## 1 Introduction  
Gansu province is one of the major potato production areas in China. The hills and mountains scattered in Gansu, however, enable only the medium and small potato harvesters to be applicable there. Potato digging shovels are the key power-consumption components in potato harvesting, whose consumption accounts for more than 27% [1]. Therefore, to improve the performance of planar potato digging shovels and, thus, reduce the draught force are of great economic and regional importance in Gansu province. The existing methods to reduce the drought force include the fluid injection [2, 3], vibration [4], electro-osmosis [3], magnetization [5], macroscopic or microcosmic surface modification [3; 6], use of different materials or coatings [7] and so on. However, these methods require higher strength and stability from the components or machinery, which is contrary to the demand for simplified and miniaturized agricultural machinery in Gansu province. [PERSON] et al. [8] and [PERSON] et al. [9] examined the characteristics of soil-burrowing animals' soil engaging organs in the 1990s and they found that the soil engaging surfaces of these animals were characterized by a spatial surface with convex domes, concave hollows, ridges, furrows or other non-smooth construction units, which was beneficial to the decrease in the compaction to soil as well as the adhesion between the soil and interface and finally the reduction in the drought force. Based on the above findings, scholars applied biomimetics method to improve the performance of soil-engaging components and obtained favorable results. For instance, the biomimetic non-smooth moldboard plugs imitating the surface morphology of dung beetle reduced the drought force by 15-18% and was used in Northeastern China [10]. By equipping some electrodes with specific shape and arrangement on the surface of soil-engaging components, the drought force decreased by 15-40%, and the components were granted lower electro-osmosis voltage and higher security [11; 12]. [PERSON] et al. [13] designed some different biomimetic subsoilers based on the macroscopic surface characters of chicken clawed toes, mole crickets clawed toes, home mouse clawed toes, and so on, and they found that the biomimetic subsoilers reduced the drought force by more than 7.1%. In view of the potential advantages of biomimetics, researchers have also developed various biomimetic potato digging shovels with excellent draught force reduction or anti-adhesion performance, such as a biomimetic convex dome shaped shovel [14], mole cricket inspired shovel [15], earthworm inspired shovel [16], wild boar inspired shovel [17], etc. In fact, the macroscopic shape, microscopic morphology and cutting edge of soil engaging components have a great effect on drought force [18]. The macroscopic surface modification is a relatively simple, reliable and low-cost method for the reduction of drought force [19]. Mole rats in Gansu province who forage and nest in the potato field have two strong forelimbs and two outward front paws, which grant them brilliant soil digging abilities. The structure of the mole rat's digit is similar to that of the potato digging shovel, and the mole rat's habitat is the same as the working environment of potato digging shovels. Therefore, the mole rat's digit is a great biomimetic prototype to improve the performance of potato digging shovels by biomimetic macroscopic surface modification method. The finite element (FE) method excels in the analysis of dynamic problems regarding material failure and large-scale deformation, which has been proven an effective approach for modeling soil-tool interaction [20; 21]. ABAQUS software (Dassault Systemes Simulia Corp., Providence, RI, USA) has provided several different yield criteria to describe the mechanical behavior of soil, such as the Mohr-Coulomb criterion [22], Drucker-Prager criterion [23], Cam-Clay criterion [24] and their extended forms.