Evaluating Schmid criterion for predicting preferential locations of persistent slip markings obtained after very high cycle fatigue for polycrystalline pure copper

Very high cycle fatigue carried out on pure copper polycrystals promotes early slip markings, labelled as slip markings of types II and III, localized close to grain or twin boundaries. In this work, we focus on whether Schmid criterion can predict the preferential sites of slip markings of types II and III and identify the active slip systems. Combining observations of slip markings and polycrystalline modeling, it is shown that considering pure cubic elastic behavior, maximum resolved shear stress as a criterion for type II slip markings preferential sites is 70% reliable criterion. Concerning slip markings of type III, the reliability falls to 30%. The role of cross slip is highlighted and a scenario rationalizing the stress amplitude conditions and sites to observe early slip markings of type II or III for copper polycrystals is proposed.     

Structural Orientation and Anisotropy in Biological Materials: Functional Designs and Mechanics

Biological materials exhibit anisotropic characteristics because of the anisometric nature of their constituents and their preferred alignment within interfacial matrices. The regulation of structural orientations is the basis for material designs in nature and may offer inspiration for man‐made materials. Here, how structural orientation and anisotropy are designed into biological materials to achieve diverse functionalities is revisited. The orientation dependencies of differing mechanical properties are introduced based on a 2D composite model with wood and bone as examples; as such, anisotropic architectures and their roles in property optimization in biological systems are elucidated. Biological structural orientations are designed to achieve extrinsic toughening via complicated cracking paths, robust and releasable adhesion from anisotropic contact, programmable dynamic response by controlled expansion, enhanced contact damage resistance from varying orientations, and simultaneous optimization of multiple properties by adaptive structural reorientation. The underlying mechanics and material‐design principles that could be reproduced in man‐made systems are highlighted. Finally, the potential and challenges in developing a better understanding to implement such natural designs of structural orientation and anisotropy are discussed in light of current advances. The translation of these biological design principles can promote the creation of new synthetic materials with unprecedented properties and functionalities.     

Estimation of anisotropic properties of nano-structured arrays by modal vibration control at microscale

Nano-structured arrays are engineered to meet the requirements of a variety of applications such as microfilters, sensors, and structural interface due to their unique mechanical characteristics, which cannot be achieved by conventional solid materials. However, it is hard to evaluate the elastic properties of nano-structured arrays owing to the discrete structure, sample size, and availability of suitable techniques. To facilitate this, we develop an advanced three-dimensional microscale vibration testing process. In the test, a specially designed three-dimensional microspecimen with tuned mass is excited by a piezoelectric actuator, and the resonance frequencies are detected by a laser device successfully. The anisotropic elastic moduli of nano-structured array composed of helical nano-springs are identified from a single spectrum. This array shows so strong characteristic anisotropy that the solid one hardly can attain. The microscale testing technique can be extended to other materials and microstructures.     

Experimental and numerical assessment of energy absorption capacity of thin-walled Al 5083 tube produced by PTCAP process

The energy absorption capacity of the Al5083 thin-walled tube produced by parallel tubular angular pressing (PTCAP) process was evaluated. Also, microstructure, mechanical properties, and anisotropy coefficients were studied in the peripheral and axial directions. Results showed that values of energy absorption decreased with processing pass increasing and the values for the unprocessed, first and second passes were obtained to be 167, 161.4 and 160.7 J, respectively. The differences between the simulation results for the energy absorption values and their experimental values for the unprocessed, the first and the second PTCAP passes samples are about 5%, 10%, and 13%, respectively. The energy absorption capacity was related to the anisotropy coefficient and microstructure. The results demonstrated that grain refinement occurred and ultimate tensile strength (UTS) and microhardness after the first and second PTCAP passes were enhanced, while the increase rate in the first pass was much severer. Also, by applying PTCAP, the deformation modes were altered, such that the deformation mode of the annealed tube was quite symmetrical and circular while for the first and second passes there have been triple and double lobes diamond. The results of the numerical simulation for the deformation mode of the annealed and PTCAPed tubes were consistent with the experimental results. The deformation mode of tubes is dependent on their mechanical properties and variation of the mechanical properties during PTCAP process.     

纳米压痕法研究单晶Al3Sc相的硬度和杨氏模量

使用EBSD和纳米压痕法研究毫米级块状单晶Al₃Sc对应的取向、硬度和杨氏模量。试验结果表明,选用过共晶Al-Sc合金加热至液态后缓慢冷却(60 ℃/h)可以得到毫米级单晶Al₃Sc,通过EBSD和纳米压痕法得到五个不同取向(567)、(139)、(124)、(113)和(144)单晶Al₃Sc的硬度在3.7~4.3 GPa,复合弹性响应模量在143.6~146.1 GPa。对比不同泊松比下各取向的杨氏模量理论值与试验值,发现泊松比为0.188时二者之间差异最小,对应各取向的杨氏模量试验值在157.5~160.6 GPa。     

The effect of evaporation on size and shape evolution of faceted gold nanoparticles on sapphire

We studied the size and shape evolution of about 180 faceted gold nanoparticles attached to a sapphire substrate during annealing at 950 °C in air. We employed the scanning force microscopy and interrupted annealing techniques to track the changes in size and shape of individual nanoparticles. The height of all single-crystalline nanoparticles was constant up to the longest cumulative annealing time of 65 h. The lateral dimensions of ∼20% of all nanoparticles shrunk during anneals, while all three dimensions of the remaining 80% of nanoparticles remained constant. Only the nanoparticles with the height below the average (for all particles) were laterally shrinking. We formulated a kinetic model relating the lateral shrinkage of the nanoparticles to the evaporation of Au atoms adsorbed on sapphire. We also assumed that the process controlling particles shrinkage is the slow self-diffusion of Au atoms along the lateral facets of the nanoparticles toward the substrate. The model predicted a power law dependence of the shrinkage rate on the particle height, with the exponent n = 3. The corresponding exponent determined from the experimental data was n = −2.9 ± 0.3, in excellent agreement with the theory. The low value of the effective self-diffusion coefficient along the lateral facets determined with the aid of our model (3.2 ± 0.2 × 10⁻¹⁷ m² s⁻¹) was attributed to the difficulties of step nucleation on atomically flat facets.     

Triaxiality at crack tip for combined Lankford's coefficient and degree of anisotropy subjected to mixed-mode (I/II) fracture

Structural parts used in boilers, turbines, ships, and many household purposes are manufactured through sheet metal forming processes. During manufacturing, the micro structure of the material is deformed and micro cracks along with anisotropic properties get induced. Present research deals with a thin sheet metal plate containing a central crack subjected to mixed mode (I+II) loading. With special reference to Lankford's coefficient and degree of anisotropy, the effect of anisotropic triaxiality on crack initiation angle has been investigated. The result reveals the combinations of Lankford's coefficient and degree of anisotropy for which crack initiation angle do not change.     

高能球磨结合粉末冶金法制备碳纳米管增强7055Al复合材料的微观组织和力学性能

采用高能球磨结合粉末冶金工艺制备了碳纳米管(CNT)含量(体积分数)分别为0、1%和3%的CNT/7055Al复合材料。采用OM、SEM、TEM以及拉伸实验等方法研究了CNT/7055Al复合材料的CNT分布、晶粒结构、近界面结构及力学性能,分析了复合材料的强化机制和各向异性。结果表明,CNT/7055Al复合材料为无CNT的粗晶区与富集CNT的超细晶区组成的双模态晶粒结构;CNT在Al基体的超细晶区中分散良好,CNT-Al界面干净清洁,界面反应产物少;3%CNT/7055Al复合材料沿挤压方向的抗拉强度达到816 MPa,但延伸率仅为0.5%。细晶强化和Orowan强化是CNT/7055Al复合材料主要的强化机制。由于CNT沿不同方向的增强效率不同以及粗晶条带组织的存在,复合材料表现出比基体合金更强烈的各向异性,在垂直挤压方向的拉伸性能要弱于沿挤压方向的拉伸性能。     

基于偶极横波测井资料预测储层压裂缝几何参数

利用偶极横波测井资料算出泊松比、弹性模量和最小水平主应力等岩石力学参数,基于IVERSON模型首先估算射孔层段压裂缝延伸高度,通过工区储层各向异性系数与裂缝宽度的回归关系估算裂缝宽度,根据椭球体积模型并结合压裂液的利用率及加砂量估算裂缝的径向延伸长度。将该方法优化编程,用于处理工区压裂前后井的测井资料,对比分析预测结果与实际压裂效果,两者较为一致,表明该方法效果明显,值得推广应用。