Experimental study on AE behavior of hydraulic concrete under compression

The research work of this paper is to quantitatively evaluate the ongoing damage degree of existing concrete structures being loaded by applying acoustic emission (AE) monitoring. On the basis of some approximations and simplifications, the relation between the AE parameter (AE event) and the scalar damage parameter for concrete structures under uniaxial compression was derived, and was modeled as a simply linear correlation by combining the rate process theory and the traditional parameter-based technique of AE, Continuous Damage Mechanics (CDM) and nonlinear regression analysis. Meanwhile, the AE event-based stress-stain relation was also modeled too. The measured data of AE monitoring system and the strain dynamical strain system used in this work confirms significantly the effectiveness of the AE event-based damage evolution assessment for existing hydraulic concrete structures. The results of the experiments show that the AE event-based method permits a fast and effective in situ assessment of the ongoing damage phenomena in hydraulic concrete structures.     

水泥砂浆在主动围压下的动态力学性能

水泥砂浆等脆性材料的力学性能与所受围压的大小密切相关。为了研究水泥砂浆在围压下的动态力学性能,研制了适于分立式Hopkinson压杆加载的主动围压装置,最大预加载主动围压大于30 MPa。实验得到了水泥砂浆在不同围压、不同应变率下的轴向应力应变曲线,发现材料在围压作用下抗压强度和韧性大大提高并且整体进入了伪塑性,而材料的应变率效应也是显著的。     

Nonlinear behavior of matrix-inclusion composites under high confining pressure: application to concrete and mortar

This paper is devoted to a micromechanics-based simulation of the response of concrete to hydrostatic and oedometric compressions. Concrete is described as a composite made up of a cement matrix in which rigid inclusions are embedded. The focus is put on the role of the interface between matrix and inclusion which represent the interfacial transition zone (ITZ). A plastic behavior is considered for both the matrix and the interfaces. The effective response of the composite is derived from the modified secant method adapted to the situation of imperfect interfaces. To cite this article: T.H. Le et al., C. R. Mecanique 336 (2008).     

Experimental and theoretical investigation of the failure behavior of a reinforced concrete target under high-energy penetration

The dynamic mechanical properties of concrete and reinforced concrete targets subjected to high-speed projectile impact loading have a significant influence on the impact resistance of protective structures.In this study,high-speed projectile penetration and perforation of concrete and reinforced concrete structures was carried out to determine the high-energy impact loading.The failure behaviors of projectile penetration and perforation of the concrete and reinforced concrete targets were investigated,and the destruction characteristics of the targets were measured.An analytical model was established using the principle of minimum potential energy.The results show that the theoretical predictions are consistent with the experimental data,indicating that the energy method is effective for predicting the dynamic mechanical properties of concrete and reinforced concrete targets under high-speed projectile penetration.     

Experimental testing of the fracture of concrete and reinforced concrete plates under impact

This paper describes the results of experimental studies on penetration of cylindrical projectiles into concrete and reinforced concrete at impact velocities reaching 0.5 km/s. An algorithm is proposed for calculating the depth of penetration of a projectile, making it possible to find the depth of penetration of high-strength steel projectiles with a mass of up to 13.5 kg into concrete on the basis of measurements of the specific work required to remove concrete using projectiles with a mass of up to 8 g.     

组合式十字形钢管混凝土柱偏压力学性能试验研究

设计了一种由槽钢和方形钢管拼焊形成的组合式十字形钢管混凝土柱,将其灵活地布置在框架结构的中节点,可使柱肢与填充墙等厚,有效地提高建筑使用面积。共制作了6根组合式十字形钢管混凝土柱试件,考虑了偏心距和长细比两种变化参数。通过对其进行偏心受压试验研究,考察了试件的破坏形态和荷载-挠度曲线,并分析了其在不同偏心距和长细比下的荷载-应变曲线发展规律。结果表明:组合式十字形钢管混凝土柱中钢管和槽钢对混凝土的约束作用强,表现为较高的延性系数;偏心距或长细比越大,试件的极限承载力及弹性刚度越小,且偏心距越大延性越好,长细比对延性影响不显著;在受拉侧纵向应变基本上符合平截面假定,在受压侧纵向应变不符合平截面假定。     

不同高应变率冲击荷载作用下混凝土试样的能量耗散分析

针对混凝土材料在冲击荷载作用下能量耗散和破碎程度关联性难确定的问题,采用分离式霍普金森压杆(SHPB)完成了混凝土试块冲击试验,通过对冲击后碎块的筛分试验和应力波传播过程中应变的测试,得到了试块的能量耗散规律和破碎特征。结果表明,(1) 入射应力波的加载时间约为380 μs,入射能、反射能和吸收能随冲击气压的增大而增大;同一气压下,反射能最早达到平衡; (2) 平均应变率越大,入射能和吸收能也越大;当平均应变率为300/s左右时,能量利用率为最低值; (3) 随着入射能的增加,吸收能的增长率越来越大,当入射能达到2100 J时,吸收能近乎线性增长; (4) 随冲击气压增大,试样由拉裂破坏逐渐转移为压碎破坏,且随着材料吸收能的增大,平均破碎尺寸越来越小。当吸收能大于700 J时,吸收能对试样的平均破碎尺寸减小的影响较小。     

Experimental investigation of the behavior of extra high strength steel

This paper comprises a study concerning the mechanical behavior of extra high strength steel. This is investigated by means of biaxial testing of flat cross-shaped specimens in the full σ₁-σ₂ plane, a concept developed earlier at Steel Structures, Luleå University of Technology. Furthermore, new specimen designs had to be developed to enable testing of a material with high yield strength and low ultimate over yield strength ratio, such as the extra high strength steel Weldox 1100. The tests are performed in two steps: one initial loading followed by unloading and a subsequent loading in a new direction. The test results, containing data from 15 biaxial tests, are characterized by a slightly anisotropic initial yield criterion where the proof stress in compression is consequently somewhat higher compared to the results in tension. In the subsequent loading the observed phenomena are a Bauschinger effect in the direction opposite the initial loading direction and that the transition from elastic to plastic state in subsequent loadings is gradual and direction-dependent.     

Numerical simulation: The dynamic behavior of reinforced concrete plates under normal impact

This investigation deals with the use of the finite element method on the reinforced concrete structural dynamic response and failure behavior when subjected to the projectile impacts of different velocities, using the test conducted in [S.J. Hanchak, M.J. Forrestal, E.R. Young, J.Q. Ehrgott, Perforation of concrete slabs with 48 MPa (7 ksi) and 140 MPa (20 ksi) unconfined compressive strengths, Int. J. Impact Eng. 12 (1992) 1–7]. The Johnson–Holmquist concrete material constitutive law model is employed to simulate the large strains, high strain states and high pressures to which the concrete is subjected. The projectile impact velocity ranges from 381 m/s to 1058 m/s. Numerical simulations demonstrate that the Johnson–Holmquist concrete material constitutive model can describe the different failure modes without any predefined defects in the element mesh, and normally obtain good agreement between the numerical simulations and test results.     

粗骨料粒径对混凝土动态压缩行为的影响研究

粗骨料作为混凝土材料组成最主要的部分,对混凝土力学性能和破坏模式有着很重要的影响。为了研究粗骨料平均粒径对混凝土动态力学性能的影响规律,针对不同平均粗骨料平均粒径（6、12、24 mm）的混凝土和砂浆材料进行了一系列SHPB试验,得到了不同应变率下各试件的应力-应变曲线,并对每种材料的动态增长因子（dynamic increase factor,DIF）与应变率的对数进行了线性拟合。结果表明:砂浆和混凝土材料的抗压强度具有明显的应变率效应,其动态抗压强度随着应变率的增加而逐渐增大,应力-应变曲线呈现相似的变化趋势;在相同的动态应变率条件下,平均粗骨料粒径为12 mm的混凝土的动态抗压强度最大,这与准静态条件下砂浆抗压强度最大截然不同;不同粗骨料粒径混凝土材料的应变率强化系数均大于砂浆材料,且随着粗骨料无量纲尺寸的增大,混凝土材料的应变率强化因子呈现先增大后减小的趋势。     

Numerical modeling of ice behavior under high velocity impacts

In this work a constitutive relation for ice at high strain rates and an algorithm for its numerical integration are developed. This model is based on the Drucker–Prager plasticity criteria, which allows a different behavior in tension and in compression. In addition a failure criteria, based on pressure cut-offs, is implemented to describe the ice damage. In order to validate the constitutive model, numerical simulations were compared with experimental results, in which ice cylinders were impacted against a steel plate, allowing the measurement of the contact load. Three different numerical solvers are used in order to analyze its performance to appropriately modeling the ice behavior.     

The combined effect of electric forces and confinement ratio on the bubble rising

In this work, the combined effect of electrohydrodynamic forces and domain confinement on the formation of a toroidal bubble is numerically studied. The numerical scheme is the Volume of Fluid (VOF) method and the surface tension and electric forces are implemented using the Continuum Surface Force (CSF) and leaky dielectric models, respectively. It is found that both domain confinement and electric forces are influential on the formation of a toroidal bubble. For smaller confinement ratios, larger electric forces are required to pierce the bubble. Moreover, the influence of both electric forces and confinement ratio are presented and discussed for bubble vertical velocity, terminal Reynolds number, velocity streamlines and side-wall shear stress.     

STRENGTH CRITERION FOR PLAIN CONCRETE UNDER MULTIAXIAL STRESS BASED ON DAMAGE POISSON＇S RATIO

A new unified strength criterion in the principal stress space has been proposed for use with normal strength concrete （NC） and high strength concrete （HSC） in compressioncompression-tension, compression-tension-tension, triaxial tension, and biaxial stress states. The study covers concrete with strengths ranging from 20 to 130 MPa. The conception of damage Poisson＇s ratio is defined and the expression for damage Poisson＇s ratio is determined basically. The failure mechanism of concrete is illustrated, which points out that damage Poisson＇s ratio is the key to determining the failure of concrete. Furthermore, for the concrete under biaxial stress conditions, the unified strength criterion is simplified and a simplified strength criterion in the form of curves is also proposed. The strength criterion is physically meaningful and easy to calculate, which can be applied to analytic solution and numerical solution of concrete structures.     

混凝土高温动态压缩力学性能实验

利用一种新的快速加热混凝土的方法和SHPB实验系统对自制混凝土进行了不同温度下的动态 压缩初步实验,发现了混凝土在高温下的动态压缩力学性能的规律性:在高温动态压缩条件下,温度变化是影 响混凝土力学性能的主要因素,应变率的影响是次要因素。另外,该混凝土高温动态压缩破坏可以分为2种 模式(裂纹模式和破碎模式)     

基于三维Hopkinson杆的混凝土动态力学性能研究

混凝土、岩石类材料在复杂应力状态下的动态力学性能研究一直备受关注,但鉴于动态实验的复杂性,对真三轴应力状态下材料的动态加载一直未曾实现。本文中研制了一套真三轴静载作用下混凝土、岩石类材料的“三维Hopkinson杆”动态力学实验系统,为冲击载荷作用下材料动态各向异性特性的研究提供了一种有效的实验测试技术。该系统采用液压伺服控制对立方体试件施加三向独立的0~100 MPa真三轴静载,再利用分离式Hopkinson压杆对试件施加冲击动载,具体研究了C30混凝土材料在不同真三轴静载条件下的动态压缩性能,得到了不同条件下X、Y、Z方向上的动态应力应变关系。     

Dynamic behavior of concrete

Concrete and cement-paste specimens, representing a model of the actual structural material and of its adhesive component, respectively, were subjected to static and dynamic tests. Static tests on virgin specimens were carried out in order to evaluate the strength, elasticity and Poisson's ratio of the materials. The dynamic experiments were conducted in order to ascertain the response of the specimens to the propagation of one-dimensional pulses. Transient loading was accomplished by the central longitudinal impact of a 1/2-in.-diam steel sphere on a ballistically suspended 3/4-in.-diam Hopkinson bar of the material at an initial velocity of about 3260 or 1650 ips. The shocked specimens were also subsequently examined to determine whether changes in static material properties had occurred as a result of passage of the waves. Both static and dynamic tests yielded consistent results for a number of specimens cast and cured in identical fashion. Comparison of the properties of the virgin and the shocked specimens indicated little, if any, shock damage. While some minor grain damage was observed in microscopic examination of thin sections taken from some of the shocked specimens, other sections did not indicate any visible cracking of the grains. The wave-propagation process appeared to occur without dispersion and relatively little attenuation, indicating that the material could be represented on a macroscopic scale as an “elastic” substance with a small structural-damping coefficient. The obvious inhomogeneities of the concrete affected the gage response whenever a gage was mounted directly over a piece of aggregate. The dynamic response of the materials has been compared with the response of several types of rocks.     

高温下混凝土的本构模拟及破坏分析

针对已建立的高温下混凝土中化学-热-水力-力学耦合过程分析的分级数学模型,发展了混凝土的化学-热-水力-力学(CTHM)耦合本构模型。在已有的Willam-Warnke弹塑性屈服准则基础上发展了考虑脱水和脱盐引起的材料损伤及化学塑性软化、塑性应变硬化/软化和吸力硬化的广义Willam-Warnke本构模型,模拟高温下混凝土的材料非线性行为。为保证全局守恒方程的Newton迭代过程的二阶收敛率,导出了非线性化学-热-水力-力学(CTHM)耦合本构模型的一致性切线模量矩阵。数值结果显示了本文所发展的化学-热-水力-力学(CTHM)耦合本构模型在模拟高温下混凝土中复杂破坏过程的能力和有效性。