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Highlighting delayed elastic processes at zero stress in a polymer glass

Abstract : The present study highlights the existence of a post-evolution of the fracture and damage in glassy polymers at zero-stress relaxation evidencing that the dynamic is not frozen below the glass transition but that it releases at zero stress in the plastic regime from past stresses. The observation of a further evolution of the glass without applied stress (at zero stress) is unexpected and inaccessible using conventional tensile tests since they are not designed to enable a zero-stress measurement. Non-contact methods such as confocal microscopy and autocorrelation analysis are used to examine polymethylmethacrylate samples plastically deformed (up to 8% elongation rate) and subsequently studied in the unloaded state. The zero-stress evolution is morphologically characterized by a nucleation of new microfractures and an evolution of the existing cracking. The analysis of the image correlation indicates that the strain field continues to evolve revealing an intermittent retraction of the displacement field at time scales up to several days. Different temperatures from −75 °C up to −45 °C below the glass transition have been tested and prove that the retraction process is thermally activated. The retraction process (about 1% − 2% of the deformation rate) implies that plastic deformation does not relax completely from anterior stresses but exhibits a delayed elasticity. These properties have profound consequences impacting the future physical and mechanical properties of the material.
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Submitted on : Monday, June 22, 2015 - 3:28:44 PM
Last modification on : Saturday, May 1, 2021 - 3:52:17 AM

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Souad Mbarek, Patrick Baroni, Laurence Noirez. Highlighting delayed elastic processes at zero stress in a polymer glass. International Polymer Processing, 2015, 64 (10), pp.1303-1308. ⟨10.1002/pi.4950⟩. ⟨hal-01166331⟩

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