Indentation Fracture Resistance Vs Conventional Fracture Toughness of Carbon Nanotube/Alumina Nanocomposites

Multiwalled carbon nanotube (MWCNT)/alumina (Al₂O₃) nanocomposites were fabricated using two varieties of CNT to access the effect of morphological variation of the filler on fracture resistance (K _R)/toughness (K _IC) of studied specimens. Special attention was also given to compare K _R and K _IC values for tracing out the primary selection criterion of appropriate indentation fracture (IF) equation used in relatively faster and simpler ‘direct crack measurement’ (DCM) technique to evaluate K _R values close to stringent ‘single edge notched beam’ (SENB) derived K _IC data. While K _IC was calculated using the unique expression suitable for specimens tested under four-point flexure, K _R values were evaluated using a series of IF equations suitable for Palmqvist and/or median crack systems. As far as change in K _R and/or K _IC of nanocomposites was concerned, it was noticed that for longer/thicker CNTs having relatively higher internal bamboo structures, much lower amount (0.15 vol pct) was adequate to achieve the highest improvement in K _R (~87 pct) or K _IC (~50 pct) over pure Al₂O₃ (^Laugier K _R ≈ 3.83 MPa-m^0.5; K _IC ≈ 3.48 MPa-m^0.5) than that required for smaller/thinner CNTs (≥0.3 vol pct). On contrary, resistance to fracture up to 1.2 vol pct CNT loading was much enhanced in specimens fabricated with smaller/thinner CNTs over those fabricated using longer/thicker CNTs. Comparatively better morphology, adequate CNT dispersion, and higher population of bridging elements in specimens containing smaller/thinner CNTs were the key factors behind such toughness retention.