Compliance of Fasteners in Metal-Composite Joints

Abstract

The greatest difficulties in creating a new structure or upgrading an old one are associated with designing joints and connecting parts of the structure, especially composite and metal ones. Nowadays, the problem of designing and improving such joints is timely and is being actively studied so that the best solutions are found. The article presents the research carried out to determine the main parameters of elements of mechanical joints made of polymer composite materials, as factors affecting the requirements for precision of connection compliance estimation. There has been found the influence of the number of fastener rows on estimating the maximum forces which act in the joint, as well as the influence of the ratio of the connection compliance to the compliance of the part. A field of possible states of the joint has been built. Within the field, the following areas have been identified: the area of high sensitivity to precision of estimating the parameters of the joint elements, requiring experimental confirmation and more accurate models; the area of average sensitivity of the joint to precision of estimating the parameters of its elements, permitting the use of known dependences; and the area of low sensitivity, where simplified models can be used. In the course of the developed model testing, joint samples were tested to evaluate the force connection compliance, and a reference variant was calculated using the Douglas formula. The reference variant of the joint sample was chosen based on the principle of equal compliances of the parts to be joined, presented by a series of samples assembled from a composite package of 80 layers and two surface layers made of 3.98 mm-thick steel. The parts were connected with steel bolts with a diameter of 6 and 8 mm. Numerically, the ratio of the connection compliance to the part compliance is 85...96 for the experimental data, and 9...10 for the calculated data. This indicated that the joint was in the transition area between high and medium sensitivity of the joint to the accuracy of compliance estimation, which requires continuous refinement of the connection parameters.