黏土中爆炸成坑地冲击耦合效应实验研究

为获得黏土中爆炸成坑体积与耦合地冲击能量的关系,采用10.5 g TNT厘米级球形炸药球作为爆炸源,在$\varnothing $1500 mm×1490 mm分层式爆炸装置中开展了变埋深条件下的爆炸实验,利用3D扫描设备记录不同埋深下弹坑的真实体积,并通过动态土压力传感器测得地冲击传播衰减规律。实验结果表明：随埋深增大,耦合至黏土中的有效地冲击能量急剧增大,装药中心下方的有效弹坑体积与耦合至黏土中的有效地冲击能量基本呈正比关系,当装药比例埋深与封闭爆炸条件下爆炸空腔半径相当时,耦合至黏土中的有效地冲击能量基本达到饱和。结合实验结果给出了黏土中爆炸耦合地冲击能量分配随装药比例埋深的变化规律,建立了地下爆炸等效封闭当量计算方法,为地下工程抗爆设计提供了理论依据。

     

Experimental study on vibrations of a nonwoven fabric cylindrical filter

Unexpected vibrations on a cylindrical filter made of nonwoven fabrics were clarified experimentally. Two types of filter with length L=1.8 m and 3.7 m, both 45 cm in diameter and 1.08 mm in thickness, were used. This is a new type of aeroelastic vibration phenomenon because the filter is a closed cylindrical vessel. In addition, the flow velocity of inner air was very slow and inner air flowed out slowly from the filter surface. The velocity distribution of air flow from a fan duct as well as the frequency and amplitude of the filter vibration were measured for two types of filter. By setting up a roll core panel at the outlet of the fan duct, we could rectify the outflow and suppressed the vibration of the shorter filter with L=1.8 m. However, this method was not adequate for the longer filter with L=3.7 m, and we could suppress the vibration by shielding the inner surface of the filter at the top region.     

温度、压力对甲烷-空气混合物爆炸极限耦合影响的实验研究

为了研究不同初始条件对甲烷-空气混合物爆炸极限的影响,利用容积为20 L的爆炸罐,在不同初始温度(25~200 ℃)和初始压力(0.1~1.0 MPa)条件下测定了甲烷-空气混合物的爆炸极限。实验结果表明,随着初始温度和初始压力的升高,爆炸上限升高,爆炸下限降低,爆炸极限范围扩大。在实验温度和压力范围内,常压/常温条件下,爆炸上限和下限与初始温度/初始压力呈线性相关。爆炸上限与初始温度的相关性受初始压力的影响,其与初始压力的相关性也与初始温度有关。然而,初始压力/初始温度对爆炸下限的影响与初始温度/初始压力的相关性并不显著。初始温度和初始压力对爆炸极限的耦合影响比单一因素对其的影响大,且相较而言,其对爆炸上限的影响更为显著。本文中绘制了影响曲面来描述初始温度和初始压力如何影响甲烷-空气混合物的爆炸极限。     

Experimental study of the initial growth of a localized turbulent patch in a stably stratified fluid

We present a laboratory experiment of the initial growth of a turbulent patch in a stably stratified fluid. The patch is created due to a localized source of turbulence, generated by a horizontally oriented and vertically oscillating grid much smaller than the tank size and far from solid boundaries. Synchronized and overlapping particle image velocimetry(PIV) and planar laser induced fluorescence (PLIF) measurements capture the evolution of the patch through its initial growth until it reached a maximum size. The simultaneous measurements of density and velocity fields allow for a direct quantification of the distribution of kinetic energy, buoyancy and degree of mixing within the patch. We can also relate the propagation speed of the turbulent/non-turbulent interface and its thickness to the properties of the turbulent fluid inside the evolving patch. The velocity measurements in this setup indicate significant transient effects inside the patch during its growth. A local analysis of the turbulent/non-turbulent interface provides direct measurements of the entrainment velocity w_e as compared to the local vertical velocity and turbulent intensity at the proximity of the interface. The detailed information about the growth of localized sources of turbulence in stratified environment might be of use in stealth design of autonomous underwater vehicles.     

泡沫铝泡孔动态变形特性研究

在SHPB实验中采用应变冻结法将试件冻结于设定的压缩应变,然后观察内部泡孔的变形情况并讨论其变形机理。分别对同一相对密度(40%)、不同基体材料的两种泡沫铝以及两种相对密度(19%和8%)、相同基体材料的泡沫铝进行实验,讨论了基体材料的本构关系、泡沫的相对密度对变形模式的影响。利用ANSYS/LS DYNA软件对泡沫铝在动态压缩时泡孔的变形进行数值模拟计算,进一步说明泡沫铝泡孔变形的模式与基体材料本构关系及相对密度的关系。     

An experimental study of the adhesion between clay and steel

Many practical problems in Soil Mechanics require a detail knowledge of the shearing resistance to relative sliding at the interface between soils and structural materials. This study presents a complete set of parameters for the shearing resistance between two different clays and a smooth steel surface. Results are given for the behaviour of clay samples tested in the shear box under quick undrained, consolidated undrained and consolidated drained conditions. These parameters are compared with those obtained for the internal shear strength.The drained and undrained results for both clays show that clay to steel initially has a higher shear strength than clay.to clay. Its subsequent behaviour depends upon whether the clay is over or normally consolidated. Some adhesion factors are suggested.     

Experimental investigation of initial and subsequent yield surfaces for laminated metal matrix composites

The aim of this work is to construct yield surfaces to describe initial yielding and characterize hardening behavior of a highly anisotropic material. A methodology for constructing yield surfaces for isotropic materials using axial–torsion loading is extended to highly anisotropic materials. The technique uses a sensitive definition of yielding based on permanent strain rather than offset strain, and enables multiple yield points and multiple yield surfaces to be conducted on a single specimen. A target value of 20 × 10⁻⁶ is used for Al₂O₃ fiber reinforced aluminum laminates having a fiber volume fraction of 0.55. Sixteen radial probes are used to define the yield locus in the axial–shear stress plane. Initial yield surfaces for [0₄], [90₄], and [0/90]₂ fibrous aluminum laminates are well described by ellipses in the axial–shear stress plane having aspect ratios of 10, 2.5, and 3.3, respectively. For reference, the aspect ratio of the Mises ellipse for an isotropic material is 1.73. Initial yield surfaces do not have a tension–compression asymmetry. Four overload profiles (plus, ex, hourglass, and zee) are applied to characterize hardening of a [0/90]₂ laminate by constructing 30 subsequent yield surfaces. Parameters to describe the center and axes of an ellipse are regressed to the yield points. The results clearly indicate that kinematic hardening dominates so that material state evolution can be described by tracking the center of the yield locus. For a nonproportional overload of (σ, τ) = (500, 70) MPa, the center of the yield locus translated to (σ, τ) = (430, 37) MPa and the ellipse major axis was only 110 MPa.     

Stress and fabric in granular material

It has been well recognized that, due to anisotropic packing structure of granular material, the true stress in a specimen is different from the applied stress. However, very few research efforts have been focused on quantifying the relationship between the true stress and applied stress. In this paper, we derive an explicit relationship among applied stress tensor, material-fabric tensor, and force-fabric tensor; and we propose a relationship between the true stress tensor and the applied stress tensor. The validity of this derived relationship is examined by using the discrete element simulation results for granular material under biaxial and triaxial loading conditions.     

Appell equations in terms of quasi-velocities and quasi-accelerations

The new forms of Appell equations in terms of quasi-velocities and quasi-accelerations for 1st order and 2nd order non-holonomic systems are presented. And the results obtained are extended to variable mass systems. Finally an example is given.     

Experimental study of turbulence-induced coalescence in aerosols

In this paper, measurements of the rate of aerosol coalescence in a well characterized turbulent flow are presented. The time dependence of the aerosol droplets’ mean radius upon initiation of flow in an oscillating grid generated turbulence chamber is determined using a phase-Doppler method. Together with a measurement of the aerosol number density from a light attenuation probe, the observed rate of change of the aerosol droplets’ mean radius can be related to the rate constant for the coalescence of two droplets. The Kolmogorov shear rate, which is the primary parameter in theories predicting coalescence rate, is determined from measurements of the root-mean-square fluctuating velocity and the integral length scale. Our experimental results are compared with theoretical predictions, obtained by solving of the population balance equation. Various expressions are considered for the coalescence rate constant to be used in the population balance equation. First, we considered various combinations of ideal coalescence rate constants, i.e. obtained theoretically neglecting particle interactions. Our data are then found to be in good agreement with theoretical predictions that take into account the simultaneous effects of turbulent shear induced and Brownian motion induced coalescence. Second, our results are compared with a theory that considers the effects of turbulent shear and Brownian motion as well as the non-continuum hydrodynamic and van der Waals interparticle interactions. The measured experimental values are generally 50–100% higher than those predicted by this theory. This discrepancy could be explained by the small polydispersity of the aerosol which may result in coalescence induced by differential sedimentation and turbulent acceleration.     

Strain localization and fabric evolution in sand

Strain localization is frequently observed in sand and is considered an important precursor related to major geohazards such as landslides, debris flow and failure of relevant geo-structures. This paper presents a numerical study on strain localization in sand, with a special emphasis on the influence of soil fabric and its evolution on the initiation and development of shear band. In particular, a critical state sand plasticity model accounting for the effect of fabric and its evolution is used in the finite element analysis of plane strain compression tests. It is found that the initiation of shear band is controlled by the initial fabric, while the development of shear band is governed by two competing physical mechanisms, namely, the structural constraint and the evolution of fabric. The evolution of fabric generally makes the sand response more coaxial with the applied load, while the structural constraint induced by the sample ends leads to more inhomogeneous deformation within the sand sample when the initial fabric is non-coaxial with the applied stress. In the case of smooth boundary condition, structural constraint dominates over the fabric evolution and leads to the formation of a single shear band. When the boundary condition is rough, the structural constraint may play a comparable role with fabric evolution, which leads to symmetric cross-shape shear bands. If the fabric is prohibited from evolving in the latter case, a cross-shape shear band pattern is found with the one initiated first by the structural constraint dominating over the second one. In all cases, significantly larger dilation and fabric evolution are observed inside the shear band than outside. The simulated shear band orientation coincides with the Roscoe’s angle for cases with high confining pressure and lies in between the Roscoe’s angle and Arthur’s angle for the low confining pressure cases.     

Experimental study of microhole growth and coalescence in ductile materials

The growth and coalescence of two microholes in copper foil were studied experimentally byin situ tensile tests under a scanning electronic microscope. Two microholes of 15–35 μm in diameter were arranged in different distances and orientations. It was found that the mechanisms of microhole evolution were represented by slipping band creation, and then crack initiation and propagation along the slipping bands in ligament. The process of microhole growth and coalescence was influenced by the inter-center distance and orientation of microholes. The critical surface of microholes at coalescence is about 2–2.5 times that of the initial one. The variation of both the inter-center distance and orientation depends on the initial angle.