Failure behaviour of metallic sleeves in riveted CFRP joints

The excellent specific and mechanical properties of fibre reinforced plastics (FRP), especially in respect of substituting of primary metal structures, caused an increasing demand in the aircraft construction industry. The material requirements demanded from conventional rivet connections, for example the lockbolt system, increased contemporaneously. Unfortunately, rivet holes of the mating parts interfere with the natural fibre patterns and thereby causing certain imperfections in the material. In addition, the fitting of the lockbolt could lead to undesirable delamination and fibre disruptions around the corner of the rivet hole. This has a further adverse impact on the joint strength. The insertion of a steel sleeve leads to a radial expansion of the sleeve during the fitting of the lockbolt. Thus, critical shear stress can be avoided during the bolt fitting. The increased complexity of the whole joint structure requires compliance with strict process limits, otherwise damages to the sleeve and, consequentially, to the entire compound are likely. The aim of this study is a detailed characterization of the sleeve material and of the entire process including the process limits. The finite element method is used as an important tool for assisting in the select correct process parameters. Based on the results of tensile tests, a certain material model on the basis of one failure criterion was defined for the steel sleeve and then implemented in the simulation of the bolt fitting. Within the setting of an unfavourable combination of process parameter, the numerical results evidence a realistic failure of the sleeve when compared with results from experimental tests.