Finite element analysis and contact modelling considerations of interference fits for fretting fatigue strength calculations

This paper presents a 3D finite element study of an interference fit assembly subjected to bending. The results accuracy and the solution convergence are governed by mesh size and contact algorithms options. Their influences were investigated to evaluate displacements and stresses near the contact edge where fretting fatigue failure occurs. Four contact algorithms were tested: Penalty function, Augmented Lagrangian, Normal Lagrange and Pure Lagrange. Performance criteria such as precision and time were highlighted and specific convergence control parameters were found. Finally, best practice rules for interference fit FEA are specified.     

形态滤波在单频周跳探测中的应用

为了实现北斗导航过程中历元间周跳的精准探测,提出了一种基于形态滤波和奇异值分解(SVD)的周跳检测方法.利用相位减伪距法构造周跳检测量作为原始周跳信号.进而构造Hankel矩阵对原始信号进行SVD,得到具有不同分辨率的分量信号,并对得到的分量信号进行形态滤波.使用峭度准则选择峭度值较大的分量进行信号重构,以达到滤除噪声干扰,突出周跳特征的目的.实验结果表明:此方法可以对小周跳进行精确的探测,有效提高了周跳的探测精度.     

局部凹陷和偏心对接对海洋立管疲劳寿命影响的数值分析

针对涡激振动导致的干涉碰撞及焊接时产生的偏心会降低海洋立管疲劳寿命的问题,用S-N法计算局部凹陷、偏心对接分别作用以及二者共同作用下海洋立管的疲劳寿命.在偏移位置、局部凹陷位置和过渡区域长度确定的情况下,偏移幅值和局部凹陷幅值越大,过渡区域的应力越大,寿命越小;在偏移幅值、局部凹陷幅值和过渡区域长度确定的情况下,过渡区域距海洋立管中部越近对过渡区域的应力及寿命影响越大.     

Low cycle fatigue of single crystal silicon thin films

Silicon will continue to be the critical structure material for micromechanical components for years to come so that reliability must be a key concern. Consequently, in order to ensure reliability design approaches must account for fatigue behavior. This work is aimed at studying the fatigue of single crystal silicon (SCS) thin films by a specially designed method. The films were tested using cantilever microbeam deflection with respect to the effect of loading conditions. To conduct a fatigue test under cyclic strain would be more realistic because many MEMS applications, such as micro-accelerometers and micro-filters, consist of beams vibrating in the same mode. A micro-force testing machine with a micro-probe and specially designed fixtures is used to contact and load the beams under the cyclic strain. Before the fatigue test, the failure strain _f of beams in the flexural test is achieved as the testing criterion. In fatigue testing, various percentages of failure strain _f, two times of the strain amplitude, are adopted. First of all, fatigue strain–life cycle (S/N) curve is achieved. Further, the curve of fatigue force detected on the SCS versus fatigue life is measured. SEM is also employed to observe the fracture modes of films under fatigue. Based on the SEM observation and force/life curve, the failure mechanism of the fatigued SCS films under the cyclic strain is proposed. This provides a viable method to evaluate the reliability of the SCS.     

Real-time indicators and influence factors of muscle fatigue in push-type work

Muscle fatigue is a significant cause of musculoskeletal injury and can easily induce unsafe behaviour. Push-type work is a common type of physical work, and if not designed appropriately, may lead to muscle fatigue. Previous studies on muscle fatigue mainly focus on investigating continuous force exertion, and in most of them, a constant muscle force is assumed, thereby ignoring the fluctuations present in exertion. In this study, bolt hole drilling was chosen to represent typical push-type work, and the muscle fatigue from this work was examined. The experimental system designed in this study monitored the muscle force in real time. In the experiments, different thrust angles (15°, 45°, and 75°), different relative force values (20% MVC, 40% MVC, and 60% MVC; MVC: maximum voluntary contraction) and different working time intervals (0 s, 30 s, 60 s, and 90 s) were considered. The results demonstrate that there is a significant positive correlation between the rating of perceived exertion (RPE) and muscle force attenuation (r = 0.786, p = 0). The cubic regression model (Y = − 0.00071x³ − 0.024x² − 0.334x + 1.146, R² = 0.639) fits the data most closely. Therefore, force attenuation can be used as a real-time indicator of muscle fatigue. In addition, the relative force value and thrust angle have a significant impact on the RPE score, whereas the working time interval has no major effect on it. This study provides a new method for evaluating muscle fatigue and a basis for the design of push-type work to reduce fatigue-induced accidents and musculoskeletal injuries.     

Elastic analysis of pin joints

The advent of large and fast digital computers and development of numerical techniques suited to these have made it possible to review the analysis of important fundamental and practical problems and phenomena of engineering which have remained intractable for a long time. The understanding of the load transfer between pin and plate is one such. Inspite of continuous attack on these problems for over half a century, classical solutions have remained limited in their approach and value to the understanding of the phenomena and the generation of design data. On the other hand, the finite element methods that have grown simultaneously with the recent development of computers have been helpful in analysing specific problems and answering specific questions, but are yet to be harnessed to assist in obtaining with economy a clearer understanding of the phenomena of partial separation and contact, friction and slip, and fretting and fatigue in pin joints. Against this background, it is useful to explore the application of the classical simple differential equation methods with the aid of computer power to open up this very important area. In this paper we describe some of the recent and current work at the Indian Institute of Science in this last direction.     

超密集网络中基于干扰协调的资源分配算法

超密集网络中,严重的小区间干扰制约了终端用户的数据速率,针对该问题,该文提出一种基于干扰协调的资源分配方案。该方案分为两个模块：第一模块基于毫微微接入点(Femtocell Access Points, FAPs)间的干扰程度,将干扰强的FAPs分到同一簇内,同簇内的FAPs共享频带资源,通过FAPs间的协作使不同簇之间实现频谱的复用;第二模块基于最大功率和最低速率的公平性准则进行最优功率分配,动态分配资源。仿真结果表明,该算法在超密集网络场景下能够有效控制FAPs间的干扰,最大化系统吞吐量。     

Slipping of the foot on the floor when pulling a pallet truck

Workers pulling pallet trucks are likely to slip when pulling and stepping on a low-friction floor. This study investigated the slipping of male participants when pulling a pallet truck, walking backward, and stepping on either a dry, wet, or glycerol-contaminated vinyl surface. The weight of the load on the truck was either low (0 kg), medium (295 kg), or high (568 kg). A motion-tracking system was used to collect the three-dimensional coordinates of the markers on the shoes. It was found that subjects might slip either upon landing of the leading foot on the toe (slip I) or before taking off of the lagging foot on the heel (slip II). The results indicated that the slip distances for both types of slip were significantly affected by the load and surface conditions and their interactions. Micro-slips (slips between 0.1 and 3 cm) and midi-slips (slips between 3 and 10 cm) were more common in slip I than in slip II. On glycerol-contaminated surfaces, the probabilities of a slide, or a slip more than 10 cm, for both slips I and II were over 40%. The implications of the results were discussed.     

Spectral analysis of heart rate variability as an indicator of driver fatigue

The effect of distance driven on three physiological variables taken to be indicators of fatigue was investigated on a 340 km highway circuit with eight inexperienced drivers as subjects. The physiological variables used were spectral values of heart rate variability in the 0.05-0.15Hz region (0.1 Hz HRV) supplemented by standard deviation of heart rate mean (S.D. HRV) and heart rate mean (HR). The analysis showed a significant relationship between 0.1 Hz HRV and distance driven while S.D. HRV and HR showed no direct relationship. The reason for this differential effect on the three physiological variables was discussed, and it was concluded that 0.1 Hz HRV seems to be a sensitive indicator of driver fatigue.     

High-cycle fatigue of single-crystal silicon thin films

When subjected to alternating stresses, most materials degrade, e.g., suffer premature failure, due to a phenomenon known as fatigue. It is generally accepted that in brittle materials, such as ceramics, fatigue can only take place in toughened solids, i.e., premature fatigue failure would not be expected in materials such as single crystal silicon. The results of this study, however, appear to be at odds with the current understanding of brittle material fatigue. Twelve thin-film (~20 μm thick) single crystal silicon specimens were tested to failure in a controlled air environment (30±0.1°C, 50±2% relative humidity). Damage accumulation and failure of the notched cantilever beams were monitored electrically during the "fatigue life" test. Specimen lives ranged from about 10 s to 48 days, or 1×106 to 1×1011 cycles before failure over stress amplitudes ranging from approximately 4 to 10 GPa. A variety of mechanisms are discussed in light of the fatigue life data and fracture surface evaluation     

Damage tolerance based design optimisation of a fuel flow vent hole in an aircraft structure

This paper investigates the design optimisation of a fuel flow vent hole (FFVH) located in the wing pivot fitting (WPF) of an F-111 aircraft assuming a damage tolerance design philosophy. The design of the vent hole shape is undertaken considering the basic durability based design objectives of stress, residual (fracture) strength, and fatigue life. Initially, a stress based optimised shape is determined. Damage tolerance based design optimisation is then undertaken to determine the shape of the cutout so as to maximise its residual strength and fatigue life. For stress optimisation, the problem is analysed using the gradient-less biological algorithm and the gradient-based nonlinear programming methods. The optimum designs predicted by the two fundamentally different optimisation algorithms agree well. The optimum shapes of the vent hole are subsequently determined considering residual strength and fatigue life as the distinct design objectives in the presence of numerous 3D cracks located along the vent hole boundary. A number of crack cases are considered to investigate how the crack size affects the optimal shapes. A semi-analytical method is employed for computation of the stress intensity factors (SIF), and an analytical crack closure model is subsequently used to evaluate the fatigue life. The 3D biological algorithm is used for designing the cutout profiles that optimise residual strength and fatigue life of the component. An improved residual strength/fatigue life (depending on the optimisation objective) is achieved for the optimal designs. The variability in SIF/fatigue life around the cutout boundary is reduced, thereby making the shape more evenly fracture/fatigue critical. The vent hole shapes optimised for stress, residual strength, and fatigue life are different from each other for a given nature and size of the flaws. This emphasises the need to consider residual strength and/or fatigue life as the explicit design objective. The durability based optimal vent hole shapes depend on the initial and final crack sizes. It is also shown that a damage tolerance optimisation additionally produces a reduced weight WPF component, which is highly desirable for aerospace industries. The design space near the ‘optimal’ region is found to be flat. This allows us to achieve a considerable enhancement in fatigue performance without precisely identifying the local/global optimum solution, and also enables us to select a reduced weight ‘near optimal’ design rather than the precise optimal shape.     

A Phenomenological Discrete Brittle Damage-Mechanics Model for Fatigue of MEMS Devices With Application to LIGA Ni

Fatigue initiation and failure of various microelectromechanical systems (MEMS) is of significant importance as they gain widespread acceptance in sensors and electronics. This paper presents an approach for utilizing available experimental fatigue data to evaluate the fatigue lives of MEMS components. The approach is based on a phenomenological discrete material representation in which a domain is represented by a collection of rigid elements that interacts via springs along their boundaries. The principles of continuum damage mechanics are used to degrade the spring stiffnesses as brittle damage occurs when the domain is subjected to fatigue loading. The model utilizes experimental stress–life data for LIGA Ni to identify the material properties used in the model. The proposed model captures the statistical distribution of material properties and geometrical randomness of the microstructure commonly observed in a wide variety of MEMS. Consequently, simulations that account for the variability in fatigue life can be readily performed. The model is applied to a dog-bone-shaped specimen to evaluate the influence of material heterogeneity and material flaws on fatigue crack initiation life and scatter. The ability of the model to predict the fatigue life of different types of MEMS devices and loading conditions is also demonstrated by simulating the fatigue stress–life behavior of a MEMS resonator support beam. $hfill$[2008-0087]      

优先级顶协议的形式化验证

优先级顶协议是一种优先级驱动的抢占式调度协议,它具有无死锁和单阻塞的性质.Dang Van Hung 和Philip Chan在文献[6]中形式地规范和验证了这两个性质.但他们没有对状态函数HiPripcp明确定义,这使得验证的细节较难理解.为了解决这个问题,提出了一种新的方法来验证优先级顶协议单阻塞的性质.通过时段演算的方法,对优先级顶协议进行了规范,并给出了状态函数HiPripcp的明确定义.根据优先级顶协议的规范,形式地验证了该协议的单阻塞性质.采用的验证方法更少地依赖于HiPripcp,这使得验证的细节更易于理解.此外,提出的验证方法可被应用于实时数据库系统中类似协议的形式化验证.     

Fatigue constrained topology optimization

We present a contribution to a relatively unexplored application of topology optimization: structural topology optimization with fatigue constraints. A probability based high-cycle fatigue analysis is combined with principal stress calculations in order to find the topology with minimum mass that can withstand prescribed variable-amplitude loading conditions for a specific life time. This allows us to generate optimal conceptual designs of structural components where fatigue life is the dimensioning factor. We describe the fatigue analysis and present ideas that make it possible to separate the fatigue analysis from the topology optimization. The number of constraints is kept low as they are applied to stress clusters, which are created such that they give adequate representations of the local stresses. Optimized designs constrained by fatigue and static stresses are shown and a comparison is also made between stress constraints based on the von Mises criterion and the highest tensile principal stresses. The paper is written with focus on structural parts in the avionic industry, but the method applies to any load carrying structure, made of linear elastic isotropic material, subjected to repeated loading conditions.     

基于正交水印的盲水印检测技术

提出一种稳健的盲水印检测技术.利用水印信号与主数据之间的正交性,水印检测时不需要使用原始数据并且可以彻底消除主数据噪声的干扰,从而大大提高了水印检测器的稳健性.无论是根据Neyman-Pearson准则还是最小错误概率准则,理论分析表明本文检测器在性能上可以取得很大改善.利用对水印加权的分组技术,盲水印检测器在性能上逼近非盲水印检测器.各种失真下的实验结果表明这种盲水印检测技术是有效的.     

Topology optimization for minimum weight with compliance and simplified nominal stress constraints for fatigue resistance

This work investigates a simplified approach to cope with the optimization of preliminary design of structures under local fatigue constraints along with a global enforcement on the overall compliance. The problem aims at the minimization of the weight of linear elastic structures under given loads and boundary conditions. The expected stiffness of the optimal structure is provided by the global constraint, whereas a set of local stress–based constraints ask for a structure to be fatigue resistant. A modified Goodman fatigue strength comparison is implemented through the same formalism to address pressure–dependent failure in materials as in Drucker–Prager strength criterion. As a simplification, the Sines approach is used to define the equivalent mean and alternating stresses to address the fatigue resistance for an infinite life time. Sines computation is based on the equivalent mean and alternate stress depending on the invariants of the stress tensor and its deviatoric part, respectively. The so–called singularity phenomenon is overcome by the implementation of a suitable qp-relaxation of the equivalent stress measures. Numerical examples are presented to illustrate the features of the achieved optimal layouts and of the proposed algorithm.     

直升机尾减速器连接寿命的确定

按照新的适航规章确定直升机尾减速器连接寿命,并给出结构检查周期.利用有限元分析结合Miner线性累积损伤理论对结构连接按照设计目标寿命进行详细尺寸定义;利用结构疲劳试验结合Miner线性累积损伤理论给出结构连接寿命;载荷谱分别选用有限元计算疲劳载荷谱和实测疲劳载荷谱,同时按照相应的准则将材料的平均S-N曲线转换为计算安全S-N曲线和试验安全S-N曲线.     

基于光纤光栅的悬臂结构疲劳裂纹实验研究

疲劳以及裂纹对机械结构的寿命和生产安全都有着重要的影响。针对叶片等悬臂类结构设计了一种采用光纤光栅进行疲劳测量的方法,通过对比实验证实了裂纹对结构疲劳过程的影响。实验结果表明：完整叶片结构试件的固有频率为124.2 Hz,经过5×106次循环后固有频率和光栅波长均无明显变化;而有0.1 mm深度裂纹的固有频率为123.3 Hz,经过5×106次循环后固有频率变为86.68 Hz,证实了裂纹对疲劳寿命的影响。     

应急通信网络中基于粒子群优化的信道分配算法

当前应急通信亟需解决的问题,在于如何快速有效地满足突发性增长的网络需求,以保证网络传输质量。无线mesh网络以其部署快速、结构灵活、鲁棒性等优点,成为了新一代灾后应急通信网络架构的优秀解决方案。文中提出了一种基于粒子群算法的信道分配优化算法PWCA,在降低全局网络干扰的前提下,考虑了不同链路对整体网络表现的影响,通过其邻接链路的数量来决定信道分配的优先级。在迭代优化的过程中,该算法利用信道分离度细分了不同信道之间的干扰程度以作为优化的评判标准。实验结果表明,该算法可以显著降低网络干扰,保障网络性能,相比传统的粒子群信道分配算法,其在优化速度以及多节点网络环境下的表现均有明显提升。     

Fatigue life prediction under cyclic thermal loads using the boundary elements method for two-dimensional problems

This study describes a sub-domain boundary element procedure for predicting the fatigue life of elastic media under cyclic transient thermal loads. The procedure assumes an initial crack in a two-dimensional medium, and evaluates the crack extension along a path defined by the maximum circumferential stress criterion. Partial closure may occur on the growing crack faces due to thermal loading. Appropriate thermal and mechanical boundary conditions are imposed on the numerical model to account for the contact state. The analysis requires solutions to the equations of quasi-static thermo-elasticity, and uses an iterative procedure to detect the contact region. We investigate cases of pure opening or mixed mode fracture, demonstrating the effects of crack closure on the predicted fatigue life. We compare the obtained results with those of other publications.