基于Lagrangian分析法的梯度泡沫金属动态力学行为研究

采用Lagrangian分析法,对梯度泡沫金属在高速冲击下的变形机理和应力响应进行研究。基于3D-Voronoi技术,构建了5种不同密度梯度的泡沫金属细观有限元模型,并进行了高速冲击下的Taylor数值实验,得到不同密度梯度泡沫金属的质点速度分布规律。采用Lagrangian分析法并结合数值实验结果,研究了高速冲击下密度梯度参数对泡沫金属的局部应变分布、应力分布以及冲击波传播与衰减规律的影响。结果表明:负密度梯度泡沫金属比正密度梯度泡沫金属具有更强的抵抗变形能力,且密度梯度参数越小,变形程度越小;负密度梯度泡沫金属的局部压实应力呈线性减小,最大局部压实应力随着密度梯度参数的减小而增大,在冲击端附近可以承受更大的载荷;正密度梯度泡沫金属的局部压实应力分布呈平台状,其最大局部压实应力小于负密度梯度泡沫金属。     

二阶层级自相似四边形蜂窝动力压缩行为数值模拟

运用ABAQUS对在冲击载荷作用下二阶自相似四边形蜂窝面外方向的动态压缩行为进行了数值模拟,分析了其在冲击载荷作用下的动态压缩过程,并对孔径尺寸、冲击速度和基体材料屈服强度对其压缩模式及应力-应变关系的影响进行了研究。数值模拟表明:孔径较小的二阶自相似四边形层级蜂窝在冲击载荷作用下更易出现整体坍塌的压缩失效模式;随着基体材料屈服强度增大,平台应力值随之增加,但对平台长度没有影响;随着冲击速度的增加,冲击端平台应力值和平台长度都有所增加。     

内凹负泊松比蜂窝结构的面内双轴冲击响应

利用有限元模拟方法研究了内凹负泊松比蜂窝结构的面内双轴冲击响应。用节点扰动方法建立了具有不同规则度的内凹负泊松比蜂窝结构,并将其在不同冲击速度下的变形模态、应力-应变曲线和能量耗散能力与规则蜂窝进行了对比分析。结果表明,冲击速度是内凹蜂窝结构变形模态最主要的影响因素。此外,在双轴冲击下,由于不规则度的引入,延长了应力-应变曲线的平台阶段,抑制了结构的各向异性程度,从而使结构的变形特征从局部密实转向整体密实。在能量吸收能力方面,结构的不规则性导致了密实化阶段的滞后,因此在相同的压缩程度下,其塑性耗散能低于规则模型。     

缺陷对金属蜂窝材料面内冲击性能的影响

 利用显式动力有限元ANSYS/LS-DYNA,数值研究了缺陷（胞元缺失）的分布位置及其尺寸对金属蜂窝材料面内冲击性能的影响。考虑到理想六边形蜂窝材料在不同冲击速度下的变形特征,将试件划分成9个子区域,讨论了缺陷集中位置、缺陷尺寸和冲击速度对蜂窝材料面内冲击变形模式和能量吸收性能的影响。研究发现,蜂窝材料的面内冲击性能依赖于缺陷的分布位置和缺陷尺寸,且在中低速时表现出较高的敏感性,但冲击速度的增加将弱化缺陷分布不均匀性的影响。由于缺陷的存在,蜂窝材料的能量吸收能力明显降低,但与缺陷分布位置相比,蜂窝材料单位体积所吸收的能量更敏感于缺陷尺寸。研究结果将为多胞材料的安全性评估及能量吸收设计提供理论指导和依据。     

退火铝合金中Portevin-Le Chatelier效应的数值模拟研究

通过综合考虑沉淀动力学和动态应变时效机理，建立了一套具有明确物理内涵的唯象本构模型.并在对不同加载应变率下的单轴拉伸实验的数值模拟中，得到了3种类型的Portevin-Le Chatelier效应应力曲线.计算结果与实验数据的较好吻合，验证了该模型的有效性. 关键词： Portevin-Le Chatelier效应 动态应变时效 数值模拟     

多阶式层级梯度蜂窝结构的共面冲击响应

为改善蜂窝结构共面的力学性能,基于传统六边形蜂窝结构,建立了六边形层级蜂窝结构,并利用层级蜂窝代替传统六边形蜂窝部分胞元层,复合成一种新型多阶式层级梯度蜂窝结构。利用显式动力学有限元方法研究了层级梯度蜂窝的共面在不同冲击速度作用下的冲击响应特性和能量吸收能力。研究结果表明：层级梯度蜂窝的变形模式与塑性坍塌强度和冲击速度有关;层级梯度蜂窝冲击端和固定端在不同冲击速度作用下的名义应力-应变曲线均与其变形模式有关;不同的复合方式会导致层级梯度蜂窝具有不同的平台应力和比吸能,且在高速冲击时其平台应力比传统六边形蜂窝提高45.4%～63.8%,能量吸收提升10.8%～34.1%。相对密度会影响层级梯度蜂窝的能量吸收能力。     

红外测温法研究Al-Mg合金中的Portevin-Le Chatelier效应

试件塑性变形过程伴随着机械能向热能的转化.利用红外测温法，通过分析红外热像仪采集的温度场图像，系统研究了Al-Mg合金中的Portevin-Le Chatelier （PLC）效应.在不同应变率下，实验得到了三类锯齿形应力-应变曲线，分析了相应情况下试件温度变化曲线的异同及其原因，探讨了三种类型PLC变形带的空间传播特性.研究发现，试件表面的温升随着应变率的增加而增加；PLC带的倾角转向发生在试件的两端或者带外的温度最高处. 关键词： Portevin-Le Chatelier效应 红外测温 Al-Mg合金     

爆炸冲击作用下铝蜂窝板失稳研究

基于非线性动力学软件AUTODYN,对铝蜂窝夹芯板在爆炸冲击载荷作用下的失稳过程进行三维数值模拟。通过建立不同参数规格的铝蜂窝夹芯板的实体模型,研究其前、后面板在TNT爆炸冲击作用下的动态响应及塑性阶段的失稳变形。计算结果表明:随着铝蜂窝板前、后面板厚度,夹芯层铝箔高度和厚度的增加,铝蜂窝板在爆炸冲击载荷下的最终残余变形量均明显减小,抵抗变形的能力增强;对于夹芯层胞元形状不同但相对密度相同的铝蜂窝板,因冲击载荷下单轴压缩"惯性效应"差异较小,其面板在爆炸载荷作用下的最终残余变形量差异不大。     

飞秒激光冲击AZ31B镁合金过程的数值模拟

采用有限元分析法对飞秒激光冲击AZ31B镁合金进行数值模拟,研究了激光冲击处理对镁合金变形过程的影响,分析了单脉冲激光冲击下材料内部的位移、动能、应力和应变的分布情况,得到了材料的瞬态速度和应变率变化过程.仿真结果表明,单脉冲飞秒激光冲击镁合金产生的塑性变形,可在材料表面形成微米级凹坑,中心点处最大位移为34μm,最大变形速度390m/s;在冲击初期,材料表面的应力和应变主要分布在冲击区域中心节点和边缘附近,并且得到镁合金的最大应力和最大应变率分别为955 MPa和1.8×106 s-1.研究结果能够为深入分析飞秒激光与镁合金作用时材料变形参量的变化规律提供数值理论依据.     

球形孔开孔泡沫铝的力学特性及准静态压缩变形机制

对胞孔形态和尺寸较为一致的球形孔泡沫铝开展静-动态压缩实验,利用数字图像相关法研究了泡沫铝在准静态压缩过程中的宏观和介观变形机理。结果表明:球形孔泡沫具有明显的应变率效应,随着应变率的增加,平台应力及屈服强度增加,吸能效率也有所提高。由于胞元壁厚不均匀和孔壁缺陷的随机分布,泡沫铝在压缩过程中会出现多条局部变形带,单个胞孔表面在孔壁缺陷处也会出现应变集中带。胞元孔的变形模式主要有3种,轴向压缩、剪切、扭转加剪切复合变形,且整体变形带处的孔壁破坏模式以剪切变形为主,孔壁的变形模式又与孔壁自身厚度以及加载方向有关。     

Investigation on ultrasonic volume effects: Stress superposition,acoustic softening and dynamic impact

Conventional high power ultrasonic vibration has been widely used to improve manufacturing processes like surface treatment and metal forming. Ultrasonic vibration affects material properties, leading to a flow stress reduction, which is called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition due to oscillatory stress, acoustic softening by easier dislocation motion and dynamic impact leading to extra surface plastic deformation. However, most researches ignored the stress superposition for the convenience of measurement, and few studies considered ultrasonic dynamic impact since the relatively low ultrasonic energy in macro scale. The purpose of this study is to investigate the characteristics and mechanisms of different ultrasonic volume effects in micro-forming. A 60 kHz longitudinal ultrasonic-assisted compression test system was developed and a series of ultrasonic-assisted compression tests at different amplitudes on commercially pure aluminum A1100 in micro-scale were carried out combining the surface analysis by SEM, EDX and micro-hardness test. Three different ultrasonic volume effects, stress superposition, acoustic softening and dynamic impact, were confirmed in the ultrasonic-assisted compression tests. In order to quantitatively predict stress superposition, a hybrid model for stress superposition is developed considering the elastic deformation of experimental apparatus in practice, the evolution of the modeling results fitted well with the experimental results. With low ultrasonic amplitude, stress superposition and acoustic softening occurred because vibrated punch contacted with the specimen all the time during compression. However, with higher amplitude, due to the extra surface plastic deformation by larger ultrasonic energy, forming stress was further reduced by the ultrasonic dynamic impact. A possible method to distinguish the effects of dynamic impact and acoustic softening is to analyze the waveform of the oscillatory stress in the process. In the case of ultrasonic dynamic impact effect, a higher amount of oxidation was observed on the specimen surface, which could be the result of local heating by surface plastic deformation and surface friction when the vibrated punch detached from the specimen. The findings of this study provide an instructive understanding of the underlying mechanisms of volume effects in ultrasonic-assisted micro-forming.     

超空泡射弹水下侵彻靶板三相耦合数值模拟

超空泡射弹侵彻问题的实质是特殊水下结构受到高速冲击载荷作用下的动态响应。对12.7 mm口径超空泡射弹侵彻典型水下目标壳体的毁伤效果开展研究,基于LS-DYNA有限元分析软件建立水环境中超空泡射弹垂直侵彻曲面靶板的等效模型,探讨射弹侵彻过程中动能侵彻和气泡溃灭对靶板联合毁伤效果,获得了靶板在各阶段的应力变化和结构变形规律。结果表明:侵彻靶板前,射弹着靶速度为200 m/s时的头部表面水介质压力峰值达768 N,靶板表面有明显下凹变形;侵彻靶板时,伴随着射弹动能侵彻和气泡溃灭冲击,水介质造成的影响不足动能侵彻的2%;侵彻靶板后,在靶板正面形成峰值速度为42 m/s的水射流进一步作用于破口;靶板整体弯曲变形,在200~300 m/s范围内,随着射弹着靶速度的增加,靶板弯曲形变量减小;靶板局部发生延性穿孔,射弹在水环境中具有更好的破口效果,射弹速度变化对破口尺寸影响不大。     

Analysis of the factors that cause unstable deformation and loss of plasticity in neutron-irradiated copper

Copper is used as an example to analyze the effect of radiation on the stress-strain curves and deformation stability of radiation-hardened metals. The analysis is based on an equation that describes the evolution of the dislocation density with deformation in a plastically deformed material. Deformation instability in the initial stage of the stress-strain curve is caused by strong deformation localization at the microscopic level as a result of the transformation of immobile radiation defects (vacancy and interstitial loops) into mobile dislocations. The channeling of a large number of dislocations along slip planes causes the appearance of a yield drop and a yield plateau in the stress-strain curves. The critical conditions for their appearance, as well as the theoretical irradiation-dose dependences of the yield-plateau length and the uniform strain to necking, are found.     

Elastic vibrations of a cylindrical nanotube with the effect of surface stress and surface inertia

Vibration frequency analysis of nanostructures may be essential for study of their thermal conductivity and mechanical characterization. Given the high surface-to-volume ratio, the elastic vibrations of an infinitely long cylindrical nanotube have been studied by considering both the effects of surface stress and that of surface inertia within the framework of surface elasticity. The phonon dispersion and the resonant frequencies for the specific vibration modes have been calculated. Numerical results have indicated that the surface stress and the surface inertia have equally important effect on the vibration behavior of the nanotube that may depend on the vibration modes as well. Due to the surface effect, the vibration modes of lower order by the classical elasticity may be indeed the modes of higher order. The surface effect on the low-frequency Raman shift has also been found.     

无氧铜的准等熵压缩性

 利用递变冲击阻抗材料叠合而成的组合飞片，在二级轻气炮上对无氧铜进行了准等熵压缩性测量，加载时间约达1 μs。用激光速度干涉仪连续记录了不同厚度处无氧铜样品自由面速度随时间的变化过程，并通过拉格朗日波分析技术计算得到40 GPa下的准等熵的应力-应变曲线。结果表明：在低应力区，无氧铜的准等熵压缩线位于冲击绝热线之上；到32 GPa以上，准等熵压缩线才回落到冲击绝热线之下。这个现象与Barker、Chhabildas等对铝与钨的实测现象是一致的，它表明：在低应力区，材料的冲击强化效应与加载速率密切相关。     

基于范德瓦尔斯表面张力模式液滴撞击疏水壁面过程的研究

液滴撞击疏水壁面过程的研究在介观流体力学和微流体作用材料科学的研究中具有重要的理论意义和工程价值. 论文在SPH方法中引入范德瓦尔斯状态方程处理液滴表面张力, 考虑流体粒子之间远程吸引, 近程排斥的内部作用力, 提出了流体粒子与疏水壁面粒子间势能函数与表面张力相结合的作用模式. 通过模拟真空条件下两个静止的等体积液滴相互融合的过程, 验证了计算模式在模拟液滴的表面张力中的有效性. 采用该模式模拟的液滴撞击疏水壁面过程, 不仅能够有效地模拟液滴撞击壁面后的变形过程, 而且清晰地模拟出液滴的回弹、腾空以及二次撞壁现象的完整过程. 模拟结果与液滴撞击疏水壁面的实验结果以及VOF模拟结果符合较好, 表明本文所提出的表面张力和疏水壁面作用力处理模式对模拟液滴撞壁过程具有实际应用价值.     

Modeling on debonding dynamics of pressure-sensitive adhesives

We propose a simple mechanical model describing viscoelasticity and cavitation during the debonding process in pressure-sensitive adhesives (PSA). Our calculation qualitatively reproduces typical stress-strain curves in the probe-tack test, such as the steep stress maxima and the following plateau region. It is shown that in the thin-film geometry the stress-strain curve is essentially determined by the cavities created by the large negative pressure. Effects of pre-existent air bubbles due to surface roughness are also discussed.     

Variation of the ultrasonic velocity in al under plastic deformation

Variation of the velocity of ultrasound propagation in polycrystalline aluminum under plastic deformation is studied. The dependences of the velocity of ultrasound on the strain and the actual stress are found to consist of three distinct stages. The study of the complex shapes of these dependences allows one to reveal additional stages in the parabolic stress-strain curve of the plastic flow, these features being impossible to observe by conventional methods. The behavior of the ultrasonic velocity observed in the experiment is explained by the changes in the defect structure of the material under deformation.     

Modelling of spall damage in ductile materials and its application to the simulation of plate impact on copper

A statistical model of dynamic spall damage due to void nucleation and growth is proposed for ductile materials under intense loading, which takes into account inertia, elastic-plastic effect, and initial void size. To some extent, void interaction could be accounted for in this approach. Based on this model, the simulation of spall experiments for copper is performed with the Lagrangian finite element method. The simulation results are in good agreement with experimental data for the free surface velocity profile, stress record behind copper target, final porosity, and void concentrations across the target. The influence of elastic-plastic effect upon the damage evolution is explored. The correlation between the damage evolution and the history of the stress near the spall plane is also analyzed.