Particle scale features in shearing of glass ballotini

Glass beads, when immersed in a liquid having the same refractive index, provide an idealised granular material which can be used both for full‐field determination of displacements and hence strains and also, by viewing under polarised light, information concerning the internal stresses within the granular assembly. A simple test box has been used to subject this granular material to rotation of principal strain axes and to encourage the development of a shear mechanism.Results from image analysis are presented to show detail of shear strains and volumetric strains. The images obtained using polarised light have also been subjected to digital manipulation. The nature of the patterns that are revealed is subtle and appears to occur at a scale quite different from the strain patterns.