Laboratory studies of the dynamic behaviour of grass, straw and polystyrene tube during high-speed cutting

Abstract

Experiments were conducted to study the behaviour of grass and straw stems when cut by sharp and blunt blades at speeds between 5 and 35 m/s; comparative experiments were done with polystyrene tube. Measurements were made of the force and energy during cutting using a piezoelectric force transducer mounted behind the blade on a rotating disc. Stem deflection modes during cutting were recorded photographically by using high-speed film in a camera with an open shutter and firing flash guns sequentially by electronic control, with delay periods ranging from 3 to 425 μs. The lowest speed for efficient cutting (the critical speed) for grass and polystyrene tube was 25–30 m/s. The minimum cutting energy per unit dry matter content for grass with a sharp blade was 55 mJ/mm 2 based on stem wall area. At speeds below the critical speed there was a large stem deflection before cutting was complete, which resulted in long stubble lengths and high cutting energy (up to 400–500 mJ/mm 2 in grass). There was no evidence of a critical speed, however, when cutting straw. The mean peak force for grass per unit dry matter content was 18 N/mm of stem diameter, which was 2.6 times greater than when cutting straw. When cutting above the critical speed for grass (inertia cutting) cutting times were as low as 0.3 ms for the sharp blade, and the stem deflection during cutting was equal to about three stem diameters. For straw, however, the deflection before cutting was complete, represented 8–10 stem diameters, with times of about 1.0 ms. For inefficient cutting at low speeds (c. 5 m/s), cutting times ranged up to 15 ms with stem deflections at the blade of up to 32 diameters for a cutting height of 40mm. The difference between stubble length and nominal cutting height for inertia cutting was about 2 mm for sharp blades but ranged up to 50 mm for inefficient cuts at low speeds. Grass stems showed the widest range of deflection mode immediately after cutting. This varied from stem translation and rotation at low speeds, (c. 5 m/s) to marked bending at intermediate speeds (15–25 m/s) and inertia cutting, with little stem displacement, at higher speeds (>25 m/s). Straw exhibited marked stem translation and rotation with little bending at speeds below 25 m/s but marked bending at higher speeds. Polystyrene tube showed a marked bending over its length at speeds up to 25 m/s, above which cutting was in the inertial mode. The minimum cutting energy for grass stems was three times that required for quasi-static cutting in shear. At low cutting speeds (<12m/s) about 65% of the energy was utilized in overcoming friction. For straw, however, the frictional component was generally relatively low (5–10%) and stem kinetic energy was equal to about 20% of the total energy input. The specific cutting energy was twice as large for the blunt blade as for the sharp blade in grass and straw. The difference between stubble length and nominal cutting height was generally about three times greater when using the blunt blade.