Fretting fatigue life prediction using the extended finite element method

In this work, an efficient procedure to predict fatigue lives in fretting fatigue problems is presented. This is accomplished by means of a combined initiation-propagation approach in which the extended finite element method (X-FEM) is used. The procedure is verified by modelling several fretting fatigue tests available in the literature. The application of the X-FEM enables to numerically evaluate the stress intensity factors (SIFs) for cracks of different lengths emanating at the end of the contact zone and to estimate the propagation life corresponding to each of the tests. This propagation life is combined with the initiation life calculated using a multiaxial fatigue criterion (Fatemi-Socie). The predicted lives are then compared with the reported experimental lives, showing that the consideration of the crack-contact interaction through the numerical models tends to improve the life estimation when compared with a fully analytical approach. The procedure can be applied to more general fretting problems for which analytical solutions are not available.     

Numerical study on failure behavior of open-hole composite laminates based on LaRC criterion and extended finite element method

Journal of Mechanical Science and Technology - A new numerical model combining LaRC failure criterion and extended finite element method (XFEM) is created to describe the failure behavior in...     

Quarter elliptical crack growth using three dimensional finite element method and crack closure technique

Shape evolution of a quarter-elliptical crack emanating from a hole is studied. Three dimensional elastic-plastic finite element analysis of the fatigue crack closure was considered and the stress intensity factor was calculated based on the duplicated elastic model at each crack tip node. The crack front node was advanced proportional to the imposed effective stress intensity factor. Remeshing was applied at each step of the crack growth and solution mapping algorithm was considered. Crack growth retardation at free surfaces was successfully observed. A MATLAB-ABAQUS interference code was developed for the first time to perform crack growth on the basis of crack closure. Simulation results indicated that crack shape is sensitive to the remeshing strategy. Predictions based on the proposed models were in good agreement with Carlson’s experiments results.     

Fretting fatigue crack initiation behavior using process volume approach and finite element analysis

This study investigates crack initiation behavior by incorporating fretting fatigue process volume. Three critical plane based fretting fatigue crack initiation parameters are characterized by computing their averaged values over the process volume and then comparing with their counterparts obtained from the localized approach. Two approaches are used: first one involves the computation of parameter at several points over a rectangular region and then its average, and second approach computes the average state of stress/strain over a radial region from which the averaged parameter is calculated. Both approaches require pre-determination of a critical location at or around which the process volume needs to be placed. Effects of size and location of process volume on the averaged value of parameters are studied in detail. Two radii of cylindrical pad are analyzed to investigate the effect of severity of stress gradient on process volume approach. Effects of finite element mesh refinement are also investigated. Averaged value of parameter decreases with the increase of process volume size. This decrease is higher when the process volume is located in the region that is away from the contact zone. Further, a parameter based on normal stress on the critical plane shows more dependence on the size of process volume than that based on shear stress or on a combination of both shear and normal stresses. Orientation of crack initiation changes within a range that is well within the scatter band of experimental observations as the process volume size increases. Averaged value of parameter for a pad with higher stress gradient has a larger reduction with the increase of process volume size than that with a lower stress gradient. Process volume size has less effect on the averaged value of parameter with coarser finite element mesh. Finally, the localized approach provides a conservative value of fretting fatigue crack initiation parameter compared to its counterpart based on the process volume.     

Effect of crack on the impact response of plates by the extended finite element method (X-FEM)

The dynamic response of cracked isotropic plates subjected to impact loading is studied in this paper. The impact properties of cracked plate are compared with the virgin ones to predict the eventual presence of discontinuities in plates. The extended finite element method (X-FEM) is employed in the mathematical modeling of the impact problem, wherein the effects of shear deformation is considered. Conventional finite element without any discontinuity is initially conducted in the numerical implementation. Enriched functions are then added to the nodal displacement field for element nodes that contain cracks. The effects of crack length and crack position on contact force and on plate deflection are analyzed. Results show that the maximal contact force decreases as the deflection increases with increasing crack length a. The effect of crack position on the dynamic response is less pronounced when the crack is near the fixed end.     

Vibration analysis of composite pipes using the finite element method with B-spline wavelets

A finite element formulation using the B-spline wavelets on the interval is developed for modeling the free vibrations of composite pipes. The composite FRP pipe element is treated as a beam element. The finite pipe element is constructed in the wavelet space and then transformed to the physical space. Detailed expressions of the mass and stiffness matrices are derived for the composite pipe using the B-spline scaling and wavelet functions. Both Euler-Bernoulli and Timoshenko beam theories are considered. The generalized eigenvalue problem is formulated and solved to obtain the modal characteristics of the composite pipe. The developed wavelet-based finite element discretization scheme utilizes significantly less elements compared to the conventional finite element method for modeling composite pipes. Numerical solutions are obtained to demonstrate the accuracy of the developed element, which is verified by comparisons with some available results in the literature.     

基于有限元法的风机叶片裂纹分析及处理

叶片断裂和裂纹是旋转机械中普遍存在的一种安全隐患.本文应用大型有限元ANSYS软件,对某一型号的叶片裂纹的进行应力分布及轴向位移计算,通过分析叶片应力-时间函数曲线,得出应力集中加快裂纹扩展结论.同时,分析了导致出现裂纹和叶片断裂的多种因素,并提出了避免类似现象发生的若干建议.     

Dynamic and acoustic characteristics of bell type structure using finite element method

Dynamic and acoustic characteristics of the bell type structure were analyzed numerically. The finite element method with 3-D general shell element was used to identify the natural frequencies and mode shapes of the structure. Mode shapes and stress distributions of a transient dynamic analysis were effectively displayed by using computer graphic technique. The results of this numerical study were in good agreement with those obtained from the experimental test of fast Fourier transform analyer. Vibrational modes, which effect the acoustic characteristics of the typical bell-type structure were found to be the first flexural mode (4-0 mode) and the second flexural mode (6-0 mode). Asymmetric effects by Dangjwas and acoustic holes gave rise to beat frequencies, and the Dangjwa was found to be most effective. When the impact load was applied to the bell, the stress concentration occured at the rim part of the bell. It was found that the bell type structure should be designed thickly at the rim part in order to prevent failure from impact loads.     

Evaluation of gas turbine rotor dynamic analysis using the finite element method

Rotors in general have complex geometries which make analytical modeling of the rotor to determine its dynamic behavior difficult. For this purpose, strong approaches such as finite element method are used to analyze the system. Finite element method saves time and money by allowing reductions in equations while solving the equation systems. Besides, it is possible to obtain faster solutions by using software that can solve these equations. Investigating the Campbell diagrams and deformations caused by critical speeds is very important in investigation of the dynamic behaviors of rotors. For this reason, obtaining the Campbell diagrams of the rotating systems and determining the critical speeds are very useful for us to observe the system behaviors. There are several programs based on finite element method to obtain these data and diagrams. In this study, a program named Dynrot was used to make dynamic analysis and the evaluation of the results and how the software was used are presented in the study. For this purpose, a gas turbine rotor with certain geometrical and mechanical properties is modeled and its dynamic analysis was made by Dynrot program.     

球笼式等速万向节滚道接触应力的有限元分析

球笼式等速万向节的截面形状影响钢球与沟道的接触应力.通过在三维造型软件Pro/E中分别建立圆弧型、双心弧型、椭圆型三种滚道截面形状的球笼式等速万向节的简化模型,导入有限元分析软件ANSYS对其承受极限转矩时的滚道接触应力进行有限元分析,通过和赫兹接触理论计算值进行比较,其相对误差较小.计算结果表明,内滚道接触应力大于外滚道;圆弧型内滚道接触应力大于其他类型滚道,而其外滚道接触应力稍小于其他类型滚道,双心弧型和椭圆型内外滚道接触应力相同;有限元分析结果基本与理论值相符,运用有限元分析方法分析其接触应力是可行可信的.     

Vibration analysis of delaminated composite beams and plates using a higher-order finite element

In order to analyze the vibration response of delaminated composite plates of moderate thickness, a FEM model based on a simple higher-order plate theory, which can satisfy the zero transverse shear strain condition on the top and bottom surfaces of plates, has been proposed in this paper. To set up a C⁰-type FEM model, two artificial variables have been introduced in the displacement field to avoid the higher-order derivatives in the higher-order plate theory. The corresponding constraint conditions from the two artificial variables have been enforced effectively through the penalty function method using the reduced integration scheme within the element area. Furthermore, the implementation of displacement continuity conditions at the delamination front has been described using the present FEM theory. Various examples studied in many previous researches have been employed to verify the justification, accuracy and efficiency of the present FEM model. The influences of delamination on the vibration characteristic of composite laminates have been investigated. Especially the variation of ‘curvature of vibration mode’ (i.e., the second-order differential of deflections in vibration mode) caused by delamination has been studied in detail to provide valuable information for the possible identification of delamination. Furthermore, two approaches have been investigated to detect a delamination in laminates by employing this information.     

Reduced stress shielding with limited micromotions using a carbon fibre composite biomimetic hip stem: a finite element model

Total hip arthroplasty (THA) enjoys excellent rates of success in older patients, but younger patients are still at risk of aseptic loosening and bone resorption from stress shielding. One solution to the stress shielding problem is to use a hip stem with mechanical properties matching those of cortical bone. The objective of the present study was to investigate numerically the biomechanical performance of such a biomimetic hip stem based on a hydroxyapatite (HA)-coated carbon fibre composite. A finite element model (FEM) of the biomimetic stem was constructed. Contact elements were studied to model the bone-implant interface in a non-osseointegrated and osseointegrated state in the best way. Three static load cases representing slow walking, stair climbing, and gait in a healthy individual were considered. Stress shielding and bone-implant interface micromotions were evaluated and compared with the results of a similar FEM based on titanium alloy (Ti-6Al-4V). The composite stems allowed for reduced stress shielding when compared with a traditional Ti-6Al-4V stem. Micromotions were slightly higher with the composite stem, but remained below 40 microm on most of the HA-coated surface. It is concluded that a biomimetic composite stem might offer a better compromise between stress shielding and micromotions than the Ti-6Al-4V stem with the same external geometry.     

Numerical analysis of stress shielding of platedhuman femur using 3-dimensional finite element method

Conventional compression bone fracture plates sometimes cause osteoporosis under the plate due to their high rigidity. In order to prevent the osteopenia, many researchers have attempted various types of bone plates. To meet the same purpose, a new concept bone plate which have a vicoelastic washer between plate and screw head is introduced. The washer is made of a biocompatible polymer (ultra high molecular weight polyethylene, UHMWPE). This research was performed first to establish a more realistic and detailed plated-bone system using finite element method and second to investigate the effect of the UHMWPE washer on the stress shielding comparing with the conventional plate model. Three-dinensional finite element meshes of the human femur with the conventional and new concept bone plate were generated and the comparative stress analysis of the stress shielding was performed with static half-stance loading condition. The results of analysis showed that the new and conventional bone plate transfer the stress through the bone average 15% and 10% that of the intact bone respecively. However, the local stress in the bone under the new bone plate have increased about 13-150% that of the conventional bone plate depending on the region. Earlier preliminary animal studies showed some promising results. It is suggested that the in vivo and FEM results support the feasibility of the new concept bone plate.     

Evaluation of strength variation of domes and cylinders with different winding angle of type III high pressure storage vessel

Journal of Mechanical Science and Technology - Type III vessels are used to store gases at high pressure and, for efficient use, should be as light as possible without compromising stability....     

Dynamic analysis of a car chassis frame using the finite element method

An automobile chassis structure has been analysed dynamically using the finite element method, and the results have been compared with those obtained experimentally. The first nine natural resonances of the frame up to 100 Hz have been examined.

Two basic methods of predicting the dynamic response have been used involving different computing techniques. Since computing costs are of prime importance in any practical application of the finite element method, the modes of operation of the programs have been briefly described and running times are compared for a number of structural idealizations.

The results presented show for a relatively simple structure that simple idealization concepts can give good results.     

有限元法在结构分析中的应用

主要介绍了有限元法的现状及发展趋势,有限元法的技术路线及有限元分析软件ANSYS。     

Thermal and stress analyses in an end-pumped Nd:YAG slab laser using finite element method

The increasing application on high laser power in industries required studies in the portion of pump radiation absorbed by laser media that exchanges to heat. Heat may cause thermal stress, stress birefringence and thermal lens effects. These effects can destroy the optical properties of the laser medium, decrease the beam quality and may lead to medium break. In this paper, the thermal and stress analyses of continuous and pulsed end-pumped Nd:YAG slab laser are studied using finite element method. Heat deposited in the slab is removed by cooling water, flowing on the largest faces of the slab, which surrounds the active media. The temperature and stress distributions of the end-pumped Nd:YAG slab are defined by coupled field methods in the ANSYS commercial finite element software. The value of maximum temperature and stress in the slab which is affected by an end-pumping are calculated. Finally the maximum pump-power range which can be applied to the slab is determined using the limit of maximum stress in the slab. The analyses are done in from transient to steady-state regimes for continuous pumping. Results show that deposited heat due to the pulsed pumping acts like a mechanical impact.     

Elasto-plastic co-indentation analysis of a bounded solid using finite element method

The present investigation deals with the elastic and elasto-plastic finite element analyses of the simultaneous indentations of a bounded solid by two flat plane rigid smooth punches under plane-strain conditions. It also extends the earlier work of Meguid et al. [1], in which the present problem was investigated experimentally using etching techniques and theoretically using the upper-bound theorem. The extension of that work, reported herein, is devoted mainly to studying the effect of the interference ratio, the height and the strain-hardening characteristics of the bounded solid upon the plastic zone development, the indentation pressure-punch displacement relationships and the unloading residual stresses using the finite element method. Some of the results are compared with the earlier findings of Meguid et al. [1] of the same problem. The importance of the present contact problem is realised in many engineering applications, e.g. impact treatment processes, friction and wear, adhesion and surface finish studies.     

Numerical simulation of fine-blanking process using a mixed finite element method

In order to achieve a more intensive understanding of the forming mechanism of the fine-blanking process, a numerical simulation has been carried out by using a mixed displacement/pressure (u/p) finite element method. According to the special requirement of the fine-blanking technique, the major process attributes, such as the vee-ring, the ejector and the edge radii of tools, have been taken into account in the finite element model. The punch–die clearance was set to 0.5% of the thickness of the workpiece. To verify the effectiveness of the simulation, the equivalent strain on the sheared surface of a SS400 steel specimen has been determined experimentally. The experimental values of the equivalent strain have been estimated by measuring the relative displacements of the local grids pre-etched on the meridian plane of the specimen. The results of the finite element simulation are in proper agreement with the experimental findings. The distributions of the shear stress and the equivalent plastic strain have been computed for discussion. Moreover, a diagram of the blanking force versus the punch penetration has also been constructed. In order to investigate the fracture mechanism in the fine-blanking process, the concept of damage mechanics has been applied. By using a void growth model, the evolution of damage at different stages of the fine-blanking has been evaluated. It has been realized that the compressive hydrostatic stress built up by the fine-blanking fixture plays an important role to suppress the initiation of macrocracks.     

某型磁悬浮列车结构动力学有限元数值计算及优化分析

对某型磁悬浮列车的结构动力学特性进行了有限元数值计算,获得了列车结构体的动力学基本品质特性,即固有频率与振动模态.在此数值分析工作基础上,研究分析了车体结构的低频敏感部位与非敏感变量,由此建立了车体结构的优化数学模型,通过数值优化分析计算,在车体底盘梁式构件减重20%的条件下以及在车厢梁式构件减重10%的条件下,结构体固有频率特性没有明显改变,达到了保持结构基本动力学品质指标,而结构有一定幅度减重的工作目的.