Volume strain
So-called “7 markers” method, patented option of VideoTraction system, is a device for characterizing local mechanical behavior of a structure that may present non-homogeneous finite deformations. For a ductile material (polymer, metallic or other), it is important to precisely characterize constitutive law in terms of deformations and strains at the level of a fairly small representative volume compared to the characteristic length of the deformation inhomogeneities.In the current state of the art, and particularly in case of a uniaxial tensile test, we often encounter difficulties linked to the too large size of the reference volume chosen for the deformation measurements. The mechanical constitutive laws obtained then only measure the average macroscopic response of the sample and do not adequately express the local properties of material, particularly when diffuse necking appears. In cases where detailed mapping of deformations is carried out (notably in correlation analysis of complex speckles on the surface of a test piece), the long processing time of images generally does not allow real-time control of a chosen component of the deformation field during a test.
According to the invention, device makes it possible to remedy these drawbacks. It is indeed based on optical measurement analysis of the relative displacements of a set of markers applied before the start of test to sample surface in the zone where deformation gradients are expected. Number of markers, their relative arrangement and their distances are adapted to sample initial geometry as well as to deformation inhomogeneities severity that may appear.
