EXPERIMENTAL INVESTIGATION ON BOLTED JOINTS IN NATURAL FIBER BASED COMPOSITES

Abstract

The development of biodegradable and environmentally sustainable composite materials has received a lot of attention in recent years due to the growing awareness of the rapid depletion of nonrenewable resources and anticipated dependency on renewable resources. Therefore, the application spectrum of natural fibers as a reinforcement in polymer composites has increased at an unprecedented rate over the last few years, owing to their low cost, sustainable nature, low density, and comparable specific mechanical properties to that of synthetic fibers. These materials are being used for the fabrication of a wide variety of products in various sectors such as automobile, construction, packaging, etc. Although every attempt is made to manufacture the NFRPCs products as single components of near-net shape, depending on the complexity, size, and application of the product, assembly of the individual components becomes an inevitable operation. The bolted joints are most commonly used for the assembly of individual components due to convenient repairs, inspection, and disassembly. Additionally, drilling is an indispensable machining operation that is repeatedly performed to make holes to facilitate the assembly operation through bolted joints. It is also well understood that the performance of bolted joints is highly dependent on the quality of the drilled hole. Drilling of NFRPCs is a rather intricate task compared to metals because of the anisotropic and heterogeneous nature of fibers. The drilling tool passes alternatively through the fiber and matrix layer, both of which have distinct properties. These issues subject the drill bit to experience variable thrust force and torque and subsequently lead to drilling-induced damage (around the hole periphery), such as thermal degradation, fiber peel-up, pullout, and delamination. The undesirable damage caused by the drilling process deteriorates the performance of bolted joints in NFRPCs. As a result, greater consideration must be given to the choice of drilling parameters and drilling tools in addition to the type of fiber and matrix materials that are used in NFRPCs in addition to their mechanical and thermal properties. Therefore, the major research focus around the world is to minimize the drilling induced damage by optimizing the drilling parameters and developing new drill tool geometries and drilling techniques, specifically customized for the drilling of NFRPCs. Both traditional and non-traditional optimization techniques can be applied to obtain the optimal values of drilling parameters using input and response variables of experiments. However, researchers have mostly utilized traditional optimization techniques, which generate only local optimal solutions, which are less efficient when compared to non-traditional optimization techniques, such as genetic algorithms, particle swarm optimization, and teaching learning-based optimization. Thus, there exists a research opportunity to optimize the drilling variables using various non-traditional optimization techniques and find the global optimal solution within in reasonable number of iterations, for damage-free drilling of NFRPCs. An exhaustive literature survey on natural fibers indicated that plant-based natural fibers extracted from the stem of the plant possess superior mechanical properties. Accordingly, the initial phase of the current research study is focused on the development and characterization of the flax and kenaf fibers reinforced polypropylene composites. The physical, chemical, and thermal characteristics of both fibers have been discussed in detail. The bidirectional flax, kenaf, and interwoven hybrid flax/kenaf reinforced composites were fabricated using compression molding, and the physical, thermal, and mechanical behavior of developed composites have been studied. The effect of hybridization has been analyzed on both static and dynamic mechanical behaviour of developed composites. The flax/PP composites demonstrated superior mechanical performance over the other developed composites, therefore further investigation was accomplished on the flax/PP composites. Furthermore, in the second phase, conventional drilling of flax/PP composite laminates has been carried out using five different drill tool geometries under various combinations of spindle speed and feed rate. The influence of these factors has been analyzed on the thrust force and push down delamination factor. Additionally, three different optimization techniques (genetic algorithm, particle swarm optimization and teaching learning-based optimization) have been implemented for the optimization of drilling variables for ensuring the minimum thrust force. A comparative analysis of damage characteristics of the drilled hole using SEM has also been explored. In the third phase, the effect of drilling induced damage has been analyzed on the ultimate bearing strength of double lap shear bolt loaded flax/PP composite laminates. Besides, the suggested values of drilling variables and drill tool geometry for minimum drilling induced damage and maximum ultimate bearing strength have been further ultilized for analyzing the effect of geometric and fastening parameters on the ultimate bearing strength and failure modes of flax/PP composite laminates. Finally, a comparative analysis of tensile failure load of single lap shear joints fabricated using only mechanical joining, fusion bonding (ultrasonic and hot plate joining), and hybrid joining (mechanical/ultrasonic joining and mechanical/hot plate joining) has been proposed for selecting a most feasible method of joining the flax/PP composites, depending upon the joint performance required. In the case of the mechanical and hybrid joining, the effect of the bolt size and number of bolts (single and double) on the tensile failure load was also investigated. Additionally, the fusion bonding effect on the crystallinity and thermal degradation behavior of the joint was analyzed using thermogravimetric analysis and X-ray diffraction, respectively. The work being reported as the outcomes of the present research initiative will help in extending the thorough understanding of drilling and joining behaviour NFRPCs. A variety of new questions and prospective areas for work in future have arisen during the investigation, indicating that this area of research requires exhaustive studies to further enhance the understanding and knowledge in this field.