Development of an enhanced microstructure–level machining model for carbon nanotube reinforced polymer composites using cohesive zone interface
Carbon nanotube (CNT)–reinforced polymer composites have found numerous applications including their use in light–weight structures, micro–fluidic devices, textiles, actuators, and bio–medical implants. The presence of CNTs in the polymer matrix has been found to influence the machining responses of the composite. An attempt has recently been made to understand the machinability of CNT–polymer composites though developing a microstructure–level finite element machining model that considers two distinct phases, viz., the CNT phase and the polymer phase, thereby assuming perfect interfacial bonding. However, subsequent experimental investigations revealed that the failure mechanism during machining is primarily governed by the CNT–polymer interface. Since the strength of the CNT–polymer interface is critical to the load transfer between the CNT phase and the polymer phase, it should be explicitly modeled in microstructure–level finite element machining model. The research in this thesis aims to incorporate the CNT–polymer interface in the microstructure–level finite element machining model to better understand the machinability of CNT–polymer composites. The CNT–polymer interface is represented by the cohesive zone model (CZM) that is characterized by two parameters, viz., interfacial strength and interfacial fracture energy. The CZM parameters for the CNT–polymer interface are obtained through nanoindentation tests, and then implemented in the machining model. The values for the interfacial strength and the interfacial fracture energy for a 2 wt.% CNT–polyvinyl alcohol (PVA) composite sample were estimated to be 42 MPa and 0.018 J/m2, respectively, at room temperature and strain rate of 0.025 /sec. These interfacial parameters were then validated for the 1 wt.% and 4 wt.% CNT–PVA composites. The results indicate that the interfacial parameters of the interface are independent of the CNT loading in the weight fraction range of 1–4%. An enhanced microstructure–level finite element machining model for CNT–PVA composites is developed by considering the CNT–PVA interface as the third phase in addition to the PVA phase and the CNT phase. To account for variable temperature and strain rate over the deformation zone during machining, temperature– and strain rate–dependent mechanical properties for the interface and the polymer material are also obtained and considered in the model. The results show that the model can predict cutting forces within 6% of the experimental values for the machining conditions used in this study. The machining simulation results reveal that a large rake angle better facilitates the material removal process during micromachining and produces lower cutting force than a smaller rake angle. A parametric study is performed to investigate the effect of interfacial strength on surface/subsurface damage of CNT–PVA composites. Subsurface damage is reduced when interfacial strength increases. However, very high interfacial strength or perfect interfacial bonding can result in more surface/subsurface damage because CNTs can bend excessively in the cutting direction, causing more local deformation and higher stress in CNT–PVA composites.
Carbon nanotube–polymer Composites, Carbon nanotube–polymer interface, microstructure–level finite element machining model
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign