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Predicting steady state interface debonding in a two-layer coated system under biaxial loading and bending.

Roberts, S J; McCartney, L N; Wright, L (2003) Predicting steady state interface debonding in a two-layer coated system under biaxial loading and bending. NPL Report. MATC(A)143

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In this report, it is described how the effective macroscopic strains may be predicted for an undamaged two-layer system, consisting of isotropic materials and subjected to combined in-plane biaxial loading, out-of-plane bending and thermal residual stresses. The theory makes use of the stress and deformation fields within the coated system and takes account of thermal stresses arising from mismatched elastic and thermal expansion coefficients. The model also takes into account anti-clastic bend deformation.
It is then shown how energy balance principles can be used to predict the steady state interfacial debonding associated with through-thickness cracks in a coated system, taking full account of the effects of biaxial loading (in and out-of-plane) and of thermal residual stresses.
For the case of a titanium nitride coating applied to a steel substrate, energy release rate calculations have been made and compare well with finite element solutions. Preliminary predictions have been made of the steady state debonding stresses that will lead to spallation for various coating thicknesses, interfacial fracture energies and stress-free temperatures. Increasing the coating thickness reduces significantly the debonding stresses.
For a chromium nitride coating applied to a stainless steel substrate, there is a much larger thermal expansion mis-match between coating and substrate that leads to very high compressive stresses in the coating. This results in much higher debonding stresses and an insensitivity to the interfacial fracture energy. In particular a threshold debonding stress is observed below which it is impossible to form a steady state debonding crack and hence spallation.

Item Type: Report/Guide (NPL Report)
NPL Report No.: MATC(A)143
Subjects: Advanced Materials
Advanced Materials > Materials Modelling
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/2818

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