基于应力路径试验的小麦粮堆力学特性和应力应变关系模型

Mechanical properties and stress strain model for bulk wheat based on stress path test

粮仓中粮堆在装卸时存在着复杂的应力路径,为了得出复杂应力路径对粮堆模量和临界状态特性的影响规律,以及粮堆应力应变关系模型,该文在侧向应力50~300 k Pa下,进行了常规三轴压缩(conventional triaxial compression,CTC)、等p压缩(constant mean normal stress compression,CMS)、三轴主动压缩(reduced triaxial compression,RTC)三轴应力路径试验,分析了应力路径和侧向压力对模量的影响和粮堆临界状态特性;修正岩土体三次曲线模型,建立了适于描述仓内小麦粮堆应力应变的模型,并通过应力路径试验结果和文献试验结果对模型的适用性进行验证。研究结果表明:各应力路径下初始模量、割线模量E50均随着侧向应力呈幂函数增长;CTC、CMS试验的割线模量E50比初始模量发生较大的降低,而RTC试验没有明显降低。在参考压力(大气压力)下,对于初始模量,CTC试验的结果最大,RTC试验的结果最小;对于割线模量E50,CTC试验的结果最小,RTC试验的结果最大。CTC试验的初始模量、割线模量均随着侧向应力增长最慢,而RTC试验的结果均随着侧向应力增长最快。不同应力路径和侧向应力下,试验的破坏点均落于同一临界状态线上,小麦粮堆临界状态应力比为0.976。修正三次曲线模型反映了粮堆强度、峰度系数和峰值应变等特性,并通过8个参数进行计算;通过应力路径试验结果和文献试验结果对该模型进行了验证。研究结果可为粮仓装卸料压力、变形的计算提供更符合实际应力路径条件的参量,建立的修正三次曲线模型可用于粮堆应力和变形的数值模拟,为粮仓的设计提供参考。

Total amount of stored grain in China is about 200 million tons,which is of great importance to ensure the quality and safety of grain during storage period,and to modernization of agriculture engineering.When a storage cycle is ended,grain will be discharged from bins,and then loaded in for another storage cycle.Vertical stress increases in loading process,while decreases in discharging process,and different stress path will be generated in the bulk grain,resulting in complicated pressure and frictional force on bin wall,which will cause crack,failure or even overturn of bin structures.It is vital to study stress strain relation and strength properties of bulk grain under different stress paths.Geotechnical constitutive models have been used in grain bin simulation,however,the feasibility of these models for bulk grain has not been validated.In this study,stress path triaxial test was utilized to simulate complicated load path in grain bins.Wheat purchased from grain depot in Zhengzhou City,Henan Province was used in this study.Specimen had water content of 10.9%,bulk density of 0.85 g/cm3,diameter of 61.8 mm and height of 125 mm,the average axial length of wheat kernel was 4.5 mm,and the ratio of specimen diameter to kernel axial length was 13.7,which was larger than the minimum specified ratio for triaxial test.Three types of stress paths were investigated in this study:conventional triaxial compression(CTC),constant mean normal stress compression(CMS),and reduced triaxial compression(RTC).In CTC test,radial stress was set as constant,while axial stress increased simultaneously;in CMS test,radial stress decreased,while axial stress increased simultaneously to keep mean normal stress constant;in RTC test,axial stress was set as constant,and radial stress decreased simultaneously.Stress path triaxial test procedure was as follows:After bulk wheat specimen was mounted on triaxial apparatus,isotropic stress was applied from 0 to prescribed value(50,100,150,200,250,300 kPa);then CTC,CMS or RTC stress path was applied on specimen,and stress strain result was recorded until axial strain reached 30%.Initial modulus,secant modulus and critical state properties were determined from stress strain curve,strength and stress strain parameters were determined,and finally new model was proposed to depict stress strain relation for bulk wheat under different stress paths.Test results show that,under the same radial stress,CTC test has the largest initial modulus,and RTC test has the lowest value.Under all stress paths,initial modulus and secant modulus are found to be in a power function growth with the ascent of radial stress.In CTC test and CMS test,secant modulus is significantly lower than initial modulus;while in RTC test,secant modulus is not significantly reduced compared with initial modulus.Under reference pressure(atmospheric pressure),CTC test has the largest initial modulus of 24.1 MPa,and RTC test has the lowest value of 7.9 MPa;while initial modulus of RTC test has the largest increasing rate,CTC test has the lowest increasing rate.Under reference pressure,CTC test has the lowest secant modulus of 7.7 MPa,and RTC test has the largest value of 9 MPa;secant modulus of RTC test has the largest increasing rate,and CTC test has the lowest increasing rate.Deviator stress increased during shearing process,and specimen failed at the peak point on q-p(deviator stress-mean normal stress)plane.Under all stress path and radial stress situations,failure point fell on the same critical state line.Critical state line for bulk wheat has straight line form,and critical state stress ratio is 0.976.New modified cubic curve model for bulk wheat grain under different stress path conditions was proposed.In the model,strength conforms to Mohr-Coulomb failure criterion;crest reduction coefficient has linear relation with residual strength ratio;peak axial strain has linear relation with radial stress.Model parameters comprised apparent cohesion,internal friction angle,residual apparent cohesion,residual internal friction angle,parameter a and k for crest reduction coefficient,and parameter b and m for peak axial strain.Calculated results show that the proposed model can simulate the results of all stress paths under different confining stress levels.The model can reflect strain softening and strain hardening properties of bulk wheat;peak shear strength and residual shear strength can be determined;and the simulated stress and strain curve coincide well with the test results.The result of this paper provides more accurate parameters for grain bin load calculation considering the stress path conditions,and the new model can be used to estimate stress and deformation of bulk wheat,and to improve the designing method of grain bins.