Estimating bond damping and bond Young's modulus for a flexible wheat straw discrete element method model

Abstract

A method to calculate the local damping coefficient and the bond Young's modulus of a flexible fibre for use in the discrete element method (DEM) was proposed and validated. Segments of harvested wheat straw were clamped on one end while the other end was deflected a set distance, released, and allowed to vibrate freely. This cantilever beam motion was captured by a high-speed camera (960 fps). The red-band of the images were isolated and used to calculate the x-section height (mm) along the stem in time. This data was then fit to a non-linear function, which conforms to beam theory. The global bond damping coefficient, as a function of x-section height, was then calculated and found to be in the range of −0.5 to −0.2. The cantilever beam experiment was then repeated in the DEM software LIGGGHTS, where the wheat straw was modelled as a single line of spheres connected by stiff-flexible bonds. A design of experiment (DOE) was ran varying bond Young's modulus and local bond damping, to determine the linear relationships with the global bond damping coefficient and the frequency of oscillation of the DEM particle respectively. With the proposed method the DEM local bond damping coefficient and the DEM bond Young's modulus were calibrated with relative errors of 0.9% and 1.8% respectively to laboratory estimated values. Utilising the linear relationships found from the DEM simulations, the bond Young's modulus was found to be in the range of 0.42–4.84 GPa.