Fracture mechanics of carbon fibre reinforced plastics to Ti-alloy adhesive joints

Abstract

Adhesive bonding has emerged as an appealing technique to join carbon fibre-reinforced plastics (CFRP) to other structural parts. The advantages that adhesive bonding offers include an even stress distribution, weight saving and superior fatigue resistance when compared to more traditional methods of joining. However, despite these advantages, the uncertainties regarding their durability have confined them largely to use in secondary structures. In the present work, a fracture mechanics methodology has been followed using both experimental and FE methods to predict the service-life of a CFRP-titanium alloy adhesive joint intended for use in a turbofan application. The methodology utilises the concept of the cohesive zone model to evaluate the performance of a simplified but representative structure, i.e. a Ti-to-CFRP tapered double-lap joint. The adhesive bondline was modelled by a layer of newly-developed cohesive elements, the kinematics and topology of which have been optimised to improve the mixed-mode behaviour and reduce the mesh-dependency. Their damage evolution has been enhanced to incorporate high-cycle fatigue degradation. Additionally, a simplified version of this formulation, specifically designed to predict only the fatigue threshold, has also been developed. To determine the various input parameters required for the models, a series of fracture mechanics specimens manufactured with a commercial film adhesive were tested quasi-statically in various modes and in mode I fatigue. Various data reduction schemes were evaluated and a version of corrected beam theory employing an effective crack length approach was found to be optimum for all tests. The fracture energies determined in the various modes were partitioned according to the theories proposed by Williams (Global) and by Davidson’s crack tip element singular field (CTE/SF) and non singular field (CTE-NSF) theories. The CTE-NSF partitioning strategy was found to be most suitable for the system under investigation. Fatigue tests were performed under wet and dry conditions, to investigate the effect of moisture on the joint performance. The fatigue results were fitted to a modified version of the Paris law and the required fatigue parameters were determined. The response of the various test specimens was simulated using the numerical scheme and good agreement with the experimental results was obtained. Significantly, the results obtained with a quadratic version of the cohesive element have been found to be independent of the element size, at least with respect to the global response. Finally, both the quasi-static and fatigue responses of the double lap joints were simulated using the cohesive element formulation and conservative predictions of the service life were obtained, in accordance with expectation, as only mode I fatigue data (lower bound Gc ) was inputted into the model.