Testing and evaluation for fatigue crack propagation of Ti-6Al-4V/ELI and 7050-T7452 alloys at high temperatures

This paper seeks to evaluate crack propagation properties and residual lives of metallic alloys subjected to fatigue loading at room and high temperatures. Fatigue crack growth tests were performed on Ti-6Al-4V/ELI and 7050-T7452 subjected to constant-amplitude and actual randomspectra loading at room temperature of about 25 ℃ and at high temperatures of 250 ℃ and 150 ℃ to determine their crack growth properties and residual lives. The damage mode and mechanisms at high temperature were compared with those at room temperature on the basis of the results of fractographic analysis. Temperature-dependent residual lives under actual random-spectra load history were evaluated based on a modified accumulation damage rule accounting for the load interaction.Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.     

A theoretical model of semi-elliptic surface crack growth

A theoretical model of semi-elliptic surface crack growth based on the low cycle strain damage accumulation near the crack tip along the cracking direction and the Newman–Raju formula is developed. The crack is regarded as a sharp notch with a small curvature radius and the process zone is assumed to be the size of cyclic plastic zone. The modified Hutchinson, Rice and Rosengren(HRR) formulations are used in the presented study. Assuming that the shape of surface crack front is controlled by two critical points: the deepest point and the surface point.The theoretical model is applied to semi-elliptic surface cracked Al 7075-T6 alloy plate under cyclic loading, and five different initial crack shapes are discussed in present study. Good agreement between experimental and theoretical results is obtained.     

Transonic wing stall of a blended flying wing common research model based on DDES method

Numerical simulation of wing stall of a blended flying wing configuration at transonic speed was conducted using both delayed detached eddy simulation(DDES) and unsteady Reynolds-averaged Navier-Stokes(URANS) equations methods based on the shear stress transport(SST) turbulence model for a free-stream Mach number 0.9 and a Reynolds number 9.6 × 10~6. A joint time step/grid density study is performed based on power spectrum density(PSD) analysis of the frequency content of forces or moments, and medium mesh and the normalized time scale0.010 were suggested for this simulation. The simulation results show that the DDES methods perform more precisely than the URANS method and the aerodynamic coefficient results from DDES method compare very well with the experiment data. The angle of attack of nonlinear vortex lift and abrupt wing stall of DDES results compare well with the experimental data. The flow structure of the DDES computation shows that the wing stall is caused mainly by the leeward vortex breakdown which occurred at x/x_(cr)= 0.6 at angle of attack of 14°. The DDES methods show advantage in the simulation problem with separation flow. The computed result shows that a shock/vortex interaction is responsible for the wing stall caused by the vortex breakdown. The balance of the vortex strength and axial flow, and the shock strength, is examined to provide an explanation of the sensitivity of the breakdown location. Wing body thickness has a great influence on shock and shock/vortex interactions, which can make a significant difference to the vortex breakdown behavior and stall characteristic of the blended flying wing configuration.     

GROWTH BEHAVIOR OF SHORT FATIGUE CRACKS FOR ALUMINUM- LITHIUM ALLOY 8090

The growth behaviors of short through cracks (0.2 < △a < 2.2mm) and long cracks are compared using CT type specimens in aluminum-lithium alloy 8090 T651. It is found that the short cracks grow much more than long ones and are observed to grow at the stress intensity ranges far below the long crack threshold. The distinction of growth bahavior between short and long cracks is attributed to the difference of their crack closure effect. The growth behavior of short cracks can be rationalized with that of long ones in terms of effective stress intensity ranges. The upper demarcation value of short through cracks for aluminum-lithium alloy 8090 is presented.     

Analysis Approach to Durability Based on Material Initial Fatigue Quality and S-N Curve

Based on probabilistic fracture mechanics approach, a new concept of material initial fatigue quality (MIFQ) is developed. Then, the relation between S-N curve and crack propagation curve is studied. From the study, a new durability analysis method is presented. In this method, S-N curve is used to determine crack growth rate under constant amplitude loading and evaluate the effects of different factors on durability and then the structural durability is analyzed. The tests and analyses indicate that this method has lower dependence on testing, and higher accuracy, reliability and generality and is convenient for application.     

Introduction of fretting-contact-induced crack closure: Numerical simulation of crack initiation and growth path in disk/blade attachment

Fretting fatigue crack initiation and growth in titanium alloy dovetail assembly was investigated by the Finite Element Method(FEM). Firstly, contact stress was calculated precisely with an elastic–plastic material model. Secondly, the location and angle of crack initiation were determined by the parameter of the maximum shear stress range on the critical plane, and the angle of crack growth was predicted by the Maximum Tangential Stress(MTS) criterion, which showed agreement with experimental observation. Finally, the fretting-contact-induced crack closure behavior was simulated by the node release technique in software ABAQUS with both elastic and elastic–plastic material models. The simulation shows that the variation of the contact status between fretting surfaces will result in crack closure even for the elastic material model. The fretting crack closure ratio decreases as the crack grows out of the contact area and it has great impact on the effective range of Stress Intensity Factor(SIF) as well as the crack growth rate.     

机械连接件中螺栓孔角裂纹应力强度因子数值计算(英文)

The three-dimensional finite element method is used to solve the problem of the quarter-elliptical comer crack of the bolt-hole in mechanical joints being subjected to remote tension. The square-root stress singularity around the corner crack front is simulated using the collapsed 20-node quarter point singular elements. The contact interaction between the bolt and the hole boundary is considered in the finite element analysis. The stress intensity factors （SIFs） along the crack front are evaluated by using the displacement correlation technique. The effects of the amount of clearance between the hole and the bolt on the SIFs are investigated. The numerical results indicate that the SIF for mode I decrease with the decreases in clearance, and in the cases of clearance being present, the corner crack is in a mix-mode, even if mode I loading is dominant.     

Numerical study of separation on the trailing edge of a symmetrical airfoil at a low Reynolds number

This study focuses on the trailing-edge separation of a symmetrical airfoil at a low Rey-nolds number. Finite volume method is adopted to solve the unsteady Reynolds-averaged Navier-Stokes (RANS) equation. Flow of the symmetrical airfoil SD8020 at a low Reynolds number has been simulated. Laminar separation bubble in the flow field of the airfoil is observed and process of unsteady bubble burst and vortex shedding from airfoil surfaces is investigated. The time-dependent lift coefficient is characteristic of periodic fluctuations and the lift curve varies nonlinearly with the attack of angle. Laminar separation occurs on both surfaces of airfoil at small angles of attack. With the increase of angle of attack, laminar separation occurs and then reattaches near the trailing edge on the upper surface of airfoil, which forms laminar separation bubble. When the attack of angle reaches certain value, the laminar separation bubble is unstable and produces two kinds of large scale vortex, i.e. primary vortex and secondary vortex. The periodic processes that include secondary vortex production, motion of secondary vortex and vortex shedding cause fluctuation of the lift coefficient. The periodic time varies with attack of angle. The secondary vortex is relatively stronger than the primary vortex, which means its influence is relatively stronger than the primary vortex.     

Numerical Simulation of Unsteady Flow Around Forward Flight Helicopter with Coaxial Rotors

Three-dimensional unsteady Euler equations are numerically solved to simulate the unsteady flows around forward flight helicopter with coaxial rotors based on unstructured dynamic overset grids. The performances of the two coaxial rotors both become worse because of the aerodynamic interaction between them, and the influence of the top rotor on the bottom rotor is greater than that of the bottom rotor on the top rotor. The downwash velocity at the bottom rotor plane is much larger than that at the top rotor plane, and the downwash velocity at the top rotor plane is a little larger than that at an individual rotor plane. The downwash velocity and thrust coefficient both become larger when the collective angle of blades is added. When the spacing between the two coaxial rotors increases, the thrust coefficient of the top rotor increases, but the total thrust coefficient reduces a little, because the decrease of the bottom rotor thrust coefficient is larger than the increase of the top rotor thrust coefficient.     

Residual Strength of Stiffened LY12CZ Aluminum Alloy Panels with Widespread Fatigue Damage

Experimental and analytical investigations on the residual strength of the stiffened LY12CZ aluminum alloy panels with widespread fatigue damage （WFD） are conducted. Nine stiffened LY12CZ aluminum alloy panels with three different types of damage are tested for residual strength. Each specimen is pre-cracked at rivet holes by saw cuts and subjected to a monotonically increasing tensile load until failure is occurred and the failure load is recorded. The stress intensity factors at the tips of the lead crack and the adjacent WFD cracks of the stiffened aluminum alloy panels are calculated by compounding approach and finite element method （FEM） respectively. The residual strength of the stiffened panels with WFD is evaluated by the engineering method with plastic zone linkup criterion and the FEM with apparent fracture toughness criterion respectively. The predicted residual strength agrees well with the experiment results. It indicates that in engineering practice these methods can be used for residual strength evaluation with the acceptable accuracy. It can be seen from this research that WFD can significantly reduce the residual strength and the critical crack length of the stiffened panels with WFD. The effect of WFD crack length on residual strength is also studied.     

恒幅及飞行模拟载荷下厚度对裂纹增长的影响

不同厚度试样的疲劳裂纹扩展试验及断口微观分析表明,恒幅加载低应力比飞行模拟载荷条件下具有明显的厚度影响。考查了模型对厚度影响的预测能力。Newman的模型具有预测厚度影响的潜力,广义改进的wellenborg模型不含有厚度影响的考虑,即使通过调整约束系数α的方法也不能计及厚度的影响。厚度对裂纹增长影响的物理原因及模型化有待于进一步研究。     

发动机试车台推力测量系统中心加载现场校准技术研究

介绍了发动机试车台推力秤中心加载校准方法,并与平面加载校准方法进行了试验比较,进而得出相应结论。     

谱载下裂纹扩展的计算及实验分析

本文针对裂纹扩展的Willenborg-Chang模型,提出了如何合理地选择参数,怎样对残余塑性区尺寸进行修正及随机加载谱的处理。通过实验和计算结果的对比,说明上述想法的正确性,并提出Willenborg-Chang模型更适用于较长裂纹的寿命估算。     

高负荷风扇级环境下叶片反问题设计

在对国内外各类叶片三维反问题深入分析的基础上,针对高效、高负荷风扇的设计需求,提出了级环境下风扇叶片三维反问题设计的技术思路。以高压比单级风扇为例,利用数值模拟、流动分析等技术手段,采用从基元截面、单排到单级环境逐步深入的方式,对级环境下三维反问题设计方法的可行性进行了验证,初步探索了级环境下叶片载荷分布规律,进一步发展并完善了高效、高负荷风扇叶片的三维反问题设计技术。     

非比例加载下GH4169高温多轴疲劳行为研究

利用薄壁管拉扭疲劳试样, 在高温控制应变循环加载下研究高温合金GH4169的多轴疲劳行为.高温多轴疲劳试验采用比例与非比例加载路径.在试验过程中,利用数据采集系统全程记录拉与扭的应力响应值以研究比例与非比例拉扭加载下的循环特性.研究结果表明,高温拉扭非比例加载下,疲劳寿命有明显降低;拉与扭应力响应多为循环软化,无明显的循环稳定现象.在低频加载下,由于高温蠕变效应加大,使循环软化速度加快,表现为曲线下降明显.在整个疲劳过程中,扭转应力响应分量均显示循环软化现象,而拉压应力响应分量的硬化与软化特性取决于应变加载参数.     

离心压缩机叶片载荷分析

本文通过对常用叶片载荷型式的分析,推荐一种较理想的叶片载荷型式,并展示高效叶轮的设计实例。 根据欧拉方程,对于理想流体的绝热流动,单位质量流体通过叶轮所获得的功可用理论能量头h_(th)表示:     

嵌入式系统Bootloader分析及DSP56F800串口加载功能实现

简要分析嵌入式系统Bootloader和基于飞思卡尔DSP56F800串口加载功能的实现。可以让客户在无程序源代码的情况下方便地实现产品的程序加载工作。     

叶轮机通流计算的时间推进方法

本文首次将时间推进思想应用于通流计算,采用四级龙格-库塔方法求解,根本上解决了目前出现的轴向流动跨音高负荷叶轮机设计困难,初步工作取得了较好结果。作者们预期,时间推进通流计算方法还将弥补现有许多通流计算方法的不足,将在工程设计中发挥更大作用。      

修正的有效系数法

 提出了一种修正的有效系数法。该方法无需将真实载荷谱从绝对值最大的载荷处截断,即可进行构件危险部位的局部应力应变响应模拟,从而得到构件危险部位真实的局部应力应变响应和真实的局部应力 (或局部应变 )谱。用该方法分析了一个缺口试件在随机谱载下的裂纹形成寿命,结果比常规的有效系数和雨流法更接近试验结果。说明该方法比常规的有效系数法更加合理,也更具实用性。     

直升机疲劳和静力试验伺服控制系统的研究

本文主要详细的介绍了用于直升机动部件结构的疲劳和静力试验中全数字式的多通道伺服控制系统的总体设计及各功能模块的划分。