Halloysite is a fine clay mineral consisting of tubular particles with a multi-layered wall structure. Our previous work has shown that the addition of 2.3 wt% halloysite nanotubes (HNTs) to an epoxy (EP) increased its Charpy impact strength by 400% (Ye et al. 2007). A two-phase structure, namely a HNT-rich particle phase and an epoxy-rich matrix phase, was found in the EP/HNT nanocomposite. Toughening mechanism study reveals that this unique microstructure is necessary for the effective toughening of epoxy, because HNT-rich particles deflect the main crack propagation, and slow down fracture process. It is proposed that the major toughening mechanism is massive micro-cracking with nanotube bridging in front of crack tip under impact loading. To prove this proposition, the quasi-static...[ Read more ]

Halloysite is a fine clay mineral consisting of tubular particles with a multi-layered wall structure. Our previous work has shown that the addition of 2.3 wt% halloysite nanotubes (HNTs) to an epoxy (EP) increased its Charpy impact strength by 400% (Ye et al. 2007). A two-phase structure, namely a HNT-rich particle phase and an epoxy-rich matrix phase, was found in the EP/HNT nanocomposite. Toughening mechanism study reveals that this unique microstructure is necessary for the effective toughening of epoxy, because HNT-rich particles deflect the main crack propagation, and slow down fracture process. It is proposed that the major toughening mechanism is massive micro-cracking with nanotube bridging in front of crack tip under impact loading. To prove this proposition, the quasi-static fracture behavior of the same EP/HNT nanocomposites was investigated by K_Ic methodology under different loading rates. A low improvement in K_Ic with a limited amount of micro-cracks was observed, which implies that massive micro-cracking under impact loading is necessary to the effective toughening of epoxy by HNTs. To understand the toughening mechanisms in the static fracture process, the tensile behavior of EP/HNT nanocomposites was studied under low speed. The tensile test results suggested that the strain at break of epoxy was improved by the introduction of HNTs, which contributed to the improvement in K_Ic . Nanotube bridging/pull-out/breaking in the main crack propagation has been found as the major toughening mechanisms for the toughening in K_Ic.

Carbon fiber-reinforced epoxy (EP/CF) composites have been widely used in many areas, including aerospace, automobile, marine, military, etc., due to their unique properties, such as high strength, high modulus and lightweight. However, the strength in the through-thickness direction is quite low because there are no fibers oriented in the thickness direction to sustain transverse loads, which results in low damage resistance. This limitation generally leads to interlaminar failures, such as delamination. Considering that the HNT crack-bridging capability and the low damage resistance of conventional EP/CF composites are largely caused by the propagation of internal defects (e.g., microcracks) under external loadings, we used the EP/HNT nanocomposite as the matrix in the fabrication of the CF composite in the present study. It was expected that the EP/HNT/CF hybrid composites would benefit from the high impact toughness due to the presence of the HNTs, leading to a new class of CF composites.

The microstructure of the EP/HNT/CF hybrid composites was examined using SEM. The mechanical properties and the failure mechanisms of the hybrid composites were studied. At a HNT loading of 2 wt%, a 25% enhancement in the Izod impact strength has been achieved, with the flexural strength and flexural modulus changed slightly. The toughening mechanisms of the new hybrid composites have been studied by investigating the interlaminar properties using three classical fracture mechanics testing methods, including short-beam shear tests, double-cantilever-beam tests and end-notched flexure tests. It was shown that the addition of HNTs to the composites increased the interlaminar shear strength and the fracture resistance under Mode I and Mode II loadings greatly. Furthermore, the thermo-mechanical properties, including the storage modulus and the glass transition temperatures, were also improved. The morphological study of the hybrid composites revealed that HNTs were non-uniformly dispersed in the epoxy matrix, forming a unique microstructure with a large number of HNT-rich composite particles enveloped by a continuous epoxy-rich phase. A study of the fracture mechanism uncovered the important role of this special morphology during the fracturing of the hybrid composites.