Volumetric strain measurement of polymeric materials subjected to uniaxial tension

Abstract

A novel method for measuring and calculating volumetric strain in circular cylindrical uniaxial tension samples made from polymeric materials is proposed. It is shown that special considerations must be taken when calculating volumetric strain when a sample is in a postnecking state. Solely based on surface data, the key feature of the proposed correction is that it allows for an inhomogeneous distribution of longitudinal strain through the diameter of the sample, where a more traditional approach would be to assume a homogeneous distribution. These two approaches are evaluated by applying them to data from a close-to-incompressible steel sample. Whereas the proposed method indicates only a small positive increase in volume, the assumption of a homogeneous distribution results in substantial negative volumetric strains. Applying the two methods to tension samples made from HDPE and PVC, where plastic dilatation is nonlinear, again shows an initial negative volumetric strain for HDPE with the assumption of a homogeneous longitudinal strain. The proposed method predicts close-to-zero early-stage volumetric strain for the same test. The differences are more subtle for samples of PVC. Micrographs obtained with scanning electron microscope show that the dilatation of PVC is related to voiding of the material around filler particles, while the underlying mechanism for HDPE is less clear. The results indicate that earlier reports of negative volumetric strain in polymers subjected to uniaxial tension might be artefacts of the implicit assumption made when calculating the volumetric strain.

Influence of Silica Specific Surface Area on the Viscoelastic and Fatigue Behaviors of Silica-Filled SBR Composites

“…The true strain was obtained at the surface of the median section of the sample along in the radial and longitudinal directions. The logarithmic volume strain, which is defined as the logarithmic ratio between the current volume and the original volume of the sample at the median section, was computed using the equation [ 26 ]: where Δ V and V 0 are the difference in the volume at a given time and initial volume, respectively, and λ L and λ R are the stretch ratio along the longitudinal and radial directions, respectively.…”

“…The true strain was obtained at the surface of the median section of the sample along in the radial and longitudinal directions. The logarithmic volume strain, which is defined as the logarithmic ratio between the current volume and the original volume of the sample at the median section, was computed using the equation [26]:…”

“…In the case of filled elastomers, the filler–filler network is responsible for the material’s non-linear viscoelastic behavior [ 8 ]. In a number of studies, the macroscopic strain field of the samples was characterized using the digital image correlation technique (DIC) [ 24 , 25 ], which helped access the volume change upon deformation [ 26 , 27 ]. In these works, the volume change was correlated to the crack growth and the fatigue properties.…”

The Tensile Behaviour of Highly Filled High-Density Polyethylene Quaternary Composites: Weld-Line Effects, DIC Curiosities and Shifted Deformation Mechanisms

“…Notable in the prior study is the use of three-dimensional DIC, allowing the volumetric deformation of the system to be estimated. This suggested the presence of substantial particle debonding—resulting in the increase of volumetric strain with an increase in filler loading—although such suggestions must be taken with care, given the dangers of assuming homogeneous distributions of strain through a tensile sample [ 33 ]. It has been established that, at least in the case of uncompatibilised materials, a substantial portion of the late-stage deformation of a composite specimen may be ascribed to debonding.…”

A technique for in situ X-ray computed tomography of deformation-induced cavitation in thermoplastics

“…When voids are present in a tensile specimen at a certain deformation stage, numerous techniques can be applied to quantify the void volume fraction, the morphology and spatial distribution of the voids. A practical approach is to determine the average void volume fraction within a cross-section of the specimen based on its external geometry [1,28]. The drawback of this method is that it relies on assumptions on the kinematics of the specimen, which may be violated when pronounced cavitation is present [16].…”