Cutting Speed

Cutting speed is the core factor that affects the cutting process and is the easiest to control in cutting operations. With the increase of strain rate from 10⁻⁸ s⁻¹ to greater than 10⁷ s⁻¹, the material response can be divided into three sections: static response, quasi-static response, and dynamic response (B Wang et al. Reference Wang, Liu, Cai, Luo, Ma, Song and Xiong2021). These responses of a material under different loading conditions represent different mechanical properties (Wang et al. Reference Wang, Liu, Su, Song and Ai2015). Achieving a strictly static cutting response is generally impractical. Cutting at a speed to obtain a quasi-static response can be termed “quasi-static cutting” (Kamandar et al. Reference Kamandar, Massah and Khanali2018). With the speed to obtain dynamic response, the cutting experiment is used to measure the values and predict the deformation as well as failure process of materials during actual operation (Wang et al. Reference Wang, Liu, Su, Song and Ai2015), which can be termed “impact cutting.”

Quasi-static Cutting

Increasing the strain rate can enhance the strength of a material (Melkote et al. Reference Melkote, Grzesik, Outeiro, Rech, Schulze, Attia, Arrazola, M’Saoubi and Saldana2017; Sirigiri et al. Reference Sirigiri, Gudiga, Gattu, Suneesh and Buddaraju2022). To reduce the influence of cutting speed on the physical properties and obtain the physical properties of the stem at static state, the quasi-static cutting should be selected. As shown in Figure 2, during quasi-static cutting, ultimate cutting stress and specific cutting energy decrease as the cutting speed increases (Rabbani et al. Reference Rabbani, Sohraby, Gholami, Jaliliantabar and Waismorady2015; Song et al. Reference Song, Zhou, Jia, Xu, Zhang, Shi and Hu2022a; Y Wang et al. Reference Wang, Yang, Zhao, Liu, Ma, He, Zhang and Xu2020; Wu et al. Reference Wu, Guan, Tang, Chen, Luo and Xie2009). It is considered that with an increase in cutting speed, the compression process of the stem would be reduced, and the point where the stem would be compressed would gradually exhibit elastic or brittle behavior (Dowgiallo Reference Dowgiallo2005; Song et al. Reference Song, Zhou, Jia, Xu, Zhang, Shi and Hu2022a). The reduction of the compression process and brittle behavior would contribute to reduced cutting resistance and energy consumption.

Figure 2. The effect of cutting speed on ultimate cutting stress and Specific cutting energy in a cutting test using sisal (Agave sisalana Perrine) leaves (Song et al. Reference Song, Zhou, Jia, Xu, Zhang, Shi and Hu2022a).

Impact Cutting

Brittleness is the property of a material breaking without plastic deformation when subjected to a force. The strength of materials increases with an increase of strain rate, including yield strength and ultimate tensile strength, while the toughness of a material decreases as the strain rate increases (Yang and Zhang Reference Yang and Zhang2019). When yield strength is greater than ultimate tensile strength in the cutting process, brittle fracture instead of plastic deformation would occur, leading to material embrittlement (Zhou et al. Reference Zhou, Shimizu, Muroya and Eda2003). Even if brittle materials have high material strength, they will absorb less energy during a brittle fracture compared to fracture without material embrittlement. Mechanical response of materials under different loading conditions is shown in Figure 3. Ignoring temperature parameters, the strain rate is ranked in descending order as ${\dot \varepsilon _4}$ , ${\dot \varepsilon _3}$ , ${\dot \varepsilon _2}$ , ${\dot \varepsilon _1}$ . The dashed EDCBA represents the critical condition for material failure, which can be referred to as the fracture trajectory. The solid lines OA, OB, OC, OE represent the stress–strain curves at different strain rates, and the material fractures at points A, B, C, and E. According to the different characteristics of the material, the entire region is divided into elastic zone, stable plastic zone, and unstable plastic zone. The dashed lines OE and OD represent the critical conditions for material transformation. When strain rate increases, the stress–strain curve would transition from OA to OE, which means that the cutting resistance and material deformation would decrease (Wang et al. Reference Wang, Liu, Su, Song and Ai2015).

Figure 3. Mechanical response of materials under different loading conditions (Wang et al. Reference Wang, Liu, Su, Song and Ai2015).

Embrittlement or brittle behavior not only occurs in metal; plants can also exhibit brittle-like behavior during cutting processes. Xu et al. (Reference Xu, Zhang, Sun, Wang, Liu, Li, Guo and Li2016) found that the head and root of cucumber (Cucumis sativus L.) cane with high water content presents the characteristics of a brittle material, while the middle of cane with low water content presents the characteristics of plastic material. Song et al. (Reference Song, Zhou, Xu, Jia and Hu2022b) make the point that during the cutting process, with the increase of cutting speed, the deformation of a plant changes from plastic to brittle. This change would reduce the resistance of crop materials to the blade and the cutting energy consumption.

Stress and strain states propagate in the form of waves, which are stress waves. During the loading process, the propagation and reflection of stress waves can lead to material embrittlement (Yang and Zhang Reference Yang and Zhang2019). According to the propagation state of stress waves in materials, stress waves have different propagation modes, such as elastic waves, plastic waves, and viscoelastic waves. The propagation velocities of elastic waves and plastic waves are shown in Equations 1 and 2, respectively (Yang and Zhang Reference Yang and Zhang2019). Zhou et al. (Reference Zhou, Shimizu, Muroya and Eda2003) indicated that when the cutting speed is greater than the static plastic wave propagation speed v _p, the plastic flow of the material decreases, and the material would be embrittled.

([1]) $${v_e} = {\left({{E}\over{{{\rho _0}}}}\right)^{{1}\over{2}}}$$

([2]) $${v_p} = \left({{1}\over{\rho _0}} \cdot {{d\sigma}\over{d\varepsilon}}\right)^{{1}\over{2}}$$

Here, v _e is the propagation speed of elastic wave (m s^–1), v _p is the propagation speed of static plastic wave (m s^–1), E is the Young’s modulus of the material (Pa), dσ/dε is the slope of the tangent line corresponding to stress point σ_p on the stress–strain curve, and ρ₀ is the density of materials (kg m⁻³).

The propagation and reflection process of the compressive wave with a wavelength λ is shown in Figure 4. In Figure 4A, the compressive wave approaches the free surface of the sample; in Figure 4B, the incident compressive wave and the reflected tensile wave from the surface overlap and form a composite wave, while tensile stress appears in the overlapping area (blue part); and in Figure 4C, the composite wave is tensile, representing the critical state. Before the critical state, the stress on the sample is partially compressive; and after the critical state, the stress on the sample is completely tensile. In Figure 4D, the overlapping area and tensile stress gradually decrease; in Figure 4E, the incident compression wave fades, and the reflection process of the stress wave ends (Wang Reference Wang2011; Yang and Zhang Reference Yang and Zhang2019). The generation of composite waves contributes to embrittlement and fragmentation, so excessively high cutting speeds may reduce cutting quality (Wang et al. Reference Wang, Liu, Su, Song and Ai2015).

Figure 4. Mechanical response of materials under different loading conditions (Yang and Zhang Reference Yang and Zhang2019).

Liu et al. (Reference Liu, Fan, Cai and Wang2015) conducted an impact tests on titanium alloy, which is ductile material. Figure 5 shows the section of the sample and the stress field during the impact process. No plastic deformation was found in regions 1 and 3 of the section, indicating that the ductile material underwent embrittlement during the impact process. Simulation shows that there are significant tensile stresses in regions 1 and 3, indicating that material embrittlement is closely related to stress waves.

Figure 5. The section of the sample and the stress field during the impact process (Liu et al. Reference Liu, Fan, Cai and Wang2015).

Constitutive Models

A constitutive model is a mathematical model that reflects the macroscopic physical properties. It describes the interrelationships between specific continuous medium kinematic quantities, dynamic quantities, and thermodynamic states. The constitutive model reflects the inherent properties of a research object, which would vary with the specific object and motion conditions being studied.

The Johnson Cook model, or J-C model, is applied to forecast the behavior and failure of materials. The J-C model can reflect the interaction between material stress and strain under conditions of large deformation and high strain rate. Due to its simple form and less input in predicting material constants, the J-C model is widely used to predict the flow behavior of materials (Sirigiri et al. Reference Sirigiri, Gudiga, Gattu, Suneesh and Buddaraju2022). It can be applied also to predict the behavior of plant stems. On one hand, plant stems would also undergo plastic deformation until material failure and fracture during cutting and destruction, which is similar to the behavior of metals. On the other hand, plant stems also exhibit significant strain hardening effect and strain rate strengthening effect in the destruction process, consistent with the main factors affecting flow stress in the J-C model. Therefore, the J-C model can be used as a constitutive model for plant stem cutting (J Liu et al. Reference Liu, Zhao, Zhang and Zhao2020). The flow stress equation is shown in Equation 3.

([3]) $$\sigma = (A + B{\varepsilon ^N})(1 + C\ln {\dot \varepsilon ^ * })(1 - {T^ * }^M)$$

Here, σ is equivalent stress (Pa); ε is equivalent plastic strain; ${\dot \varepsilon ^ * }$ is the dimensionless strain rate; T* is the dimensionless temperature; A is the yield strength of sample with the influence of strain rate and temperature; B is strain hardening coefficient; C is strain rate hardening coefficient; N is strain hardening exponent; and M is the thermal softening coefficient. There are three parenthetical expressions on the right side of the flow stress equation. From left to right, they represent strain hardening effect, strain rate strengthening effect, and temperature effect, which can influence flow stress.

The dimensionless strain rate ${\dot \varepsilon ^ * }$ and the dimensionless temperature T* can be calculated based on Equations 4 and 5.

([4]) $${\dot \varepsilon ^*} = {{\dot \varepsilon } \over {{{\dot \varepsilon }_0}}}$$

([5]) $${T^*} = {{T - {T_r}} \over {{T_m} - {T_r}}}$$

Here, $\dot \varepsilon $ is strain rate; ${\dot \varepsilon _0}$ is reference strain rate; T is the temperature at deformation (C); T _r is the reference temperature at deformation (C); and T _m is the material melting temperature (C).

When cut, plant stems would not heat up significantly, and the stem temperature would be approximately equal to room temperature, that is, T = T _r. When predicting the cutting process of plant stems, the J-C model can be simplified as Equation 6 (J Liu et al. Reference Liu, Zhao, Zhang and Zhao2020).

([6]) $$\sigma = (A + B{\varepsilon ^N})(1 + C\ln {\dot \varepsilon ^ * })$$

Because the J-C model does not indicate the failure of the material, the J-H constitutive model is proposed as better predicting the behavior and failure of brittle materials (Holmquist and Johnson Reference Holmquist and Johnson2011; Johnson and Holmquist Reference Johnson and Holmquist1994), which can also reflect the behavior of materials under large deformation and high strain rate conditions. The J-H model can be described by Equation 7.

([7]) $${\sigma ^ * } = \sigma _i^ * - D\left( {\sigma _i^ * - \sigma _f^ * } \right)$$

Here, σ* is equivalent normalized strength of materials (Pa); σ_i* is normalized strength of intact materials (Pa); σ_f* is normalized fracture stress (Pa), and σ_f* is not greater than σ_f*_max; σ_f*_max is a dimensionless fracture strength of sample; D is the damage factor of the material (0 ≤ D ≤ 1), and when the material is intact, D = 0. The dimensionless expression of σ* is shown in Equation 8.

([8]) $${\sigma ^*} = {\sigma \over {{\sigma _{HEL}}}}$$

Here, σ_HEL is the equivalent stress at Hugoniot elastic limit (Pa).

The expression of normalized strength of intact materials σ_i* is shown in Equation 9, and the expression of normalized fracture stress σ_f* is shown in Equation 10.

([9]) $$\sigma _i^ * = a{({p^ * } + {t^ * })^n}(1 + c\ln {\dot \varepsilon ^ * })$$

([10]) $$\sigma _f^ * = b{({p^ * })^m}(1 + c\ln {\dot \varepsilon ^ * })$$

Here, a, b, c, m, and n are the material constants to be determined; ${\dot \varepsilon ^ * }$ is the dimensionless strain-rate; p* is dimensionless pressure; t* is dimensionless maximum tensile strength. The expressions of dimensionless pressure p* and dimensionless maximum tensile strength t* are shown in Equations 11 and 12.

([11]) $${p^*} = {P \over {{P_{HEL}}}}$$

([12]) $${t^*} = {t \over {{P_{HEL}}}}$$

Here, P is actual pressure (Pa); P _HEL is the pressure at Hugoniot elastic limit (Pa); and t is the maximum tensile strength of sample (Pa).

Wedge Angle and Sliding Cutting Angle

The sharpness of the blade is an important factor affecting the cutting process and has a significant impact on cutting force, cutting energy, cutting quality, and blade life (McCarthy et al. Reference McCarthy, Hussey and Gilchrist2007; Schuldt et al. Reference Schuldt, Arnold, Kowalewski, Schneider and Rohm2016). As shown in Figure 6, the wedge angle γ is the angle between the front and rear cutting surfaces of the cutting edge. As the wedge angle decreases, the sharpness of the blade increases (McCarthy et al. Reference McCarthy, Annaidh and Gilchrist2010). However, an excessively small wedge angle can lead to insufficient strength at the blade tip, resulting in cutting stability issues (McCarthy et al. Reference McCarthy, Annaidh and Gilchrist2010). One other factor that significantly affects the sharpness of the blade is the radius of the blade tip (Schuldt et al. Reference Schuldt, Arnold, Kowalewski, Schneider and Rohm2016). The blade would be sharper when the radius of the blade tip is smaller. Due to the significant variation in blade tip radius with blade wear in practical operations, this article will not discuss the impact of blade tip radius on cutting plant stems.

Figure 6. Schematic diagram of sliding angle (Song et al. Reference Song, Zhou, Jia, Xu, Zhang, Shi and Hu2022a).

During the cutting process, the sliding cutting angle is a crucial parameter affecting cutting resistance. The sliding cutting angle has been widely used in agricultural production related to cutting, and it is an important performance index of some agricultural machinery (Huang et al. Reference Huang, Tan, Tian, Zhang, Ji, Liu and Shen2023).

The sliding cutting angle is defined as the angle between the absolute velocity direction of the blade motion and the normal velocity direction, as shown in Figure 7 (Tian et al. Reference Tian, Xia, Wang, Song, Yan, Li and Wang2021), where V is actual velocity of blade motion, V _n is normal velocity of blade motion, V _t is tangential velocity of blade motion, and α is the sliding cutting angle. Angle α shall be between 0° and 90°, and when angle α is 0°, the cutting method is considered to be cross-cutting (Song et al. Reference Song, Zhou, Xu, Jia and Hu2022b).

Figure 7. Schematic diagram of sliding angle (Tian et al. Reference Tian, Xia, Wang, Song, Yan, Li and Wang2021).

Sliding cutting can effectively reduce the ultimate stress of material tension, tensile fracture, and shear failure (Qian et al. Reference Qian, Ma, Xu, Li, Huo and Li2023). Some studies suggest that a larger sliding cutting angle leads to lower cutting resistance (Hu et al. Reference Hu, Xu, Yu, Lu, Han, Chai, Wu and Zhu2023; Rabbani et al. Reference Rabbani, Sohraby, Gholami, Jaliliantabar and Waismorady2015; W Wang et al. Reference Wang, Wang, Zhang, Lv and Yi2022b), while other studies indicates that the relationship between them is not linear (Cao et al. Reference Cao, Yu, Tang, Zhao, Gu, Liu and Chen2023; Huan et al. Reference Huan, Pang, Li, Wang and You2024; Zhang et al. Reference Zhang, Ni, Liu, Hu, Zhang and Fu2024). An increase of sliding cutting angle would reduce the actual wedge angle of the blade, making the blade sharper and reducing the normal stress during cutting, which is conducive to reducing the cutting force. On the other hand, an increase of the sliding cutting angle would increase the cutting displacement of the blade and the length of the blade put into cutting work, which is not conducive to reducing the cutting energy. When the sliding cutting angle is small, reducing the cutting force in the cutting process has a greater impact on the total cutting energy than reducing the cutting displacement. However, when the sliding cutting angle is large, although the ultimate shear stress decreases, the cutting will consume more energy due to the friction resistance between the blade surface and the stem (Song et al. Reference Song, Zhou, Jia, Xu, Zhang, Shi and Hu2022a). This is essentially a comparison between two effects caused by reducing cutting angle, and the values of both sides change with the cutting object.

Moisture Content

Because the stem is considered to be a composite material composed of cells (Du et al. Reference Du, Hu and Buttar2020), the lack of water would affect the structure and the physical properties of the cells, thus impacting the physical properties of the stem (Rabbani et al. Reference Rabbani, Sohraby, Gholami, Jaliliantabar and Waismorady2015). Moisture content would have a significant impact on the toughness and strength of the stem and would greatly impact the cutting process (Cao et al. Reference Cao, Yu, Tang, Zhao, Gu, Liu and Chen2023; Tang et al. Reference Tang, Li, Zhang, Wang and Li2020, Reference Tang, Liang, Zhang, Wang, Zhang and Li2021). Moisture content is among the characteristics of the cutting object, the stem itself, but moisture content is relatively easy to predict and control in agricultural production.

The influence trend of stem moisture content on cutting resistance can be divided into two types. First, when the stem is fresh and the plant cells do not die in a large range during cutting, the cutting resistance decreases with the increase of moisture content (Chandio et al. Reference Chandio, Li, Ma, Ahmad, Syed, Shaikh and Tunio2021; Du et al. Reference Du, Hu and Buttar2020; Kumar et al. Reference Kumar, Mahapatra, Behera, Pradhan, Swain, Rath and Sahni2023; Taghinezhad et al. Reference Taghinezhad, Alimardani and Jafari2013). Second, when the stem has been dried or the plant cells have died in a large range before the experiment, the cutting resistance increases with the increase of moisture content (Charee et al. Reference Charee, Sudajan, Junsiri, Laloon and Oupathum2021; Galedar et al. Reference Galedar, Jafari, Mohtasebi, Tabatabaeefar, Sharifi, O’Dogherty, Rafiee and Richard2008; Oyefeso et al. Reference Oyefeso, Akintola, Afolabi, Ogunlade, Fadele and Odeniyi2021).

The increase of water content would make the fresh plant cells fuller and would further increase the Young’s modulus of the stem. The increase of water content would also reduce the ultimate shear stress of fresh stems. In summary, increasing the moisture content would increase the brittle material characteristics of fresh stems.

Generally, there are several ways to dry plants, including freeze drying, relative humidity convective drying, infrared drying, microwave drying, and ultrasonic drying. All these drying methods would create a porous microstructure in the dried product and have great effects on plant cells and tissues (Guo et al. Reference Guo, Wu, Guo, Ding, Pan and Ma2020b; Osae et al. Reference Osae, Essilfie, Alolga, Bonah, Ma and Zhou2020; Rashid et al. Reference Rashid, Ma, Jatoi, Safdar, El-Mesery, Sarpong, Ali and Wali2019). The effects can be seen in Figure 8: the arrows refer to the pores and cavities caused by drying. Freeze drying can produce ice crystals in cells, resulting in voids in the cell structure and cell collapse. In relative humidity convective drying, the synergistic effect of temperature and relative humidity will make plants create micropores, and the dried product will be softer than the non-dried one (Xu et al. Reference Xu, Tiliwa, Yan, Azam, Wei, Zhou, Ma and Bhandari2022). Infrared drying and microwave drying will lead to irreversible cell rupture and destruction of cell structure, which will lead to the loss of plant tissue integrity and severe cell collapse (Osae et al. Reference Osae, Essilfie, Alolga, Bonah, Ma and Zhou2020; Wu et al. Reference Wu, Guo, Guo, Ma and Zhou2021). It can be determined that ultrasonic drying can change the five textural properties of the target: hardness, elasticity, cohesion, adhesion, and stickiness (Xu et al. Reference Xu, Tiliwa, Yan, Azam, Wei, Zhou, Ma and Bhandari2022). In summary, drying can reduce the material strength of plant stems by altering their cell and tissue structure.

Figure 8. Longitudinal microstructure of carrot (Daucus carota L.). (A) Fresh; (B and D) infrared drying; (C and E) ultrasound-assisted infrared drying (Guo et al. Reference Guo, Wu, Guo, Ding, Pan and Ma2020b).

Material Preparation and Test Methods

Plant cutting is a complex process that involves many variables. To measure whether a cutting process meets the indicators required by the operation, it is necessary to select some variables to characterize important characteristics of the cutting process.

The cutting process evaluation indices shown in this paper are ultimate cutting stress, specific cutting energy, and cutting quality. Ultimate cutting stress represents the maximum resistance received by the blade when cutting, and the cutting resistance is one of the core indicators of much cutting equipment (Tian et al. Reference Tian, Zhang, Ji, Huang, Liu and Shen2023). Because cutting resistance is affected by the diameter of the plant stem, the effect of diameter can be removed by using shear stress compared with shear resistance. Specific cutting energy represents the work done by the cutting system during the cutting process and the resistance of the blade in the cutting process. Similar to the ultimate cutting stress, specific cutting energy also removes the influence of diameter. Cutting quality is the evaluation index for the flatness of the section produced by cutting when harvesting crops, such as sugarcane (Saccharum officinarum L.), which can be seen in Figure 9. Improving the cutting quality can reduce the harvest losses and stubble damage rates of crops. Moisture content, which is an important physiological and mechanical index of plant cannot be used directly to represent the cutting process, but can indirectly affect the process by affecting the mechanical properties of plant stems. The moisture content of fresh plant stems can vary, but the variation range is small.

Figure 9. The sugarcane stubble damage condition: (A) undamaged stubble; (B) slight damage; (C) moderate damage; (D) severe damage; and (E) uprooting (Qian et al. Reference Qian, Ma, Xu, Li, Wang, Yang and Wang2024).

Ultimate Cutting Stress and Specific Cutting Energy

To calculate ultimate cutting stress and specific cutting energy, it is necessary to select the data acquisition instrument according to the cutting speed. To obtain a quasi-static response, the texture analyzer can be used for the cutting test, as shown in Figure 10A (Liu et al. Reference Liu, Wang, Ma, Wang, Du, Shi, Ni and Mao2022; Tang et al. Reference Tang, Liang, Wang, Zhang and Wang2022). The cutting displacement cutting resistance curve can be obtained with a cutting test using the texture analyzer; the ultimate cutting stress can be obtained using Equation 13, and specific cutting energy can be obtained using Equation 14. To obtain dynamic response, a pendulum-based cutting test setup or other homemade cutting test machine can be used. A pendulum-based cutting test setup can be seen in Figure 10B. The calculation of ultimate cutting stress and specific cutting energy is the same as that used for quasi-static cutting.

Figure 10. Test instruments: (A) texture analyzer and (B) pendulum-based cutting test setup.

([13]) $$\tau = {{{F_{\max }}} \over A}$$

([14]) $${E_p} = {1 \over A}\int\limits_0^s {Fds} $$

Here, τ is ultimate cutting stress (MPa); F _max is the maximum force the blade received (N); A is the area of the section caused by cutting (mm²); E _p is specific cutting energy (×10⁻³ J mm⁻²); s is blade displacement during the cutting process (m); F is instantaneous force received by the blade during the cutting process (N).

Cutting Quality

When measuring cutting quality, taking sugarcane as an example, if the crack on a stem stub exceeds one node of sugarcane or the stub is pushed down or pulled out, the stub should be considered damaged. If the section of a stem stub is flat and exhibits no tearing, the stub can be considered to be qualified stub. The damage rate for measuring cutting quality can be calculated according to Equation 15 (Qian et al. Reference Qian, Ma, Xu, Li, Huo and Li2023).

([15]) $${C_P} = {{{N_P}} \over {{N_A}}}$$

Here, C _P is stubble damage rate (%); N _P is the number of damaged stem stubs produced by the test; N _A is the number of stem stubs produced by the test.

Moisture Content

Moisture content is an important physical parameter that needs to be measured using instruments. Many devices can be selected to assist in the measurement of moisture content. The use of hyperspectral technology is efficient and nondestructive and has been widely used in the measurement of agricultural products (Lu et al. Reference Lu, Sun, Yang, Wu, Zhou and Shen2018). However, to improve accuracy, the moisture content of the test points needs to be the same as that of the whole plant sample (Sun et al. Reference Sun, Tian, Wu, Dai and Lu2020). Hyperspectral technology is suitable for measuring the moisture content of smaller parts such as leaves, seeds, or slender stems (Lu et al. Reference Lu, Sun, Yang, Wu, Zhou and Shen2018; X Zhou et al. Reference Zhou, Sun, Mao, Wu, Zhang and Yang2018). The use of a constant-temperature oven is accurate, but time-consuming, destructive, and cumbersome (Sun et al. Reference Sun, Tian, Wu, Dai and Lu2020). The constant-temperature oven would use convective air or infrared or microwave to promote the loss of moisture in plant samples and finally end the drying when the sample mass no longer decreases. The moisture content measured with a constant-temperature oven can be calculated using Equation 16.

([16]) $${M_C} = {{{m_A} - {m_P}} \over {{m_A}}}$$

Here, M _C is moisture content (%); m _P is the mass after drying (g); and m _A is the mass before drying (g).

Bionic Cutting Blade Design

In recent years, the application of physiology as well as morphology in agriculture has been increasing. Different biological characteristics have been incorporated into the design of cutting tools based on observing cutting processes in nature and analyzing low-resistance, anti-wear, and anti-friction characteristics,. These characteristics are successful examples of the cutting mechanisms evolved by various organisms. This effectively enhances the performance of cutting tools and the quality of the cutting process (Yu et al. Reference Yu, Han, Zhang and Zhang2021). The use of bionics in agricultural components includes the design of openers, plows, and cutting blades to reduce resistance and power consumption (Tian et al. Reference Tian, Li, Zhang, Chen, Shen and Huang2017).

In bionic cutting blade design, the first step is to determine the required cutting blade features and the suitable bionic prototype. Especially for the blade design for plant cutting, the blades need to be low resistance and anti-friction. The material properties and cutting mechanisms of different materials vary from one another, and different bionic prototypes need to be selected according to the cutting object requirements (Yu et al. Reference Yu, Han, Zhang and Zhang2021). For example, the common pest Locusta migratoria manilensis, which mainly feeds on plants, has highly developed chewing mouthparts. The mandibles of the Oriental migratory locust are an important and efficient tool for cutting plants and a suitable bionic prototype (Hu et al. Reference Hu, Xu, Yu, Lu, Han, Chai, Wu and Zhu2023). Otidognathus davidis larvae feed on bamboo (Bambusa spp.), which has high strength and hardness. Bamboo weevil larvae can easily cut the thick, tough fibers bamboo. The cutting ability mainly comes from the larva’s sturdy and sharp mandibles and can serve as a good bionic prototype in the design of plant cutting blades (Tong et al. Reference Tong, Xu, Chen and Li2017).

It is necessary to extract features from bionic prototypes, usually insect mandibles or animal claws. First, observe and record bionic prototypes through scanning electron microscopy, 3D microscopy, or nano-computed tomography (CT) scanners. Then, extract contour data from raw images through image post-processing software (Moyer and Bemis Reference Moyer and Bemis2017; J Wang et al. Reference Wang, Guan, Gao, Zhou and Tang2020; Zhang et al. Reference Zhang, Ni, Liu, Hu, Zhang and Fu2024). In the end, based on actual needs, the extracted data should be reproduced on the cutting blade in a specific size. This can be the reproduction of the cutting part curve of the bionic prototype by the blade edge curve design in a two-dimensional perspective (Cao et al. Reference Cao, Yu, Tang, Zhao, Gu, Liu and Chen2023; Guan et al. Reference Guan, Fu, Xu, Jiang, Wang and Cui2022), or the reproduction of the cutting part curve of the bionic prototype by the blade specific design in a three-dimensional perspective (Li et al. Reference Li, Chen and Chen2016). The design process for a bionic cutting blade can be seen in Figure 11.

Figure 11. The design process for a bionic cutting blade inspired by Oriental migratory locust (Cao et al. Reference Cao, Yu, Tang, Zhao, Gu, Liu and Chen2023).

Serrated Cutting Blade Design

The design of serrated blade is a common improving design scheme for ordinary blade. A serrated blade design would not change the blade curve of the original blade design but would replace the original blade edge with a row of serrations.

The design of a serrated blade is similar to that of a saw. The serrations will produce local high-pressure points on the cutting object, which will produce cutting object failure (Gan et al. Reference Gan, Mathanker, Momin, Kuhns, Stoffel, Hansen and Grift2018). After the stem fails at high-pressure points, the displacement of serrations, which would already be embedded in the stem, would strengthen the tensile stress and shear stress of the blade on the stem. Because shear and tensile failure of stem fibers are the main sources of stem failure (Tang et al. Reference Tang, Liang, Wang, Zhang and Wang2022), strengthening tensile stress during cutting can encourage stem failure and reduce energy consumption (Gan et al. Reference Gan, Mathanker, Momin, Kuhns, Stoffel, Hansen and Grift2018).

There are many specific designs for serrations, including designs with triangular protrusions like saws (Mello and Harris Reference Mello and Harris2003; J Wang et al. Reference Wang, Wang, Li, He, Lu and Liu2021), designs with strip-shaped serrations (Liu et al. Reference Liu, Mathanker, Zhang and Hansen2012; Momin et al. Reference Momin, Wempe, Grift and Hansen2017), and designs that use the curve obtained from a bionic prototype as the blade curve at the serrated gap (X Chen et al. Reference Chen, Xu, Zhang, Tan, Ding and Tagar2024). A blade with strip-shaped serrations is shown in Figure 12.

Figure 12. Serrated blade design with strip-shaped serrations (Gan et al. Reference Gan, Mathanker, Momin, Kuhns, Stoffel, Hansen and Grift2018).

Equal Sliding Cutting Angle Blade Design

The influence of sliding cutting angle on the cutting process has been described earlier. To solve the problem of uneven force and large fluctuations on the blade during the cutting of rattan straw, an equal sliding angle cutting blade design has been proposed (Guo et al. Reference Guo, Zhang, Xu, Li, Chen and Wu2014). The equal sliding angle cutter not only reduces cutting power consumption and improves cutting efficiency but also extends blade life (Song et al. Reference Song, Zhang, Xie, Chen and Liu2024; Zheng et al. Reference Zheng, He, Li, Diao, Wang and Zhang2016).

As shown in Figure 13, a polar coordinate system is established with point O as the pole, and the blade rotates around point O as the center. When the blade curve AB rotates dδ, any point D on the blade curve would changes to point M, and vector r would become r′. Set dr as the difference between r and r′. When dδ approaches 0, arc MD can be regarded as a straight line, and the lengths of arc ND can be regarded the same as line MN. According to the definition of a sliding angle, in triangle ABC, the relationship between sliding angle τ and polar angle δ can be expressed by Equation 17:

([17]) $$\tan \tau = {{ND} \over {MN}} = {{rdr} \over {dr}}$$

Figure 13. Schematic diagram of the equal sliding cutting angle blade curve (P Liu et al. Reference Liu, He, Li, Li, Wang, Lu, Zhang and Li2020).

If the sliding angle is constant, the polar coordinate equation of the blade curve is shown in Equation 18).

([18]) $$r = C{e^{{\delta \over {\tan \tau }}}}$$

Here, C is constant.

The polar coordinate equation is a logarithmic spiral equation. In a logarithmic spiral curve, if any ray passing through the pole O intersects with a curve, the angle between the tangent of the curve at the intersection and the ray is a fixed value, which is the slip angle τ. Therefore, the equal sliding angle blade can also be referred as a logarithmic spiral blade.

The equal sliding angle cutting blade has a significant effect on cutting stems with low fiber content. However, banana (Musa acuminata Colla) straw has a large amount of fiber. If the equal sliding angle cutting blade is used to cut banana straw, due to inertia, the fiber of the banana straw easily slides from the high cutting speed blade tip to the low cutting speed blade base and then wraps around the blade shaft. Therefore, in this case, the variable sliding cutting angle blade is needed (Zhang et al. Reference Zhang, Wang, Li and Liang2018).