Institute Of Mechanics,Chinese Academy of Sciences
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Reaction induced elastoplastic deformation and interlayer cracking during oxidation in copper nanowires
No full textCopper nanowires have attracted significant attention for their potential applications in optics, electronics, and catalysis. However, the oxidation of nanowires in service devices can result in severe interlayer cracking, which compromises structural reliability. A detailed investigation is needed to accurately characterize the coupled processes of oxidation, plastic deformation and internal stress, as well as to assess the failure risk of interlayer cracking in nanowires. Here we developed a unified chemo-mechanical coupling model which incorporated oxidation process, oxygen ion diffusion, large plastic deformation and interlayer cracking. A robust finite element program was implemented to model the oxygen ion concentration and stress distribution during oxidation. Cohesive elements were employed to simulate the interlayer cracking behavior of copper nanowires. The results showed that shifts circumferential stress on the nanowire surface from compression to tension. Additionally, higher energy release rates shift the fracture location closer to the nanowire center. These findings advance the understanding of mechanical mechanisms underlying hollow nanostructure formation through oxidation, with implications for other fields involving chemo-mechanical coupling
Seismic efficiency: From hydraulic fracturing-acoustic emission laboratory experiments of shale based on energy budget
No full textFluid injection-triggered earthquakes have been documented worldwide and quite a number of events have significant moment magnitudes (Mw >= 3). Seismic efficiency (n), defined as the ratio of injection volume to net seismic moment release in hydraulic fracturing operations, is a crucial parameter to evaluate seismic hazard. However, a quantitative assessment of seismic and non-seismic (aseismic) energy release is a key aspect of understanding the intrinsic properties of cracking rocks. Therefore, we develop a novel n model based on the hydraulic-fracturing-propagation energy budget and performed laboratory experiments on hydraulic fracturing in shale by injecting distilled water at different rates under pseudo-triaxial stress conditions with simultaneous monitoring of acoustic emission (AE). We estimate AE energy accurately with absolute value correction of sensors using a laser Doppler vibrometer, and the dissipation of the potential energy using displacement and pressure sensors. The results show that the proposed n model can evaluate the induced seismic characteristics effectively compared with field data and the injection rate controls the change of n to some extent. Moreover, there is a log-linear relationship between seismic efficiency and injection efficiency (ratio of AE energy and injection energy), which may provide an experiential method for evaluating seismicity during the early phase of hydraulic fracturing
Advancing the mechanical performance of chemically complex alloys through strategically engineered bamboo-inspired multi-stage heterostructures
No full textInnovative design in heterostructure materials has emerged as a pivotal strategy to address the strength-ductility trade-off in metals and alloys. Inspired by the hierarchical structures found in bamboo, this study engineered a bamboo-like heterogeneous microstructure in a (FeCoNi)86Al7Ti7 chemically complex alloy (CCA) through a multi-step thermomechanical processing route. The bio-inspired triple heterostructures, featuring hierarchical grain sizes and multiscale, multi-morphology precipitates, significantly enhance the balance between strength and ductility, achieving nearly 2 GPa ultra-high tensile strength while maintaining good uniform plastic deformation. During deformation, L12 nanoprecipitates contribute to precipitation strengthening through the shear mechanism, while L21 submicron precipitates within the grains do so via the Orowan looping mechanism. L21 precipitates at the grain boundaries (GBs) act as reinforcement phases in the composite material. The bamboo-like heterostructure also alters dislocation accumulation by constraining deformation between coarse and ultrafine grains, influenced by the surrounding ultrafine grains and the diverse behaviors of precipitates. This pronounced back-stress strengthening across the matrix significantly enhances the strain-hardening capacity, thereby ensuring uniform plastic deformation. Overall, this novel approach demonstrates superior mechanical properties and offers a promising strategy for overcoming the strength-ductility trade-off in advanced alloys
Explosive fragmentation of brittle granular materials
No full textThis study experimentally investigates the dynamic fragmentation behaviors of brittle granular materials subjected to explosive loadings, employing a concentric shell configuration. The setup consisted of a high-explosive sphere surrounded by a densely packed shell of dry glass spheres. To minimize reflection enhancement, the particle shells were confined within thin-walled glass casings, effectively simulating air-exposed conditions. This configuration allowed rarefaction waves reflected from the outer surface of the particle shell to significantly influence particle fragmentation, particularly in thinner shells. A specialized fragment-collecting apparatus was designed to prevent collision-induced damage to particle fragments, enabling the recovery of most fragments with preserved post-test morphologies following the explosion tests. A comprehensive analysis was conducted on the breakage extent and pulverization degree of the fragmented brittle particles, utilizing metrics such as breakage index, fragmentation volume fraction, and fractal dimension. These parameters exhibited significant variations as the particle shell thickness increased from a dimension comparable to the explosive radius to several times that radius. Notably, the thinnest particle shell underwent near-total particle crushing, evidenced by a fractal dimension of up to 3.2, indicating intense fractal crushing. When the shell thickness increased to 3.75 times the explosive radius, the fragmentation volume fraction was nearly halved, and the fractal dimension decreased significantly. These variations in fragmentation behaviors highlight the impact of divergent blast waves, which impart transient explosive loadings with rapidly decaying overpressures on the particles. The experimental results elucidate the relationship between explosive fragmentation and transient explosive loadings, providing estimations for the radii of pulverized and fractured spherical zones. Particles fragmented by explosive loadings exhibit a markedly higher fractal dimension compared to those fractured by quasi-static loadings, even when fragmentation volume fractions are similar. This suggests distinct breakage mechanisms between the two loading conditions
Velocity dependence of adhesive wear due to asperity plowing
No full textAdhesive wear at sliding contact interfaces is governed by the failure of microscale asperities, influenced by material properties, interfacial adhesion, and sliding velocity. This study investigates the velocity-dependent wear mechanisms at the asperity level using coarse-grained molecular dynamics simulations across a spectrum of materials with tunable adhesion potentials. Our results reveal two distinct regimes: 1) Plasticity-dominated regime. At lower velocities, asperity interactions primarily result in plastic smoothing, leading to a decrease in the wear rate with increasing velocity. This behavior arises due to velocity-dependent contact forces, which exert a stronger influence than the relatively weak velocity dependence of wear volumes. 2) Fracture-dominated regime. Beyond a critical velocity, localized fracture generates debris particles, causing wear volumes to surge and wear rates to increase by orders of magnitude. The transition between these regimes is driven by competition between strain rate hardening and inertial effects. These findings clarify the non-monotonic velocity dependence of wear rates in tribological systems and provide predictive criteria for mitigating wear in engineering applications
Numerical Investigation of the Influence of Nanoparticle Shape on Nanofluids Thermocapillary Convection Instability
No full textFor disclose the effect of nanoparticle shape on flow regime and critical condition of thermocapillary convection, the nanofluid thermocapillary convection with different shaped nanoparticles are investigated in this paper, and the oscillation characteristics of thermocapillary flow are analyzed. The results indicate that, the nanoparticles can significantly alter the oscillatory characteristics of thermocapillary convection instability, and the influence of platelet shaped nanoparticle is the strongest and followed by cylinder, blade, brick, and sphere nanoparticles. The main frequency of oscillatory thermocapillary convection decreases with the increase of sphericity, and the size of hydrothermal wave propagation angle at free surface is as follows: sphere > brick > cylinder > blade > platelet
Study on impact resistance and energy dissipation mechanisms of metallic glass nanofilms
No full text非晶合金（金属玻璃，Metallic glasses，MGs）因其独特的长程无序原子结构，展现出优异的力学和物理性能，在航空航天、电子信息、新能源等高科技领域具有重要的应用价值。特别是非晶合金薄膜材料，凭借其独特的快速能量耗散机制和优异的抗冲击性能，在冲击防护领域展现出巨大的应用潜力，为新一代防护材料的开发提供了新的研究方向。本文采用激光诱导微颗粒冲击测试（Laser-induced micro-particle impact testing，LIPIT）技术，结合全原子分子动力学（Molecular dynamics，MD）模拟与量纲分析，系统研究了非晶合金薄膜在动态冲击载荷下的力学响应与能量耗散机制，并提出了高防护性能设计方法。主要研究进展包括：
（1）揭示了单层非晶合金薄膜的动态尺寸效应，建立了防护性能的跨尺度相似律。通过LIPIT实验，发现了由于单层非晶合金薄膜随厚度增加而出现的韧-脆变形转变机制，展现出显著的动态尺寸效应。通过量纲分析，确定了影响单层非晶合金冲击防护性能的无量纲参数。结合MD模拟结果，建立了单层非晶合金薄膜防护性能的跨尺度相似律，为单层非晶合金薄膜冲击防护性能预测提供了理论依据。
（2）通过同种非晶合金薄膜叠层设计，克服了单层薄膜的尺寸效应。基于侵彻形貌表征和MD模拟结果，揭示了多层非晶合金薄膜在冲击载荷下的主要能量耗散机制，获得了多层薄膜临界速度和变形模式之间的关系。获得了氧含量对叠层非晶合金薄膜界面强度的影响规律，提出了影响界面强度的竞争机制，进而阐明了氧含量对多层非晶合金薄膜冲击防护性能的影响规律。
（3）基于MD模拟，研究了G相与L相非晶合金薄膜的冲击防护性能，揭示了G相薄膜在冲击过程中存在额外的能量耗散通道，从而显著提升了其防护性能。发现了G相非晶合金薄膜与L相薄膜相同的动态尺寸效应。随着薄膜厚度的增加，G相和L相薄膜的变形模式从锥形变形过渡到冲塞破坏，均表现出明显的尺寸效应。获得了G相薄膜的临界厚度及其影响机制。
（4）通过非晶合金薄膜与聚合物薄膜的叠层设计，进一步提升了材料的冲击防护性能，展现出优于传统金属防护材料的吸能特性。MD模拟揭示了能量耗散的主要机制，包括界面滑移、聚合物分子链拉伸与非晶合金锥形变形等。明确了不同尺度下非晶合金薄膜与聚合物薄膜的叠层设计优化方案及其耗能机制。</p
Heterogeneous grain structure delivers work hardening and high ductility in a VCoNi alloy with ultrahigh yield strength
No full textStrengthening a metal through cold working or grain refinement can significantly increase its yield strength by several times or even more, while it inevitably leads to a dramatic loss of ductility. The problem is the difficulty of the dislocation multiplication and accumulation within a uniformly-grained structure under uniaxial loading, resulting in the deficiency of work hardening capability. To address this limitation, we have employed the heterogeneous grain structure (HGS) as a microstructural strategy to enhance work hardening to improve ductility. Through thermo-mechanical processing, two types of the face-centered-cubic-structured single-phase HGSs are produced in a VCoNi alloy. The first HGS is composed of recrystallized grains, spanning ultrafine grains (<1 μm in grain size) and fine grains (>1 μm), along with twinned grains of ~200 nm in size, while the second remains a part of deformed structure within the matrix of recrystallized grains. Upon straining, these HGSs exhibit synergistic work hardening, combining the forest dislocation-mediated work hardening with geometri-cally necessary dislocations-based hardening, accompanied by strain partitioning among grains of varying sizes. Furthermore, both HGSs undergo dynamic reinforcement during tensile deformation through grain refinement, particularly evident in the transformation from twinned grains to ultrafine grains, which is more obvious during cryogenic deformation. As a result, the first HGS shows uniform ductility of 19 % and 29 % at yield strength of 1.6 and 1.8 GPa during ambient (298 K) and cryogenic (77 K) deformation, respectively. The second HGS achieves enhanced yield strengths to 1.9 and 2.3 GPa at these temperatures, retaining considerable ductility of 10 % and 14 %. These strength-ductility combinations outstrip those in conventional alloys and multi-principal element alloys.</p
Impact of surface ablation on the evolution of Mack instability modes in hypersonic boundary layers
No full text高超声速临近空间飞行器在大气层中做长时间巡航时，表面往往面临严峻的气动加热，进而出现表面烧蚀或微烧蚀的现象。这一现象对飞行器的气动力与气动热（特别是边界层转捩）有重要影响。由于转捩过程中表面摩阻和热流往往突增数倍，对转捩的有效预测是飞行器气动性能设计的关键。然而，目前尚无有效的理论模型来刻画烧蚀效应对转捩的影响。一般，表面烧蚀往往表现为凹凸不平的几何形貌、壁面不均匀热斑和质量引射等效应，同时，高焓气体所带来的热、化学非平衡效应也对流动有重要影响。本文通过渐近分析与数值计算相结合的方法，围绕上述因素进行理论建模，旨在从原理上揭示烧蚀形貌对转捩的影响机理。
（1）建立了刻画烧蚀效应对平均流影响的高焓三层结构理论。基于考虑化学非平衡（Chemical Non-equilibrium，简称 CNE）效应的可压缩 Navier-Stokes 方程组，开展了高雷诺数渐近分析研究，确定了法向三个渐近分层及其主导控制方程，并通过求解底层的非线性边界层方程定量刻画由烧蚀形貌、不均匀热斑以及质量引射所引起的位移函数。
（2）通过多尺度分析，建立了边界层中的失稳模态（Mack 模态）与平均流修正相互作用的高焓局部散射理论。通过引入幅值放大因子，定量刻画烧蚀效应对失稳模态的作用效果。该理论表明，烧蚀效应对 Mack 模态演化的影响包含两个因素，分别是由烧蚀引起的凹坑附近的局部散射效应和由质量与能量注入引起的下游 Mack 模态增长率持续修正效应。
（3）高焓局部散射理论的数值结果表明，当 Mack 模态的频率低于某一临界阈值时，烧蚀效应会促进 Mack 模态的增长；而对于频率较高的 Mack 模态，烧蚀则表现为抑制效应。该临界频率与 Mack 第二模态频段内的最不稳定频率接近。</div
抑制水下航行体初生空化的粗糙带参数优化设计
No full text抑制水下航行体初生空化对减小空化带来的不利影响具有重要意义。本文针对通过改变物面粗糙性来抑制初生空化的方法，开展基于代理模型方法的粗糙带参数优化设计研究。首先，采用数值计算的方法分析粗糙带参数对水下航行体头部空化初生特性的影响机制，并给出粗糙带各参数的初始设计范围，然后，采用代理模型方法进行参数选优分析。结果表明：航行体头部表面设置粗糙带可改变其表面压力分布规律，粗糙带前后边界会引起压力的小幅波动，可改变最小压力值从而影响空化初生特性；通过代理模型的敏感度分析可知，相比于粗糙带的位置和宽度，其高度对初生空化数的影响较大，最终获得的优化结果经数值计算验证可明显降低初生空化数，达到较好的抑制空化初生的效果