Topology optimization of flexure hinges with a prescribed compliance matrix based on the adaptive spring model and stress constraint

This paper focuses on the configuration design of flexure hinges with a prescribed compliance matrix and preset rotational center position. A new method for the topology optimization of flexure hinges is proposed based on the adaptive spring model and stress constraint. The hinge optimization model is formulated by maximizing the bending displacement with a spring while optimizing the compliance matrix to a prescribed value. To avoid numerical instability, an artificial spring is used as an auxiliary calculation, and a new strategy is developed for adaptively adjusting the spring stiffness according to the prescribed compliance matrix. The maximum stress of flexure hinge is limited by using a normalized P-norm of the effective von Mises stress, and a position constraint of rotational center is proposed to predetermine the position of the rotational center. In addition, to reduce the error of the stress measurement, a simple but effective filtering method is presented to obtain a complete black-and-white design. Numerical examples are used to verify the proposed method. Topology results show that the obtained flexure hinges have the prescribed compliance matrix and preset rotational center position while also meeting the stress requirements.     

Effect of fiber-reinforcement on the mechanical behavior of sand approaching the critical state

Several types of ground improvement methods that employ fiber-reinforcement have been developed in recent years. A series of consolidated drained triaxial compression tests has been conducted here to examine the effect of short fibers on the mechanical properties of Toyoura sand. Sand with 0%, 0.2%, 0.4%, and 1% fiber contents, prepared to yield random distribution, was sheared under several confining pressures and controlled via their initial relative densities. The test results showed that the maximum and residual deviatoric stresses increased, whereas the volumetric expansion decreased with an increase in fiber content. Although the stress ratio η (=q/p′) and specific volume changed depending on the fiber content and confining pressure with shear progression, they each reached the same values for a definite fiber content at the end of shearing, independent of initial relative density. In other words, the unique critical state line can be found for a definite fiber content. Moreover, the greater the fiber content, the larger the slope of the critical state line at the end of shearing. Additionally, as the length of fibers shortened with the same percentage of fiber inclusions in sand, the deviatoric stress and the stress ratio decreased, approaching the shear-strain-volumetric response of unreinforced sand.     

Numerical simulation analysis and experimental validation on wear/fracture mechanisms of polycrystalline cubic boron nitride superabrasives in high-speed grinding

In this study, a two-dimensional finite element model is proposed to investigate the wear/fracture mechanisms of polycrystalline cubic boron nitride (PCBN) superabrasives in high-speed grinding process. The special geometric microstructures of PCBN grains are constructed by using the classic Voronoi tessellation technique, and cohesive elements are embedded into the geometric model of PCBN grains as the potential crack propagation paths for simulating the wear/fracture behaviours of PCBN grains under grinding loads. The effects of uncut chip thickness per grain (a_gmax) on the stress distribution characteristics and wear/fracture behaviours of PCBN grains during grinding are discussed in detail. Results show that the wear behaviour of PCBN grains during grinding mainly occurs around the grain vertex region; however, the fracture behaviour, leading to the quick failure of PCBN grains, is prone to appear around the grain–filler bonding interface, which is usually on the opposite side of the in-feed direction. Moreover, to separate the PCBN grains from the macro-fracture during grinding, the uncut chip thickness per grain should be kept smaller than 1.0?µm to prevent the unfavourable fracture behaviour from appearing around the grain–filler bonding interface. Furthermore, the corresponding single-grain grinding trials are performed to validate the numerical simulation results by evaluating the wear/fracture morphologies of the PCBN superabrasives in the actual grinding operation.     

Elastoplastic stress–strain relationship of Si2N2O–Si3N4 ultrafine-grained ceramics based on microhardness test and finite element simulation

A method for obtaining the stress–strain relationship of ceramic materials was proposed on the basis of the relationship between the maximum load and the indentation size obtained by microhardness test. The microhardness testing process of Si₂N₂O–Si₃N₄ ultrafine-grained ceramics was simulated using ABAQUS finite element software. The stress–strain relationship curve of the material was obtained by repeatedly modifying and comparing the experimental and simulation results. The hardness testing principle and elastic–plastic theory were comprehensively applied in this work in accordance with the geometrical characteristics of the Vickers diamond indenter. The theoretical formula for calculating the stress–strain relationship of hard and brittle materials using microhardness experimental data combined with finite element simulation was deduced. The elastic–plastic area division principle for calculating yield stress was proposed. The accuracy of the theoretical formula was verified by comparing the theoretical and simulation results.     

Study on internal stress damage detection in long-distance oil and gas pipelines via weak magnetic method

Weak magnetic stress detection is an important issue in oil–gas pipeline internal detection area. In order to verify the characteristics of weak magnetic stress internal detection signals, we built herein a magneto-mechanics equivalent model having a balanced magnetic field. First, we calculated the relationship between the stress and the weak magnetic signals; consequently, the analysis propagation laws of the weak magnetic signals with non-magnetic saturation were pointed out. Finally, the theoretical model was validated by a systematic experimental research. The analytical results show that a one-to-one linear link between the weak magnetic signals and the stress concentration is clear. Instead, the change of the weak magnetic signals with the liftoff is nonlinear, therefore we are proposing the Boltzmann liftoff correction factor whose degree of adaptability of the equivalent model can reach the value of 94%. It is possible to note that when the liftoff is in the approximate linear stage, the relevance ratio and the recognition rate of the magneto-mechanics curve show a high-quality. This conclusion is important in the engineering field for the set of the liftoff.     

基于四参数随机生长模型的页岩储层应力敏感分析

页岩储层的应力敏感性是影响其后期开发效果的关键因素，从微观的角度深入认识其应力敏感机理及其影响因素对页岩气的开发具有重要意义。借助四参数随机生长模型构建了不同孔隙度和不同孔隙大小分布的岩心样本，利用弹性力学理论模拟了不同有效应力作用下各个岩心孔隙半径的分布变化及其对岩心固有渗透率的影响，深入分析了孔隙大小及其形状因子与上述两者之间的关系。结果表明，导致页岩应力敏感的直接原因是有效应力作用下孔隙面积的减小及孔隙位置的迁移。有效应力的增大使得各孔隙半径均有减小，孔隙半径的减小比例分别与孔隙初始面积和孔隙的形状因子呈正相关关系和负相关关系。在相同的孔隙度条件下，孔隙半径越均匀，平均孔隙半径越小，应力敏感性越强。有效应力的增加使得岩心固有渗透率呈指数型下降且孔隙度越小、固有渗透率越低的岩心，其应力敏感性越强，孔隙度对固有渗透率的影响大于孔隙半径均匀性的影响。     

Using a pulsed electric current to reduce the susceptibility of high-strength steel to the stress corrosion cracking caused by hydrogen

Under the tensile loading, the damage of metals in the corrosive medium is the most destructive and harmful. In this study, the stress corrosion cracking behavior of H-charged high-strength steel in 3.5 wt% NaCl solution after electropulsing treatment was investigated. The experimental results from elongation, yield strength, fracture morphology, and polarization curves all demonstrate the positive effect of the pulsed processing, as it reduced the susceptibility of steel to stress corrosion cracking by removing hydrogen by electropulsing. The reduction in hydrogen content of the pulsed high–strength steels was attributed to electromigration and increased system free energy, which drove the hydrogen atoms in the steel to de–trap and reduced the susceptibility to stress corrosion cracking.     

The influence of stresses on ageing kinetics of 3Y- and 4Y- stabilized zirconia

Hydrothermal ageing is one of the most important limiting factors for the use of yttria-stabilized zirconia ceramics in contact with water-containing environments. It consists in the transformation of tetragonal phase to monoclinic phase, initiates on the surface of zirconia in the presence of water, and leads to roughening and potentially to micro-cracking and loss of integrity. The present work seeks to explore the influence of applied and residual mechanical stresses on the ageing kinetics of 3Y- and 4Y-TZP. Residual stresses were obtained by rough polishing. A subsequent Annealing step was employed for the preparation of samples free of residual stresses. All samples were submitted to in situ 3-points bending tests in water vapour atmosphere inside an autoclave at 134 °C, allowing surfaces with a mechanical stress gradient to be exposed to hydrothermal ageing. The evolution of the monoclinic fraction with time and stress was then analyzed using Mehl-Avrami-Johnson equation.