**4. Discussion**

The cutting mechanical properties of the stalks of Chinese little greens were investigated by numerical simulation technology, and then the optimal cutting combination parameters were obtained. Finally, the simulation results were verified to be sensitive and reliable by physical experiments. The results of this study showed that the sliding–cutting angle, oblique angle, and the average cutting speed all had a significant effect on the maximum cutting stress, and the optimal cutting combination parameters were 29◦, 38◦, and 500 m/s, respectively. Results indicated that dynamic simulation techniques are simple and effective compared to traditional physical testing methods.

We found that the oblique edge angle had the most significant effect on the maximum cutting stress, reaching the optimum value at 38◦. When the oblique angle was 35~38◦, the cutting stress was proportional to the oblique angle. This is because the squeezing and rubbing effect of the stalk fibers on the cutter become stronger when the blades are gradually cut into the stalk, and the resistance of the cutting blade to destroy the stalk fibers increases, so stress concentration is prone to occur. Furthermore, the cutting stress was inversely proportional to sliding angle. This result is comparable to the results of the study by Cui et al. [6] and Zhang et al. [7], which proved that the stalk of Chinese little greens has similar cutting mechanical properties to that of lettuce and rice. However, a sliding angle that is too large will cause problems such as unstable clamping and cutting failure. The cutting stress is smaller for high-speed cutting progress, and it was in agreement with millet stalk [40]. In addition, the cutting power consumption was proportional to the cutting speed in some studies [15,18]. This result may be due to the inconsistent physical properties of apple tree branches and greens. It also may be because the cutting-stress speed curve is a U-shaped curve. When cutting progresses at low speed, the cutting blades generate impact kinetic energy, and the impact force at the moment of contact with the stalk will cause the fiber structure to rupture and reduce the cutting resistance. When a certain threshold is reached, however, the cutting stress increases with the increase in cutting speed.

However, it is worth noting that the explicit dynamic analysis technology still has certain limitations, such as an inability to realize the simulation of continuous and reciprocating cutting progress, which has a certain error with the actual operating conditions. In addition, mechanical vibration during cutting also affects simulation accuracy. Considering the short harvest period for vegetables and the cultivation mode of facility agriculture, the effect of constitutive parameters among plant materials, such as differences in moisture content on cutting resistance, was also not considered in this study. Further studies would help to optimize cutting parameters according to material constitutive parameters.
