Browsing by Author "Marquis, Gary, Prof., Aalto University, Department of Applied Mechanics, Finland"

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Shear fatigue of adhesively bonded frictional interfaces in high-strength steel
(2014) Hurme, Susanna
School of Engineering | Doctoral dissertation (monograph)
This thesis presents new experimental data, physical observations and a new analytical model relevant for the design of bonded/bolted hybrid joints in high-strength steel plates subjected to cyclic shear loading. Hybrid joints can have improved fatigue endurance as compared to welded connections of similar geometries but, thus far, adequate research data has not been available. A procedure, linking the interface characterisation and the stress analysis of the full-scale joint, is presented in this thesis. The procedure can be readily adopted for product development involving the bonded/bolted hybrid joint. Further understanding of the physical processes of failure in the bonded and clamped interface is obtained by a scanning electron microscopy study of the adhesive layer at different stages of fatigue life. Failure of the bonded/bolted joint occurs at the bonded interface where the total strength is a combination of cohesive and frictional forces. In this thesis, a new experimental method is introduced for characterising the idealised bonded and clamped interface under shear fatigue loading. The experimental method is based on the modified napkin ring specimen, which involves a bonded annular contact interface subjected to a well-defined and uniform stress state of constant normal stress and alternating shear stress. Experiments are used to identify the fatigue failure processes in the bonded and clamped interface under different clamping stresses, as well as the fatigue strength using a statistical method. Fatigue strength of the interface is then utilised to estimate the fatigue strength of a full-scale bonded/bolted double lap joint. The interface stress state in the full-scale joint is estimated using the finite element method using the cohesive zone model defined as contact definition at the interface. Parameters for the cohesive law are obtained from quasi-static fracture experiments on the modified napkin ring specimen. The damage state in the adhesive is shown to consist of several microstructurally small cracks of different lengths and orientations. An exponential distribution which evolves with the number of fatigue cycles is suggested for describing the true damage state in the material. However, the evolution of damage, i.e., the growth, interaction and coalescence of the short cracks is a complicated phenomenon with insufficient theoretical understanding in the field of fracture mechanics. Therefore, a phenomenological fatigue damage evolution model is developed where damage is measured directly from the change in the compliance of the modified napkin ring specimen. The cohesive zone law is re-formulated to include fatigue damage. The model is necessary for a more accurate fatigue assessment of the hybrid bonded/bolted joint.