Study of hardening and cyclic plastic behaviour around the crack tip of C(T) specimen made by as‐received and annealed copper

The purpose of this research is determining experimentally the characteristics of tension and cyclic plastic behaviours of as‐received and annealed coppers and studying distribution of stress/strain field near the crack tip. Samples made by pure copper were annealed at 420°C for 40 minutes in electric furnace. To determine the properties of the cyclic plastic behaviour, proper tests with symmetric strain‐controlled conditions were performed on standard samples. Chaboche nonlinear hardening model was used to determine the cyclic plastic behaviour of both materials. According to results, annealing process creates isotropic hardening in the copper and also changes its initial kinematic hardening behaviour. Effects of the annealing and hardening on the variations of the stresses and strains around the crack tip were investigated. Also, ratcheting and mean stress relaxations versus number of cycles, inside the plastic region, were studied.     

界面性能对SiCp/6061Al复合材料棘轮行为的影响

利用复合材料细观有限元分析方法,对SiC颗粒增强6061Al合金复合材料的单拉行为、单轴棘轮行为进行数值模拟。模拟中讨论了耦合自由边界、界面结合状态对复合材料棘轮行为的影响;同时,分析了基体和界面的微观变形特征及其演变规律。选取1组合理的微结构参数,对复合材料的棘轮行为进行数值模拟,并通过与实验结果的比较,检验有限元模型的合理性。结果表明：耦合边界很大程度改善了模拟结果;界面结合状态越好,即界面弹性模量、屈服强度和硬化模量越高,产生的棘轮变形越小;具有合理参数值的弱界面模型给出的棘轮变形预测结果比完好界面模型的结果更接近于实验值。     

不同温度下MDYB-3有机玻璃的棘轮效应

对在不同温度和应力状态下, MDYB-3有机玻璃的棘轮效应进行了实验和分析.运用棘轮应变与应力-应变滞回环能量的对应关系和高聚物塑性变形的应力促进热激活理论,建立了一种新的适用于高聚物材料的稳态棘轮应变增长率的计算模型,揭示了温度、载荷频率、平均应力和应力幅值对棘轮应变演化的影响.将该模型应用于不同温度下MDYB-3有机玻璃疲劳过程中的棘轮变形增长行为的描述,理论预测结果与试验结果吻合良好.     

SiCp/6061Al合金复合材料循环变形行为的有限元模拟

在细观分析模型的基础上,利用ABAQUS有限元程序对T6热处理后不同体积分数的颗粒增强SiCp／6061Al合金复合材料的单调拉伸行为和单轴循环变形行为进行了有限元数值模拟;同时,通过与相应的实验结果的比较对模拟效果进行了评价．模拟结果表明：由于采用了能够描述基体循环棘轮行为的新的本构模型,利用有限元法模拟可对颗粒增强金属基复合材料的循环变形行为,特别是循环棘轮行为进行合理的描述．模拟结果还揭示了复合材料循环变形行为在细观层次上的不均匀性和复杂性．     

Phenotypic Selection of Magnetospirillum magneticum (AMB‐1) Overproducers Using Magnetic Ratcheting

Magnetosomes, magnetic nanoparticles (MNPs) encapsulated in lipid membranes produced by magnetotactic bacteria (MTB), have superior properties such as a narrow size distribution, making them of potential significant value for biomedical and industrial applications. However, the slow growth rate and genetic complexity of MTB have thus far limited large scale production of biologically synthesized MNPs. This problem is compounded by a lack of a platform to select MTB of interest. Here, the development of a magnetic ratcheting system that facilitates automated, live, and quantitative isolation of Magnetospirillum magneticum (AMB‐1) based on their phenotype, i.e., number of MNPs is described. Using repeated (five) cycles of random chemical mutagenesis and magnetic selection, AMB‐1 overproducers that biomineralize on average 2.2‐fold more MNPs (≈25) than the widely available strain AMB‐1 are generated. The size, shape, and magnetic properties of the MNPs of the overproducers are also similar to the control AMB‐1, supporting the utility of the platform for enriching MTB with overproducer phenotypes.     

Effect of stress amplitude on uniaxial ratcheting of aluminum alloy 2124‐T851

This paper presents the latest research results of ratcheting behavior of aluminum alloy 2124‐T851 under uniaxial loading. All fatigue tests were carried out at the Technical University of Ostrava mainly under block loading in order to determine the effect of stress amplitude and mean stress on the strain response of the material. Experimental results show that the critical value of the axial deformation is close to the ductility of the material investigated. The aim of this paper also is to draw important conclusions in terms of ratcheting modelling for the material investigated.     

Ratcheting‐fatigue behaviour of bainite 2.25Cr1MoV steel with tensile and compressed hold loading at 455°C

The ratcheting behaviour of a bainite 2.25Cr1MoV steel was studied with various hold periods at 455°C. Particular attention was paid to the effect of stress hold on whole‐life ratcheting deformation, fatigue life, and failure mechanism. Results indicate that longer peak hold periods stimulate a faster accumulation of ratcheting strain by contribution of creep strain, while double hold at peak and valley stress has an even stronger influence. Creep strains produced in peak and valley hold periods are noticeable and result in higher cyclic strain amplitudes. Dimples and acquired defects are found in failed specimen by microstructure observation, and their number and size increase under creep‐fatigue loading. Enlarged cyclic strain amplitude and material deterioration caused by creep lead to fatigue life reduction under creep‐fatigue loading. A life prediction model suitable for asymmetric cycling is proposed based on the linear damage summation rule.     

中碳贝氏体钢的室温单轴循环变形行为研究

为了了解中碳贝氏体钢支承辊的接触疲劳失效机制,对中碳贝氏体钢材料在室温单轴循环加载下的应变循环特性和棘轮行为进行了实验研究。讨论了材料的循环软/硬化特性及其对单轴棘轮行为的影响,同时揭示了该材料棘轮行为的平均应力、应力幅值及其加载历史的依赖性。实验研究表明:材料的循环软/硬化特性具有明显的应变幅值依赖性,进而导致材料在不同的应力水平下出现不同的棘轮行为。研究得到了一些有助于该类材料循环变形行为本构描述的结论。