNT laminates. The ratio of interphase thickness to CNT bundle size was higher in FCNT than in CNT, confirming a better stiffening effect due to the nano reinforcement of FCNT laminates. Weibull moduli of mode I initiation and propagation and mode II initiation for laminates with CNT in all laminas, FCNT, are higher than reference and CNT laminates, confirming a higher crack energy consistency. The Mode I initiation and propagation and mode II initiation fracture energies of CNT and FCNT were (15% and 10%), (38% and 44%), and (1.7% and 9.6%) higher than those without CNT. Mixing 1wt% CNT with all laminas expanded the mixed-mode I-II fracture envelope function by 21%. Unlike FCNT laminates, CNT laminates do not considerably improve the mixed-mode I-II fracture toughness. Crack deflection and a more arduous crack path due to the rigid CNT bundles increased the crack energy and delayed the crack propagation.