Study on the Fracture Characteristics and Mechanism of Used Drip Irrigation Tape Under Different Stretching Speeds

Abstract

The crushing of used drip irrigation tape is a crucial step in the recycling and reuse of drip irrigation tapes. Incomplete crushing and low efficiency are among the main factors restricting its reprocessing. Investigating the fracture characteristics and the mechanism of fracture during the crushing process is key to solving this problem. Therefore, this study constructs a stretching fracture platform to investigate the influence of stretching speed on the fracture characteristics and reveals the fracture mechanism by analyzing fracture morphology, force-displacement curves, fracture energy, and microstructure. The results show that as the speed increases, the limit strain decreased from 117.7% to 38.7%, and the fracture location always occurs at the junction between the necked and non-necked area, the fracture mode transitions from ductile fracture to brittle fracture, the deformation mode shifts from being dominated by elastoplastic deformation to being dominated by elastic deformation, and the mechanical response curve changes from five stages to three stages. When the stretching speed increases from 60 mm/s to 70 mm/s, a jump phenomenon is observed in macroscopic and microscopic. As the speed increases, the total energy absorbed by the drip irrigation tape decreases from 1.29 × 10⁻² J/mm³ to 0.39 × 10⁻² J/mm³. Brittle fracture primarily absorbs energy for the disintegration and fracture of lamellae in the spherulites at the fracture surface. Ductile fracture primarily absorbs energy for the extension of the fibrous structure, and the mechanical properties of the necked area are strengthened, which leads to the fracture location always occurring at the junction between the necked and non-necked area.

1. Introduction

Approximately 33,300 hectares of farmland in Xinjiang utilize drip irrigation technology, requiring about 40,000 tons of drip irrigation tape annually. The extensive application of drip irrigation technology continually increases the demand for drip irrigation tape [1,2]. The recycling and remanufacturing of drip irrigation tape benefits farmers’ income while reducing environmental pollution caused by residual drip irrigation tape in the soil. The steps involved in the remanufacturing of used drip irrigation tape include crushing, cleaning, and the preparation of regenerated materials [3]. The crushed used drip irrigation tape must meet the requirement of being easy to clean, the crushing effect directly impacts the quality of remanufactured drip irrigation tape. Currently, the main methods for crushing drip irrigation tape include double-roller tearing, rotary cutting, and shearing [4]. Rotary cutting is prone to tool blockage due to entanglement, and shearing may cause over-crushing, resulting in environmental issues. The double-roller tearing type crushing method uses differential speed pulling and toothed blade (Annotation 1) cutting to crush the drip irrigation tape (Annotation 2), as shown in Figure 1. This method crushes the drip irrigation tape into uniform sizes with high efficiency. However, during the crushing process, stretch forces cause plastic deformation of the drip irrigation tape, resulting in necking. This makes the width of the drip irrigation tape narrower, leading to missed shearing cutting, which reduces the crushing effect of the tearing shear crusher. Moreover, necking causes sand and soil to embed in the fiber layer, complicating the subsequent cleaning process. If the cleaning is not thorough, some soil impurities will remain and enter the remanufacturing process, affecting the mechanical characteristics of the remanufactured drip irrigation tape. In fact, necking is primarily caused by the difference in speeds of the double rollers. Therefore, revealing the mechanical characteristics, plastic necking, and microscopic deformation mechanisms of breakage of used drip irrigation tape under different stretching speeds is crucial for improving the crushing efficiency of the tearing load and the quality of remanufactured drip irrigation tape.

The main component of the drip irrigation tape is polyethylene [2]. It is a flexible hose with a flattened circular cross-section, possessing good flexibility and the ability to undergo elastic deformation. Under continuous stress, it will undergo irreversible plastic deformation. Polyethylene is a common polymer material known for its light weight, low cost, excellent electrical and thermal insulation properties, and resistance to chemical corrosion [5,6]. It is widely used in vital national infrastructure projects such as water supply, gas distribution, and oil and gas transportation [7]. Therefore, scholars have conducted extensive research on the mechanical response of polyethylene materials under external loading. Reis [8,9,10,11] and others studied the mechanical characteristics of polyethylene materials during the stretching process. Their research shows that with increasing speed, significant differences are observed in the macroscopic fracture modes and mechanical response indicators. Tang [12,13] used molecular dynamics simulations to elucidate the view that stretching induces orientation order in the microstructure. Parsons [14] and others analyzed the formation and propagation of crack damage zones in polyethylene, they prepared high-density polyethylene and more fracture-resistant ethylene copolymer bulk samples by pressing and compared the differences in damage zones, the results showed that the improved fracture resistance is related to the deformation of the microstructure under shear forces. Pritchard [15] and others prepared polyethylene samples with varying numbers and lengths of short-chain branches and confirmed that the fracture characteristics of polyethylene are significantly related to molecular weight. Gosch [16,17,18,19,20,21,22] and others studied the fracture mechanisms of polyethylene under tensile and compressive forces, revealing that the microscopic damage of polyethylene is driven by both internal crystal transformation and molecular chain movement. Nasiri [23,24,25,26,27,28,29,30,31,32] and others studied the damage of polyethylene material pipelines over time, they proposed three failure modes for polyethylene pipes: ductile failure, quasi-brittle failure, and brittle failure. In conclusion, scholars have studied the fracture characteristics and mechanisms of polyethylene materials from different perspectives, which has helped in understanding the damage of polyethylene materials. However, it is evident that most of these studies focus on macro and micro analysis of polyethylene materials using standard samples. Drip irrigation tape, as a polyethylene thin-layer material formed by hot-melt molding, exhibits significant differences in mechanical properties compared to standard bulk polyethylene samples. Additionally, its mechanical properties and microstructure change after one usage cycle [33,34,35]. As shown in Figure 2a,b, electron microscope scanning images of new and used drip irrigation tape are presented, significant microstructural differences can be observed between the new and used tapes, with the new tape being smoother and the used tape displaying a rough texture. Furthermore, this study found that during the stretching process of used drip irrigation tape, the fracture always occurs at the junction between the necked and non-necked area. Clearly, the above studies fail to reveal the breakage mechanism of drip irrigation tape. Therefore, to improve the crushing effect and efficiency of the crusher under tearing load, it is crucial to investigate the fracture characteristics and the underlying mechanism of used drip irrigation tape under different stretching speeds.

In this study, a stretching testing platform was built to perform stretching experiment on used drip irrigation tape at different speeds. By statistically analyzing its fracture mechanical characteristics, the effect of stretching speed on fracture characteristics was revealed, including fracture morphology, force-displacement curves, and fracture energy, the microstructure of the used drip irrigation tape in the fracture extension area and the area adjacent to the fracture surface was analyzed after stretching, revealing the mechanical response behavior and the corresponding fracture mechanism. The research results can provide theoretical support for the optimization design of the crusher parameters under tearing load, and also provides a reference for understanding the fracture mechanisms of thin-layer polyethylene materials under external loads.

2. Materials and Methods

2.1. Drip Irrigation Tape Samples

The experiment material selected was the single-wing labyrinth-type drip irrigation tape conforming to the GB/T 19812.1-2017 standard [36]. The collection site was located in the cotton field of the 12th Regiment of Alaer City, Xinjiang Uygur Autonomous Region (81°18′31″ E, 40°29′21″ N), and one production cycle was used (six months). Considering the influence of soil conditions and terrain variations on the mechanical performance of drip irrigation tape, to ensure the representativeness of the samples, the nine-square grid sampling method (3 × 3 grid, total size 1200 cm × 1200 cm) was used to select drip irrigation tape samples. Choosing tapes that were properly installed and undamaged. After cleaning and air-drying, the tapes were used as experiment materials for used drip irrigation tape, with a length of 1200 mm. As shown in Figure 3, the drip irrigation tape stretching sample has clamping areas at both ends, each with a length of 200 mm, and the stretching area has a length of 800 mm.

2.2. Experiment Apparatus

To test the mechanical and fracture characteristics at different stretching speeds, this study constructed an experiment platform (stretching load accuracy ±0.05 mm/s, data acquisition frequency 50 Hz) for uniaxial stretching experiment on used drip irrigation tape. The experimental apparatus is shown in Figure 4, where label 1 represents the information processing system (Dell G15), label 2 represents the guide roller, label 3 represents the speed-adjustable motor (6IK200RGN-CF, with a reduction ratio of 7.5), label 4 represents the winding roller (diameter 55 mm), label 5 represents the digital sampling controller, label 6 represents the used drip irrigation tape, label 7 represents the clamping fixture, label 8 represents the S-type sensor (Dongguan Lupu E-commerce Co., Ltd., Dongguan, China, HP-500N/external, with an accuracy of 0.5%), and label 9 represents the speed controller. One end of the drip irrigation tape is fixed on the clamping fixture, which consists of two bolts, and the other end is fixed to the winding roller connected to the output shaft of the speed-adjustable motor. A guide roller is placed between the winding roller and the clamping fixture, and the motor speed can be adjusted under the action of the speed controller. The S-type sensor is installed beneath the clamping fixture, and is connected to the digital sampling controller, which displays the tensile force in real-time, the collected data is simultaneously transmitted to the software (S-type sensor companion software) interface of the information processing system. The drip irrigation tape is aligned with the S-type sensor to ensure the accuracy of the measurement results.

2.3. Stretch Test

To perform stretching test on drip irrigation tape at different speeds, speeds ranging from 20 to 90 mm/s were selected, this range covers the common crushing differences of roller crushers [37], with a gradient interval of 10 mm/s. After adjusting the motor speed using the speed controller, one end of the used drip irrigation tape was wound on the winding roller (winding length 200 mm), and the other end was fixed in the clamping mechanism. To prevent slippage in the clamping area, the clamping area was folded to increase the thickness, with a total folding length of 200 mm. The digital sampling controller was turned on, and the motor was started. The motor applied a uniform stretching motion to the used drip irrigation tape until it broke. To minimize errors, each stretching speed was repeated 20 times, and the average value was used as the experimental result.

The used drip irrigation tape has the ability to return to its original length after unloading the force. The percentage elongation after fracture is calculated using Equation (1), as illustrated in Figure 5.

$${\displaystyle \mathrm{\phi }}={(\mathrm{L}}_1/({\mathrm{L}}_0-({\mathrm{L}}_2-{\mathrm{L}}_1)\left)\right)\times 100\%$$

(1)

In the equation, $\mathrm{\phi }$ is the percentage elongation after fracture, $\%$, ${\mathrm{L}}_0$ is the original length of the sample, mm, ${\mathrm{L}}_1$ is the necking length, mm, and ${\mathrm{L}}_2$ is the length after unloading the force, mm.

Before the stretching fracture occurs, the length of the used drip irrigation tape is gradually elongated. The limit strain at fracture is calculated using Equation (2).

$$\mathsf{\epsilon }=\left(\right(\mathrm{L}-{\mathrm{L}}_0)/{\mathrm{L}}_0)\times 100\%$$

(2)

In the equation, $\mathsf{\epsilon }$ is the limit strain (engineering strain), $\%$, and $\mathrm{L}$ is the limit length of the sample before fracture, mm.

The engineering stress is calculated using Equation (3).

$$\mathsf{\sigma }=\mathrm{F}/\mathrm{A}$$

(3)

In the equation, $\mathsf{\sigma }$ is the engineering stress, MPa, $\mathrm{F}$ is the tensile force, N, and $\mathrm{A}$ is the cross-sectional area, mm².

The yield strength refers to the critical stress value at the transition from elastic deformation to plastic deformation [38], while the tensile strength is the maximum stress that the used drip irrigation tape can withstand before fracture.

Fracture energy density represents the energy absorbed per unit volume during the fracture process of the used drip irrigation tape, equal to the area enclosed by the true stress–strain curve [39]. Elastic energy density represents the energy absorbed per unit volume during elastic deformation before the yield strength, while yield energy density represents the energy absorbed during both elastic and plastic deformation after the yield strength, the proportion of elastic energy density is the ratio of elastic energy density to fracture energy density [40,41], as shown in Figure 6. Equations (4) and (5) convert engineering stress–strain to true stress–strain [42]. The elastic energy density, yield energy density, and elastic energy density proportion were calculated using Equations (6)–(8).

$${\mathsf{\sigma }}_{\mathrm{t}}=\mathsf{\sigma }(1+\mathsf{\epsilon })$$

(4)

$${\mathsf{\epsilon }}_{\mathrm{t}}=\mathrm{l}\mathrm{n}(1+\mathsf{\epsilon })$$

(5)

In the equation, ${\mathsf{\sigma }}_{\mathrm{t}}$ is the true stress, MPa, and ${\mathsf{\epsilon }}_{\mathrm{t}}$ is the true strain.

$${\mathrm{U}}_{\mathrm{e}\mathrm{e}\mathrm{d}}=0.5{\mathsf{\sigma }}_{\mathrm{t}1}{\mathsf{\epsilon }}_{\mathrm{t}1}$$

(6)

$${\mathrm{U}}_{\mathrm{p}\mathrm{e}\mathrm{d}}={\mathrm{U}}_{\mathrm{f}\mathrm{e}\mathrm{d}}-{\mathrm{U}}_{\mathrm{e}\mathrm{e}\mathrm{d}}$$

(7)

$$\mathsf{\delta }={\mathrm{U}}_{\mathrm{e}\mathrm{e}\mathrm{d}}/{\mathrm{U}}_{\mathrm{f}\mathrm{e}\mathrm{d}}$$

(8)

In the equation, ${\mathrm{U}}_{\mathrm{f}\mathrm{e}\mathrm{d}}$ is the fracture energy density, J/mm³, ${\mathrm{U}}_{\mathrm{e}\mathrm{e}\mathrm{d}}$ is the elastic energy density, J/mm³, ${\mathrm{U}}_{\mathrm{p}\mathrm{e}\mathrm{d}}$ is the yield energy density, J/mm³, ${\mathsf{\sigma }}_{\mathrm{t}1}$ is the yield strength in the true stress–strain curve, MPa, and ${\mathsf{\epsilon }}_{\mathrm{t}1}$ is the true strain corresponding to the yield strength in the true stress–strain curve, $\mathsf{\delta }$ represents the proportion of elastic energy density, $\%$.

2.4. Scanning Electron Microscope Test of Used Drip Irrigation Tape

To investigate the changes of the used drip irrigation tape before and after fracture, scanning electron microscope (Apero S) was used to observe its surface microstructure. Samples were taken from the brittle fracture extension area, the area adjacent to the brittle fracture surface, the ductile fracture extension area and the area adjacent to the ductile fracture surface, with sample sizes of 8 mm × 8 mm, as shown in Figure 7. In the sampling locations, area 1 corresponds to the fracture extension area, while area 2 corresponds to the area adjacent to the fracture surface. To avoid contamination from dust, the selected samples were washed with distilled water and dried at room temperature in a dust-free area. The experimental procedure is shown in Figure 7, the sample was placed on the sample holder and then gold-coated in a vacuum environment, SEM micro-scanning was performed with scanning parameters as shown in

Table 1. Scanning electron microscope parameters.

USE CASE	High Voltage (HV)	Current (curr)	Detector (det)	Working Distance (WD)	Magnification (mag)	High-Wide Field (HWF)	Scale Bar
Standard	5.00 kV	50 pA	ETD (Secondary electron detection)	9.9 mm	10,000×	12.7 μm	5 μm

. The microstructure was quantitatively analyzed based on image segmentation methods.

2.5. Stretch Test of Necked and Non-Necked Used Drip Irrigation Tape

To demonstrate the mechanical property differences between the necked and non-necked area of the used drip irrigation tape, a universal testing machine (Shanghai Zhuoji Instruments Co., Ltd., Shanghai, China, WD-D3) was used for tensile testing of both areas, as shown in Figure 8. (Noted that the universal testing machine was selected because the sample length of the necked area was short, making it difficult to fix). The total length of the used drip irrigation tape tensile sample is 250 mm, with a clamping length of 25 mm and a stretching area length of 200 mm. Each sample was subjected to a tensile test at a speed of 8 mm/s until the used drip irrigation tape broke. This experiment was repeated 20 times for each sample, and the average value was used as the experimental result.

2.6. Data Processing

Statistical analysis was performed using SPSS Statistics 26 software. Inter-group differences were compared using analysis of variance (ANOVA) and t-tests, with p < 0.05 considered statistically significant.

3. Results and Discussion

3.1. Fracture Characteristics Analysis

3.1.1. Effect of Stretching Speed on Fracture Characteristics

As shown in Figure 9, the macroscopic fracture morphology of the used drip irrigation tape under different stretching speeds is presented. From the comparison in the figure, it can be observed that with an increase in speed, the necking length of the used drip irrigation tape during the stretching process gradually decreases, and the fracture location is at the junction of the necked and non-necked area. To verify the universality of the above phenomenon, statistical analysis was conducted on the percentage elongation after fracture and necking length of 20 sets of parallel samples, as shown in Figure 10. In the figure, it can be observed that with the increase in stretching speed, the percentage elongation after fracture and necking length gradually decrease. A jump occurs at a stretching speed of 60–70 mm/s, where the percentage elongation after fracture drops from 288% to 160%. According to Luo [43], damage with significant yield deformation after fracture is defined as ductile fracture, while damage with no obvious deformation is defined as brittle fracture. As shown in Figure 9, it can also be clearly seen that within the stretching speed range of 20–60 mm/s, the used drip irrigation tape exhibits significant yield deformation after fracture, the range indicated by the red scale in the figure corresponds to the irreversible deformation caused by yield. However, after fracture at 70–90 mm/s, the tape is relatively intact, with no noticeable irreversible deformation. Therefore, the used drip irrigation tape undergoes ductile fracture at 20–60 mm/s and brittle fracture at 70–90 mm/s. Additionally, from the statistical data in Figure 10, it can be observed that when the stretching speed exceeds 70 mm/s, the necking length is less than 16 mm, and the variation with increasing speed becomes smaller. However, brittle fracture does not imply that no deformation occurred during the fracture process of the drip irrigation tape.

As shown in Figure 11, the limit strain of the used drip irrigation tape under different stretching speeds is presented. Overall, the trend of its variation is consistent with the trend of the percentage elongation after fracture, both showing a gradual decrease, with a jump phenomenon occurring between 60–70 mm/s. However, a comprehensive review of Figure 9, Figure 10 and Figure 11 indicates that the deformation occurring at stretching speeds of 70–90 mm/s almost fully recovers after unloading the stretching load. Therefore, it can be concluded that with the increase in stretching speed, the deformation of the used drip irrigation tape transitions from being dominated by elastoplastic deformation to being dominated by elastic deformation. For the crushing of used drip irrigation tape, plastic deformation is clearly detrimental to its crushability. For crusher performance, plastic deformation will reduce the crushing effect of the tearing-type crusher. To further quantify the response of the used drip irrigation tape under different stretching speeds, the deformation behavior of the tape will be further clarified through its mechanical response.

3.1.2. Effect of Stretching Speed on Mechanical Response

Figure 12 and Figure 13 present the mechanical response of the used drip irrigation tape under different stretching speeds for ductile fracture and brittle fracture, and upper-case letters represent the turning points of each stage. Overall, with the increase in speed, the limit strain of the used drip irrigation tape decreases. Moreover, a comparison of the force variation under different stretching speeds reveals that at stretching speeds of 20–60 mm/s, the force initially increases, then it gradually levels off and begins to decline, followed by a slight decline, remaining stable before slightly increasing again until fracture failure occurs (Figure 12). At stretching speeds of 70–90 mm/s, compared to 20–60 mm/s, the force only shows an initial increase, followed by a steady decrease and then a slight decline until fracture failure (Figure 13). This phenomenon indicates that as the speed increases, there are differences in the mechanical response. As shown in Figure 12, at stretching speeds of 20–60 mm/s, ductile fracture can be divided into five stages: the elastic stage, yield stage, strain softening stage, cold drawing stage, and strain hardening stage [44,45]. As shown in Figure 13, at stretching speeds of 70–90 mm/s, brittle fracture is divided into three stages: the elastic stage, yield stage, and strain softening stage.

The first stage of ductile fracture is the elastic stage, where the thickness and cross-sectional area of the used drip irrigation tape remain nearly unchanged (Figure 12a, section OA, AB). The elastic stage consists of the linear elastic stage (Figure 12b, section OA) and the nonlinear hysteretic elastic stage (Figure 12b, section AB), where the stretching displacement is small but the force increases sharply. In the linear elastic stage, the force-displacement curve exhibits a linear increasing relationship, which follows Hooke’s Law. In the nonlinear hysteretic elastic stage, it deviates from the linear relationship. The second stage is the yield stage, during which uniform plastic deformation occurs over the entire used drip irrigation tape sample (Figure 12a, section BC), and the force-displacement curve shows a slight and steady decrease (Figure 12b, section BC). The third stage is the strain softening stage, where the used drip irrigation tape transitions from uniform plastic deformation to localized plastic deformation. The thickness and cross-sectional area of the tape decrease significantly, indicating the occurrence of neck formation (Figure 12a, section CD). The force-displacement curve shows a decrease, with the tensile force decreasing as the deformation increases (Figure 12b, section CD). The fourth stage is the cold drawing stage, during which the necking length of the used drip irrigation tape increases (Figure 12a, section DE, EF), and the tensile force stabilizes at the yield platform tensile force D point (Figure 12b, section DE). The fifth stage is the strain hardening stage, characterized by slight increases in force and displacement (magnified region of Figure 12b, section EF). After this stage, the macroscopic morphology of ductile fracture is observed (Figure 12a, section FG).

The first stage of brittle fracture is the elastic stage, where the thickness and cross-sectional area of the used drip irrigation tape remain nearly unchanged (Figure 13a, section OA, AB). It similarly includes the linear elastic stage (Figure 13b, section OA) and the nonlinear hysteretic elastic stage (Figure 13b, section AB). The second stage is the yield stage, during which the plastic deformation of the used drip irrigation tape is uniform (Figure 13a, section BC), and the tensile force decreases slightly and steadily (Figure 13b, section BC). The third stage is the strain softening stage, during which the cross-sectional area at the location of concentrated tensile force decreases, localized plastic deformation begins, and a slight yield phenomenon occurs (Figure 13a, section CD ①). Cavities appear at the fracture location, indicating that brittle fracture is about to occur, and no further expansion of the yield phenomenon takes place (Figure 13a, section CD ②, red dashed box area). The force-displacement curve shows a slight downward trend (magnified region of Figure 13b, section CD). As deformation increases and external force continues to be applied, the macroscopic morphology of brittle fracture is observed (Figure 13a, section DE).

Based on the above results, it is shown that as the stretching speed increases, the mechanical response of the used drip irrigation tape transitions from five stages to three stages, with deformation of the drip irrigation tape shifting from being predominantly elastoplastic to predominantly elastic. Combined with the macroscopic post-fracture states in Figure 9, it is preliminarily speculated that as the stretching speed increases, the time available for the drip irrigation tape to absorb energy from the external tensile load gradually decreases, leading to a decrease in the energy absorbed by the drip irrigation tape. Furthermore, based on the final stage changes in the mechanical responses of ductile fracture and brittle fracture in Figure 12 and Figure 13, it is preliminarily speculated that the mechanical properties of the drip irrigation tape are strengthened after necking, causing the fracture location to consistently occur at the junction between the necked and non-necked area. Therefore, to further reveal the fracture mechanism and verify the above hypothesis, the following analysis will focus on the fracture energy consumption of the drip irrigation tape under different stretching speeds and the microstructure of different areas of the tape after tensile fracture.

3.2. Analysis of the Fracture Mechanism

3.2.1. Effect of Stretching Speed on Energy Density

To obtain the energy consumed during fracture of the drip irrigation tape at different stretching speeds, the fracture energy density, yield energy density, and elastic energy density ratios for 20 test groups were statistically analyzed, as shown in Figure 14 and Figure 15. As shown in the figures, with an increase in stretching speed, both the fracture energy density and yield energy density decrease, while the proportion of elastic energy density gradually increases. This phenomenon is consistent with the hypothesis, proving that as the speed increases, the energy absorbed by the drip irrigation tape decreases, and the deformation mode shifts from being elastoplastic to being predominantly elastic. This results in the drip irrigation tape undergoing primarily elastic deformation during high-speed tensile fracture, and the tape quickly regains its original length due to the disappearance of external loads after fracture, forming a macroscopic brittle fracture phenomenon, leading to a reduction in necking length and a decrease in limit strain.

From a material perspective, during the stretching process, crack initiation and crack propagation consume fracture energy, with most of the fracture energy primarily used for crack initiation [46]. As shown in Figure 14, when the stretching speed is low, the fracture energy density is higher, and energy is consumed in forming high-density cracks, the fracture energy density used for crack propagation is relatively lower, hindering crack growth, which induces the necking phenomenon, manifested as ductile fracture. At higher speeds, the fracture energy density is lower and accompanied by rapid crack propagation. Compared to ductile fracture, the fracture resistance of the drip irrigation tape decreases, leading to brittle fracture.

Based on the above analysis, it has been proven that as the stretching speed increases, the time for the drip irrigation tape to absorb the energy from the external load gradually decreases, leading to a reduction in the energy absorbed by the drip irrigation tape. At the same time, based on the fracture energy density, yield energy density, and elastic energy density ratios, the mechanisms of ductile fracture and brittle fracture are revealed. Moreover, based on the mechanical responses in Figure 12, it was observed that the tensile force increased before the ductile fracture occurred. Therefore, it is speculated that this phenomenon is due to necking altering the microstructure of the drip irrigation tape, resulting in the strengthening of the mechanical properties in the necked area, causing the fracture to consistently occur at the junction between the necked and non-necked area.

3.2.2. Microstructural Analysis of Used Drip Irrigation Tape

To validate the hypothesis that the mechanical properties at the necked area are strengthened, and to further elucidate the fracture mechanism of the drip irrigation tape from a microstructural perspective, scanning electron microscope (SEM) observations were performed on different locations of the tape after tensile fracture. As shown in Figure 16a–d, these correspond to the brittle fracture extension area, the area adjacent to the brittle fracture surface, the ductile fracture extension area (necked area), and the area adjacent to the ductile fracture surface. Polyethylene is a semi-crystalline solid, where the smallest crystalline unit is the lamellae. The stacking of these layered lamellae can form larger spherulites, while the amorphous areas are mainly composed of loosely arranged disordered areas [47,48]. Compared with the new drip irrigation tape (Figure 2a), the used drip irrigation tape exhibits surface undulations due to gradual weathering and collapse of certain microstructures during use (Figure 2b). Compared to the unstretched used drip irrigation tape (Figure 2b), all areas of the used drip irrigation tape after tensile fracture exhibit varying degrees of structural changes. Firstly, compared to the unstretched used drip irrigation tape (Figure 2b), the brittle fracture extension area of the stretched and fractured tape shows the presence of a fibrous structure (Figure 16a, red solid box), most areas still retain the original morphology that has not undergone stretching (Figure 16a, red dashed box). In the area adjacent to the brittle fracture surface, a relatively higher concentration of lamellae is present (Figure 16b, red solid circle), along with an increased occurrence of fibrous structures aligned parallel to the stretching direction (Figure 16b, red solid box). In the ductile fracture extension area, a large number of elongated fibrous structures are observed, with the fibers being significantly longer and overall exhibiting a densely packed state (Figure 16c, red solid box). The area adjacent to the ductile fracture surface also exhibits fibrous structures, but these structures are less prominently aligned in the stretching direction compared to those in the area adjacent to the brittle fracture surface (Figure 16d, red solid box), and more features remain the original morphology that has not undergone stretching (Figure 16d, red dashed box).

A comparison of the microstructures between the ductile fracture extension area (Figure 16c) and the brittle fracture extension area (Figure 16a) reveals that the fibrous structures in ductile fracture are finer, more elongated, and show stronger orientation, corresponding macroscopically to the expansion of the neck and resulting in high limit strain. In contrast, the fibrous structures in brittle fracture are shorter than those in ductile fracture, indicating significantly lower plastic deformation at the microstructural level, which manifests macroscopically as a lower limit strain.

The energy density map in Figure 14 indicates that the energy absorbed during brittle fracture is relatively low. Combined with the microstructure in Figure 16a,b, the brittle fracture extension area shows the presence of fibrous structures, with some areas maintaining their original state, but in the area adjacent to the brittle fracture surface, both the lamellae and fibrous stretching structures significantly increase compared to the extension area (as shown in Figure 17, the fibre-like structure proportion in the brittle fracture extension area is 41%, the area adjacent to the brittle fracture surface is 85%). This is due to the relatively fast stretching speed, which results in a shorter energy transfer time, most of the energy absorbed by the drip irrigation tape is consumed during the disintegration and fracture of lamellae in the spherulites at the fracture surface, while a small portion of the energy is absorbed by the extension area, leading to some deformation.

By comparing the microstructures of the ductile fracture extension area (Figure 16c) and the area adjacent to the ductile fracture surface (Figure 16d), it can be observed that the fibrosing degree in the ductile fracture extension area is greater than that in the area adjacent to the ductile fracture surface (as shown in Figure 17, the fibre-like structure proportion in the ductile fracture extension area is 97%, the area adjacent to the ductile fracture surface is 67%). Combined with the energy density map in Figure 14, it can be concluded that, during low-speed stretching, most of the energy absorbed by the drip irrigation tape is used for the extension of fibrous structures, consuming energy to produce macroscopic necking plastic deformation. As a result, the area adjacent to the ductile fracture surface maintains a relatively well-preserved original morphology, while the extension area exhibits a densely packed and highly fibrous state. The mechanical response curve in Figure 12 shows an increase in tensile force before fracture, additionally, research by Lv [44] indicates that highly oriented fibrosing strengthens the mechanical properties of polyethylene materials. Therefore, it is preliminarily concluded that the fracture location consistently lies at the junction between the necked and non-necked area, due to the enhanced mechanical properties in the necking area.

Comprehensive analysis of scanning electron microscope microstructures, mechanical response patterns, and fracture energy density has proven that brittle fracture absorbs less energy, primarily involving elastic deformation, the microstructure in the area adjacent to the brittle fracture surface shows fibrosing and lamellae, and the mechanical response at the final stage shows a decrease. Ductile fracture absorbs more energy, primarily involving elastoplastic deformation, the microstructure of the ductile fracture extension area exhibits a densely packed and highly fibrous state, and the mechanical response at the final stage shows an increase. Based on the concept that highly fibrosed structures enhance the mechanical properties of polyethylene materials, it is speculated that the mechanical properties of the drip irrigation tape are improved in the necking area, to confirm that the fracture location consistently lies at the junction between the necked and non-necked area due to this phenomenon, we compared the mechanical response curves during the stretching process of the necked and non-necked area.

3.2.3. Analysis of the Mechanical Characteristics of the Necked and Non-Necked Drip Irrigation Tapes

As shown in Figure 18, the mechanical response curves during the stretching process of the necked and non-necked drip irrigation tapes are displayed. It can be observed that the curve for the necked area of the drip irrigation tape initially shows a rapid increase, and it largely maintains elastic deformation before fracture, the fracture force is significantly higher than the maximum response force in the non-necked area. A t-test was conducted on the fracture force of the neck and non-neck areas, and the t-test results are shown in

Table 2. The results of the t-test for the fracture force of the necked and non-necked areas of the drip irrigation tape.

	F	Significance	t	Degrees of Freedom	Significance (Two-Tailed)	Mean Difference	Standard Error Difference
Assuming equal variances	306.52	<0.001	−3.37	586	<0.001	−10.34	3.06
Not assuming equal variances			−1.77	85.17	0.081	−10.34	5.85

, indicating a significant difference (p < 0.05). These results further demonstrate that the mechanical properties of the necked area of the used drip irrigation tape are strengthened, leading to the fracture location consistently occurring at the junction between the necked and non-necked area.

3.3. Prospect

The above research has clarified the fracture characteristics and mechanism of the drip irrigation tape under different stretching speeds. As the stretching speed increases, significant differences are observed in the macroscopic fracture mode, mechanical characteristics, fracture energy density, and microstructure. A jump phenomenon occurs when the stretching speed is within the range of 60–70 mm/s. The current issue with the crusher is that the spacing of the crushing teeth is not optimal. Therefore, to improve the operational efficiency of the crusher and reduce the soil impurities in the recycled drip irrigation tape, the necking phenomenon during the crushing process can be minimized by increasing the differential speed of the rollers. Based on this, with the roller speed differential set at 70 mm/s as the central variable, and considering the arrangement of the toothed blade (Annotation 1 in Figure 1) and the circular washers (Annotation 3 in Figure 1), as well as the feeding amount as the other two test factors, a parameter optimization experiment was conducted to achieve high crushing efficiency and effectiveness, and to reduce the difficulty of subsequent cleaning operations.

4. Conclusions

This study constructs a stretching test platform to perform a stretching experiment on used drip irrigation tape at different speeds. By analyzing the mechanical characteristics and microstructure, the fracture characteristics and the mechanism of fracture formation are revealed. The main conclusions are as follows:

• As the stretching speed increases, the limit strain decreases from 117.7% to 38.7%, the fracture location consistently occurs at the junction between the necked area and the non-necked area, the fracture mode transitions from ductile fracture to brittle fracture, and the deformation mode changes from predominantly elastoplastic deformation to predominantly elastic deformation. When the stretching speed increases from 60 mm/s to 70 mm/s, there is a noticeable jump in the macroscopic fracture mode, mechanical characteristics, fracture energy density, and microstructural features.
• As the stretching speed increases, the mechanical response curve transitions from the elastic stage, yield stage, strain softening stage, cold drawing stage, and strain hardening stage to the elastic stage, yield stage, and strain softening stage.
• As the stretching speed increases, both the yield energy density and fracture energy density show a decreasing trend, while the proportion of elastic energy density increases. The fracture energy density decreased from 1.29 × 10⁻² J/mm³ to 0.39 × 10⁻² J/mm³, the yield energy density decreased from 1.02 × 10⁻² J/mm³ to 0.23 × 10⁻² J/mm³, and the proportion of elastic energy density increased from 21.03% to 40.95%. As the speed increases, the energy absorbed by the used drip irrigation tape decreases, and its ability to recover its original length after unloading increases. Due to the energy absorbed during brittle fracture, which is consumed in the disintegration and fracture of lamellae in the spherulites at the fracture surface, the microstructure of brittle fracture shows a significant increase in lamellae and fibrous stretching structures in the area adjacent to the brittle fracture surface, compared to the brittle fracture extension area. Energy absorption during ductile fracture occurs in the extension of the fibrous structure. The ductile fracture microstructure in the extension area exhibits a densely packed and highly fibrous state, which leads to the enhancement of the mechanical properties in the necked area. The stretching test further demonstrates that the mechanical properties of necked used drip irrigation tape are improved compared to those of non-necked used drip irrigation tape, which results in the fracture location consistently occurring at the junction between the necked and non-necked area.