穴盘水稻秧苗茎秆蠕变与应力松弛特性的试验研究
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摘要
为深入了解穴盘水稻秧苗茎秆拉伸过程中的力学特性,该文利用微控电子万能试验机对穴盘水稻秧苗茎秆常规拉伸、蠕变与应力松弛特性进行测试与分析。试验表明:常规拉伸一般有2次断裂,1次断裂前,应力-应变为线性关系,没有明显的屈服过程,且平均断裂应力大小随加载速率的增加而呈线性增加;拉伸蠕变和应力松弛过程分别利用伯格斯四元件模型和麦克斯韦五元件模型进行描述。通过对曲线进行拟合,得到相关的模型参数,基于选用的流变模型和本构方程,结合秧苗茎秆的生物体特点,分别对蠕变速率、蠕变柔量和应力松弛速率、应力松弛时间进行分析,结果表明:蠕变和应力松弛过程都是弹性动力与黏滞阻力之间彼此牵制的过程;蠕变过程产生与蠕变时间和初始应力呈正相关的塑性应变,应力松弛过程导致茎秆大分子链发生变化,均对秧苗茎秆造成一定损伤;秧苗茎秆内部含有柔嫩与粗壮2种组织结构。研究结果可为秧苗机械拔取的损伤评估和相关仿真分析提供参考。
In order to understand the mechanical property of rice seedlings stem raised in cell tray in the stretching process, the general tensile, creep and stress relaxation property of rice seedlings stem were tested with JK-100 KE micro-control electronic universal testing machine(UTM) and analyzed. The test variety of rice was Huahang No.31, planting in plastic cell tray in the field in Qilin North of South China Agricultural University. After growing for 25 days in the field, the seedlings had 3 to 5 leaves, and the seedlings with straight stalks and good growth were selected for the experiment. In experiment process, two ends of seedling stem samples were fixed in upper and lower clamps of the UTM respectively, and the original distance between 2 clamps was 20 mm. The frequency of data acquisition was 1/s. In the general tensile test, the loading rates were 2, 4, 6, 8, 10 and 12 mm/min, respectively, and 20 repetitions were carried out under each loading rate. In the creep test, the loading speed was 0.1 N/s. When the stress reached about 1.5 MPa, the stress began to be maintained, the test was ended after being maintained for 1 200 s, and the tensile creep curve of the seedling stem was obtained. The creep test was repeated 20 times. In the stress relaxation test, the loading speed was 0.6 mm/min. When the strain reached about 2.5%, the strain began to be maintained. The test was ended after being maintained for 1 200 s, and the tensile stress relaxation curve of the seedling stem was obtained. The stress relaxation test was repeated 20 times. The results of general tensile test showed that the seedling stalks usually ruptured 2 times due to the structural features of the covering leaf sheath. And before the first rupture, the stress-strain curve was linear. The fracture strain was 5.2%-9.6%, the fracture stress was 3.1-7.3 MPa, and the modulus of elasticity was 0.46-0.93 MPa. And the average fracture stress increased linearly with the increase of loading rate. In this study, the tension creep and stress relaxation process were described by the Burgers 4-element model and the Maxwell 5-element model, respectively. The tensile creep and stress relaxation testing data were fitted with MATLAB curve fitting tool, and the relevant rheological parameters were obtained, and the constitutive equations of creep and stress relaxation of rice seedling stems raised in cell trays were established. The creep model fitting parameters were that: delay elastic modulus Ek was 2.893-6.115 MPa, delay viscosity coefficient ηk was 308-817 MPa·s, delay time Tk was 106-175 s, instant elastic modulus Ek0 was 0.5-1.0 MPa, viscosity coefficient ηkv was 5 329-13 750 MPa. The stress relaxation model fitting parameters were that: decay elastic modulus Em1 and Em~2 were respectively 0.066-0.144 MPa and 0.051-0.120 MPa, decay viscosity coefficients ηm1 and ηm~2 were 1.71-5.19 MPa·s and 16.29-63.89 MPa·s respectively, relaxation time Tm1 and Tm~2 were 26.03-41.95 s and 260.1-534.2 s respectively, instant elastic modulus Em0 was 0.335-0.601 MPa. Based on the selected rheological models and constitutive equations, and considering biological characteristics of the seedling stems, the parameters, including the creep rate, creep compliance, stress relaxation rate, stress relaxation time, were analyzed. The results showed that there are 2 kinds of tissue structure in seedling stem, that is, tender and strong, which could be reflected by the Maxwell bodies with 2 different stress relaxation times. And the processes of creep and stress relaxation were both the process of the transition from unbalanced structure and matter structure to the balance state in rice seedling stem, and also the process of mutual containment of elastic dynamic force and viscous resistance to achieve the equilibrium state. In the creep process, the unrecoverable plastic strain was generated, and under the same conditions, it was increased with the increase of initial stress and creep time, and the proportion of the plastic strain with the total strain increased with creep time, which had no relationship with the initial stress size. What's more, the stress relaxation process resulted in microscopic damage, including the broken or extended macromolecular chains, increased or decreased bond angle and so on. This research provide a reference for damage assessment and simulation analysis of seedlings pulled up from plastic cell trays. In the future, the relationship between the rheological properties of rice seedlings stem and the levels of stress and strain should be further studied.
In order to understand the mechanical property of rice seedlings stem raised in cell tray in the stretching process, the general tensile, creep and stress relaxation property of rice seedlings stem were tested with JK-100 KE micro-control electronic universal testing machine(UTM) and analyzed. The test variety of rice was Huahang No.31, planting in plastic cell tray in the field in Qilin North of South China Agricultural University. After growing for 25 days in the field, the seedlings had 3 to 5 leaves, and the seedlings with straight stalks and good growth were selected for the experiment. In experiment process, two ends of seedling stem samples were fixed in upper and lower clamps of the UTM respectively, and the original distance between 2 clamps was 20 mm. The frequency of data acquisition was 1/s. In the general tensile test, the loading rates were 2, 4, 6, 8, 10 and 12 mm/min, respectively, and 20 repetitions were carried out under each loading rate. In the creep test, the loading speed was 0.1 N/s. When the stress reached about 1.5 MPa, the stress began to be maintained, the test was ended after being maintained for 1 200 s, and the tensile creep curve of the seedling stem was obtained. The creep test was repeated 20 times. In the stress relaxation test, the loading speed was 0.6 mm/min. When the strain reached about 2.5%, the strain began to be maintained. The test was ended after being maintained for 1 200 s, and the tensile stress relaxation curve of the seedling stem was obtained. The stress relaxation test was repeated 20 times. The results of general tensile test showed that the seedling stalks usually ruptured 2 times due to the structural features of the covering leaf sheath. And before the first rupture, the stress-strain curve was linear. The fracture strain was 5.2%-9.6%, the fracture stress was 3.1-7.3 MPa, and the modulus of elasticity was 0.46-0.93 MPa. And the average fracture stress increased linearly with the increase of loading rate. In this study, the tension creep and stress relaxation process were described by the Burgers 4-element model and the Maxwell 5-element model, respectively. The tensile creep and stress relaxation testing data were fitted with MATLAB curve fitting tool, and the relevant rheological parameters were obtained, and the constitutive equations of creep and stress relaxation of rice seedling stems raised in cell trays were established. The creep model fitting parameters were that: delay elastic modulus Ek was 2.893-6.115 MPa, delay viscosity coefficient ηk was 308-817 MPa·s, delay time Tk was 106-175 s, instant elastic modulus Ek0 was 0.5-1.0 MPa, viscosity coefficient ηkv was 5 329-13 750 MPa. The stress relaxation model fitting parameters were that: decay elastic modulus Em1 and Em~2 were respectively 0.066-0.144 MPa and 0.051-0.120 MPa, decay viscosity coefficients ηm1 and ηm~2 were 1.71-5.19 MPa·s and 16.29-63.89 MPa·s respectively, relaxation time Tm1 and Tm~2 were 26.03-41.95 s and 260.1-534.2 s respectively, instant elastic modulus Em0 was 0.335-0.601 MPa. Based on the selected rheological models and constitutive equations, and considering biological characteristics of the seedling stems, the parameters, including the creep rate, creep compliance, stress relaxation rate, stress relaxation time, were analyzed. The results showed that there are 2 kinds of tissue structure in seedling stem, that is, tender and strong, which could be reflected by the Maxwell bodies with 2 different stress relaxation times. And the processes of creep and stress relaxation were both the process of the transition from unbalanced structure and matter structure to the balance state in rice seedling stem, and also the process of mutual containment of elastic dynamic force and viscous resistance to achieve the equilibrium state. In the creep process, the unrecoverable plastic strain was generated, and under the same conditions, it was increased with the increase of initial stress and creep time, and the proportion of the plastic strain with the total strain increased with creep time, which had no relationship with the initial stress size. What's more, the stress relaxation process resulted in microscopic damage, including the broken or extended macromolecular chains, increased or decreased bond angle and so on. This research provide a reference for damage assessment and simulation analysis of seedlings pulled up from plastic cell trays. In the future, the relationship between the rheological properties of rice seedlings stem and the levels of stress and strain should be further studied.
引文
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[7]王玉兴,罗锡文,唐艳芹,等.气力有序抛秧机输秧机构动态模拟研究[J].农业工程学报,2004,20(2):109-112.Wang Yuxing,Luo Xiwen,Tang Yanqin,et al.Dynamic simulation of the rice-seedling feeding mechanism of paddy seedling pneumatic throwing transplanter[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE),2004,20(2):109-112.(in Chinese with English abstract)
[8]原新斌,张国凤,陈建能,等.顶出式水稻钵苗有序移栽机的研究[J].浙江理工大学学报,2011,25(5):749-752.Yuan Xinbin,Zhang Guofeng,Chen Jianneng,et al.Development on rice plotted-seeding sequential transplanter of ejection type[J].Journal of Zhejiang Sci-Tech University,2011,25(5):749-752.(in Chinese with English abstract)
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