STRAIN MEASUREMENT IN FABRICS. PART II: A NON-CONTACT METHOD OF DETERMINING FINITE STRAINS - THEORETICAL BASIS

Deformations in the plane of a fabric, because of their nature, are almost invariably expressed as "percentage extensions", i.e., as changes in length per unit original length. Such measures of strain only exhibit tensor transformation behaviour when the strains are infinitesimally small; this is rarely the case with textile fabrics. With the growth of engineering end-uses and related design techniques, plus the drive to achieve the virtual reality catwalk, there is a need for deformations of fabrics to be expressed as true finite strains. A method of calculating such strains from experimental data is described, based on the finite element stress analysis methods widely used in engineering. The method involves measuring the displacements during deformation of a grid of points marked on the fabric.