木薯块根拔起的最大应力数值模拟及试验

Numerical simulation test of maximum stress of tuber in cassava lifting

为了探明木薯拔起时块根最大应力的影响规律和确定块根不被拔断的允许最大拔起力,论文采用FEM(finite element method)和SPH(smoothed particle hydrodynamics)的耦合方法及二次回归旋转设计方法,通过构建土壤-块根-茎秆系统的数值模拟计算模型,进行木薯块根拔起数值模拟试验,测定各因素组合条件下的块根最大拔起力和块根最大应力,建立块根最大应力与拔起速度、块根的大小、长短和生长深度及土壤的软硬程度的多因素耦合数学模型,研究了各影响因素及交互作用对块根最大应力的影响规律,同时,通过块根最大拔起力和块根最大应力的散点图,研究了块根最大拔起力和块根最大应力的相关性,确定了块根不被拔断的允许最大拔起力,且2014年12月底在广西武鸣县某木薯种植地,采用随机抽样方法,进行了木薯块根最大拔起力和块根拔断率的田间试验和统计分析,对块根不被拔断的允许最大拔起力进行了验证,最大拔起力小于0.98 k N时,块根拔断率为2.5%。结果表明,块根最大应力与各影响因素的多因素耦合数学模型的F检验在0.000 1水平上显著,精度较高,可用于块根最大应力的影响分析;块根最大应力随拔起速度的增大呈先增大,后减小的变化,随生长深度、块根长度和土壤硬度的增大而增大,随块根直径的增大而减小;块根最大应力与最大拔起力相关性不强,块根允许最大拔起力约为0.98 k N。

In order to discover the influence law of maximum stress of cassava tuber, and obtain the allowable maximum lifting force without the breakage of cassava tuber during cassava tuber lifting, the numerical simulation model of soil,cassava tuber and cassava stem system was established by adopting explicit dynamics simulation software LS-DYNA. In the numerical simulation model, large deformations and fractures of the soils close to the tubers occurred and small deformations occurred in most parts of the rest. Thus, the coupling method of FEM（finite element method） and SPH（smoothed particle hydrodynamics） was applied in the numerical simulation model. The SPH method was used in inner soil layer where large deformations occurred and the FEM method was used in the outer soil layer where small deformations occurred. The coupling between the inner and outer soil was realized by ＂nodes-surface＂ in LS-DYNA. And MAT_FHWA_SOIL was selected as soil material model in the numerical simulation model, because it takes account of the influence of moisture content, strain softening, strain rate effect, void ratio, and pore-water pressure, and obeys the modified Mohr-Coulomb yield criterion. Numerical simulation tests of cassava tuber lifting were carried out by using the numerical simulation model and the quadratic regression rotation design method. There were 5 experiment factors and 2 experiment indices in the quadratic regression rotation design tests. The experiment factors were the lifting velocity, the length,dimension and growth depth of the tuber and the soil hardness, respectively. The experiment indices were the maximum lifting force and the maximum stress of tuber which were measured under different factor combination conditions. According to the numerical simulation tests results, the multi-factor coupling mathematic models between the maximum stress of the tuber and the lifting velocity, the length, dimension and growth depth of the tuber and the soil hardness were established by using statistical software SPSS. Based on the coupling mathematic models, the different influencing factors and their interactions on the maximum stress of the tuber were investigated, the relationship curves were drawn by using mathematics software Math CAD, and the corresponding influence laws were obtained by the relationship curves. And based on the numerical simulation tests results, the scatter diagram of the maximum lifting force and the maximum stress of tuber was plotted by using mathematics software Math CAD, and the correlation between them was studied by the scatter diagram.According to the scatter diagram, the allowable maximum lifting force was obtained under which there was no breakage of tuber. Moreover, the allowable maximum lifting force under which there was no breakage of tuber was verified by cassava tuber lifting tests, which were carried out in the cassava planting field in Wuming County, Guangxi Zhuang Autonomous Region in the end of December 2014 by adopting the method of random sampling. Meanwhile, the maximum lifting force and the breakage rate of tuber were analyzed by statistical method. When the allowable maximum lifting force was less than0.98 k N, the breakage rate of tuber was 2.5%. The result showed that the multi-factor coupling mathematic models between the maximum stress of the tuber and the lifting velocity, the dimension and growing depth of the tuber and the soil hardness had high precision, because the F test of the multi-factor coupling mathematic models was significant at 0.000 1 level. The multi-factor coupling mathematic models could be used in the effect analysis of the maximum stress of tuber. With the increasing of lifting velocity, the maximum stress of tuber increased at first and then decreased. With the increasing of growing depth, tuber length and soil hardness, the maximum stress of tuber increased. But the maximum stress of tuber decreased when tuber′ s diameter increased. There was little correlation between the maximum stress of tuber and the allowable maximum lifting force. The allowable maximum lifting force was 0.98 k N.