| Abstract [eng] |
Carbon fibre reinforced composites are widely used in industries for their high in-plane specific stiffness and specific strength. But these materials like other materials are prone to damages. These damage mechanisms are unique in the case of these materials and they are fibre breakage, matrix cracking, fibre-matrix de-bonding and delamination in case of laminated composites. De-lamination could be considered as a contribution from other damage mechanisms like matrix cracking, fibre fracture and de-bonding, hence de-lamination is a serious issue in composites. Factors leading to de-lamination are weak fibre/matrix interface and brittle nature of the resins. Researches have revealed that the de-lamination problem could be reduced to some extend by altering the internal structure of a composite material. Introduction of carbon nanotubes is one widely used approach. It has been shown that CNTs bridge crack front thereby resisting crack formation. In this work, we made six double cantilever beam specimens (ASTM 5528). The first three i.e. 1.𝑛 (𝑛=1,2,3) were without nanotubes, while the other three, 2.𝑛 (𝑛=1,2,3) were with nanotubes in the epoxy system, the carbon nanotubes were of multi-wall type with no functionalisation. A fixed amount of 0.3% carbon nanotubes were dispersed in the matrix by stirring in a small vacuum chamber. De-lamination process was simulated using a double cantilever beam in the opening mode (mode I), coupled with acoustic emission (AE) signal registration. Post experiment specimen 1.3 and 2.3 were analysed under SEM. The influence of carbon nanotubes on the inter-laminar fracture toughness was studied. Latter modal acoustic emission analysis was carried out by generating wavelet transforms using the AGU-Vallen wavelet solver. The two main modes A0 (anti-symmetric or flexural mode) and S0 (symmetric or extensional mode) were studied and a classification was tried on the basis of fracture mechanism prevalent in composite materials. Class A signals signified matrix cracking, Class B that of fibre fracture and Class C was synonymous with fibre/matrix de-bonding. Additionally two more classifications were carried out, with signals with slightly different waveform pattern, and hence Class AA was for matrix cracking and Class BB for fibre fracture. The results showed a tremendous increase in inter-laminar fracture toughness𝐺𝐼𝑐, but at the expense of a reduced load bearing capacity. While classification of signals by carrying out a modal acoustic emission analysis was found to be more promising than conducting a peak frequency analysis. |
