title: Analysis of adhesive joints with mechanically interlocking microstructured adherends
Creators: name: Hamilton, Alexander Andrew William
Publisher: name: University of Glasgow
Year: 2023

description: Limited understanding of surface behaviour relative to bulk properties makes interfaces a point of notable uncertainty and weakness within engineering structures. Although previous research has touched on the notion of improving the mechanical performance of adhesive interfaces via surface patterning, there is significant scope for further exploration. Moderate success has been achieved using laser-based techniques to produce structuring, but this approach lacks control over surface geometry and pattern morphology. The key feature of this work stems from the fact that accurate geometries at the sub-millimetre scale have not really been explored for adhesive bonding applications. Using the James Watt Nanofabrication Centre (JWNC) at the University of Glasgow, novel adhesive joints having interlocking micron sized features have been developed and analysed. Various micro-fabrication strategies were studied with a micro-imprinting/injection moulding protocol identified as the optimal strategy. A new route to the fabrication of micro-structured surfaces using injection moulding has been established by developing a flexible mould insert produced by adopting a nanoimprint process. The approach is rapid and more versatile. For example, Bosch process produced masters having sidewall scalloping can even be used as the flexibility of the mould insert permits ejection of the parts even with scalloping. Utilising this fabrication approach, polycarbonate single lap joints were tested in tension using a custom test rig. Results indicate that the presence of the micro-structured features enables load carrying via localised feature bending yielding substantially greater mechanical strength (95.9%) and work-to-failure (162%) compared to the unstructured (planar roughened) adhesive joint baseline, although it should be noted that this testing setup largely mitigated Mode I peel stresses during testing. Finite element (FE) modelling was used to demonstrate that most of the load carrying capacity originates from bending of the interlocked features (rather than from the adhesive). This result even opens up the possibility of adhesive-less joints (as long as the joint can be held together). A statistically guided optimisation protocol was then performed (using the FE model) to ascertain the optimal micro-feature geometries. The final part of the work centred on determining the viability of 3D-printing micro-structured joints as a more economical and versatile manufacturing approach. Investigations into maximum printing resolution were conducted as well as mechanical tests to elucidate the implications of microstructuring on printed joints. The main focus within this research was the ability to tailor bond-line compliance as a means to alter the stress distribution within the single lap joint, with results demonstrating that a more optimum combination of joint mechanical properties is possible by tailoring to place more compliant features near the edges of the joints. A final proof-of-concept study was performed using tensile butt joints as the joint configuration. Here, micro-structured interfaces again enabled greater joint strength through providing greater active bonding area as well as providing more load carrying resistance via the stronger shear mode (via the feature sidewalls).