磁驱动准等熵压缩下LY12铝的强度测量

高压高应变率加载下材料的强度研究一直是冲击动力学的一个难题,目前动态载荷下材料的高压强度测量主要是基于平板撞击技术,冲击温升和应变率效应对材料强度的影响难以分离. 基于小型磁驱动加载装置CQ-4,开展了磁驱动准等熵压缩下LY12 铝的声速和强度测量的实验研究,讨论了考虑加载-卸载过程时磁驱动压缩实验的负载电极设计、实验样品设计、数据处理与分析等内容,并获得了12 GPa 压力范围沿加载-卸载路径的声速变化和峰值压力点的强度数据.      

压剪复合加载下聚丙烯屈服行为的实验研究

为研究聚丙烯在压剪复合加载下的变形行为和屈服规律,设计了一种压剪复合加载的试件(Shear-Compression Specimen,SCS),分析其构型参数对变形行为的影响,并分别对倾斜角度为5°,15°,30°,45°,60°的SCS进行了准静态加载实验,对SCS压缩变形过程中测量区的压缩应力和剪切应力的演化规律进行了分析。结果发现:聚丙烯在压剪复合加载下屈服规律受静水压影响明显,偏离von-Mises屈服准则;而Hu-Pae屈服准则可较好地描述聚丙烯在不同压剪复合加载条件下的屈服规律。此外,对加载速率影响屈服的规律进行研究,发现聚丙烯在压剪复合加载下的屈服行为具有应变率敏感性。     

磁驱动等熵压缩实验构形的磁流体力学计算模拟

在一维弹塑性反应流体力学拉格朗日编码SSS 的基础上扩充编制成功的一维磁流体力学计算编码SSS/MHD,可以计算模拟磁驱动等熵压缩（ICE） 实验中多介质、多空腔的不同厚度平行样品构形（台阶靶） 运动,并且把磁流体计算与模拟脉冲功率驱动源的集中参数外电路方程组直接耦合,不依赖于实验当时测量的驱动电流（电压） 数据,直接根据原始数据对实验样品的力学和电磁学运动过程和各种参数剖面进行数值模拟. 介绍了对于纯铝、钽和塑料粘结炸药等3 种性能不同材料的台阶靶样品ICE 实验的计算模拟,得到了与激光速度干涉测量的不同厚度样品后自由面（或光学窗口界面） 速度历史十分符合的结果,对于实验结果的分析和实验设计的改进具有重要意义. 旨在为磁驱动实验系统的校核、设计和实验构形改进提供有一定预测能力、功能比较完善的计算模拟研究手段.      

MAGNETO-HYDRODYNAMICS MODELING OF CONFIGURATIONS IN MAGNETICALLY DRIVEN ISENTROPIC COMPRESSION EXPERIMENTS

在一维弹塑性反应流体力学拉格朗日编码SSS 的基础上扩充编制成功的一维磁流体力学计算编码SSS/MHD,可以计算模拟磁驱动等熵压缩（ICE） 实验中多介质、多空腔的不同厚度平行样品构形（台阶靶） 运动,并且把磁流体计算与模拟脉冲功率驱动源的集中参数外电路方程组直接耦合,不依赖于实验当时测量的驱动电流（电压） 数据,直接根据原始数据对实验样品的力学和电磁学运动过程和各种参数剖面进行数值模拟. 介绍了对于纯铝、钽和塑料粘结炸药等3 种性能不同材料的台阶靶样品ICE 实验的计算模拟,得到了与激光速度干涉测量的不同厚度样品后自由面（或光学窗口界面） 速度历史十分符合的结果,对于实验结果的分析和实验设计的改进具有重要意义. 旨在为磁驱动实验系统的校核、设计和实验构形改进提供有一定预测能力、功能比较完善的计算模拟研究手段.     

压剪复合冲击下氧化铝陶瓷的剪切响应实验研究

通过对92.93%氧化铝陶瓷进行倾斜板碰撞实验,研究了多晶陶瓷材料在压剪复合冲击下的非弹性变形响应和剪切波传播规律。压剪复合冲击实验由57 mm开槽气体炮驱动铜飞片对陶瓷靶板加载,通过试件内埋植的电磁速度计来测量内部质点速度历程。将纵向粒子速度从感应电动势曲线中分离后得到横向粒子速度历程,发现在压剪复合冲击下由于材料剪切刚度的降低而引起的剪切波衰减。冲击软回收试件的扫描电子显微镜（SEM）观察表明,冲击载荷低于屈服强度时,多晶氧化铝陶瓷中存在沿晶界、气孔的微裂纹成核与扩展,在高于屈服强度的冲击加载下进一步产生了穿晶微裂纹,微裂纹系统导致了材料在卸载后的显著的体积膨胀。     

一种新的高应变率复合压剪实验技术

提出了一种新的基于Hopkinson杆实验技术的在102~103s-1高应变率下实现压剪复合加载的实验装置,并给出了相应的理论分析和数值模拟。为了获取材料在复杂应力下的本构关系,借助斜飞片冲击实验的思想,对Hopkinson杆进行改造,将入射杆的末端改进为截锥形,以便在试样中同时产生压缩和剪切应力。利用有限元分析软件LS-DYNA对试样中的应力波传播进行模拟计算,并利用改进装置进行了初步实验。计算和分析结果表明,利用所设计的装置可以实现对试样的动态压剪复合加载,获得材料在高应变率复杂应力加载下的本构响应,进而建立材料在复杂应力状态下本构行为的描述。     

磁驱动等熵压缩和高速飞片的实验技术（续2）

4磁压加载等熵压缩实验的数据处理方法 4．1磁驱动实验测量技术 磁驱动实验与通常冲击动力学实验的差别首先在于加载手段的不同，不是纯力学方式（即通过驱动器与样品的直接接触施加作用力）加载，而是利用经过样品的电流与其自身产生磁场相互作用的电磁力无接触地加载，其优点是磁压力的高低只与导体样品表面电流密度有关。在不考虑磁扩散的前提下，把流经导体样品表面的电流密度及其分布测量准确，就能准确的给定磁压力。对于简单的电极构形，通常用罗柯夫斯基线圈测量流过的脉冲电流。     

动载荷下材料的屈服强度变化

描述了冲击加载和准等熵加载下获得材料动态屈服强度的方法,并以钨合金材料为研究对象,通过VISAR测试技术测量了钨合金材料在冲击加载下的屈服强度随压力的变化,并把实验结果与文献报道的准等熵加载下的材料强度变化的实测结果进行了比较。结果表明,准等熵加载下材料的屈服强度明显高于冲击加载下的材料屈服强度,并认为这主要是由于两种加载方式下材料的温升效应不同所造成的。     

基于 Hopkinson 压杆的动态压剪复合加载实验研究

本文提出了一种可用于研究压剪复合加载下，材料动态力学性能的实验装置；该装置主要是基于 Hopkinson 压杆，通过添加一个带倾斜端面的垫块，实现压剪复合加载。本文分析了该实验装置的基本数据处理方法，并利用有限元分析验证了此分析方法的可行性；然后利用该装置对常规金属材料进行了相同冲击速度，不同倾斜角度（分别为 0、30、45 度）下的一系列实验。实验结果表明，该装置能实现压剪复合加载，并且能得到材料的动态屈服面，为研究材料在复杂应力状态下的动态力学性能提供了新的实验方法。     

磁驱动平面准等熵加载装置、实验技术及应用研究新进展

利用脉冲大电流装置产生随时间变化平滑上升的磁压力,实现对平面、柱面等不同结构样品的磁驱动准等熵（斜波）压缩,为极端条件下材料动力学研究提供了一种偏离Hugoniot状态热力学路径的加载手段。本文从磁驱动准等熵加载装置、实验技术、数据处理方法等方面综述了磁驱动准等熵加载技术研究近十年的新进展,评述了利用磁驱动准等熵加载技术和方法开展极端条件下材料高压状态方程、高压强度与本构关系、相变与相变动力学等方面研究的进展情况,展望了磁驱动准等熵加载技术发展及其在材料动力学、武器物理和高能量密度物理等方面的应用前景。     

High-pressure strength of aluminum under quasi-isentropic loading

Under shock loading, metals typically increase in strength with shock pressure initially but at higher stresses will eventually soften due to thermal effects. Under isentropic loading, thermal effects are minimized, so strength should rise to much higher levels. To date, though, study of strength under isentropic loading has been minimal. Here, we report new experimental results for magnetic ramp loading and impact by layered impactors in which the strength of 6061-T6 aluminum is measured under quasi-isentropic loading to stresses as high as 55 GPa. Strength is inferred from measured velocity histories using Lagrangian analysis of the loading and unloading responses; strength is related to the difference of these two responses. A simplified method to infer strength directly from a single velocity history is also presented. Measured strengths are consistent with shock loading and instability growth results to about 30 GPa but are somewhat higher than shock data for higher stresses. The current results also agree reasonably well with the Steinberg–Guinan strength model. Significant relaxation is observed as the peak stress is reached due to rate dependence and perhaps other mechanisms; accounting for this rate dependence is necessary for a valid comparison with other results.     

Submicrosecond strength of ultrafine-grained materials

We present the results of measuring the strength properties of metals and alloys with face-centered cubic lattice (copper, aluminum), body-centered cubic structure (Armco iron, tantalum), hexagonal close-packed structure (titanium and titanium alloy BT6) in the original coarse-grained and submicrocrystalline state under shock-wave loading. The grain dimension of the materials under study was changed by intensive plastic deformation. The influence of the grain dimensions on the dynamic yield stress does not always agree with the data of low-rate test even in sign, which is interpreted in the framework of general laws of the strain rate influence on the metal and alloy flow stress. As the grain dimension decreases, there is an increase in the compression rate in the plastic shock wave, a small increase in the fracture strength (spall strength), and an increase in the spall fracture rate.     

晶粒度对纯铝动态力学性能的影响

通过热处理方式制备了3种不同晶粒度的纯铝样品。利用分离式霍普金森压杆加载装置测量了3种纯铝在应变率102～104 s-1范围内的应力应变曲线。实验曲线表明,纯铝的屈服应力和流动应力随晶粒度的增大而减小;根据实验数据拟合了纯铝在动态下的Hall-Petch公式。同时采用一级轻气炮对3种纯铝进行了层裂实验,结果表明晶粒度对纯铝的层裂特性几乎没有影响。     

Anisotropic hardening and non-associated flow in proportional loading of sheet metals

Conventional isotropic hardening models constrain the shape of the yield function to remain fixed throughout plastic deformation. However, experiments show that hardening is only approximately isotropic under conditions of proportional loading, giving rise to systematic errors in calculation of stresses based on models that impose the constraint. Five different material data for aluminum and stainless steel alloys are used to calibrate and evaluate five material models, ranging in complexity from a von Mises’ model based on isotropic hardening to a non- associated flow rule (AFR) model based on anisotropic hardening. A new model is described in which four stress–strain functions are explicitly integrated into the yield criterion in closed form definition of the yield condition. The model is based on a non-AFR so that this integration does not affect the accuracy of the plastic strain components defined by the gradient of a separate plastic potential function. The model not only enables the elimination of systematic errors for loading along the four loading conditions, but also leads to a significant reduction of systematic errors in other loading conditions to no higher than 1.5% of the magnitude of the predicted stresses, far less that errors obtained under isotropic hardening, and at a level comparable to experimental uncertainty in the stress measurement. The model is expected to lead to a significant improvement in stress prediction under conditions dominated by proportional loading, and this is expected to directly improve the accuracy of springback, tearing, and earing predictions for these processes. In addition, it is shown that there is no consequence on MK necking localization due to the saturation of the yield surface in pure shear that occurs with the aluminum alloys using the present model.     

闭孔泡沫铝的多轴唯象受压本构参数

基于显微计算机断层扫描影像信息, 逆向重建闭孔泡沫铝试件的三维细观有限元模型, 定量研究闭孔泡沫铝在多轴压缩载荷作用下的大变形力学行为. 讨论了泡沫金属唯象弹塑性本构参数的确定方法, 根据计算结果确定了3 个有代表性的泡沫材料本构模型的本构参数, 并验证了这些本构模型在描述多轴压缩应力状态下的精度. 研究表明, 对于单轴压缩, 3 个本构模型的屈服面均有很好的精度;对于静水压缩, 有限元软件"ABAQUS"的可压缩泡沫本构模型屈服面会发生严重偏离, 陈-卢本构模型"屈服面" 略微低估静水压缩的屈服应力, 而体积强化本构模型的屈服面有很好的精度.      

Tensile and mixed-mode strength of a thin film-substrate interface under laser induced pulse loading

Laser induced stress waves are used to characterize intrinsic interfacial strength of thin films under both tensile and mixed-mode conditions. A short-duration compressive pulse induced by pulsed-laser ablation of a sacrificial layer on one side of a substrate is allowed to impinge upon a thin test film on the opposite surface. Laser-interferometric measurements of test film displacement enable calculation of the stresses generated at the interface. The tensile stress at the onset of failure is taken to be the intrinsic tensile strength of the interface. Fused-silica substrates, with their negative nonlinear elasticity, cause the compressive stress wave generated by the pulse laser to evolve a decompression shock, critical for generation of the fast fall times needed for significant loading of surface film interfaces. By allowing the stress pulse to mode convert as it reflects from an oblique surface, a high amplitude shear wave with rapid fall time is generated and used to realize mixed-mode loading of thin film interfaces. We report intrinsic strengths of an aluminum/fused silica interface under both tensile and mixed-mode conditions. The failure mechanism under mixed-mode loading differs significantly from that observed under pure tensile loading, resulting in a higher interfacial strength for the mixed-mode case. Inferred strengths are found to be independent, as they should be, of experimental parameters.     

Finite ramp time correction method for non-linear viscoelastic material model

In this paper, a simple formula for the prediction of stress (strain) in the relaxation (creep) period is derived for a non-linear viscoelastic material model which takes into account the finite ramp time. Usually, it is assumed that the ramp time is small and, therefore, loading can be described via Heaviside step function. This assumption, when applied to the material parameters identification process, can lead to a large errors in the values of the approximated material parameters. Especially, for the materials which undergo significant stress decay in the beginning of relaxation the assumption of infinite small ramp time can induce severe errors. With the help of the derived formula more reliable material parameter identification can be accomplished. The proposed method is tested with numerical simulations and compared with analytical results, Heaviside step loading case and method described by Nordin and Varna. Simulations show good agreement with analytical results.     

Numerical study of rate-dependent strength behavior under ramp and shock wave loading

The main objective of the current study was to gain a detailed understanding on the rate-dependent strength behavior under ramp and shock wave loading. A forward, numerical-simulation-based cause and effect analysis was used to address the research objective. The apparent strength associated with shock and ramp wave loadings with different risetimes and shapes was investigated. It was shown that intrinsic material strength could vary with pressure, temperature, and deformation history, but the apparent strength, which was larger than the intrinsic strength, was a result of the interaction between the rate sensitivity of the strength and the rate of the external loading. The degree of interaction led to different levels of mechanical and thermal dissipations and their partition, which was manifested by different temperature, stress, and deformation histories.     

Quasi-static crush behavior of aluminum honeycomb specimens under non-proportional compression-dominant combined loads

The quasi-static crush behavior of aluminum 5052-H38 honeycomb specimens under non-proportional compression-dominant combined loads is investigated by experiments. A test fixture was designed such that dominant compressive and shear loads with respect to the strongest material symmetry direction can be controlled and applied independently. Compression-dominant combined loads and pure compressive loads were applied in different sequences to induce non-proportional combined loads. The experimental results show that the normal crush and shear strengths in combined loading regions and the normal crush strengths in pure compressive loading regions of the non-proportional combined loads are quite consistent with the existing phenomenological yield criterion based on the experimental normal crush and shear strengths under proportional combined loads. The experimental results also indicate that the sequence of loading paths for the non-proportional combined loads does not affect the crush strengths when compared with those obtained under proportional combined loads. In addition, the experimental results indicate that the non-normality plastic flow behavior of honeycomb specimens under non-proportional combined loads is consistent with that under proportional combined loads. Finally, specimens crushed under non-proportional combined loads show developments of different stacking patterns of folds in different loading regions of the non-proportional combined loads. The experimental results suggest that the incremental stress–strain relation for the transition loading path within the so-called yield surface from one yield state to another may be related to the displacement increments that correspond to the change of microscopic folding mechanisms instead of the usual elastic relation according to the classical plasticity theory.