The influence of the substrate’s stiffness on the liquid shim effect in composite-to-titanium hybrid bolted joints

Abstract

Cast in place liquid shim is usually used to fix the gap/mismatch problems, which occur during assembly process of large aircraft structures. The effect of liquid shim on performances of the assembled structures is influenced not only by thickness and mechanical properties of the shim, but also by the location where the shim is used, that is to say the stiffness of the substrates may have impact on the shim’s effect. To study the usage of shim in composite-to-titanium bolted joints, a three-dimensional finite element method is introduced, and the method incorporates the progressive damage of composite materials, elastic-plastic property of the titanium alloy, super-elastic property of the liquid shim, contact relationships between the joint elements, and real assembly conditions of the mechanical joints. After validating through comparing with the experimental results, the modeling method is adopted to simulate the tensile response of the bolted joints with shims. Furthermore, both the influence of liquid shim layer thickness on the mechanical behaviors of composite-to-titanium bolted joints and the influence of the substrate stiffness on the liquid shim effect are studied in detail. Based on the analysis of the results, it can be concluded that the maximum load, initial joint stiffness and design load of the joints decrease with the increase of liquid shim layer’s thickness; and the effect of liquid shim layer relies heavily on the stiffnesses of the substrates and will reduce when the substrates become stiffer.