2.2.1. Coefficient of Restitution

In this study, we mainly measured the coefficient of restitution between phloem and Q235A steel, xylem and Q235A steel, and phloem and xylem. The coefficient of restitution is a parameter that measures the ability of an object to return to its original shape after a collision. It is defined as the ratio of the normal relative separation velocity after a collision to the relative approach velocity before collision (i.e., the ratio of the highest rebound height (*h*') and the initial drop height (*h*) during the collision between the test object and the material) [33]. When an object falls freely and collides with the test object, the object bounces freely after the collision, and only gravity works during the falling and rising process. The formula for calculating the coefficient of restitution is as follows (2):

$$c = \left| \frac{v\_2' - v\_1'}{v\_1 - v\_2} \right| = \left| \frac{-\sqrt{2gh'}}{\sqrt{2gh}} \right| = \sqrt{\frac{h'}{h}} \tag{2}$$

where *v*<sup>1</sup> and *v*<sup>2</sup> are the velocities of the test object and the material before collision, m/s; *v*1 and *v*<sup>2</sup> are the velocities of the test object and the material after collision, m/s; *h* and *h* are the initial drop height and the highest rebound height, mm; and *g* is the acceleration due to the fact of gravity, m/s2.

The maximum rebound height of the coefficient of restitution can be measured using a high-speed camera system. The experimental process was recorded by connecting the high-speed camera, HiSpec5 (Fastec Imaging Inc., San Diego, CA, USA) to a computer. The camera was set to a frequency of 500 Hz, with a resolution of 1280 × 1024 pixels, and a sampling rate of 2 ms. Considering that the ramie stalk has a lower density, the air resistance significantly impacts the experimental results when the falling speed is high. Therefore, the object to be measured was dropped freely from an initial height (h) of 205 mm, collided with the contact material, and the highest rebound height (h ) was captured and recorded. The experimental process is shown in Figure 2.

**Figure 2.** Measurement of the restitution coefficient: (**a**) test equipment; (**b**) keyframe of the rebound's highest point.

The rebound height (h ) was recorded by capturing the keyframe when the tested object reached the highest position after the collision, from which the coefficient of restitution was calculated. Each group of tests was repeated ten times to analyze the positive collision process between the phloem and Q235A steel, xylem and Q235A steel, and phloem and xylem, the coefficient of restitution was then calculated according to Equation (2), and the average value was taken.
