活性粉末混凝土的层裂性能研究

采用60%的超细工业废渣取代水泥制备了一种生态型的活性粉末混凝土(RPC),采用分离式霍普金森压杆装置对不同纤维掺量的RPC材料进行了层裂性能实验.研究得出了入射波强度和冲击次数对层裂过程中应力波传播的影响规律.按一维弹性波理论编写了入射压缩波和反射拉伸波在试件自由端附近相互作用的程序,计算出试件自由端附近拉应力的分布,由试件的层裂位置得到材料的层裂强度.结果表明,随着入射波强度的增加和冲击次数的提高,材料的拉伸损伤逐渐增加,反射拉伸波的强度逐渐降低.RPC材料层裂强度和破坏形态具有明显的应变率效应,层裂强度和破坏程度随着应变率的提高而增加.通过纤维的增强作用,层裂裂缝的宽度和深度都降低了.     

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。     

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。     

混杂纤维高强混凝土层裂试验研究

采用Φ74 mm直锥变截面霍普金森压杆,对尺寸为Φ70 mm×600 mm,掺钢纤维和聚丙烯纤维的C80级混杂纤维高强混凝土(HFRHSC)试块,进行应变率为1~30 s-1的层裂试验。通过加置波形整形器和万向头,得到了适合层裂试验的理想电压时程波形,试验发现钢纤维和聚丙烯纤维的掺入可显著提高混凝土的层裂强度;HFRHSC层裂强度随应变率提高而提高;钢纤维对层裂强度的增强效应大于聚丙烯纤维的增强效应;混凝土层裂断面与试块自由端的距离基本呈现随应变率提高而减小的趋势,层裂次数随应变率的提高而增多,素混凝土试块的层裂破坏面较为平整,而HFRHSC试块的层裂面凹凸不平。     

钢纤维超高强混凝土动态力学性能

采用分离式霍普金森压杆装置对不同纤维体积率的钢纤维超高强混凝土进行不同应变率的冲击压缩试验,结果表明钢纤维超高强混凝土是应变率敏感材料,并测出其应变率敏感阀值,当应变率超过阀值后,钢纤维超高强混凝土的强度、韧度与弹性模量都随纤维体积率的增加而显著提高,在高应变率下,超高强混凝土基体成粉碎性破坏,而钢纤维超高强混凝土呈现出“裂而不散”的破坏形态。     

软材料的霍普金森压杆测试新技术

提出了一种用于软材料动态压缩性能测试的霍普金森压杆改进方案。该方案采用了长杆直接撞击试件的加载方式并在透射杆中使用半导体应变片。通过对测试过程中试件内应力波传播的分析,提出了实现软材料动态性能测试时试件内应力尽快均匀的最佳加载方式以及实现此种加载的加载整形技术。该方案能解决由于杆-试件之间波阻抗失配导致的应变信号微弱而测试困难的问题,与高聚物霍普金森压杆相比则具有更高的可靠性。该方案能实现对软材料的大变形、近似恒应变率及大应变率范围动态压缩测试。最后给出了一个试验性装置的测试结果。     

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。     

湿热环境下Carbon/Epoxy复合材料层合板动态压缩性能

随着纤维增强复合材料的广泛应用, 研究其在湿热环境下的动态力学性能具有重要的理论研究意义与工程应用价值。首先对碳纤维增强环氧树脂基(Carbon/Epoxy)复合材料层合板试件进行了湿热处理, 其后采用分离式霍普金森压杆(SHPB)技术开展了干/湿态试件高应变率压缩实验并对实验结果进行分析。结果表明: 材料脱(吸)湿过程呈现出两段式特点, 存在二次脱(吸)水现象; Carbon/Epoxy复合材料层合板的强度在垂直铺层方向具有显著的应变率敏感性, 随着应变率从1 500 s-1增加至6 000 s-1, 其强度增加近3倍, 与此同时应变率对其弹性模量的影响却非常微弱; 此外, 湿热处理有助于提升该材料的动态力学性能, 经20 d吸湿后材料动态强度有最大12.45%的增幅, 吸湿使得材料动态强度的上升在应变率较低时比较明显。      

活性粉末混凝土冲击压缩性能实验研究

通过对活性粉末混凝土材料进行Hopkinson压杆冲击压缩实验,得到不同钢纤维含量的活性粉末混凝土在不同应变率下的应力-应变全曲线,并给出了不同应变率下材料的动态压缩强度和动态增长因子,实验结果表明,活性粉末混凝土具有应变率敏感性,钢纤维的掺入部分提高了材料的冲击压缩性能。     

混凝土恒定围压下冲击加载实验装置研制

研制可实现混凝土、岩石及土等非均匀材料恒定围压下轴向冲击压缩加载实验装置,研究复杂应力状态下高应变率力学性能。该装置通过围压油缸及轴向油缸对试件施加静水压,利用大直径分离式Hopkinson压杆(SHPB)进行轴向冲击加载,测量恒定围压下材料动态应力-应变曲线。该装置具有冲击加载前试件处于静水压状态、轴向冲击加载过程中试件围压保持恒定不变等优点。结果表明,围压对混凝土材料动态力学性能影响明显。     

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.     

Study on Dynamic Behavior and Tensile Strength of Concrete Using 1D Wave Propagation Characteristics

Concrete structures are usually fractured under dynamic loadings, so it is important to have a clear knowledge of their dynamic behavior and tensile strength. First, the principle of one-dimensional (1D) stress wave reflection and superposition at free surface is briefed, and the spalling test method based on the Hopkinson bar is presented. Then, the attenuation law of stress wave is explored and the dynamic tensile/compressive moduli of concrete are evaluated according to the wave propagation experiment. Lastly, the influences of strain rate on the spalling tensile strength and failure patterns of concrete are further analyzed. The testing results demonstrate that the attenuation of stress wave accords with an exponential law when propagating in the concrete bar. The difference between the dynamic elastic moduli of concrete in tension and in compression is minor. Spalling tensile strength is sensitive to strain rate, and there is an obvious linear correlation between dynamic increase factor (DIF) and strain rate in a log-log plot for strain rate above 1.0/s; a single fracture occurs at low strain rate, while multiple fractures are detected with increasing strain rate.     

Study on energy properties and failure behaviors of heat-treated granite under static and dynamic compression

Abstract

The material testing machine and the split Hopkinson pressure bar (SHPB) were adopted, respectively, to conduct the static and dynamic compression tests on granite specimens heat treated by different temperatures. The effects of strain rate and heat-treatment temperature on the mechanism of energy evolution of the specimen during deformation and failure process were studied. The results show a significant strain rate effect on the granite, with the energy dissipation density increasing with increasing impact velocity (or strain rate), regardless of the treatment temperature. The specimens heat treated at 300?°C and 700?°C have the minimum and maximum energy dissipation densities, respectively. The specimen in the SHPB tests easily broke into pieces or even powder; while under static compression, only macroscopic fracture surfaces and spalling phenomenon on the specimen were detected. The energy dissipation density is inversely proportional to the compressive strength of the specimen. The rate of energy dissipation change is defined, which can be used to identify the stages in the deformation process of rock and to determine the position of the failure point in the stress-strain curve. For both the dynamic and static compression tests, the value of energy utilization ratio is relatively low, with a maximum value of about 35%.     

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.     

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.     

纤维高强混凝土的动态力学性能试验研究

为研究纤维高强混凝土材料在冲击荷载下的动态压缩性能,采用大尺寸φ75mm Hopkinson压杆,对三种纤维含量的钢纤维高强混凝土、PVA纤维高强混凝土试件进行了三种应变率范围的冲击压缩试验,得到了它们在较高应变率范围内的动态应力-应变关系。试验表明纤维高强混凝土材料为应变率敏感性材料,在较高应变率范围内纤维高强混凝土材料的动态应力-应变关系是与应变率相关的。纤维高强混凝土材料的破坏应力和破坏应变随应变率的增大而增大。钢纤维和PVA纤维对混凝土耗能能力的改善和提高表现在材料达到峰值应力后开始破坏的过程中。同时也对两种纤维高强混凝土材料的纤维增韧特性及耗能机理也进行了分析和探讨。     

A testing technique for concrete under confinement at high rates of strain

A testing device is presented for the experimental study of dynamic compaction of concrete under high strain rates. The specimen is confined in a metallic ring and loaded by means of a hard-steel Hopkinson pressure bar (80 mm diameter, 6 m long) allowing for the testing of specimens large enough regarding the aggregate size. The constitutive law for the metal of the ring being known, transverse gauges glued on its lateral surface allow for the measurement of the confining pressure. The hydrostatic and deviatoric responses of the specimen can then be computed. The proposed method is validated by several numerical simulations of tests involving a set of four different concrete-like behaviours and different friction coefficients between the cell and the specimen. Finally, three tests performed with the MB50 concrete at three different strain rates are processed with the method and are compared with literature results for the same material under quasi-static loadings.