Dynamic tensile strength and failure mechanisms of thermally treated sandstone under dry and water-saturated conditions

To study the tensile strength and failure mechanisms of rock with hydro-thermal coupling damage under different loading rates, a series of static and dynamic splitting tests were conducted on thermally treated sandstone under dry and water-saturated conditions. Experimental results showed that high temperatures effectively weakened the tensile strength of sandstone specimens, and the P-wave velocity declined with increasing temperature. Overall, thermal damage of rock increased gradually with increasing temperature, but obvious negative damage appeared at the temperature of 100 °C. The water-saturated sandstone specimens had lower indirect tensile strength than the dry ones, which indicated that water−rock interaction led to secondary damage in heat-treated rock. Under both dry and water-saturated conditions, the dynamic tensile strength of sandstone increased with the increase of strain rate. The water-saturated rock specimens showed stronger rate dependence than the dry ones, but the loading rate sensitivity of thermally treated rock decreased with increasing treatment temperature. With the help of scanning electron microscopy technology, the thermal fractures of rock, caused by extreme temperature, were analyzed. Hydro-physical mechanisms of sandstone under different loading rate conditions after heat treatment were further discussed.     

Strengthening boron carbide through lithium dopant

The high strength of boron carbide (B₄C) is essential in its engineering applications such as wear-resistance and body armors. Here, by employing density functional theory simulations, we demonstrated that the strength of B₄C can be enhanced by doping lithium to boron-rich boron carbide (B₁₃C₂) to form r-LiB₁₃C₂. The bonding analysis on r-LiB₁₃C₂ indicates that the electron counting rule (or Wade's rule) is satisfied in r-LiB₁₃C₂ whose formula can be written as r-Li⁺(B₁₂)^2-(CB⁺C). The shear deformation on r-LiB₁₃C₂ indicates that its ideal shear strength is larger than that of B₄C because of the existing of Li dopant. The failure process of r-LiB₁₃C₂ under ideal shear deformation initiates from breaking the icosahedral-icosahedral B-B bonds. Then these B atoms react with the middle B in the C-B-C chain, resulting in the disintegration of icosahedral clusters and brittle failure. More interesting, the nanotwinned r-LiB₁₃C₂ is even stronger than r-LiB₁₃C₂ because of the directional nature of covalent bonding at the twin boundaries. This suggests that the nanotwinned r-LiB₁₃C₂ has a significant enhanced strength compared to B₄C. Our simulation results illustrate the deformation mechanism of Li-doped boron carbide and its nanotwinned microstructure. We proposed to improve the strength of boron carbide by doping Li into B₁₃C₂ and increasing its twin densities.     

厚松散层薄基岩坚硬顶板工作面覆岩破坏电法监测

掌握导水裂缝带发育高度对于厚松散层薄基岩煤矿工作面安全开采具有重要的意义。在煤层采动影响前,在工作面巷道向煤层工作面顶板施工1个仰孔,布置孔中电极电缆,形成钻孔电法监测系统。在巷道中连接并行电法仪器和钻孔电缆,数据采集方式称为AM法。随采煤工作面位置逐渐接近并进入钻孔控制范围,监测电极电流值和视电阻率值发生变化。结果表明:对潘北煤矿厚松散层薄基岩坚硬顶板工作面电法监测显示,弯曲下沉带电极电流值和视电阻率值较为稳定,受采动影响程度较小;导水裂缝带内,电极电流值明显下降,视电阻率值明显升高;顶板高度0~40 m采动超前影响范围可达410 m左右;工作面坚硬顶板砂岩地层为控制覆岩破坏的关键层,采空区上方坚硬顶板岩层垮落滞后工作面9~16 m;工作面导水裂缝带高度为37 m,导水裂缝带未发育到基岩面,风化砂质泥岩裂隙在采动应力作用下存在闭合现象。     

喷丸对25CrNi2MoV钢滚动接触疲劳性能的影响

目的提高25CrNi2MoV钢的滚动接触疲劳性能。方法对25CrNi2MoV钢进行表面喷丸处理,并采用3D形貌仪、光学显微镜、显微硬度仪、X射线应力分析仪与滚动接触疲劳试验机等仪器,对试样表面形貌、表面显微组织、显微硬度、表面残余压应力与滚动接触疲劳性能等进行测试分析。结果与未处理试样相比,经喷丸处理后,试样表面形貌由磨削加工槽型向酒窝状的弹坑转变,表面粗糙度增大,表面显微硬度由503HV0.2增大到577HV0.2,增加了14.7%,表面残余压应力由-90.0 MPa增大到-758.0 MPa。当喷丸强度为0.445 mmA时,试样具有最好的滚动接触疲劳寿命,其额定寿命(L10)、中值寿命(L50)、特征寿命(L63.2)分别为4.973×10^6次、6.578×10^6次和6.945×10^6次,分别是未处理试样对应寿命的11.1倍、7.3倍和7.0倍,试样滚动接触疲劳失效形式主要为疲劳剥落。当喷丸强度为0.596 mmA时,试样表面出现微裂纹,导致滚动接触疲劳寿命降低,此时试样疲劳失效形式主要为点蚀与疲劳剥落。未处理试样疲劳失效形式主要为分层。结论喷丸处理能细化试样表层晶粒组织,增大试样表面粗糙度、表面硬度与表面残余压应力。合适强度的喷丸处理可以抑制试样表面与次表面裂纹的萌生与扩展,显著提高滚动接触疲劳性能。     

Effects of defect dipoles on tunable dielectric response in relaxor ferroelectric ceramics

Tunable dielectric materials have drawn much attention due to their wide applications including capacitors and microwave tunable devices. Ferroelectrics materials have special spontaneous polarization which can be reversibly switched by an external electric field. Therefore, tunable dielectric constant can be easily achieved in ferroelectrics. However, the study of nonlinear dielectric response induced by defect dipoles is rarely concerned. Here, we report the effects of defect dipoles on tunable dielectric response under alternative current (AC) and direct current (DC) electric field in defect dipoles introduced Pb(Lu_1/2Nb_1/2)O₃–PbTiO₃ ceramics. A modified Rayleigh model is proposed to successfully characterize dielectric nonlinearity and reveals the interaction between domain walls and defect dipoles. The defect dipoles had more sensitive effect on dielectric response under AC field than that of defect dipoles-free samples. The drop of intrinsic dielectric contribution under AC field results from the detriment effect of defect dipoles. The irreversible contribution is altered by the movements of defect dipoles under AC field, subsequently inducing the nonlinearity of dielectric response. Samples with defect dipoles have larger tunable scope of dielectric properties than that of defect dipoles-free samples. The present work discovers the potential of application of defect dipoles-tuned dielectric response ferroelectrics in devices which requires both high AC and DC biases, and help to better understand the complex dielectric response of ferroelectrics.     

浅埋深高瓦斯工作面瓦斯抽放对采空区自燃“三带”的影响研究

以五虎山煤矿010908工作面为背景,采用理论分析、数值模拟和现场实测等手段对浅埋深高瓦斯工作面瓦斯抽放对采空区自燃"三带"影响进行研究。研究结果表明:当瓦斯绝对涌出量与采空区漏风量处于均衡状态时,此时瓦斯对煤自燃将出现明显的耦合影响;当采空区漏风量小于瓦斯绝对涌出量时,采空区遗煤自燃将受到阻碍;与之相反,当漏风量大于瓦斯涌出量时,采空区遗煤自燃受瓦斯涌出量的影响较小;高位钻孔与工作面距离越远,采空区内部的漏风路径也越长,采空区氧化带、窒息带所处的区域越向采空区深部扩大,但靠近工作面一侧的氧化带范围并没有出现明显变化。     

石油工程岩石强度与变形尺度效应研究现状综述与展望

岩石强度与变形存在显著的尺度效应,致使实验室尺度的岩样强度与变形测试结果不能直接应用于工程范围的大尺度岩体或微纳米级尺度的岩屑(矿物)。实现岩石强度与变形的跨尺度表征,即基于常规岩心测试手段,结合跨尺度表征方法来预测其他尺度的岩石力学参数和变形规律对于油气高效、经济、安全开采具有重要意义。本文在充分调研了国内外关于岩石强度与变形尺度效应研究的基础上,介绍了当前研究岩石强度与变形尺度效应的主要理论、主要认识和影响因素,并对石油工程领域岩石强度与变形尺度效应的研究方向进行了展望。     

深部层状岩体巷道承载特性的物理模拟试验研究

为研究深部高应力条件下层状岩体巷道的承载特性及失稳机理,采用物理相似模拟试验方法,并借助高速摄像机、静态电阻应变仪、位移计等手段,研究了不同支护方式(无支护、锚杆支护、锚杆索支护)层状岩体巷道的变形破坏特征和应力演化规律;同时,从压力拱的角度分析了层状岩体巷道的成拱特性。研究结果表明:无支护条件下,巷道围岩变形破坏严重,顶板大面积垮落,呈现倒V型破坏区,无法形成有效的压力拱结构;采取锚杆支护和锚杆索支护后,巷道围岩应力状态得到改善,能够形成有效的压力拱,围岩变形量减小。研究成果为类似地质条件巷道支护设计提供了依据。     

Mechanical degradation of proton exchange membrane during assembly and running processes in proton exchange membrane fuel cells with metallic bipolar plates

The mechanical degradation of the proton exchange membrane (PEM) is one of the main aspects affecting the lifetime of proton exchange membrane fuel cells (PEMFCs). It was observed in our previous study that the stress/strain distribution in the PEM of fuel cells with metallic bipolar plates (BPPs) is more complex, owing to manufacturing and assembly errors of the BPPs. The present study further concentrates on the stress/strain evolution in the membrane of fuel cells throughout the assembly and running processes by a finite element model. In membranes at the joint area between the gasket and gas diffusion layers, a serious stress concentration aggravated as the misalignment displacement increases. As for the membrane in reaction area, the plastic strain reaches highest level at the center of the groove after hygrothermal loading. The maximum stress is mainly relevant to the temperature and humidity and has little concern with the misalignment. The model and results of this study offer guidance regarding the design of PEMFC. Owing to the stress concentration, an additional protection should be set in the joint area, and the assembly error should be limited within 0.05 mm.     

Effect of temperature on the performance of laterally constrained dielectric elastomer actuators with failure modes

This article analytically investigates the temperature influence on the performance of laterally constrained dielectric elastomer actuators. The effects of both temperature and stretches on the permittivity are taken into consideration. The governing equations of state are established by modeling the actuator as a thermodynamic system of three degrees of freedom. Various failure modes, including electromechanical instability, electric breakdown, loss of tension, and tensile rupture are considered to restrict the operation state and to determine the region of allowable state. Numerical calculations are performed to depict the variations of some important physical quantities with generalized coordinates and to gain insight on the influence of temperature on the critical curves of failures. It is shown that the temperature has obvious effects on the operation state and the allowable region. The present results can be used for designing and optimizing an actuator, and the present approach can be extended to other kinds of dielectric elastomer actuators with complex geometries.