MACHINING BEHAVIOR OF FIBER-REINFORCED POLYMER COMPOSITES

Abstract

The application spectrum of fiber-reinforced composites has increased at an unprecedented rate over the last few years owing to their outstanding multifunctional properties including high specific strength and stiffness, low density, good damping properties, and good chemical and corrosion resistance as compared to the conventional engineering materials. These materials are being used for making of wide variety of sophisticated engineering components used in aircraft, spacecraft, and automobile industries. Though, fiber-reinforced composites are considered as high-tech sophisticated materials, yet their selection as structural materials is often limited due to their poor machining performance. The inhomogeneous and anisotropic nature of the composites, low thermal conductivity of the composite constituents, and highly abrasive nature of the fibers renders the conventional machining techniques ineffective for machining of fiber-reinforced composites. Machining operations such as drilling, milling, turning, grinding, and boring are the most common machining operations that are usually performed on the composite parts. Among all the machining operations, drilling is an indispensable machining operation which is repeatedly and extensively performed for making of holes to facilitate assembly of several composite parts through mechanical fastening such as, bolting, riveting etc. It is well understood that the efficiency of the mechanical fastening is highly dependent on the surface quality of the drilled hole. The conventional drilling which is frequently used for making of holes in composite parts is not convenient anymore because of a plethora of challenges encountered. One of the perplexing challenges is the drilling-induced damage which majorly occurs due to the generation of thrust force and torque. The damage formed in and around the drilled hole creates an uneven surface which subsequently results in poor surface quality or rejection of viii the composite part. Furthermore, the heterogeneous nature of the composite subjects the drill bit to experience variable forces due to the difference in the properties of the matrix and reinforcement. The resultant forces result in damage of composites in the form of delamination, fiber pull-out, spalling, splintering etc. Therefore, the major research focus around the world is to minimize the drilling-induced damage by optimizing the cutting parameters, developing newer tool point geometries and drilling techniques dedicated to the drilling of fiber-reinforced composites. Therefore, there exists a research opportunity to conceptualize and develop cost-effective high-quality machining methods for composite laminates. In the last few decades, researchers have tried to conceptualize, develop, and commercialize the unconventional or advanced routes of processing with an objective of reducing the damage that is induced during drilling of fiber-reinforced composites. Advancements in the unconventional machining processes offer an opportunity to process these materials economically, thus realizing the full potential of the fiber-reinforced composites. The research and development work in the area of unconventional machining processes may lead to conceptualization of an optimal process that is suitable for damage-free drilling of a wide variety of composite parts. In the present research endeavour, the drilling behavior of the synthetic and natural fiber-reinforced composites has been experimentally investigated. The drilling behavior of the developed composites has been explored using three different drilling techniques namely, conventional drilling, modulation-assisted drilling, and rotary-mode ultrasonic drilling. One of the fundamental differences between the conventional drilling, modulation-assisted drilling, and rotary-mode ultrasonic drilling is: (a) in conventional drilling, the interaction between the tool and work material is direct which continues till the complete drilling operation culminates, (b) in modulation-assisted drilling, the interaction of the tool with the ix work material is pulse-intermittent, and (c) in rotary-mode ultrasonic drilling, there is no direct contact between tool and work material. The conventional drilling of the composite laminates has been carried out using three different drill point geometries under various combinations of spindle speed and feed. The characteristics of the formed chips and the behavior of thrust force and torque signals have been analysed and mapping of thrust force and torque has been proposed. Influence of drill point geometry, feed, and spindle speed on drilling forces has been investigated experimentally. A comparative analysis of damage characteristics of drilled hole using SEM has also been explored. The modulation-assisted drilling of the composite laminates has been carried out under different experimental conditions. The influence of the feed, frequency of rotation, drill point geometry, frequency of modulation, and modulation amplitude on the drilling-induced thrust force and subsequently damage has been experimentally investigated. The cutting mechanism has been emphasized in order to understand the drilling behavior of composite laminates. The rotary-mode ultrasonic drilling process has been conceptualized and developed in order to produce damage-free, clean-cut holes in the composite laminates. The performance of the developed process has been analysed in terms of material removal rate, tool wear rate, surface roughness, and damage produced during drilling of the composite laminates. The influence of various input parameters including power rating, slurry concentration, and abrasive size on the drilling characteristics of composite laminates has been experimentally investigated. The work being reported as the findings of the present research initiative will help in extending the understanding of drilling behavior of both natural and synthetic fiberreinforced polymer composites. Many new questions and potential areas for work in future x have arisen during the investigation, which shows that this area of research requires exhaustive studies to further enhance the understanding and knowledge in this field.