加氢反应器裙座支撑区的机械应力分析

加氢反应器是石油产品加工的重要设备,由于其工作在高温高压的场合下,机械应力很大,所以对其裙座支撑区的机械应力分析就尤为重要,在设计时不应忽略.利用ANSYS软件建立了加氢反应器裙座支撑区有限元模型,并对其进行机械应力分析.     

Effect of adherend's rigidity on the shear strength of single lap adhesive joints

The present paper compares the tensile shear strength of single-lap joints with different adherends. Three materials were combined in the single lap joints: a carbon/epoxy laminated composite, a high elastic limit steel and the 6082-T6 aluminium alloy. The shear strength of joints was influenced by the adherend stiffness and the highest shear strengths were obtained using high stiffness adherend materials. The overlap length influenced the shear strength in different ways depending on the adherend materials. Numerical analysis concluded that the increase in the rigidity of the adherends decreases the rotation of the specimen and promotes a more uniform distribution of stresses in the glue. In joints with distinct materials, the less stiff material was found to determine the strength of the appropriate joint.     

Joint Strength Optimization of Adhesively Bonded Patches

Aircraft face damage from impact with objects or birds or due to ageing that leads to fatigue cracks. The conventional methods of repairing aircraft metallic structures generally include the use of a plate joined by screws or rivets. Although these methods are efficient in the short term, they introduce stress concentrations leading to the initiation of new cracks that are difficult or impossible to detect by non-destructive methods. For these reasons, it is necessary to develop new methods to improve the behaviour of the structure (especially for long term) and its manufacture cost. One of the solutions that have been studied by the aeronautical industry is the use of patches bonded with structural adhesives. However, adhesively bonded patches have problems of stress concentration at the edges where crack initiation is prone to occur. This problem can be reduced by the use of a taper and a spew fillet at the end of the patch and by the use of a mixed adhesive technique where a ductile adhesive is placed at the edges of the patch. Double strap specimens from 3 mm thick 6063-T6 aluminium alloy sheet were analysed. Aluminium and straps (or patches) with an internal taper, an adhesive spew fillet, and dual adhesives were experimentally tested. The results obtained were explained by a finite element analysis. A taper angle is beneficial only for the brittle adhesive. The use of two adhesives is advantageous for the taperless configuration.     

Finite element stress analysis and strength evaluation of epoxy-steel cylinders subjected to impact push-off loads

The stress wave propagations and stress distributions in epoxy-steel cylinders in which the outside surface of a solid cylinder (steel) is adhered to the inside surface of a hollow cylinder (epoxy resin) subjected to impact push-off loads were analyzed using the finite element method (FEM). The impact push-off loads were applied to epoxy-steel cylinders on a solid cylinder by dropping a weight. The FEM code employed was ANSYS/LS-DYNA. It was found that the maximum principal stress occurs at the upper edge of the interface, where the rupture initiates in epoxy-steel cylinders under the impact push-off loads. Besides, it was also found that the normal stress near the upper edge of the interface increases as the rigidity and the initial impact velocity increase; meanwhile it decreases as the diameter and the height of the solid cylinder increase. The strength of epoxy-steel cylinders increases as the rigidity of the solid cylinder increases, and the diameter and the height of the solid cylinder decrease. In addition, it was observed that the characteristics of the joints subjected to the impact push-off loads are opposite to those of the joints subjected to the static push-off loads. Furthermore, experiments were carried out to measure the strain response of epoxy-steel cylinders subjected to impact and static push-off loads. Fairly good agreements were observed between the numerical and the measured results.     

柔性石墨金属波齿复合垫片金属骨架结构参数的研究

在金属波齿复合垫片的金属骨架上,施加一定的压紧应力(45 MPa).在三组的不同结构参数下,采用有限元法对三种不同齿尖厚度做对比分析,了解它的密封性能.分析结果表明,在齿尖部位,随着其厚度的增加,波齿环上的应力出现迅速均化的现象,整体变形减小.可知,适当的齿尖厚度,能够使波齿垫片的承载能力得到极大的提高.     

温度对风机基础内力的影响分析

通过对风机基础在高温作用下的内力分析,研究风机基础开裂的问题,并对比不同围护墙厚度的风机基础的抗开裂性能,为风机基础的设计提供依据。     

Numerical optimization of proton exchange membrane fuel cell cathodes

A fuel cell gradient-based optimization framework based on adaptive mesh refinement and analytical sensitivities is presented. The proposed approach allows for efficient and reliable multivariable optimization of fuel cell designs. A two-dimensional single-phase cathode electrode model that accounts for voltage losses across the electrolyte and solid phases and water and oxygen concentrations is implemented using an adaptive finite element formulation. Using this model, a multivariable optimization problem is formulated in order to maximize the current density at a given electrode voltage with respect to electrode composition parameters, and the optimization problem is solved using a gradient-based optimization algorithm. In order to solve the optimization problem effectively using gradient-based optimization algorithms, the analytical sensitivity equations of the model with respect to the design variables are obtained. This approach reduces the necessary computational time to obtain the gradients and improves significantly their accuracy when compared to gradients obtained using numerical sensitivities. Optimization results show a substantial increase in the fuel cell performance achieved by increasing platinum loading and reaching a Nafion mass fraction around 20-30 wt.% in the catalyst layer.     

Understanding and control of adhesive crack propagation in bonded joints between carbon fibre composite adherends II. Finite element analysis

Carbon fibre composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. However, such bonded joints must be shown to behave in a predictable and reliable way. A major aspect of this is to demonstrate that the progress of cracks through the bonds is well understood. The simulation work presented here complements the experimental work presented in Part I. The observed failure processes and their sequence are successfully described and modelled.     

A rapid test to measure adhesion between optical fibers and ethylene–vinyl acetate copolymer (EVA)

Optical fibers are transported on a drum and on some occasions it is essential to fix them and to keep them fixed to the drum. Adhesives are used in such situations to obtain the best and durable fixation. This research shows a new approach for the examination of the adhesion effects and forces between the optical fibers and the adhesive during a tensile test. A solution of ethylene–vinyl acetate (EVA) copolymer in toluene was used as the adhesive for optical fibers and applied using specially designed apparatus. The testing procedure consisted of optical fiber characterization using optical microscopy for measurement of geomertrical parameters; tensile test to obtain the strength of adhesion that keeps the fibers together; nanoindentation of the optical final layer; SEM for precise geometry measurements and insight in the adhesive aspect after the test, and FTIR analysis to find out the chemical composition of polymer protective coating to have an obtain information about chemical environment for the adhesive. The optical fiber with outstanding mechanical characteristics was used to examine adhesives. A sample of two fibers connected with EVA was subjected to the tensile test to examine the adhesion forces. Finite element modeling was used to simulate the behavior and stress distribution of the adhesion layer. This method could assist in reaching a conclusion about the quality of the adhesion obtained in an experimental procedure.     

受压容器开口补强的优化设计

将有限元应力分析方法与设计方法相结合 ,提出了基于受压容器口补强的极限分析理论和优化设计思想 ,采用有限元前处理程序 ,自动生成常见球罐和柱罐的优化力学模型 ,并在屏幕上或用汇图仪绘制出罐体的三维图形。建立起受压容器开口补强优化设计的教学模型。根据此问题的特殊性 ,采用改进后的 0 .6 1 8法进行优化计算 ,优化过程中将模型划分成可变域和不可变域两部分 ,利用计算机的顺序文件存贮不变域单元刚度阵 ,提高了有限元重分析的计算效率     

Biomechanical three-dimensional finite element analysis of monolithic zirconia crown with different cement thickness

Limited information is available on the optimal cement thickness of monolithic zirconia crowns. This study was designed to evaluate the stress distribution in the posterior monolithic zirconia crowns with different cement thicknesses under masticatory force and maximum bite force using three-dimensional finite element analysis. The prepared and unprepared mandibular right first molar models were scanned and exported to the computer-aided design system. Solid models of monolithic zirconia crowns, which were cemented on prepared teeth were generated. Four models were fabricated applying different cement thicknesses (100 µm, 200 µm, 400 µm, and 600 µm). The solid models were imported into the finite element analysis software and meshed into tetrahedral elements. Four three-dimensional finite element models were simulated under masticatory force and maximum bite force: vertical (axial), angular (45°) and horizontal loads of 280 N at 5 points; vertical load of 700 N at 8 points were loaded, respectively. The stress distribution varied with the different cement thicknesses and directions of applied loads. The monolithic zirconia crowns with cement thicknesses exceeding 200 µm had wider distributions of peak maximum principal stress under the same loading conditions. Monolithic zirconia crowns have more stress concentrations on the occlusal surfaces, while the cement layers have more stress concentrations on the cervical areas. Thicker cement layers were associated with more concentrated stresses on the buccal and lingual cervical areas. The test results show that the cement thickness plays an essential role in the success of monolithic zirconia restorations in terms of reducing cement wash-out. Cement thickness of 100 µm is recommended for monolithic zirconia crowns.     

Strength of epoxy adhesive-bonded stainless-steel joints

The strength of stainless-steel joints bonded with two epoxy adhesives was investigated. The experimental programme included tests on single-lap and butt joints, as well as thick-adherend and napkin ring shear tests. Results suggested that the tensile and shear strengths of the epoxy adhesives were quite similar. However, finite element (FE) analyses raised doubts on the true adhesive strengths, due to the complex stress state in joint tests and pressure-dependent adhesive behaviour. In spite of some uncertainties, FE analyses showed that failure could be fairly well predicted by a maximum shear strain criterion.     

Failure analysis of complex bonded structures: Experimental tests and efficient finite element modelling by tied mesh method

This paper aims at assessing the accuracy and applicability of an efficient finite element (FE) computational method for the prediction of the post-elastic response of large and complex bonded structures. In order to overcome the limitations found in the technical literature, such as the use of user defined elements, the present work assesses the applicability of a reduced computational technique, named Tied Mesh (TM) method, previously presented by the authors. The method is based on standard modelling tools, describes the adherends by semi-structural elements (plates or shells), and the adhesive by means of a single layer of cohesive elements. The benchmarks for the TM method are the force-displacement curves obtained by experimental tests on a complex, industrial-like structure. A square thin-walled beam is considered, which made of two different portions butt joined by overlapping thin plates on each side. Two different geometries of the beam are loaded by a three point bending fixture up to failure, thus originating a complex stress field on the bonded region. The comparison with the experimental data shows a good accuracy of the proposed TM method in terms of structural stiffness, maximum load (error below 10%) and post-elastic behaviour up to the collapse of the structure. The numerical precision and the computational speed make the TM method very useful for the efficient analysis of complex bonded structure, both in research and industrial world.     

ON NONISOTHERMAL FLOWS OF BINGHAM PLASTICS

A numerical scheme based on the Galerkin's finite element method is used to analyze nonisothermal flows of Bingham plastics. These fluids exhibit yield stresses, below which the material does not flow. This condition is enforced by introducing the bi-viscosity model in which the plug behaviour is approximated by a highly viscous fluid. The bi-viscosity model approaches the ideal Bingham plastic model if the pre-yield viscosity becomes large. This numerical method is applied to solve three problems namely the Graetz-Nusselt, recirculating flow, and sinusoidal channel flow problems for various values of the yield stress. The size of the plug as well as the velocity and temperature fields correspond closely with values available in literature.