| In the mesoscopic scale,the contact between polycrystalline grains is not simply face-to-face bodning,but a certain transition exists which is called as the grain boundary and is a kind of crystal defect.Such kinds of crystal defects may cause to polycrystalline fracture.There is few work about the grain in the frame of the background of multiple crystal grains and body centered cubic structure,and there exist few related work about the simulation of grain boundary by adding cohesive unit layer to any two grains.Moreover,the high cost and difficult convergence also exist when using Voronoi model to simulate polycrystalline,so this dissertation focuses on discussing the mechanical analysis of the simplified polycrystalline model with the cohesive unit layer.The main contents of this dissertation are as follows:(1)Firstly,a new method is used to reproduce the simplified polycrystalline geometric model based on Voronoi model.The key point lies in calculating the length between the central point and the seed point of the element(C3D8)and classifying the elements.Then,the mechanical properties of the model are analyzed.Two kinds of material models,on the macro and mesoscopic scales are used,and different material parameters are assigned to different grains and the analyzing results of two models are compared.The results show that the stress nephograms of the geometric models are quite different from that of the corresponding single crystal models.Compared with the single crystal models,the stress nephogram of the polycrystalline models are more irregular and complexed.The mechanical response curves of all the models are similar to each other,but the stress-strain curves of the structural internal elements’ nodes of the polycrystalline model have cyclic reversion--the stress-strain curves of the internal elements’ integration points have not this reversion.The analysis results of the simplified models are similar to those of Voronoi models with the same number of grains.Therefore,the mechanical analysis of Voronoi model can be replaced by that of the simplified polycrystalline model.Finally,the mechanical responses of the simplified polycrystalline model with different numbers of grain in the same volume are compared.(2)Based on the simplified polycrystalline geometry model,cohesive element layer is constructed between grains to simulate intergranular cracks.The method of constructing the model is as follows: 1)All kinds of elements and their nodes should be numbered reasonably and strictly,and the number of nodes,cohesive elements and their nodes will start from this major number.2)The element can be enlarged or contracted by rationally changing the coordinates of nodes.3)Because the number of the grain element has a certain rule,the cohesive element generated on the surface of the model can be deleted through the regular formula;4)The master and slave surfaces with the bound constraints between the grains and the cohesive elements can be strictly determined by the regular formula;5)The same element which is the cohesive element on the grain surface or not can be judged by the rule formula,and then the element will be remained or be deleted.6)The grain element and cohesive element are redefined,and the grain-element set and cohesive-element set are classified according to the main number.After the step 4),the cohesive element layer model will be generated in the form of element.The mechanical properties are analyzed by using the same two material models as used in(1).The results show that the mechanical response of polycrystalline can not be correctly reflected with the model in element form.After compared with results of the simplified model in(1),it can be seen that the internal node of polycrystalline is more stable and the stress-strain curve has no cyclic reversion,and the stress nephogram of the two material models are similar,which indicates that the influence of the material model on the mechanical analysis results is weakened after the cohesive element layer is added into the simplified model.Finally,the influence of the geometric thickness of cohesive element layer on the mechanical properties of the model is analyzed.It is found that the fracture of the model is easier with the increase of the thickness. |
