Schomaker, R., Pedwell, R., and Knickrehm, B., "Methodology for Classification of Shim Materials," SAE Int. J. Aerosp. 7(2):255-262, 2014, doi:10.4271/2014-01-2253.
As a result of the increasing use of fibre reinforced plastic (FRP) components in a modern commercial aircraft, manufacturers are facing new challenges - especially with regards to the realisation of significant build rates. One challenge is the larger variation of the thickness of FRP components compared with metal parts that can normally be manufactured within a very narrow thickness tolerance bandwidth. The larger thickness variation of composite structures has an impact on the shape of the component and especially on the surfaces intended to be joined together with other components. As a result, gaps between the components to be assembled could be encountered. However, from a structural point of view, gaps can only be accepted to a certain extent in order to maintain the structural integrity of the joint. Today's state of the art technologies to close gaps between FRP structures comprise shimming methods using liquid and solid shims. Another option is the use of peelable shims that offer significant economic benefits compared with liquid and solid shims. However, the influence of different peelable shim materials on the bolted joint performance is still a subject of investigation - especially with regards to chemical resistance.Therefore a larger investigation campaign has been initiated in order to establish a comprehensive description methodology for shims that would enable an optimised application-related selection of shim types and materials in terms of mechanical, chemical manufacturing performance and costs.