Mixed-Mode Fatigue Disbond on Metal-to-Metal Interfaces

1. Mixed-Mode Fatigue Disbond on Metal-to-Metal Interfaces PhD Candidate: Daniel Bürger Department: ASM Section: Structural Integrity and Composites Supervisor: C. D. Rans Promoter: R. Benedictus Start date: 01-10-2010 Funding: Brazilian Air Force Preliminary results Surface characterization The main crack path is determined by Mode II loading (crack close to the upper surface). Mixed-Mode surfaces presents a mix of Mode I and Mode II characteristics. According to the surface features MM can be divided in three regions: 0%-30%: Mode I dominated 40%-70%: Mixed-Mode 80%-100%: Mode II dominated Formation model of rollers and vertical cracks in Mode II and Mixed-Mode • Motivation • Adhesive bonding provides a promising alternative to riveted connections, providing a more efficient mode of load transfer and eliminating local stress concentrations. However, the widespread application of bonding for safety-critical structures is currently limited by the lack of robust fatigue and degradation prediction methods. • Failure in a bonded interface is classified according to the type of loading, with loading resulting in a peeling mode of deformation (Mode I) and loading resulting in sliding shear deformation (Mode II). Most joint designs experience a combination of both loading modes called Mixed Mode (MM). Empirical disbond models for pure Mode I and pure Mode II are well-known in the literature. However, Mode I and Mode II effects cannot yet be combined satisfactorily in a MM theoretical model. As a result, the most accurate MM models are based on curve fitting experimental data obtained at different MM ratios. This approach results in an excessive experiments number, which is both costly and time consuming. • Next step of the research • Based on the fractured surfaces, develop a Mixed-Mode fatigue disbond model to predict disbond at different mode ratios. Mode I surface Mode II surface Mixed-Mode 70% surface Loading modes Disbond growth data Aerospace Engineering Objective The research program aims at developing a mechanistic mixed-mode loading fatigue disbond growth model for metal-to-metal bonded interfaces. Experimental procedure The tests were based on standards DCB and MMB. Fatigue experiments (R=0.1) were conducted on aluminium bonded samples at different mode ratios (GII/GTotal), from 0% to 100% and the fractured surfaces were analysed. Preliminary conclusions Most of the available Mixed-Mode model do not predict accurately mixed-mode disbond. Even models based on fitting experimental parameters can predict a results with large errors from the experimental results. Fractographic analysis of fatigue disbonded metal-to-metal interfaces revealed similar features to fractographic analysis of fatigue delamination on composites. The crack path is dominated by the presence of mode II. The carrier supports a part of Mode II loading, reducing the Mode II loading and reducing the mode ratio on the adhesive. The fractured surface showed three distinct regions as the mode ratio changes: Mode I dominated, Mode II dominated, and Mixed-Mode (both Mode I and Mode II features coexists without visible interaction). Specimen dimensions and tests • Publications • D. Bürger, C.D. Rans, R. Benedictus, (2013) “Characterization of Mixed-Mode Fatigue Failure on Metallic Bonded Joints”, 17th ICAF Symposium. • D. Bürger, C.D. Rans, R. Benedictus, (2013) “Mixed-Mode Fatigue Disbond of Metal-to-Metal interfaces: Effect of the Adhesive Support”, 2nd Int. Conf. on Structural Adhesive Bonding.