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New method of measuring elastic properties of coatings: part I - theoretical approach.

Roberts, S J; McCartney, L N (2003) New method of measuring elastic properties of coatings: part I - theoretical approach. NPL Report. MATC(A)146

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In this report, it is described how the effective macroscopic strains may be predicted for an undamaged multi-layered 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 layered system and takes account of mismatched elastic and thermal expansion coefficients. The model also takes into account anti-clastic bend deformation. The resulting equations are used to predict the effective strains and in-plane stresses for a two-layer coated system.
The inverse form of the model provides a method of estimating coating properties from the global deformation behaviour of layered systems that are subject to uniaxial tensile loading or pure bending. The stress-strain behaviours for TiN coated on steel and aluminium substrates in uniaxial tension and pure bending are investigated in two of the cases presented here. There is an emphasis on determining the effects of coating and substrate thicknesses, and their moduli, on the macroscopic strain response, which can be measured during experimental testing. It is shown that the ratio of coating to substrate thickness should be as large as is practically possible to maximise the strain responses that have to be measured during testing. Lower modulus substrates are also shown to produce larger strain responses.
The derived model is also used to demonstrate the limitations of the modified Stoney equation for film stresses, which applies only to thin coatings under thermal loading.

Item Type: Report/Guide (NPL Report)
NPL Report No.: MATC(A)146
Subjects: Advanced Materials
Advanced Materials > Surface Engineering
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/2816

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