Extrinsic and Intrinsic Mechanical Responses of Confined Indented Thin Films

24 Pages Posted: 10 Aug 2020

See all articles by Owen Brazil

Owen Brazil

School of Physics, CRANN & AMBER, Trinity College Dublin

Johann P. de Silva

Trinity College (Dublin) - Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN); Trinity College (Dublin) - School of Physics

John B. Pethica

School of Physics, CRANN & AMBER, Trinity College Dublin

Graham L. W. Cross

Trinity College (Dublin) - Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN); Trinity College (Dublin) - School of Physics

Abstract

Confined geometries offer useful and experimentally amenable mechanical testing arrangements in which to study the molecular and micro-structural processes which govern plastic yield in stress environments dominated by hydrostatic pressure over shear. However, the changes to macroscopic stress - strain behaviour that result from switching from an unconfined mode such as uniaxial compression to a confined one are often overlooked and display a surprising level of complexity, even for simple elastic - plastic constitutive models. Here we report a confinement induced strain hardening effect in polystyrene thin films achieved through repeated plastic loading with a cylindrical flat punch whose diameter is many times the initial film thickness. This high aspect ratio combines with constraint provided by film material surrounding the contact to generate a state of confined uniaxial strain in the indented region, rendering the deformation one dimensional. By repeated loading into the plastic domain, we achieve a 66% increase in the confined yield stress, from 0.3 GPa to 0.5 GPa. Through finite element simulation and analytic modelling of the principal stresses and strains, we show that this effect arises not from intrinsic changes to the structure of the material, but rather residual stresses imparted during plastic loading. We contrast this effect with intrinsic changes to glassy thin films such as physical ageing and thermal cross-linking.

Keywords: Indentation, thin films, stress-strain, confinement, polymers

Suggested Citation

Brazil, Owen and Silva, Johann P. de and Pethica, John B. and Cross, Graham L. W., Extrinsic and Intrinsic Mechanical Responses of Confined Indented Thin Films. Available at SSRN: https://ssrn.com/abstract=3661923 or http://dx.doi.org/10.2139/ssrn.3661923

Owen Brazil

School of Physics, CRANN & AMBER, Trinity College Dublin

Johann P. de Silva

Trinity College (Dublin) - Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN)

2-3 College Green
Dublin, Leinster
Ireland

Trinity College (Dublin) - School of Physics

College Green
Dublin
Ireland

John B. Pethica

School of Physics, CRANN & AMBER, Trinity College Dublin

Graham L. W. Cross (Contact Author)

Trinity College (Dublin) - Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) ( email )

2-3 College Green
Dublin, Leinster
Ireland

Trinity College (Dublin) - School of Physics ( email )

College Green
Dublin
Ireland

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