Measurement of the Rolling Friction Coefficient

The coefficient of rolling friction pertains to the deformation-induced resistance when an object rolls or tends to roll without slipping on another surface [36]. The experimental setup and method for measuring the rolling friction coefficients in this study were the same as those for measuring the static friction coefficients. According to the rolling friction coefficient measurement method provided in the literature [37,38], the test object was placed radially on the test plate. The handle of the inclinometer was shaken to raise the inclination angle of the test plate, and the rolling angle was recorded when the test object showed a rolling trend. The coefficient of rolling friction coefficient was then calculated. When measuring the rolling friction coefficient between phloem–Q235A steel, a high-roundness ramie stalk was selected, and rolling was achieved by external phloem contact. When measuring the rolling trend between the phloem and xylem, the phloem was peeled off in advance and glued to the steel plate. The xylem was placed radially on the phloem plate, the inclination angle was increased, and the rolling friction coefficient was calculated.

#### *2.3. Physical Test of the Stacking Angle*

The stacking angle is a microparameter that characterizes the granular materials' flow and friction characteristics. Its numerical value is related to the material type, surface shape, and moisture content, and it is affected by the coefficient of restitution and the coefficient of friction [39]. The stacking angle test is usually used to calibrate discrete element parameters of granular materials. Therefore, we conducted a physical test of the stacking angle. The measurement results can calibrate the contact parameters between phloem and Q235A steel, phloem and phloem, xylem and Q235A steel, and xylem and xylem.

Through preliminary comparative tests, the cylindrical lifting method was found to be suitable for measuring the phloem's stacking angle, and the extraction of the partition method was suitable for measuring the stacking angle for the xylem.

Due to the ramie phloem fibers' lengthy and highly flexible nature, forming a stacking angle is difficult. We referred to the material processing method for calibrating the discrete element parameters of sugarcane leaves [40] and tobacco rods [16], and the phloem was peeled from the ramie stalks with a utility knife and trimmed into 5.6 × 5.6 mm specimens without altering the surface shape of the material. The stacking angle method for the ramie phloem was the cylinder lifting method, where a 20 g sample was placed into a steel cylinder with a diameter of 45 mm and a height of 57 mm. The cylinder was then lifted at a uniform speed of 4 mm/s using a TFW-508 mechanical universal testing machine, and the phloem sample fell onto a 250 mm × 250 mm × 2 mm (length × width × thickness) steel plate from the bottom of the cylinder. After all phloem specimens had come to a complete stop, a stable phloem material pile was formed. The Canon EOS 70D DSLR camera was used to capture the main view of the phloem material pile from 50 cm in front of the pile, as shown in Figure 4a.

**Figure 4.** Physical test of the stacking angle: (**a**) stacking angle of the phloem; (**b**) stacking angle of the xylem.

For the xylem, a ramie stalk xylem radial stacking angle measurement device with removable partitions was created based on reference [17]. The device was made of Q235A steel and had dimensions of 500 mm × 200 mm × 300 mm for the length, width, and height, respectively. To measure the xylem, the ramie stalk sheath was removed, and the phloem was peeled entirely clean. All xylem lengths were controlled at approximately 140 mm. A certain number of xylem stalks were placed into one side of the radial stacking angle

measurement device. The entire ramie xylem stalk was moved to the other side after the partition was vertically lifted, forming a radial stacking angle upon collision with the wall. The main view of the radial stacking angle formed after the movement of the xylem stalks was captured using the Canon EOS 70D DSLR camera, as shown in Figure 4b.
