Abstract

One technique for improving the fatigue life of a fastener hole in a built-up metallic structure is to use an interference fit bushing. The installation of these bushings produces residual compressive stresses around the hole that improve the fatigue life. It also causes residual strains and therefore distortion of the part. The distortion is both in-plane growth of the part and out-of-plane deflection, or cupping. The resulting distortion can cause difficulty in downstream assembly, such as misalignment of fastener holes or a misfit of mating surfaces. For small parts with only a few bushings, it is possible to predict the distortion by explicitly modeling the installation process by using a finite element approach. However, for a large part with many bushings, it is not practical to build a model with a detailed finite element representation of each bushing installation. In this work, a simplified approach is presented for predicting the global distortion of large shell-like parts that is caused by installing many interference fit bushings. The simplified approach consists of representing the bushings with equivalent springs in a shell-based finite element model of a large part. In this approach, the equivalent springs are not constructed with spring elements that are commonly found in finite element codes. Rather, the equivalent springs are constructed with a number of shell elements in two layers. The properties of the elements that make up the equivalent springs are estimated from experiments on small test coupons. The same springs, with the same calibrated parameters, are then applied to the larger part of interest. Results indicate that this approach can be used to estimate the distortion that can be expected in a large part that is cause by the installation of interference fit bushings.

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