Opening-mode micro-cracking in brittle coatings on ductile wires/rods

A simplified two-dimensional model is presented to simulate periodically distributed micro-cracking in a thin coating fully bonded to an elastoplastic aluminum wire/rod. The alumina coating which is generated by anodic oxidation is treated as an elastic material, while the ductile aluminum wire substrate is characterized by a bi-linear elastoplastic hardening model. An elastoplastic shear lag model is applied to transfer the shearing stress from the substrate layer to the thin coating. When the coated structure is subjected to different applied load, the system will undergo different stress levels and exhibit different cracking stages. Accordingly, explicit solutions corresponding to different loading stages are presented based on a generalized axisymmetric formulation. Finite element simulation is employed to verify the present elastic solution when the applied load is relatively small and the whole system is in the elastic state. Experimental characterization is conducted to validate the present elastoplastic solution when the substrate or interlayer undergoes large deformation. A versatile high-fidelity optical microscope is utilized to check the micro structure of the coating and continuously monitor the fracture development in the coating layer, through which valuable detailed micro-cracking information, such as critical applied load corresponding to crack initiation, crack pattern and crack spacing, is obtained. It shows that the presented fracture model is able to accurately capture the stress and strain distribution in the coated structure and predict the fracture initiation, infilling, and saturation in the thin coating layer.