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Title: Investigations on the Mechanical Joints Prepared From Electron Beam Cured Carbon Epoxy Nanocomposite Laminates
Authors: Kumar, Mohit
Supervisor: Saini, Jaswinder Singh
Bhunia, Haripada
Keywords: Electron Beam Curing;Mechanical Joints;Carbon Fiber;MWCNT;Failure loads
Issue Date: 17-Aug-2021
Abstract: In many development sectors, the fuel efficiency and harmful emissions from machines into the environment are the major concerns. Furthermore, these machines are made of conventional metallic materials that are susceptible to corrosion. To tackle these problems, various industries and researchers are focusing on the use of alternative lightweight materials that replace the old conventional materials to increase the strength and efficiency of the product. Therefore, a huge consideration is given to the lightweight fiber reinforced polymer (FRP) composite materials due to its high strength and stiffness to their weight ratios and good corrosion resistance properties. The curing process plays a key role in preparation of these composite materials. The thermal curing process is the recognized manufacturing technology for curing of FRP composites, which consists of several hours of material heating at high temperatures, mostly under pressure and in the presence of highly toxic amine-based hardeners, but now the industries are focusing on the alternative methods such as electron beam (EB) curing process that have evidenced a huge transformation in the field of FRP composites. In actual working conditions, the application of FRPs requires joining of materials with each other or with metallic or non-metallic components. Two types of joints i.e., adhesive joints and mechanical joints are used for joining of FRPs. Adhesive joints being permanent in nature fails to operate in the same manner after the joint is opened. However, mechanical joints are preferred due to their ease of assembly and detachability. The mechanical joints seem to be simple, but they are much more than that because of stress concentration due to drilled holes. Improper design of a joint may lead to a failure of the whole structure. So, it is necessary to investigate the behavior of joints to achieve the maximum load-carrying capacity. In numerous applications of marine and civil sectors, the FRP composites and their mechanical joints have concerns about their long-term durability under harsh environmental surroundings, such as ultraviolet (UV) radiation, moisture, elevated temperature, alkalinity, fire, etc. The main objective of the present work is to investigate the performance of EB cured carbon/epoxy composite joints under different geometric parameters and different aging environmental conditions to evaluate the bearing failure loads of the composite joints. Addition of multi-wall carbon nanotubes (MWCNTs) nanofiller into the epoxy, improved the performance of mechanical joints prepared from carbon/epoxy composite laminates. The carbon/epoxy composite laminates were prepared using EB curing process and thermal curing process by incorporating 0.1 to 0.5 wt.% of MWCNTs. The optimum wt. % of MWCNTs in the composite material was found to be 0.3 wt.% showing maximum strength properties for both curing processes. Further, investigations were carried out on neat and optimized 0.3 wt.% of MWCNTs added composite joints prepared using both the curing processes. The single pin joint configurations were prepared using geometric combinations of width to diameter (W/D) ratio and edge to diameter (E/D) ratio, both varying from 2 to 5. Incorporating MWCNTs have shown positive contribution to failure loads and failure modes of the pin joint composite specimens. The failure of the pin joint is much simpler than the bolted joints where the lateral constraints are involved in terms of compressive forces. The numerically predicted ultimate failure loads were compared with the experimental obtained results, which were within 10% of acceptable difference, providing good correlation between each other. Knowing the contribution of the MWCNTs into the pinned joint, the study was extended to bolted joints to analyze their durability under different environmental aging conditions. The composite bolted joint specimens were designed as per ASTM D5961 standard having geometric parameters i.e., W/D and E/D ratios fixed to 6 and 5, respectively. The bolt torques of 0, 2 and 4 Nm were used. The performance of bolted joints was analyzed under hygrothermal aging conditions and accelerated weathering aging conditions. For hygrothermal aging, three different water immersion temperatures i.e., 25 oC, 45 oC and 65 oC were used for duration of 10, 20 and 30 days. The water absorption studies were conducted as per ASTM D5229 standard. The statistical investigations were performed using the central composite design on different control factors i.e., temperature, duration, bolt torque and material. The immersion of composite bolted joints into water at elevated temperatures (45 oC and 65 oC), for 30 days significantly reduces the performance of composite material. The elevated temperature for prolonged duration contributes to inducing the water intake kinetics. The neat composite specimens were more susceptible to the water absorption, especially at higher water temperatures. Incorporating MWCNTs nanofiller, lowered the water absorption rate and thus increased strength retention ability. This increase corresponds to the tortuosity effect which forces water molecules to follow prolonged paths and reduces moisture absorption rate. For accelerated weathering aging, combined cyclic exposure with 8 h of UV at 60 oC and 4 h of condensation at 50 oC (at relative humidity of 100% in condensation cycle) was given to the composite specimens as per ASTM G154, for duration of 250, 500, 750 and 1000 h. Upto 250 h of aging exposure, a slight increase in strength of the joints was observed whereas after 250 h of aging exposures, strength of the joints decreased significantly. The MWCNTs added composite specimens hold the strength retention ability due to increased interfacial area and UV resistant properties that counters the aging effect. The bolt torques contributed in a positive way for unaged specimens. The increased bolt torques enables stress distribution over a wide area, which enhanced the failure loads and stiffness of the bolted joints. But under both the aging conditions, specimen exposure for long durations showed less bolt torque efficiency in neat composite specimens. Moreover, incorporating MWCNTs in the specimens shows good bolt torque effectiveness even at higher exposure duration, under similar conditions. The progressive damage analysis with characteristic curve method was used to predict the ultimate failure loads of bolted joints exposed to hygrothermal aging and accelerated aging conditions. The numerically predicted results have shown good correlation with the experimental ones.
Description: PhD thesis
Appears in Collections:Doctoral Theses@MED

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