Effects of heat treatment on the mechanical properties at elevated temperatures of plain-woven SiC/SiC composites

The effects of heat treatment on the mechanical properties of plain-woven SiC/SiC composites at 927 °C and 1200 °C in argon were evaluated through tensile tests at room temperature and at elevated temperature on the as-received and heat-treated plain-woven SiC/SiC composites, respectively. Heat treatment can improve the mechanical properties of composites at room temperature due to the release of thermal residual stress. Although heat treatment can damage the fiber, the effect of this damage on the mechanical properties of composites is generally less than the effect of thermal residual stress. Heat treatment will graphitize the pyrolytic carbon interface and reduce its shear strength. Testing temperature will affect the expansion or contraction of the components in the composites, thereby changing the stress state of the components. This study can provide guidance for the optimization of processing of ceramic matrix composites and the structural design in high-temperature environments.     

带沟槽涡轮叶片颗粒沉积特性和气膜冷却性能研究

为探究带沟槽叶片的颗粒沉积特性以及气膜冷却性能，以某型高压涡轮含有尾缘劈缝结构的涡轮叶片为原型，针对实际工况建立沉积模型，采用数值模拟方法研究了0<M≤2不同吹风比下沟槽结构对叶片表面颗粒沉积特性和叶片表面气膜冷却性能的影响规律。结果表明：沟槽结构提高了总碰撞效率，降低了总沉积效率，颗粒易沉积于气膜孔下游以外区域以及端壁两侧，沟槽导致压力面中后部颗粒沉积的区域增大；随着吹风比的增加，沟槽内部捕获效率提高，整体颗粒捕获效率降低，沟槽内部的气膜冷却性能不断下降，但沟槽下游部分区域的气膜冷却性能优于原始结构；沟槽的存在使下游附近沿展向的气膜覆盖区域变大，冷却性能提升，沿孔流向的展向平均气膜冷却效率最高可提升18%。     

碳化硅陶瓷基复合材料在航空发动机上的应用需求及挑战

随着航空发动机推重比的不断提高,急需发展轻质、高强韧、耐高温、长寿命、抗烧蚀、抗氧化的碳化硅陶瓷基复合材料(SiC matrix ceramic composites,CMC-SiC),以满足航空发动机愈加苛刻的服役要求。本文简要介绍了CMC-SiC复合材料的特点和制备方法,综述了CMC-SiC复合材料在国外先进航空发动机热端部件上的应用进展及国内的研究现状。从工程化角度,指出了国内在高性能纤维、构件设计及制备、环境障涂层、无损检测技术、考核验证方法、修复技术等方面存在的差距及需突破的关键技术,指出了今后国内的研究目标与发展方向。     

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg₂SiO₄–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg₂SiO₄ (M-M₂S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M₂S aggregates increased Si_xO_y^z− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO₂–Al₂O₃–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.     

Full-field dynamic strain reconstruction of an aero-engine blade from limited displacement responses

Blade strain distribution and its change with time are crucial for reliability analysis and residual life evaluation in blade vibration tests. Traditional strain measurements are achieved by strain gauges (SGs) in a contact manner at discrete positions on the blades. This study proposes a method of full-field and real-time strain reconstruction of an aero-engine blade based on limited displacement responses. Limited optical measured displacement responses are utilized to reconstruct the full-field strain. The full-field strain distribution is in-time visualized. A displacement-to-strain transformation matrix is derived on the basis of the blade mode shapes in the modal coordinate. The proposed method is validated on an aero-engine blade in numerical and experimental cases. Three discrete vibrational displacement responses measured by laser triangulation sensors are used to reconstruct the full-field strain over the whole operating time. The reconstructed strain responses are compared with the results measured by SGs and numerical simulation. The high consistency between the reconstructed and measured results demonstrates the accurate strain reconstructed by the method. This paper provides a low-cost, real-time, and visualized measurement of blade full-field dynamic strain using displacement response, where the traditional SGs would fail.     

Micromechanical model of time-dependent damage and deformation behavior for an orthogonal 3D-woven SiC/SiC composite at elevated temperature in vacuum

This paper presents a micromechanical model to predict the time-dependent damage and deformation behavior of an orthogonal 3-D woven SiC fiber/BN interface/SiC matrix composite under constant tensile loading at elevated temperature in vacuum. In-situ observation under monotonic tensile loading at room temperature, load–unload tensile testing at 1200 °C in argon, and constant load tensile testing at 1200 °C in vacuum were conducted to investigate the effects of microscopic damage on deformation behavior. The experimentally obtained results led to production of a time-dependent nonlinear stress–strain response model for the orthogonal 3-D woven SiC/SiC. It was established using the linear viscoelastic model, micro-damage propagation model, and a shear-lag model. The predicted creep deformation was found to agree well with the experimentally obtained results.     

基于随机过程的三维粗糙表面接触刚度研究

目的 建立三维粗糙表面法向接触刚度的有限元模型,研究法向接触刚度对粗糙度、自相关系数、弹性模量、屈服极限等参数的敏感性。方法 首先基于随机过程理论,采用二维数字滤波技术,生成满足Gauss分布和指数自相关函数的粗糙表面,建立三维粗糙表面接触有限元模型。然后根据结合面静位移与结合面受力关系推导出静刚度表达式,得到两粗糙表面的法向接触刚度。根据中心复合试验设计方法选取的样本点和有限元计算结果,建立二阶响应面模型。最终定义了临界自相关系数,并研究了临界自相关系数与其他参数的关系。结果 有限元计算得到的法向接触刚度结果合理,与试验结果相比最大误差不超过12.34%。弹性变形、塑性变形及真实接触面积随载荷的增加逐渐增大。法向接触刚度与粗糙度呈现负相关趋势,粗糙度不变时法向接触刚度随自相关系数的增大先增大后减小;法向接触刚度与弹性模量呈负相关趋势,法向接触刚度与屈服强度呈正相关趋势,且粗糙度的改变对法向接触刚度影响最大。当压力为200 MPa时,粗糙度、自相关系数、弹性模量、屈服极限分别为0.8 μm、18.91、240 GPa、355 MPa,法向接触刚度达到最大值121.53 MPa/mm,优化后接触面的法向接触刚度提高247%,并给出了临界自相关系数选取公式。结论 所建立模型正确、准确,为粗糙表面法向接触刚度计算提供一种有效方法,可为航空发动机安装边结构设计提供理论指导。     

一种压阻式压力传感器全温区温度补偿方法

针对压阻式压力传感器因热零点漂移、热灵敏度改变以及热迟滞效应引起的误差,提出一种压阻式压力传感器全温区温度补偿方法。该方法是在温升和温降全温区样本采集的数据基础上,采用最小二乘法曲面拟合原理对压阻式压力传感器进行数字补偿。通过对传感器进行实验标定和误差分析,并与常用的单一温升样本采集并进行数字补偿的方法进行对比,结果表明该方法能有效降低传感器因热迟滞效应引入的误差,提高传感器在全温区内标定点和非标定点的测试精度。同时,该方法校准参数少,计算量相对较小,对于硬件要求较低,达到在性能和成本之间的良好平衡,是一种实用性较强的在线补偿方法,具有较强的工程应用价值。     

服役工况及喷丸强化对航空齿轮钢接触疲劳性能的影响

目的 针对不同服役工况和表面处理下航空齿轮钢接触疲劳性能不明确的问题,探究不同工况参数及喷丸强化对AISI 9310航空齿轮钢接触疲劳性能的影响。方法 基于双圆盘滚动接触疲劳试验台,对AISI 9310航空齿轮钢圆盘滚子试件在2种接触应力、3种滑差率的服役工况,以及渗碳磨削、喷丸强化2种工艺下开展滚动接触疲劳性能研究。使用白光干涉仪、X射线衍射应力仪和显微硬度仪对喷丸强化前后的表面形貌、残余应力和显微硬度进行表征,探究喷丸处理后的表面完整性与滚动接触疲劳性能之间的规律。结果 当接触应力由2.5 GPa增至3.0 GPa时,AISI 9310试件的滚动接触疲劳寿命降低了64.7%;当滑差率由10%增至20%、30%时,试件的滚动接触疲劳寿命分别下降了18.9%和42.8%,同时寿命分散性有所降低。此外,研究发现喷丸强化使试件表面残余压应力提升了104.3%,并形成了残余压应力层,表面显微硬度提升了4.8%,同时使试件的沟壑状刀痕转变为凹凸不平的弹坑,表面性状宽高比提升至0.931 0,更有利于润滑油的储存,从而使得试件的滚动接触疲劳寿命提升了87.8%。结论 获取了不同接触应力、滑差率和...     

Vibration based damage detection of rotor blades in a gas turbine engine

This paper describes the problems concerning turbine rotor blade vibration that seriously impact the structural integrity of a developmental aero gas turbine. Experimental determination of vibration characteristics of rotor blades in an engine is very important from fatigue failure considerations. The blades under investigation are fabricated from nickel base super alloy through directionally solidified investment casting process. The blade surfaces are coated with platinum aluminide for oxidation protection. A three dimensional finite element modal analysis on a bladed disk was performed to know the likely blade resonances for a particular design in the speed range of operation. Experiments were conducted to assess vibration characteristics of bladed disk rotor during engine tests. Rotor blade vibrations were measured using non-intrusive stress measurement system, an indirect method of blade vibration measurement utilizing blade tip timing technique. Abnormalities observed in the vibration characteristics of the blade tip timing data measured during engine tests were used to detect the blade damage. Upon disassembly of the engine and subsequent fluorescent penetrant inspection, it was observed that three blades of the rotor assembly were identified to have damaged. These are the blades that exhibited vibration abnormalities as a result of large resonant vibration response while engine tests. Further, fractographic analysis performed on the blades revealed the mechanism of blade failures as fatigue related. The root cause of blade failure is established to be high cycle fatigue from the engine run data history although the blades were put into service for just 6 h of engine operation.