Rheological properties of a straw layer

The experiment was performed on a layer of straw whose thickness was equivalent to the load of 4 kg m−2. Every straw sample was subjected to 15 strain cycles. A preliminary analysis of the results revealed that compression and decompression curves resemble the proper rational function, where selected parameters of that function were determined by the direction of strain and the number of strain cycles. Straw was described with the use of a rheological model where it was divided into layers subjected to elastic strain and internal friction proportional to the rate of displacement and deformation. Unknown model parameters were estimated based on an analysis of the equation describing the results of the experiment and the results produced by the model simulating the rheological properties of straw. The elastic modulus was directly proportional to load, and its value decreased in successive strain cycles. Internal friction was determined based on the loss of mechanical energy. In fluid dynamics, internal friction is associated with viscosity, whereas in fibrous materials such as straw, mechanical energy is lost mainly due to sliding friction. The losses were minimized in successive strain cycles.