Effect of Washers and Bolt Tension on the Behavior of Double-Lap S2-Glass Fabric Epoxy Composite Joints

Experimental characterization of thick composite bolted joints is performed to study the effect of washer size and bolt preload on bearing properties. S2-glass fabric-epoxy composite coupons [0/90; +45/−45 @ 10 sets] of 12.5 mm thickness were tested under double shear tensile loading. Two different washer sizes and thicknesses were used in this investigation. A force washer is used to monitor the clamp load variation during the test. It has been found that the initial bolt tension (preload) and washer size have a significant effect on bearing stiffness and bearing strength of thick composite joints. For a low bolt preload, test data shows a significant clamp load increase with the joint displacement. However, the percentage increase in clamp load is reduced as the preload is increased to 50kN. The outward buckling and delamination of the laminate in the composite coupons were found to be the main cause for clamp load increase.