Abstract: There is described an apparatus for determining the behaviour of powder of granular material under stress, comprising, at least one flexible boundary arranged to abut one face of a sample of the powder or granular material, means to apply stress to the sample through the flexible boundary, a detector to detect the position of the flexible boundary as the sample is stressed, and a control to adjust the position of the flexible boundary. The detector may comprise an electrical circuit, advantageously a make and break circuit. There is also described an apparatus for determining the behavior of powder or granular material under stress, comprising at least one flexible boundary arranged to abut one face of a sample of the powder or granular material, means to apply stress to the sample through the flexible boundary, and adjusting members attached to the flexible boundary to cause it to strain as the sample strains under the applied stress.