Bulk compression characteristics of straw and hay

Compression of biological materials facilitates their transport and storage. In agriculture, straw and hay are commonly compressed with extrusion, but this process is highly influenced by changing crop conditions. A mathematical description of the compression profile of fibrous materials would be useful to compare the compression characteristics of dierent materials and to predict the energy required for compressing it to a certain density. In this study, dierent biomass modelling techniques have been reviewed to select the most useful crop compression model. It is shown that the selected crop model (Faborode model) is appropriate in describing the crop compression up to a density of 145 kg m^-3, dry matter (with R^2 > 0.8). The selected stress-deformation relation (Faborode model) involves two crop parameters which are determined for wheat straw and hay at different moisture contents and particle orientations (random and parallel stacking). These parameters allow for separating the time-dependent and the elastic compression behaviour. The relaxation properties of the samples have been estimated by fitting the stress decay over time. It is shown that a Maxwell model cannot properly describe the relaxation (R2 > 0.7) while the Peleg model resulted in reasonable fits with high R2 (> 0.8). Although the model of Peleg is accurate only after a relaxation time of one second, the parameters can be used in describing the relaxation behaviour.