上颌尖牙压低的三维有限元分析

目的:分析压低加载变化对Von mises应力分布的影响,探讨尖牙压低过程中的力学机制,为临床正畸治疗提供客观的理论依据。方法:运用已经建立的上颌尖牙及矫治器的三维有限元模型,在有限元模型中的弓丝上选择三个代表性的点:距托槽近中边缘1mm处、远中边缘1mm处、托槽中点处。在此三点上设计五种工况,模拟不同的压低情况,描绘出牙体及牙周膜的位移形变图、Von mises应力分布图。结果:不同的压低加载对牙周膜应力分布有明显的影响。①在弓丝上单独施加压低力时,牙周膜应力集中区主要位于牙周膜唇面颈1/3靠近中处;②在压低的同时施加舌向力,牙周膜应力集中区由唇侧向舌侧转移;③40g的压低与5g的舌向力共同作用时,尖牙在不发生唇舌向倾斜的情况下被压低,牙周膜应力分布较均匀。结论:当压低力与舌向力的比值等于8:1时,尖牙在不发生唇舌向倾斜的基础上被压低;当压低力与舌向力的比值大于8:1时,尖牙在压低的同时发生牙冠唇向牙根舌向的倾斜运动;当压低力与舌向力的比值小于8:1时,尖牙在压低的同时发生牙冠舌向牙根唇向的倾斜运动。

“O”型圈附着体义齿与套筒冠义齿对基牙牙根应力分布的研究

目的 分析比较“O”型圈附着体覆盖义齿与套筒冠覆盖义齿的基牙牙根应力分布特点。方法 应用三维有限元应力分析方法研究了“O”型圈附着体覆盖义齿与套筒冠覆盖义齿的基牙牙根应力分布特点。结果 垂直载荷下 ,桩核型“O”型圈覆盖义齿基牙牙根内综合应力峰值较套筒冠覆盖义齿牙根内应力峰值下降了 6 6 .7% ,应力分布更均匀 ,斜向载荷下 ,两种覆盖义齿的基牙牙根内应力峰值均较垂直载荷时高 ,但应力分布规律与垂直载荷类似。结论 “O”型圈覆盖义齿可以缓冲传递到基牙及粘骨膜的牙合力并使应力峰值降低 ,有利于口腔软硬组织的健康。尤其适用于需要利用残根作为基牙及基牙牙周健康欠佳的患者。

3D FEM analysis for layered rock considering anisotropy of shear strength

An empirical expression of cohesion （C） and friction angle （Ф） for layered rock was suggested. This expression was compared with a test result made by the former researchers. The constitutive relationship of a transversely isotropic medium and Mohr-Coulomb criterion in which C and Ф vary with directions were employed, and a relative 3D elasto-plastic FEM code was developed, in which the important thing was to adopt a search-trial method to find the orientation angle （p） of shear failure plane （or weakest shear plane） with respect to the major principal stress as well as the corresponding C and Ф Taking an underground opening as the calculation object, the numerical analyses were carried out by using the FEM code for two cases of transversely isotropic rock and isotropic rock, respectively, and the computation results were compared. The results show that when the rock is a transversely isotropic one, the distributions of displacements, plastic zones and stress contours in the surrounding rock will be non-axisymmetric along the tunnel＇s vertical axis, which is very different from that of isotropic rock. The stability of the tunnel in transversely isotropic rock is relatively low.

Study on mechanism of circumferential cracks in cold-region tunnels

There exists frost damage in cold-region tunnels. The circumferential cracks affect the function of tunnels. Using three-dimensional finite element method, the authors analyzed the mechanism of circumferential cracks in cold-region tunnels It is proved that the internal thermal stress in lining exceeds the tensile strength of concrete, which is the direct cause for circumferential cracks occurring. The laws of thermal stress in lining induced by parameters such as temperature drop, horizontal drag coefficient and length of lining are analyzed. The conclusions are valuable to the design and construction of cold-region tunnels considering the thermal stress.

Structural effects of three-dimensional angle-interlock woven composite undergoing bending cyclic loading

This paper reports the structural effects of three-dimensional(3-D)angle-interlock woven composite(3DAWC)undergoing three-point bending cyclic loading from experimental and finite element analysis(FEA)approaches.In experiment,the fatigue tests were conducted to measure the bending deflection and to observe the damage morphologies.By the FEA approach,a micro-structural unit-cell model of the 3DAWC was established at the yarn level to simulate the fatigue damage.The stress degradation at the loading condition of constant deformation amplitude was calculated to show the degradation of mechanical properties.In addition,the stress distribution,fatigue damage evolution and critical damage regions were also obtained to qualitatively reveal the structural effects and damage mechanisms of the 3DAWC subjected to three-point bending cyclic loading.

Fracture analysis of rock mass based on 3-D nonlinear Finite Element Method

Traditional fracture analysis is based on fracture mechanics and damage mechanics. They focus on the propagation of the fracture. However, their propagation criterions are not easily applied in practice and the current analysis is limited in planar problem. This paper presents a new theory that the occurrence of the unbalanced force (derived from the Deformation Reinforcement Theory) could be the criterion of the initiation of the fracture, and the distribution area and propagation of the unbalanced force could be the indication of the fracture propagation direction. By aggregate analysis with Stress Intensity Factor (SIF) criterion, the unbalanced force actually is the opposite external load that is the SIF difference incurred between the external loads and permitted by the structure. Numerical simulation and physical experiments on pre-fracture cuboid rock specimens proved that the occurrence of the unbalanced force could be the initiation of the fracture. Mesh size dependence was also considered by analysis of different mesh size finite element gravity dam models. Furthermore, the theory was applied to the feasibility analysis of the Baihetan arch dam together with physical experiments in order to evaluate the fracture propagation of dam heel. The results show that it is an effective way to use unbalanced force to analyze the fracture initiation and propagation when performing 3-dimensional nonlinear FEM calculation.