Research on the Mechanical Properties of Peanuts during the Harvesting Period

Abstract

To address the gap in research on the mechanical properties of peanuts in the harvesting period, the mechanical properties of peanut seedling vines, pods, and peduncles were studied during the peanut harvesting period. The moisture content of peanut pods, peduncles, and seedling vines was measured, yielding values of 36.03%, 66.76%, and 77.95%, respectively. The tensile characteristic parameters between the pods and peduncles, peduncles and peduncles, and the seedling vines and peduncles during the harvesting period were determined using an electronic universal testing machine at three different loading speeds: 10 mm/min, 20 mm/min, and 30 mm/min. The test results indicated that the peduncle–peduncle, peduncle–pod, and peduncle–seedling vine detachment forces were 19.91 N, 17.41 N, and 8.62 N. The mechanical properties of peanuts during the harvesting period differed from those of peanuts that were excavated and sun-dried. Peanut-digging and turning machines should be designed based on the detachment force required to separate the peduncles from the pods, which, at a loading speed of 20 mm/min, is 17 N. This examination of the mechanical properties of peanuts during the harvesting period could have significant practical implications and a lasting influence on enhancing the efficiency and quality of peanut harvesting, refining harvesting machinery design, and advancing agricultural modernization.

1. Introduction

The peanut (Arachis hypogaea L.) is an annual herbaceous plant from the family Fabaceae of the order Rosales [1]. It is the fourth-largest oilseed crop globally and one of the most significant oilseed and economic crops in China. Peanuts play a crucial role in ensuring a domestic edible oil supply and promoting food consumption diversification [2,3,4,5]. In 2022, according to data from the FAO and the National Bureau of Statistics of China, the global peanut planting area was approximately 30.54 million hectares, with China accounting for around 4.68 million hectares, representing 15.32% of the world total and ranking second globally after India. The global volume of peanut production is about 54.24 million tons, with China producing approximately 18.33 million tons, accounting for 33.79% of the world total and ranking first globally [6,7].

The research and development of peanut production machinery in China began in the 1960s. After more than 60 years of rapid development, by 2021, China’s mechanization rates for peanut cultivation, sowing, and harvesting were 81.96%, 58.65%, and 50.9%, respectively, with a comprehensive mechanization rate of 65.65% [8]. For a long time, the development of peanut-harvesting machinery in China has been very backward, seriously restricting the development of the peanut industry in China [9]. The mechanical properties during the peanut-harvesting period are among the key factors affecting the mechanical performance of peanut-harvesting machinery, so it is of great significance to study the mechanical properties in the peanut-harvesting period between seedling vines, peduncles, and pods.

Wang Chuantang proposed that the technical indicators for the mechanical harvesting of common peanut varieties include a minimum strength of 5 N for the peduncle and a minimum shell strength of 1.26 kN [10]. George contends that the strength of the peduncle affects the loss rate and divides the strength of the peduncle into four levels: low (5.69–8.44 N), medium (8.45–13.96 N), high (13.97–16.72 N), and very high (>16.72 N) [11]. Research by Xu Jing has shown that the force required to detach a seedling vine from a peduncle (8.34 N) is greater than that for detaching pods from peduncles (7.03 N), and the detachment force distribution of pod–peduncle nodes is uneven, with most varieties concentrated in the lower half, while the seedling vine–peduncle detachment force distribution is relatively uniform [12]. Chen Xiaoshu’s research shows that the tensile strength of the connection between the seedling vine and peduncle is greater than that for the pod and peduncle. The peduncle usually falls off due to insufficient tensile strength, causing significant losses via buried pods. Whether harvested manually or mechanically, the harvesting of peanut varieties requires a low rate of buried pods and pods that are difficult to crack or break; as such, suitable machine-harvested varieties with peduncles and pods possessing high-impact resistance are required [13]. Zhang Xiao’s research shows that the thickness of the pod shell and the diameter of the pod waist of non-shedding pod varieties are significantly higher than those of shedding pod varieties; the breaking strength of the pod shell, the thickness of the middle and inner parts of the pods at different maturities and water content levels, the thickness of the ovary stalk connected to the pods, and the content of structural substances in the pod shell are all significantly higher than those of shedding pod varieties [14]. Sun Yawen’s research shows that the peduncle–pod and peduncle–seedling vine detachment force for the varieties that do not easily drop their pods at different maturity levels is significantly higher than that of varieties that easily drop their pods. With a decrease in the moisture content in the peduncle, the detachment force of the peduncle shows a trend of increasing first and then decreasing, with the maximum detachment force occurring at a moisture content of 30%; the detachment force of the peduncle is the highest when a pod’s maturity level is 75% [15]. Guan Meng’s research shows that the optimal sun-drying time for peanuts before picking is 3–5 days. At this time, the moisture content of various parts of the peanut plant is basically reduced to 10–20%, and the tensile strength of the stem at a peduncle node is 10–15 N, while at the node peduncle, it is 7–9 N [16]. The above results were obtained from studies on the physical properties of peanuts after being dug up and dried, and there is no research on the mechanical/physical properties of peanuts at harvest time.

At present, there are two main operation modes for peanut-harvesting machinery in China: two-stage harvesting and combined harvesting. When operating a two-stage harvesting machine, the digging harvester lays the peanut plants with their pods facing upwards in the field, which is a position that is conducive to the rapid drying of the pods and can effectively reduce the occurrence of mold on the pods during rainy days. Therefore, both domestically and internationally, there is a demand for peanut-digging and harvesting technology with a seedling vine-turning function. The mechanical properties of peanut seedling vines during the harvesting period (namely, those acting between seedling vines and peduncles, pods and peduncles, and peduncles and peduncles) are crucial for mechanized harvesting, as these properties affect harvesting efficiency and the pod loss rate. Currently, research on the mechanical properties of peanuts after drying is conducted both domestically and internationally, yet research on the mechanical properties of peanuts during the harvesting period remains scarce. In this study, we aimed to determine the strength of peanut peduncles to assess pod–peduncle and seedling vine–peduncle detachment forces, with the main objective being to reduce peanut burial and loss rates. The study of the mechanical properties of the seedling vine–peduncle, pod–peduncle, and peduncle–peduncle connections during the harvest season is of great practical significance and can have a long-term impact on improving the efficiency and quality of peanut harvesting, reducing losses, optimizing harvesting machinery design, and promoting agricultural modernization. Through field surveys and experiments, a theoretical basis can be provided for determining parameters, such as the digging depth, angle, and clamping force for peanut harvesting, thereby reducing losses during harvesting and laying the foundation for the development of harvesting machinery suitable for peanut production in China.

2. Materials and Methods

2.1. Experimental Materials

Our experiment focused on the ‘Yuhua 40’ variety, with samples taken from the recommended upright peanut variety grown in the planting base in Ru’nan County, Zhumadian City, Henan Province. The soil in the peanut planting base was sandy, with planting taking place in May and harvesting occurring in mid to late September. The planting pattern included single-ridge double-row planting with mechanical ridging. The ridge spacing was 80 cm, the ridge surface width was 50 cm, the row spacing was 25 cm, and the ridge groove width was 30 cm. The growth depth of the peanut pods ranged from 25 to 117 mm, the distribution range of the peanut pods was 72 to 136 mm, and the uprightness distribution range of the peanut seedlings ranged from 191 to 315 mm. The peanut-planting pattern and plant morphology are shown in Figure 1.

2.2. Test Equipment

The main instruments used in this experiment include a DWD electronic universal testing machine produced by Shenzhen Sansi, a DGF30/7-IA electric hot-air-drying oven (temperature range: 0–300 °C, voltage: 220 V), an electronic balance with a capacity of 100 g and an accuracy of 0.0001 g, and a digital caliper with a range of 150 mm and accuracy of 0.01 mm. The electronic universal testing machine, as shown in Figure 2, has a rated load capacity of 5 kN, an accuracy class of 0.5, a displacement resolution of 0.01 mm, and a loading speed of 0.01–500 mm/min. The load–displacement relationship can be automatically recorded by a computer in a point-by-point manner, and the coordinates and structural parameters of each point can be read from specified files [17].

2.3. Test Methods

2.3.1. Determination of Moisture Content in Peanuts

Five parts each were taken from peanut seedling vines, pods, and peduncles. Then, the weighed peanuts were placed in a DGF30/7-IA electric hot-air-drying oven using a drying box, and the temperature was set to 105 °C for drying [17]. The mass of the parts was measured every two hours until a constant mass was reached. Next, the moisture content was calculated according to Formula (1). Finally, the arithmetic average was determined.

$$W={\displaystyle \frac{M_1-M_2}{M_1}}$$

(1)

Here, W is the moisture content, calculated by subtracting the mass of the material after drying from the mass of the material before drying and then dividing this by the mass of the material before drying, given as a percentage; M₁ is the mass of the material before drying, given in g; and M₂ is the mass of the material after drying, given in g.

2.3.2. Peanut Tensile Test Method

Three types of test samples, including peanut seedling vine–peduncle, pod–peduncle, and peduncle–peduncle samples, were used for testing. During the experiment, the selected samples were placed inside an electronic universal testing machine, and their tensile strength was tested at three different loading speeds, namely, 10 mm/min, 20 mm/min, and 30 mm/min, until they broke, as shown in Figure 3. The samples that detached at the middle position were considered valid tensile samples [18]. Five sets of appropriate tensile data were selected from the test data at the three different loading speeds of 10 mm/min, 20 mm/min, and 30 mm/min for final analysis.

3. Results

3.1. Moisture Content

According to the test results, the masses of the peanut pods, peduncles, and seedling vines remained constant after these parts were dried at 105 °C for approximately 24 h, as illustrated in

Table 1. Moisture content of peanut pods, peduncles, and seedling vines.

No.	Pod Moisture Content/%	Peduncle Moisture Content/%	Seedling Vine Moisture Content/%
1	36.65	67.87	76.8
2	36.23	64.94	78.34
3	35.51	66.78	76.59
4	36.03	66.89	78.2
5	35.71	67.32	79.81
$\overline{x}$	36.03	66.76	77.95
s	0.4	0.99	1.17

.

According to Table 1, the moisture content of the harvested pods, peduncles, and seedling vines was 36.03%, 66.76%, and 77.95%, respectively. The standard deviations of the moisture content of the peanut pods, peduncles, and seedling vines were 0.4, 0.99, and 1.17, respectively, all of which are less than five, indicating a low degree of dispersion between the sample data and high reliability [19].

3.2. Tensile Test Results and Analysis of Peanut Harvest Period: The Seedling Vine–Peduncle, Pod–Peduncle, and Peduncle–Peduncle Mechanical Properties

3.2.1. Tensile Test Results and Analysis of the Seedling Vine–Peduncle Mechanical Properties

Single-factor tensile tests were conducted on peduncles and seedling vines with a moisture content of 66.76% and 77.95%, respectively, at three different loading speeds: 10 mm/min, 20 mm/min, and 30 mm/min. The tensile force–displacement curve for the peduncles and seedling vines is shown in Figure 4, and the test data are presented in

Table 2. The results of a single-factor experiment conducted on peduncles and seedling vines.

Loading Speed/mm/min	Cohesion Force/F	Average Value/F
10	10.56	10.12
	9.45
	10.15
	10.32
	10.11
20	9.97	8.62
	8.8
	7.2
	10.15
	6.97
30	6.88	7.31
	7.46
	6.15
	7.19
	8.83
Average value/F		8.68

.

In Figure 4, it can be observed that the peduncle–seedling vine curve exhibits a linear change trend in pressure and displacement in the early stage of tension without a clear yield point. Subsequently, as the tension increases, the peduncle–seedling vine connection reaches its tensile limit, and thus, the two are rapidly pulled apart, resulting in a sudden drop in tension. Throughout the tensile process, tension fluctuates significantly with the increase in displacement as the detachment of the peduncle from the seedling vines is not instantaneous but gradually occurs as they are pulled apart from the stress concentration point.

Using IBM SPSS Statistics 24 software, a p-value test was conducted on the peduncle–seedling vine connection at a significance level of α = 0.05. The results of an analysis of variance are shown in

Table 3. A single-factor analysis of variance for the peduncle–seedling vine connection.

Sources	Sum of Squares	Degrees of Freedom	Mean Square	F	Significance
Intra-class	19.838	2	9.919	8.798	0.004
Interblock	13.530	12	1.127
Total	33.368	14

, indicating a p-value of 0.004, which is less than the significance level of 0.05. This suggests that different loading speeds have a significant impact on the tensile properties of the connection between the peduncle and seedling vines.

According to Table 2, when the loading speed is 10 mm/min, the force required to detach the peduncle from seedling vines is 10.12 N; when the loading speed is 20 mm/min, the detachment force is 8.62 N; and when the loading speed is 30 mm/min, the detachment force is 7.31 N, with an average detachment force of 8.68 N. The data distribution for the peanut peduncles and seedling vines with varying loading speeds is illustrated in Figure 5.

In Figure 5, it is evident that as the loading speed increases, the average and median values of the peduncle–seedling vine detachment force decrease, suggesting that the peduncle–seedling vine detachment force varies at different loading speeds and decreases with an increase in the loading speed. Moreover, at different loading speeds, the seedling vine–peduncle detachment force follows a normal distribution. Specifically, when the loading speed is 10 mm/min, the seedling vine–peduncle detachment force is concentrated; at 20 mm/min, it is uniform; and at 30 mm/min, the normality of the seedling vine–peduncle detachment force is poor. Therefore, the loading speed for the tensile testing of the connection between peduncles and seedling vines should be less than 20 mm/min to ensure that the maximum seedling vine–peduncle detachment force does not exceed 8.62 N.

3.2.2. Tensile Test Results and Analysis of the Peduncle–Peduncle

The peduncles with a moisture content of 66.76% were subjected to single-factor tensile tests at three different loading speeds: 10 mm/min, 20 mm/min, and 30 mm/min. The tensile force–displacement curves of the peduncles are depicted in Figure 6, and the test data are detailed in

Table 4. The peduncle–peduncle single-factor test results.

Loading Speed/mm/min	Cohesion Force/F	Average Value/F
10	34.07	27.94
	32.09
	27.71
	24.4
	21.43
20	17.58	19.91
	23.61
	25.18
	17.09
	16.18
30	20.11	17.21
	17.18
	14.41
	18.33
	16.01
Average value		21.69

.

According to Figure 6, the peduncle–peduncle connection exhibits a linear relationship between stress and displacement in the initial stage of tension without an obvious yield point. As the tension force continues to increase, the peduncle–peduncle connection reaches the ultimate tensile strength, and then the peduncles are rapidly pulled apart, resulting in a sudden drop in the tension force. Throughout the tension process, the tension force fluctuates significantly with the increase in displacement. This fluctuation occurs because the detachment of the two peduncles is not instantaneous; rather, they are gradually pulled apart from the stress concentration point.

Using IBM SPSS Statistics 24 software, a p-value test was conducted on the connections between the peanut samples’ peduncles at a significance level of α = 0.05. The results of the analysis of variance are shown in

Table 5. Single-factor analysis of variance for the peduncle–peduncle connection.

Sources	Sum of Squares	Degrees of Freedom	Mean Square	F	Significance
Intra-class	311.195	2	155.598	9.460	0.003
Interblock	197.383	12	16.449
Total	508.578	14

, indicating a p-value of 0.003, which is less than the significance level of 0.05. This result suggests that different loading speeds have a significant impact on the tensile properties of the peduncle–peduncle connection.

According to Table 4, when the loading speed is 10 mm/min, the peduncle–peduncle peeling force is 27.94 N; when the loading speed is 20 mm/min, the peduncle–peduncle peeling force is 19.93 N; and when the loading speed is 30 mm/min, the peduncle–peduncle peeling force is 17.21 N, with an average peeling force of 21.69 N. The data distribution of the peduncle–peduncle peeling force with a changing loading speed is shown in Figure 7.

From Figure 7, it can be gleaned that as the loading speed increases, the average and median values of the detachment force between the peduncles decrease. This suggests that the detachment force between the peduncles varies at different loading speeds. As the loading speed increases, the detachment force between the peduncles decreases. Additionally, the detachment force between the peduncles at different loading speeds follows a normal distribution. When the loading speed is 10 mm/min, the detachment force between the peduncles exhibits poor normality. At a loading speed of 20 mm/min, the detachment force between the peduncles shows a uniform distribution. When the loading speed is 30 mm/min, the detachment force between the peduncles displays a concentrated distribution. Therefore, the loading speed for the tensile testing of the peduncles should be greater than 20 mm/min, meaning that the minimum value of the detachment force between the peduncles should be 19.91 N.

3.2.3. Tensile Test Results and Analysis of Peduncle–Pods

Single-factor tensile tests were conducted on peduncles with a moisture content of 66.76%, and pods were used with a moisture content of 36.03% at three different loading speeds of 10 mm/min, 20 mm/min, and 30 mm/min. The tensile force–displacement curve of the peduncle–pod connection is shown in Figure 4, and the test data are presented in

Table 6. Results of the peduncle–pod single-factor experiment.

Loading Speed/mm/min	Cohesion Force/F	Average Value/F
10	19.73	18.03
	18.98
	17.5
	16.92
	17.04
20	19.67	17.41
	16.6
	18.06
	15.91
	16.8
30	14.9	15.2
	16.27
	15.93
	14.83
	14.17
Average value		16.89

.

According to Figure 8, the peduncles and pods exhibit a linear relationship between stress and displacement in the initial stage of tension without a clear yield point. Subsequently, as the tension continues to increase, the connection between the peduncle and pods reaches the ultimate tensile strength, and the two are rapidly pulled apart, resulting in a sudden drop in tension. Throughout the process of applying tension, the tension fluctuates significantly with an increase in displacement as the detachment of the peduncle from pods is not instantaneous: these parts are gradually pulled apart from the stress concentration point.

Using IBM SPSS Statistics 24 software, a p-value test was conducted on the peduncles and pods at a significance level of α = 0.05. The results of the analysis of variance are shown in

Table 7. Single-factor analysis of variance for the peduncles and pods.

Sources	Sum of Squares	Degrees of Freedom	Mean Square	F	Significance
Intra-class	21.794	2	10.897	7.235	0.009
Interblock	18.075	12	1.506
Total	39.869	14

, indicating a p-value of 0.009, which is less than the significance level of 0.05, suggesting that different loading speeds have a significant impact on the tensile properties of the peduncle–pod connection.

According to Table 6, when the loading speed is 10 mm/min, the force required to detach the peduncle from the pods is 18.03 N; when the loading speed is 20 mm/min, the detachment force is 17.41 N; and when the loading speed is 30 mm/min, the detachment force is 15.2 N, with an average detachment force of 16.89 N. The data distribution of the peduncle–pod detachment force with varying loading speeds is shown in Figure 9.

According to Figure 9, as the loading speed increases, the average and median values of the peduncle–pod detachment force decrease, indicating that the peduncle–pod detachment force varies at different loading speeds and decreases with an increase in the loading speed. Additionally, the peduncle–pod detachment force at different loading speeds follows a normal distribution. However, the normality of the peduncle–pod detachment force is poor when the loading speeds are 10 mm/min and 30 mm/min. When the loading speed is 20 mm/min, the peduncle–pod detachment force is uniformly distributed. Therefore, the recommended loading speed for the tensile testing of the peduncle–pod connection is 20 mm/min, and the peduncle–pod detachment force is 17.41 N.

4. Discussion

According to the results of the single-factor experiment, the amount of force required to detach the peduncle from the pods, the peduncles from each other, and the peduncles from seedling vines is 19.91 N, 17.41 N, and 8.62 N, respectively. Therefore, the detachment force of the peanut nodes during harvesting is ranked as follows: peduncle–peduncle > peduncle–pods > peduncle–seedling vines. This ordering indicates that peanuts generally detach at the peduncle and the seedling vine. However, in actual work, the shedding of peanut pods occurs at the contact point between the peduncle and the pods. Therefore, peanut-digging-and-turning machines should be designed based on the force required to detach the peduncles from the pods, which, along with a loading speed of 20 mm/min, is 17 N.

By comparing this with the existing literature, it can be seen that the mechanical properties of peanuts at harvest and after being dug up and dried differ. The detachment force ranking for various nodes of peanuts at harvest is peduncle–peduncle > peduncle–pods > peduncle–seedling vines; by contrast, the ranking at various nodes of peanuts after being dug up and dried is seedling vines–peduncle > pods–peduncle. This difference is due to the functional relationship between the peanut pods, peduncles, and seedling vines, as illustrated in

Table 8. Comparison of the mechanical properties of peanuts at harvest and after digging and drying.

Sources	Peduncle–Peduncle/N	Peduncle–Pods/N	Peduncle–Seedling Vines/N	Sorting
This study	19.91	17.41	8.62	peduncle–peduncle > peduncle–pods > peduncle–seedling vines
Ref. [12]		7.03	8.34	peduncle–seedling vines > peduncle–pods
Ref. [13]				peduncle–seedling vines > peduncle–pods
Ref. [16]		7–9	10–15	peduncle–seedling vines > peduncle–pods

. Hence, the moisture content of peanuts should be taken into account during the operational process.

Existing studies may not have fully considered the impact of different soil types and environmental conditions on the mechanical and physical properties of peanut harvesting. Factors such as soil moisture, temperature, texture, etc., may affect the efficiency of mechanical harvesting and the rate of fruit damage. In the future, expanding research to multiple soil and climate conditions and evaluating the best practices and adjustment strategies for mechanical and physical operations can enhance the applicability and practicality of research results. By studying the mechanical and physical properties of the harvesting period for peduncle–peduncle, peduncle–pod, and peduncle–seedling vines, a theoretical basis can be provided for the design of digger inverters to reduce losses and damage in peanut harvesting, thereby improving overall agricultural productivity.

5. Conclusions

The moisture content values of the pods, peduncles, and seedling vines of the harvested variety of peanut pods are 36.03%, 66.76%, and 77.95%. The standard deviations of the moisture content test averages for peanut pods, peduncles, and seedling vines are 0.4, 0.99, and 1.17, respectively, which are all much lower than five, indicating a small degree of dispersion between the sample data.

The detachment forces between peduncles and seedling vines, peduncles, peduncles, and pods vary at different loading speeds. The detachment forces decrease as the loading speed increases. In this study, when the loading speed was 10 mm/min, the amount of force required to detach the peduncle from the seedling vines, the peduncles from each other, and the peduncle from the pods was 10.12 N, 27.94 N, and 18.03 N, respectively. At a loading speed of 20 mm/min, the peduncle–seedling vine, peduncle–peduncle, and peduncle–pod detachment forces were 8.62 N, 17.21 N, and 17.41 N. At a loading speed of 30 mm/min, the amount of detachment force required to separate the peduncles and seedling vines, the peduncles from each other, and peduncles and pods was 7.31 N, 19.93 N, and 15.2 N, respectively.

The mechanical properties of peanuts at harvesting time are different from those of peanuts after being dug up and dried. The order of the detachment force at each node of the peanuts at harvesting time is peduncle–peduncle > peduncle–pods > peduncle–seedling vines, while the order for each node of peanuts after being dug up and dried is seedling vines–peduncle > pods–peduncle. In practical situations, the detachment of peanut pods occurs at the contact point between the peduncle and the pods. Therefore, peanut-digging and turning machines should be designed based on the detachment force between the peduncle and the pods, which, along with a loading speed of 20 mm/min, is 17 N.