Effect of washer size and tightening torque on the performance of bolted joints in composite structures

The weight and fuel savings offered by composite materials make them attractive not only to the military, but also to the civilian aircraft, space, and automobile industries. In these industries, bolting and riveting are extensively used as a primary method of forming structural joints. Therefore, the main objective of the present paper is to investigate the influence of certain factors on the strength of bolted joints in [0/±45/90]s glass fiber reinforced epoxy (GFRE) composites. These factors include the tightening torque (T = 0, 5, 10 and 15 Nm) and the washer outer diameter size (Dwo = 14, 18, 22 and 27 mm). The mechanical properties (tensile, compressive, and in-plane shear) of GFRE laminates have been determined experimentally and theoretically.The experimental results show that under the same tightening torque, the slope of load–displacement diagrams of bolted joints (stiffness) increases with decreasing washer size. Bolted joint with 18 mm washer size and 15 Nm tightening torque has the maximum strength. In the range of the investigated tightening torques, the bearing strength of bolted joint with 18 mm washer size increases with increasing tightening torque. The load–displacement curve of bolted joint specimen with finger tight, T = 0 Nm, has the lowest slope/stiffness with several knees, which may indicate an unstable development of internal damage. Most of the test specimens failed in the following sequence: (a) delamination between the 0°, ±45°, and 90° layers due to their different strains under the compressive bearing load, (b) net tension failure mode for 90° layers, (c) shear-out failure modes for 0° layers, and (d) the final failure was nearly catastrophic due to the bearing failure of ±45° layers.