| Fresh jujube is a kind of viscoelastic solid agricultural materials.It will be affected by different forms of loading(such as extrusion,collision,rub,vibration,etc),thus causing mechanical damage.In order to predict mechanical damage of fresh jujube in the process of harvesting,storing and processing,and the mechanical response under the particular state(constant deformation and load),this study researched the whole fruit compression,stress relaxation and creep experiment for four kinds of fresh eat and processing all-purpose jujube varieties,which are the most possible scale cultivation in Gansu province of less rain and arid regions.Eventually,the study got the test curves from five kinds of working conditions and clarified the correlation between the test factors and results.Fresh jujube rheological model and its constitutive equation were established according to the test results,and using the method of finite element and virtual prototype to conduct the numerical simulation analysis of compression and the stress relaxation,which broaden the scope of agricultural material mechanics research.The research results can provide some scientific bases for some links of engineering analysis,such as packaging,transportation,processing,storage,and for quality evaluation and related equipment design,and it can provide the theoretical source for the further understanding the fresh jujube structure,improving quality control of processing and reducing mechanical damage.(1)Four kinds of fresh jujube(Jingyuan Xiaokou jujube,Linze jujube,Lanzhou Tantan jujube,Dunhuang Mingshan jujube)specialty from Gansu were taken up the experimental research of compression mechanical properties on the horizontal and vertical under the five kinds of loading rate.The corresponding force-displacement curves and mechanics parameters of varieties fresh jujube,such as elasticity modulus,stiffness,fracture energy,rupture force and the relative rupture deformation,were obtained.The results showed that the crack of all varieties of fresh jujube appeared mostly in pedicle under transverse compression,while it appeared in all directions of jujube meridian under the longitudinal compression,and the fracture forms were relatively consistent;the fracture force of transverse compression ranged from 227.52 N to 342.8N,and that of longitudinal compression ranged from 276.82 N to 383.17N;the variety of jujube had a significant effect on the elasticity modulus,but the compressive direction and loading rate on the influence of elasticity modulus were not significant;when the fresh jujube was transverse compression,the rupture strength and elasticity modulus,fracture energy showed highly significant positive correlation(p<0.01),but the fracture energy was significantly positively related to elasticity modulus,relative rupture deformation(0.01<p<0.05);when the fresh jujube was longitudinal compression,the rupture strength and relative rupture deformation,fracture energy showed highly significant positive correlation in p<0.01,and it was highly significantly negative related to elasticity modulus,while the elasticity modulus was significantly negative related to fracture energy and relative rupture deformation,and fracture energy was significantly positive related to relative rupture deformation in 0.01<p<0.05.The method of finite element was used to simulate compression mechanics test process of fresh jujube in transverse and longitudinal direction.The results showed that: when the fresh jujube was pressured,stress-strain was mainly concentrated near the load point,which was consistent with the test results that fresh jujube was ruptured in head loading place;the maximum deviation between the test value of force-deformation curve and the simulation value was 11.4%.It indicated that the method of finite element can simulate and research the mechanical process of fresh jujube and widen the research methods of biomechanics.(2)The stress relaxation characteristics of experimental research under transverse and longitudinal direction was conducted in two compression displacement(2mm,3mm)about four kinds of fresh jujube specialty from Gansu.After obtaining the stress relaxation characteristic curve and selecting the appropriate mathematical model of relaxation,and then the corresponding relaxation characteristic parameters was fitted out by SPSS.The results showed that: each jujube had obvious stress relaxation phenomenon,and the relaxation stress in initial was rapidly attenuation,but it tended to be stable after balance stress;the relaxation stress of longitudinal compression was greater than transverse compression at any time,and the greater the deformation of fresh jujube,its biggest relaxation stress,the greater the relaxation stress at any time,the greater the final balance stress;the stress relaxation model of fresh jujube was better simulated using the generalized Maxwell 5-component model,its correlation coefficient can reach above 0.99;the varieties of fresh jujube,the deformation and loading direction had extremely significant effects(p<0.01)on the decay elastic modulus E1 and E2,and different varieties of fresh jujube had significant impact on the balance elastic modulus E0(0.01<p<0.05);E0 showed extremely significant positive correlation(0.01<p<0.05)with E1 and E2,and it was also extremely significant positive correlation between E1 and E2,the viscous coefficient ?2 and E1,E2,E0 was extremely significant positive correlation(0.01<p<0.05).Using the rheological model,the fresh jujube rheological process was analyzed from the angle of dynamics.The curves of test value and simulation value was compared,and the maximum deviation was 4.7% which showed that the rheological process of fresh jujube can be simulated and researched using the method of virtual prototyping,and that offered a new way for research of biomechanics.(3)The experimental study of the creep property was carried out in two kinds of load(85N,115N)under horizontal and longitudinal direction with four kinds of fresh jujube specialty from Gansu.The corresponding creep property curve was obtained.The corresponding creep parameters were fitted out by selecting the creep mathematical model.The creep properties of different fresh jujube samples and different loading forces were analyzed.The relationships among creep property parameters of fresh jujube were got through analyzing the whole fruit model parameters.The results showed that: when the fresh jujube samples was in the constant load,change trends of deformation-time relation curve were same,and creep deformation in the horizontal was greater than it in the longitudinal;when the fresh jujube in the horizontal deformation,the initial deformation of Mingshan jujube was biggest under the certain loading(115N was 4.85 mm,85N was 3.6mm),and that of Linze jujube was smallest,and the curve was bottommost(115N was 4.16 mm,85N was 3.0mm);the greater of the loading,the greater of the deformation(the deformation of 115 N at any time was greater than it of 85N);Burgers model could better simulate the creep characteristics of fresh jujube,and fitting coefficient of SPSS was more than 0.99.The varieties of fresh jujube and loading direction had an extremely significantly effect on the instantaneous elasticity of E1(p<0.01),and the influence of various factors on the delayed elasticity coefficient of E2 were not significant.The varieties of fresh jujube,loading direction and constant load had an effect on ?1,?2 were not significant(p>0.05).The instantaneous elasticity coefficient of E1 was extremely significant positive correlation with the delayed elasticity coefficient of E2(p<0.01).The instantaneous viscous coefficient of ?1 and elasticity coefficient of E1 and E2 was extremely significant negative correlation(0.01<p<0.05).The delayed time was negatively correlated with elasticity coefficient of E1 and E2,and it was positively correlated with viscous coefficient of ?1,?2,but they were all not significant. |
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