Exploiting carbon nanotubes to modulate the interfacial and coupling behaviour in carbon fiber reinforced polymer composites

Abstract

The average mechanical properties of carbon fiber reinforced polymer (CFRP) composites primarily depend on the properties of the constituents and the interaction between them. The strength of interaction is related to the extent of interface/interphase region (region having distinct properties from fiber and matrix) surrounding the fiber. Enhancing the interfacial area is one of the possible strategies to improve the interfacial strength and hence average mechanical properties of CFRPs. To this end, carbon nanotubes (CNTs) are directly grown on the surface of carbon fiber using chemical vapor deposition (CVD) method to modify the fiber/polymer interphase in this work. Grafting of CNTs on fiber surface also eliminates the adverse effects of CNT agglomeration in polymer matrix. The CNT grafted carbon fabric is later used with polymer matrix to process CNT grafted carbon fiber/polymer multiscale composites. A detailed experimental investigation is carried out to quantitatively determine the size and properties of interphase and how the existence of interphase due to the incorporation of nanotubes influences the average properties of conventional CFRPs. The effect of CNT grafting on the interfacial properties is investigated through micro-mechanical testing of single carbon fiber composites. Energy dispersive spectroscopy and nanoindentation reveal three times increase in interphase size and interfacial area after CNT incorporation in CFRP. CNT grafting not only stiffens the interphase, it also increases the interfacial shear strength (IFSS) of CFRPs by around 41% as recorded from single fiber pull-out test. However, an increase in CNT growth time inside CVD reactor decreases the interphase stiffness and IFSS due to the poor wetting caused by the presence of highly dense network of entangled nanotubes on the fiber surface. Next, the effect of matrix type on the average mechanical response of CNT grafted carbon fiber/polymer is investigated. To this end, the wetting behavior of two commonly used polymer resins, epoxy and polyester, with as-received and CNT grafted carbon fibers is studied using contact angle measurements. As compared to polyester, epoxy shows better wettability with carbon fiber which gets even better after CNT grafting on the carbon fiber surface. Apart from IFSS, interfacial fracture toughness (Gic) is another crucial parameter to ensure that CNT grafted CFRP offers high resistance to interfacial crack initiation and propagation. Moreover, it is possible that the structural composite may be subjected to varying loading environments during their life cycle. Hence, it is imperative to investigate the effect of CNT incorporation on the rate sensitivity of IFSS and Gic of CFRPs. Accordingly, micro-droplet debond tests are performed on conventional and CNT modified CFRPs at crosshead speeds varying from 0.005 to 5 mm/min. Experimental results show that interphase designing using different CNT incorporation strategies lead to a broad range of IFSS (16 – 79 MPa) and Gic (100 – 453 J m-2) for conventional CFRPs. Moreover, it is shown that simultaneous improvement in IFSS and Gic is possible with certain interphase designs which is rather difficult to achieve otherwise. It is observed that IFSS and Gic are not constant material properties but follow a power-law type relationship with the applied loading rate. The power law exponent found to be a function of interfacial microstructure. The improvements recorded in interfacial properties of CNT grafted CFRPs during micro-mechanical testing are beneficial only if they can be translated at the bulk laminate level as well. Therefore, the effect of CNT grafting and matrix type on the average mechanical properties of laminated composites is investigated. It is found that CNT grafting has the potential to improve the transverse elastic stiffness (by 60%) and the interlaminar shear strength (by 78%) of epoxy based laminates. Scanning electron microscopy of the fractured fiber surfaces indicates that different interfacial failure mechanisms operate in the presence and absence of CNTs at the fiber/polymer interface. Apart from interfacial and interlaminar properties, the existence of couplings in laminated composites poses another challenge in designing composites for structural applications. It will be interesting to see how CNT grafting affects the coupling terms in the A, B, and D matrices of CFRP laminates. Keeping this in mind, numerical simulations using commercially available finite element analysis tool Abaqus are performed to gain better insights on how the incorporation of CNT grafted laminas modulate the coupling behavior of CFRP laminates. The deformation behavior of composite cantilever beam is studied under tensile, bending and combined loading. Simulation results show that systematic use of CNT grafted laminas can maximize or minimize the coupling effect in laminated CFRPs. Based on the findings of the present work, it is concluded that modifying the interphase in conventional CFRPs using CNT grafting is an efficient strategy to tailor the interfacial and interlaminar properties along with laminate couplings. It is envisaged that the present study is a significant step ahead to design the composites as per the need of a particular application.