BEHAVIOR OF BOLT AND ADHESIVE CONNECTIONS IN GLASS FIBER REINFORCED PLASTIC MEMBERS
Authors:   , Doyle James 
Published: September 2024

Structural fasteners must be engineered to resist a large variety of loading conditions. Isotropic materials with high shear strength can be designed to withstand multidirectional loading. However, since fiber reinforced plastic materials are direction dependent, a more in-depth understanding of the effects of loading direction and fiber orientation is needed. To achieve a better understanding of load transfer and failure mechanisms in FRP fasteners, an experimental program has been devised which includes static testing by varying overlap areas and connector strengths. From the results of these tests, more accurate joint designs should be possible. Connectors such as arc welding and adhesive bonding are particularly resistant to shearing forces. Although welding is possible on thermoplastics, the FRP components made from thermosets cause permanent deformation upon heating and cooling. Therefore, adhesive joint resistance to shear loading on thermoset FRP materials has been emphasized in this research project. Optimization of the adhesive joints can be achieved by finding the effective overlapping length of members and maximum adhesive width where stresses remain constant. Experiments are conducted to show that there is a point where the ultimate strength of a joint is not increased for an increase in adhesive length. When the loading conditions cannot be accurately determined or if the material is weak in bearing, a clamping connector is required. The most effective method of creating a clamping support is through the use of nuts, bolts and washers. However, when the bolt holes are drilled, glass fibers are severed, thus weakening the material in this region. Friction connections are analyzed to determine the effect of this material destruction and magnitude of increased joint strength. After an optimum edge distance is established, an effective torque level should be determined to increase the joint efficiency. Experiments show that joint strength can be greatly improved by increasing bolt torque for an adequate edge distance (3D). Theoretically, an ultimate load increase of 60% is possible due to the high compressive strength of the thermoset components. Approximately 80 tests are conducted on FRP coupons and jointed configurations. The majority of tests were conducted to understand bolted joint behavior. Approximately 1/4 of tests are for base coupon (coupon) research, 1/2 on bolted connections, and 1/4 on adhesive joints. After the joint behavior and strength issues are understood, a computer model is used to simulate strength and failure modes. The jointed specimen configuration is replicated and analyzed using the finite element method to predict the experimental results. Stress concentrations, plotted as contour stresses, are used to determine the failure modes. Tsai-Wu failure criterion is utilized to circumscribe ultimate strength of the laminate. Final results and conclusions are made suggesting the proper connection for a given loading condition. Also, recommendations for further research is provided