Creep-rupture behaviour of notched oxide/oxide ceramic matrix composite in combustion environment

ABSTRACT

Creep-rupture tests were performed in the combustion environment on double-edge notch and centre hole oxide/oxide ceramic matrix composite specimens. The specimens were exposed to the maximum temperature of 1250?±?50°C in the notch region where the combustion flame directly impinged. Specimens were loaded to the desired creep load levels and the loads were sustained till either the specimens ruptured or a run-out time of 25?h was achieved. Optical and scanning electron microscopes were used to characterise specimen damage. The test results were compared to its counterparts in 1200°C (isothermal) laboratory air environment. At a given creep life, the applied creep stress for both the notch types was generally lower in the combustion environment than the laboratory air environment. Finite element simulations attributed lower applied creep stress in the combustion environment to the presence of thermal gradient stresses, which were not present in the isothermal laboratory air environment.     

不同小试样测量蠕变性能的比较研究

小试样蠕变试验技术,在服役构件力学性能测试中得到了广泛的发展与应用。目前已有小尺寸单轴试验、小冲杆试验、剪切冲压试验、压痕试验、三点弯试验、悬臂梁试验、固支直杆弯曲试验、圆环试验和双杆试样等方法。基于上述小试样方法,重点从试样尺寸、应力状态、关联式类型、蠕变性能、变形测量和影响因素等方面,全面地综述不同小试样测量材料蠕变性能的方法和结果的异同。通过对比发现,不同类型试样所得蠕变曲线差异较大。小冲杆试样的数据转换过程较为复杂,试验过程中影响因素较多。悬臂梁试样和圆环试样的等效标距较大,变形测量误差小。三点弯、悬臂梁和圆环试样,蠕变试验过程中存在大变形效应,可采用临界载荷法预先控制试验载荷范围。固支直杆弯曲试样方法对摩擦的敏感性较低,可获得较为准确的断裂数据。所得结论为选择合适的小试样方法测试材料的蠕变性能提供理论指导和依据。     

机床液压系统爬行原因分析及消除

全面阐述了机床液压系统出现爬行的机理、危害，分析了产生爬行的原因，指出了消除爬行的措施，为改善机床液压系统稳定性和提高机床加工精度提供了重要的参考。     

Sn60Pb40焊层在热循环过程中蠕变行为的仿真研究

文中采用ANAND焊点本构关系模型描述了Sn60Pb40焊层蠕变行为,通过MARC有限元软件模拟了电子封装器件Sn60Pb40焊层在热循环中的蠕变过程,研究了Sn60Pb40焊层的蠕变应变最大值出现的位置,分析了封装结构热失配和焊层厚度对Sn60Pb40焊层蠕变应变的影响。结果表明:焊层边角处最早发生蠕变断裂,降低封装结构热失配程度和优化焊层厚度均可减小Sn60Pb40焊层的蠕变变形。该研究结果不仅为封装结构的热匹配优化设计提供了新的思路,也为预测焊层蠕变断裂位置和优化工艺提供了技术支持。     

Interpretation of viscous deformation of Zr-based bulk metallic glass alloys based on Nabarro-Herring creep model

Superplastic-like viscous deformation of bulk metallic glass alloys around the glass transition temperature (Tg) was analyzed based on the Nabarro-Herring creep model, a classical creep model, where the diffusional motion of atoms or vacancies through the lattice (atomic configuration) is considered. The amorphous matrix of bulk metallic glasses that has a randomly-packed atomic configuration was assumed to behave in a manner similar to the grain boundary in polycrystalline metals so as to approximate the diffusivity of the major constituent element. In spite of rough approximation of the parameters in the Nabarro-Herring creep equation, a reasonable value of the diffusion path (d) could be obtained from the experimentally-obtained metal flow data, including the steady state stress and the strain rate. Due to the absence of vacancy sources such as grain boundaries in homogeneous metallic glasses, the diffusion path, which, in polycrystalline materials, generally is the average distance between vacancy sources such as grain boundaries, was considered in this work as the average distance between tunneling centers in bulk metallic glass alloys. The calculated diffusion path was comparable to the density of tunneling centers around Tg, proposed by M. H. Cohen and G. S. Grest based on free volume theory. The calculated diffusion path showed monotonous decrease with temperature over Tg for Zr-based bulk metallic glass alloys. Based on this analysis, a schematic model for viscous deformation of bulk metallic glass was proposed.     

不锈钢板翅结构钎焊残余应力及其高温蠕变松弛行为三维有限元分析

利用有限元软件ABAQUS,对不锈钢板翅结构在钎焊过程中产生的残余应力及其高温下的蠕变松弛行为进行三维有限元分析,得到了残余应力的变化规律.结果表明,由于镍基钎料BNi-2和不锈钢母材304力学性能的差异以及夹具的约束作用,钎焊后在接头处产生了较大的残余应力.在高温环境下,由于蠕变松弛,残余应力大幅度下降.在蠕变稳态阶段,钎缝处仍然存在一定的残余应力;钎角处蠕变应力和应变集中,易萌生裂纹,成为最薄弱环节.研究结果为板翅结构的高温强度设计提供参考.     

Fe-15Cr-25Ni合金晶界特征与蠕变行为

在两种不同晶界状态下研究了Fe-15Cr-25Ni合金蠕变行为。单相合金表现出典型的回复蠕变行为,而在晶界上密集析出碳化物时蠕变速度大大降低,蠕变规律也和单相合金明显不同。沿晶界析出密集排列的碳化物时,晶界附近位错密度高,亚晶细小。对两种晶界状态下的蠕变机制进行了讨论。     

Creep deformation behavior and failure diagnostic diagrams of a columnar-grained nickel-based superalloy

Uniaxial creep deformation of a columnar-grained cast Ni-based superalloy has been analyzed in the temperature range of 1035-1315 K. Two-stage heat treatment of the material resulted in an average γ′ particle size of 0.4 μm and inter-particle distance of 0.04 μm. At 1035 K, the alloy showed predominantly primary creep with very low tertiary creep strains. At temperatures above 1035 K, a pseudo-tertiary creep regimen is predominant. Faceted fracture, coarsening of γ′ particles, decohesion of the matrix around the particles, and rafting of the coarsened γ′ particles have been observed as failure modes over the temperature range studied. An analysis is presented to understand creep deformation behavior of the material in the secondary and tertiary creep stages, by expressing them as power functions of time. Failure diagnostic diagrams have also been constructed for the material over the temperature range investigated. Disclaimer: The views expressed in this paper are solely those of the authors alone and do not reflect in any way those of the American Bureau of Shipping.     

高温多轴疲劳损伤与寿命预测研究进展

介绍国内外疲劳一蠕变交互作用研究的一些进展，其中包括微观裂纹萌生及扩展机理的研究、高温多轴疲劳一蠕变实验的发展。对四种具有代表性的疲劳一蠕变损伤累积模型及寿命预测方法，即线性损伤累积模型、损伤率法、延性耗竭法和过应力法进行较为详尽的描述，并对这几种方法在多轴领域的改进和推广进行重点介绍。     

热处理对金属基短纤维复合材料蠕变性能影响的试验和理论研究

采用试验和有限元方法研究了热处理制度对金属基短纤维复合材料(MMC)蠕变性能的影响，同时考虑了2种不同热处理制度和1种铸造状态。试验结果表明，MMC的蠕变响应与热处理制度有关。在相同蠕变应力时，铸态试样有最小的最小蠕变应变率和最长的蠕变寿命，而热处理制度2(试样在550℃保温24h后随炉冷却)具有明显的最大的最小蠕变应变率和最短蠕变寿命，热处理制度1(试样在550℃保温24h后水冷)具有中等的最小蠕变应变率和中等的蠕变寿命。用单胞模型结合有限元方法模拟分析了热处理的影响，结果表明，热处理制度的影响可以归于热处理引起的残余应力和残余应变以及它们的历史，具体可以归于基体的蠕变耗散能、纤维轴向力和纤维—基体界面力。同时考虑了单胞模型的参数对上述分析结果的影响。