Hygrothermal Effects on Strength of Mechanical Joints Prepared From Glass Epoxy Composite Laminates

Abstract

The climatic conditions of the earth are rapidly changing, and every year, an increase in CO2 content is reported in the earth’s environment. On the other hand, development in the technology and growth in population has increased the consumption of natural resources of our planet. To tackle these problems, scientists and the industry are focused on alternative light-weight materials, which can improve the efficiency of the vehicles and also reduce the overall emissions in the environment. Therefore, the demand of the light-weight and high strength materials brought forth the composite materials. To manufacture and transport a big structure of a composite involves joining of the composite to composite or composite to metals and alloys. Adhesive joints being permanent may damage the primary components made of composites while opening the joints for maintenance. However, the mechanical joints facilitate easy assembly and disassembly when required and thus preferred over adhesive joints. 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. Fiber reinforced plastics (FRPs) used in marine and aerospace structures are generally exposed to high level of humidity and temperature. Consequently, the durability and performance of the structures made of FRPs have become a primary concern for composite designers. The main objective of the present work is to improve the durability and performance of mechanical joints made of glass fiber reinforced plastic (GFRP) composites. The metal insert was used at the pin-hole interface to improve the performance of the single pinned joint. Addition of nanoclay into the epoxy further enhanced the performance of the pinned joints made of glass fiber reinforced composite material. The geometric parameters, i.e., edge distance to hole diameter (E/D) ratio and width to hole diameter (W/D) ratio were varied from 2 to 5 and 3 to 6, respectively. Metal inserts reduced the stress concentration around the hole and redistributed stresses at the pin/hole interface, which eventually increased the ultimate failure load of the joint. The use of nanoclay and metal inserts certainly improved the load-bearing capacity of the pinned joint. The physics of the pinned joint is relatively simple when compared to the bolted joint as there are no out-of-plane compressive forces present in case of pinned joint. Knowing the contribution of the nanoclay into the pinned joint, the study was extended to analyze the performance of single and double lap bolted joints. Bolted joints were prepared from woven glass fiber reinforced laminates with the addition of nanoclay contents. Different geometric parameters, i.e., edge distance to hole diameter (E/D) ratio and width to hole diameter (W/D) ratio were varied over the range of 2 to 5. Different levels of torque, i.e., 0, 3 and 5 Nm were considered to analyze the effect of bolt torque on the failure behavior of the joint. Failure load of the bolted joint increased with the increase in E/D and W/D ratios. Increasing bolt torque has shown an increase in the failure load and stiffness of the bolted joint. Both the bolt torque and the nanoclay contributed to the performance of the bolted joint. To investigate the performance of the bolted joint exposed to aging environments, two different types of aging environments, i.e., hygrothermal aging (moisture, and temperature), and accelerated aging (ultraviolet radiation, moisture, and temperature), were considered. For hygrothermal aging, three different temperatures, i.e., 25, 50, and 75°C along with three different duration of exposure, i.e., 1, 2 and 3 weeks were considered. A full factorial design of experiment was conducted on important control factors, i.e., water temperature, exposure time, bolt torque, and material variation. Exposure to hygrothermal conditions degraded the material significantly and the water temperature was found to be the most significant factor. The specimens under combined attack of elevated temperature (50, and 75°C), and moisture for 3 weeks experienced a significant degradation. For accelerated aging, a maximum of 500 hours cyclical ultraviolet exposure was given to the specimens as per ASTM D1544. A full factorial design of experiment was conducted on control factors (aging time, bolt torque, and material variation) using response surface methodology. It was found that the strength of the joints prepared with and without the nanoclay content decreased with the increase in the duration of aging. However, the joints prepared with nanoclay content demonstrated higher failure loads. Progressive damage analysis along with Hashin failure criteria was performed to predict failure loads and failure modes in mechanical joints, numerically. A good agreement was obtained between the numerical predictions and the experimental findings.