Space-resolved diffusing wave spectroscopy measurements of the macroscopic deformation and the microscopic dynamics in tensile strain tests

TL;DR

This study integrates space-resolved diffusing wave spectroscopy with mechanical testing to map strain and microscopic dynamics in polymers, detecting early plastic activity with nanometer sensitivity.

Contribution

It introduces a novel combined optical and mechanical measurement technique using DWS for spatially resolved deformation analysis in turbid materials.

Findings

High agreement between optical and stereo-correlation strain maps

Detection of plastic rearrangements at early deformation stages

Sensitive measurement of displacements as small as 1 nm

Abstract

We couple a laser-based, space-resolved dynamic light scattering apparatus to a universal testing machine for mechanical extensional tests. We perform simultaneous optical and mechanical measurements on polyether ether ketone, a semi-crystalline polymer widely used in the industry. Due to the high turbidity of the sample, light is multiply scattered by the sample and the diffusing wave spectroscopy (DWS) formalism is used to interpret the data. Space-resolved DWS yields spatial maps of the sample strain and of the microscopic dynamics. An excellent agreement is found between the strain maps thus obtained and those measured by a conventional stereo-correlation bench. The microscopic dynamics reveals both affine motion and plastic rearrangements. Thanks to the extreme sensitivity of DWS to displacements as small as 1 nm, plastic activity and its spatial localization can be detected at an early stage of the sample deformation, making the technique presented here a valuable complement to existing material characterization methods.