混凝土材料层裂强度的实验研究

利用Φ74大尺寸Hopkinson压杆和混凝土长杆试件研究了混凝土材料的层裂强度及其应变率效应.入射的压缩波通过压杆透入试件并反射成拉伸波而形成层裂.实验中采取在试件上多点贴应变片,讨论了应力波在混凝土试件中传播的波形弥散和幅值衰减,并在考虑了损伤演化影响的基础上确定了试件材料的层裂强度.对某种普通混凝土在不同应变率下的测试显示层裂强度受应变率影响明显.结果表明,本文提出了一种测定混凝土层裂强度的有效方法.     

超高韧性水泥基复合材料的层裂试验研究

层裂是材料遭受冲击、爆炸等高速荷载时的一种常见破坏方式。该文利用直径80 mm的霍普金森杆实验装置,研究了超高韧性水泥基复合材料UHTCC(Ultra High Toughness Cementitious Composites)中应力波的传播特性和材料的层裂强度。通过在试件表面粘贴5组应变片,获得了在0.2 MPa、0.3 MPa、0.4 MPa、0.5 MPa打击气压下,UHTCC中应力波的传播曲线。利用高速摄影机记录层裂试验,观测了UHTCC的层裂破坏过程。由试件表面应变片测得的应力波曲线,计算了材料中的应力波波速、动态弹性模量,分析了应力波在该材料中传播的衰减规律,并计算出不同打击气压下材料的层裂强度及应变率。试验结果显示:UHTCC的层裂过程相比混凝土具有更多的韧性特征;UHTCC中的应力波峰值在0 mm～500 mm范围内衰减迅速;在同等应变率下,UHTCC与静态抗拉强度相近的混凝土相比,层裂强度高出10 MPa左右,且UHTCC的层裂强度具有明显的应变率敏感性。     

钢纤维混凝土动态抗拉强度的实验研究

基于线弹性和一维应力波假定,采用Φ75mmSHPB对钢纤维体积率Vf分别为0、0.75%和1.50%的三种混凝土材料进行了一维杆层裂实验,考虑了应力波在混凝土材料内传播时的波形弥散效应和应力幅值衰减,通过计算应变片记录的应力信号确定了材料的动态抗拉强度。结果表明,钢纤维混凝土的动态抗拉强度受应变率和钢纤维体积率的影响,本文为测试脆性材料的动态抗拉强度提供了一种有效方法。基于微观扫描技术,对钢纤维增强机理进行了分析。     

An experimental method for dynamic tensile testing of concrete by spalling

A new application of the spalling phenomenon in long specimens is reported in this paper. The new experimental technique is based on an experimental setup which consists of an air launcher of cylindrical projectiles with a Hopkinson bar as a measuring tool and a relatively long concrete specimen in contact with the bar. The incident compression wave transmitted by the Hopkinson bar into the specimen is reflected as a tensile wave causing spalling. Although such configurations have been reported in the past, the main advantage of the present approach lies in the application of the detailed analysis, based on the wave mechanics with dispersion, to extract the specimen behaviour. Such an approach leads to an exact estimation of the local failure stress in tension at high strain rates, even above 100 s⁻¹. This paper demonstrates, using two series of tests on concrete, that this experimental setup can cover one decimal order of strain rates, from ∼10 to ∼120 s⁻¹. The tests performed at high strain rates on wet and dry concrete have indicated that the tensile strength is substantially influenced by the loading rate or strain rate. The absolute value of the failure stress for wet and dry concrete is almost the same for a particular strain rate, which does not occur when subject to low strain rates in tension or compression. A brief discussion is offered on a high rate sensitivity of concrete strength in tension at high strain rates.     

轴向钢筋增强混凝土一维应力层裂实验研究

基于Φ74mmSHPB实验平台进行了混凝土及轴向钢筋增强混凝土(UDRC)杆的一维应力层裂实验,采用超高速相机拍摄实验中杆表面的实时变形情况,使用数字图像相关法(DIC)分析杆表面的位移场及应变场演化过程,探讨混凝土及增强混凝土在应力波加载过程中发生拉伸断裂(层裂)的规律,并进一步结合有限元分析了钢筋在层裂过程中的作用。结果表明:UDRC杆中应力波的传播满足一维应力假设;钢筋对UDRC发生拉伸层裂的影响可以忽略,而在混凝土基体断裂后将使结构保持完整;断裂试件中的裂纹在拉压应力波交替作用下反复张开闭合,随着应力波在杆中的衰减而趋于稳定;UDRC与混凝土的层裂强度基本相同,且具有相似的应变率增强效应;在实验加载范围内,光圆钢筋和螺纹钢筋的结构增强效果没有区别。     

Dynamic compressive and splitting tensile properties of concrete containing recycled tyre rubber under high strain rates

In order to raise the efficiency of resource utilization, recycling waste rubber particles into concrete as aggregate has been widely accepted. When the size and content of the rubber particles are appropriate, rubberized concrete can achieve many excellent properties. This study investigated the impact of rubber replacement on dynamic compressive and splitting tensile properties of concrete. The split Hopkinson pressure bar tests of rubberized concrete containing 5%, 10%, 15% and 20% volume replacement for sand were completed. The failure modes, stress curves and dynamic strength values of rubberized concrete under high strain rates were recorded. The results reveal that the dynamic compressive and splitting tensile strength of rubberized concrete decrease with increasing rubber content. Meanwhile, peak strain increases with increasing rubber content. Dynamic increase factors (DIFs) of compressive and splitting tensile strength also were calculated, where rubberized concrete shows a stronger strain rate sensitivity. The analysis of specific energy absorption illustrates that rubberized concrete with 15% rubber replacement has the best impact toughness. In addition, ratios of dynamic compressive–tensile strength of rubberized concrete were calculated, which are between 3.82 and 5.39.     

Dynamic splitting tensile properties of concrete and cement mortar

To investigate the dynamic tensile behaviours of concrete and cement mortar, a 50‐mm split Hopkinson pressure bar was applied on Brazilian disc specimens for dynamic tensile experiments, in which strain rate varied from 10^?5 to 20 s^?1. The high‐speed camera testing technique was used to capture the dynamic fractured process of the specimens at relative high strain rate. The experimental results revealed that the dynamic tensile strength of concrete specimens has a stronger strain rate effect than that of cement mortar specimens. Then three typical failure patterns of the specimens were confirmed in dynamic experiments. In addition, one‐parameter semi‐empirical relation between dynamic tensile strength and strain rate was established. Finally, the limitation of dynamic splitting experiments on Brazilian disc specimens was discussed in detail at high strain rate, in which the crack initiates from the contact point between the incident bar and specimens rather than the centre of the specimens.     

Concrete subjected to projectile and fragment impacts: Modelling of crack softening and strain rate dependency in tension

This paper deals with modelling of plain concrete in tension. The aim is to improve the accuracy of the numerical analyses for projectile and fragment impacts on concrete. A bi-linear crack softening law and a strain rate-dependent law are implemented in the hydrocode AUTODYN. Parametric studies are made, and numerical analyses are compared with experiments conducted and with experiments found in the literature. The depth of penetration is mainly dependent on the compressive strength of the concrete. However, to correctly model spalling, cracking and scabbing in concrete, the tensile strength, fracture energy, and strain rate in tension are very important. It is shown that the accuracy of the results in the numerical analyses of concrete subjected to projectile and fragment impacts was improved, when using a bi-linear softening law and the modified strain rate dependency for tension.     

Ultra‐High‐Speed Full‐Field Deformation Measurements on Concrete Spalling Specimens and Stiffness Identification with the Virtual Fields Method

Abstract: For one decade, spalling techniques based on the use of a metallic Hopkinson bar in contact with a concrete sample have been widely employed to characterise the dynamic tensile strength of concrete at strain rates ranging from a few tens to hundreds of s^?1. However, the processing method based on the use of the velocity profile measured on the rear free surface of the sample (Novikov formula) remains quite basic. In particular, the identification of the whole softening behaviour of the concrete material is currently out of reach. In the present paper, a new processing technique is proposed based on the use of the virtual fields method (VFM). First, a digital ultra‐high‐speed camera is used to record the pictures of a grid bonded onto the specimen. Then, images of the grid recorded by the camera are processed to obtain full‐field axial displacement maps at the surface of the specimen. Finally, a specific virtual field has been defined in the VFM equation to use the acceleration map as an alternative ‘load cell’. This method applied to three spalling tests with different impact parameters allowed the identification of Young's modulus during the test. It was shown that this modulus is constant during the initial compressive part of the test and decreases in the tensile part when microdamage exists. It was also shown that in such a simple inertial test, it was possible to reconstruct average axial stress profiles using only the acceleration data. It was then possible to construct local stress–strain curves and derive a tensile strength value.     

地下工程抗爆防震塌设计动力学机理讨论

在爆炸载荷下,反射拉应力波是导致混凝土防护结构震塌的主要原因。材料的动态抗拉强度是结构抗爆防震塌设计所必须考虑的主要参量,抑制或减弱拉伸冲击波的强度是防护结构设计的一个重要目标。研究了混凝土和钢纤维增强混凝土的动态力学性能以及它们结构的爆炸震塌响应。试验表明,高强混凝土C100的抗震塌能力还不如普通混凝土C40;加入体积含量2%的钢纤维可有效提高混凝土的强度以及结构的抗震塌能力;而且加入相同含量的钢纤维后,纤维增强C100混凝土的抗震塌能力与增强C40混凝土相比并无明显不同。应用一维应力波理论近似分析,揭示了抗震塌设计的动力学机理,分析表明,"三明治"复合层结构可以有效提高结构的抗震塌能力,此分析为设计具有更好抗爆能力的防护工程提供了科学依据。     

低应变率下混凝土动态拉伸破坏尺寸效应细观模拟

在混凝土静态破坏尺寸效应方面已取得了较完善的成果,而在动态破坏尺寸效应方面,包括其产生机制及对应的尺寸效应律的研究则非常匮乏。为探讨动态荷载作用下混凝土尺寸效应行为,从细观角度出发,结合混凝土细观结构特征,考虑动态加载下细观组分应变率效应的影响,建立了混凝土破坏行为研究的细观力学分析模型与方法。以双边缺口混凝土试件为例,对其在低应变率（10^-5 s^-1~1 s^-1）下混凝土动态拉伸破坏行为及尺寸效应进行细观数值模拟,并分析了应变率效应对动态破坏尺寸效应的影响。最后,结合应变率效应对强度及尺寸效应的影响规律—“强度增强效应”与“尺寸效应削弱效应”,在静态破坏尺寸效应律的基础上,建立了混凝土拉伸强度的“静动态统一”尺寸效应理论公式,并验证了理论公式的准确性和合理性。     

Frequency-dependent tensile and compressive effective moduli of elastic solids with distributed penny-shaped microcracks

This paper develops micromechanics models to estimate the tensile and compressive elastic moduli of elastic solids containing randomly distributed penny-shaped microcracks. The crack faces are open under tension and closed under compression. When the crack faces are closed, they may slide against one another following Coulomb’s law of dry friction. The micromechanics models provide analytical expressions of the tensile and compressive moduli for both static and dynamic cases. It is found that the tensile and compressive moduli are different. Further, under dynamic loading, both compressive and tensile moduli are frequency dependent. As a by-product, the micromechanics models also predict wave attenuation in the dynamic case. Numerical simulations using the finite element method are conducted to validate the micromechanics models.     

Preparation of C200 green reactive powder concrete and its static–dynamic behaviors

In this paper, a new type of green reactive powder concrete (GRPC) with compressive strength of 200 MPa (C200 GRPC) is prepared by utilizing composite mineral admixtures, natural fine aggregates, short and fine steel fibers. The quasi-static mechanical properties (mechanical strength, fracture energy and fiber–matrix interfacial bonding strength) of GRPC specimens, cured in three different types of regimes (standard curing, steam curing and autoclave curing), are investigated. The experimental results show that the mechanical properties of the C200 GRPC made with the cementitious materials consisting of 40% of Portland cement, 25% of ultra fine slag, 25% of ultra fine fly ash and 10% of silica fume, 4% volume fraction of steel fiber are higher than the others. The corresponding compressive strength, flexural strength, fracture energy and fiber–matrix interfacial bonding strength are more than 200 MPa, 60 MPa, 30,000 J/m² and 14 MPa, respectively. The dynamic tensile behavior of the C200 GRPC is also investigated through the Split Hopkinson Pressure Bar (SHPB) according to the spalling phenomena. The dynamic testing results demonstrate that strain rate has an important effect on the dynamic tensile behavior of C200 GRPC. With an increase of strain rate, the peak stress rapidly increases in the dynamic tensile stress–time curves. The C200 GRPC exhibits an obvious strain rate stiffening effect in the case of high strain rate. Finally, the mechanism of excellent static and dynamic properties gains of C200 GRPC is also discussed.     

静载和动载下不同龄期混凝土力学特性的试验研究

利用杆径为75mm的SHPB试验装置对5种不同龄期下的混凝土分别进行了冲击压缩试验,系统了解了冲击载荷对不同龄期支护混凝土力学特性的影响。为了进行对比,利用INSTRON系统也进行了相应龄期下的静载压缩试验。试验研究表明：静载下混凝土强度、割线弹性模量随龄期增长而增长,其中强度增长主要集中在龄期7d以前,割线弹性模量增长则集中在龄期14d以后,而峰值应变随龄期增长整体上呈减小的趋势;动载下混凝土强度、峰值应变以及单位体积吸收能随着龄期增长而增长,在各个龄期都表现出对应变率具有一定的敏感性,其中不同龄期混凝土的动态强度随应变率增加呈现指数函数增长趋势。不同龄期的混凝土在动载下以拉伸破坏为主,静载下基本呈现剪切破坏形式。     

超高性能混凝土动态冲击拉伸性能研究

超高性能混凝土(Ultra-high performance concrete,UHPC)作为一种具有超高物理力学性能的新型建筑材料,能显著提高军事防护工程的抗爆炸冲击能力,对保障防护工程中人员的生命安全具有重要意义。为揭示爆炸冲击波在防护工程自由面引起的动态拉伸破坏行为,利用霍普金森压杆装置(Split Hopkinson pressure bar,SHPB)对UHPC进行动态冲击拉伸试验,系统研究了粗集料种类、钢纤维掺量以及应变率对UHPC动态冲击拉伸性能的影响规律。结果表明:粗集料种类对UHPC的动态冲击拉伸强度有较显著的影响,相比于花岗岩和铁矿石,玄武岩粗集料对动态冲击拉伸性能的提高更为明显;UHPC的动态冲击拉伸强度会随着钢纤维掺量的增加而显著提高,但钢纤维掺量对UHPC动态拉伸强度的贡献存在4%的临界值;此外,UHPC表现出明显的应变率效应,当应变率为7~50s-1时,其效应最为显著。     

纤维对超高性能混凝土残余强度及高温爆裂性能的影响

采用普通原材料制备56 d龄期抗压强度为140~160 MPa的空白组超高性能混凝土、钢纤维超高性能混凝土及混杂纤维超高性能混凝土,测定其遭受高温作用后的残余抗压强度和劈裂抗拉强度,并对100%含湿量的混凝土试块进行高温爆裂试验。此外,测定大小2种加热速率对超高性能混凝土高温爆裂行为的影响。结果表明:所配制混凝土的残余抗压强度均随着目标温度的升高呈现先增大再降低的趋势,800℃高温后的残余抗压强度约为常温强度的30%。钢纤维与混杂纤维混凝土的残余劈裂抗拉强度亦呈现先升高再降低的趋势,800℃高温后的残余劈裂抗拉强度分别为常温强度的15.1%和35.4%。空白组混凝土的残余劈裂抗拉强度随着目标温度的升高而单调下降,800℃高温后的强度值约为常温强度的20.3%。7.5℃/min加热速率下,100%含湿量的3种混凝土试块均发生了严重高温爆裂,单掺钢纤维可以改善超高性能混凝土的高温爆裂,但不能避免爆裂的发生,而混杂纤维对超高性能混凝土高温爆裂的改善效果并未显著优于钢纤维。2.5℃/min加热速率下,混杂纤维可避免部分超高性能混凝土试块发生爆裂。      

骨料粒径对混凝土动态拉伸强度及尺寸效应影响分析

骨料粒径是影响混凝土力学性能及破坏机理的重要因素。从细观角度出发,将混凝土看作由骨料颗粒、砂浆基质及界面过渡区组成的三相复合材料,考虑细观组分的应变率效应,建立了混凝土动态拉伸破坏行为研究的细观力学分析模型,模拟研究了不同骨料粒径下混凝土动态拉伸破坏行为,并揭示了动态拉伸强度的尺寸效应规律。研究表明:低应变率下骨料不发生破坏,骨料粒径对混凝土动态拉伸破坏模式及拉伸强度影响显著,且拉伸强度的尺寸效应随骨料粒径的减小而削弱;高应变率下裂缝将贯穿骨料,骨料粒径的大小对混凝土动态拉伸强度及尺寸效应影响可忽略。最后,结合应变率效应的影响机制,建立了混凝土拉伸强度的"静动态统一"尺寸效应理论公式,该公式可以较好描述各骨料粒径下混凝土动态拉伸强度与试件尺寸的定量关系。     

低合金汽车结构钢动态响应模型研究

目的 研究低合金汽车结构钢不同应变速率下的动态响应行为,并提供一种简单准确的动态增长因子的预测方法,为内高压成形工艺提供参考.方法 进行高应变速率拉伸(动态拉伸)实验,并用高速摄像机和扫描电子显微镜对材料的拉伸行为和断口形貌进行表征,利用实验数据建立一系列结构钢的动态响应模型,研究不同应变速率下的响应特性.结果 随着应...     

三种建筑钢筋材料高应变率下拉伸力学性能研究

钢筋混凝土结构除承受静载荷外,在恐怖袭击、燃气管道爆炸等特殊情况下还会承受动态载荷的作用,因此,研究其组成材料之一的钢筋材料在动态载荷作用下的力学性能,对于研究钢筋混凝土结构整体在动态载荷作用下的力学响应有重要意义。该文首先利用旋转盘冲击拉伸试验系统对3种常用建筑钢筋材料(HPB235、HRB335和HRB400)在400 s^-1~2000 s^-1应变率范围内的动态拉伸力学性能进行试验研究,然后根据试验数据,分析应变率对屈服强度的影响规律,并对常用的Johnson-Cook本构模型进行修正,以获得可以更好描述这3种钢筋材料动态拉伸应力-应变关系的本构模型及相关的材料参数。研究结果表明:3种钢筋材料的屈服应力均随应变率的增大而增大,而静载屈服应力越低的钢筋对应变率越敏感;修正后的Johnson-Cook本构模型能较好地描述3种钢筋材料的动态拉伸应力-应变关系。     

二维C/SiC复合材料准静态和动态拉伸力学性能

采用分离式Hopkinson拉杆装置和电子万能试验机研究了二维C/SiC复合材料在4种应变率（0.001、0.010、90.000和350.000 s-1）下的拉伸力学性能,计算并验证了动态试验中的应力平衡状态;采用SEM分析了复合材料在不同应变率下的破坏断口和失效机制;建立了复合材料包含损伤和应变率相关的本构方程。结果表明:二维C/SiC复合材料的应力-应变曲线都表现出非线性的特征。随着应变率的增加,二维C/SiC复合材料的拉伸强度从204 MPa增加到270 MPa,增加了33%,这表明复合材料的拉伸强度具有较强的应变率敏感性。复合材料在准静态和动态加载下表现出不同的破坏模式是由材料内部界面行为的应变率效应造成的。