Dynamic failure analysis on granite under uniaxial impact compressive load

High strain-rate uniaxial compressive loading tests were produced in the modified split Hopkinson pressure bar (SHPB) with pulse shaper on granite samples. It was shown that the failure of the granite cylinder was typical tensile splitting failuremode by sudden splitting parallel to the direction of uniaxial compressive loading at different strain rates. Besides, it was concluded that not only the strength of granite increased, but also the fragment size decreased and the fragment numbers increased with the increasing strain rate. To quantitatively analyze the failure phenomena, the numerical calculation based on a dynamic interacting sliding microcrack model was adopted to investigate the influence of microcrack with the different initial crack length, crack angle, crack space and friction coefficient on the macro-mechanical properties of granite under different strain rates. Accordingly, the strain-dependency of the compression strength and the fragmentation degree of granite was explained reasonably. __________ Translated from Chinese Journal of Geotechnical Engineering, 2007, 29(3): 385–390 [译自: 岩土工程学报]     

花岗岩在单轴冲击压缩荷载下的动态断裂分析

利用脉冲整形器改进后的分离式Hopkinson压杆(SHPB)系统,对新加坡Bukit Timah地区的花岗岩圆柱形试样进行了高应变率下的单轴压缩实验。实验结果发现:随着应变率的增加,不仅花岗岩材料的抗压强度增大,而且以轴向拉伸劈裂为主要破坏形式的破碎程度也有所提高,表现为碎块的尺寸减小和数量增加。针对上述花岗岩的动态特性,采用多裂纹相互作用的动态滑移型裂纹模型定量的分析了不同应变率下,材料的微裂纹的初始长度、角度、初始裂纹间距以及裂纹面的摩擦系数等微裂纹特征对材料动态强度及破碎的影响,将岩石类材料的宏观动力学特性与其细观微结构联系起来,合理地解释了花岗岩的动态强度及破碎程度的应变率相关性。     

单轴循环冲击下岩石的动力学特性及其损伤模型研究

利用改进的大直径SHPB试验装置,对花岗岩试件进行单轴循环冲击压缩试验,分析花岗岩在循环冲击载荷下的力学特性及能量吸收规律。通过基于Weibull分布的动态统计损伤模型计算岩石的累积损伤,结合试验曲线分析岩石累积损伤的演化规律。研究结果表明：随着冲击载荷循环作用次数的增加,变形模量变小,试件的屈服应变增大,峰值应力呈降低趋势。岩石的累积比能量吸收值随着冲击次数的增加而增大,且试件破坏前其值增加缓慢,试件破坏时其值急剧增大。基于Weibull分布的动态损伤本构模型的计算曲线与试验曲线具有较好的一致性,该模型能反映岩石的强度与应变、应变率的关系。累积损伤随着循环冲击次数的增加而增大,其增加速率由小变大,试件破坏前累积损伤的增加较为平缓,其主要增量由最后一次冲击破坏产生。     

Numerical modelling of uniaxial compressive failure of granite with and without saline porewater

It is important for rock engineering design to be able to validate numerical simulations, i.e. to check that they adequately represent the rock reality. In this paper, the capability and validity of four numerical models is assessed through the simulation of an apparently simple case: the complete process of microstructural breakdown during the uniaxial compressive failure of intact crystalline rock. In addition to comparing the capabilities of the four models, the results generated by each model were compared with the experimentally determined complete stress–strain curves for the Swedish Ävrö granite for different porewater conditions. In this way, it has been possible to audit the models’ adequacy for this particular simulation task. It was found that although the models had common features, they were each idiosyncratically different and required considerable expertise to match the actual stress–strain curves (which did not monotonically increase in axial strain)—indicating that, for more complex simulations, both adequate modelling and appropriate validation are not going to be an easy task. The work was conducted within the framework of the international 2004–2007 DEmonstration of COupled models and their VALidation against EXperiments with emphasis on Thermo Hydro Mechanic and Chemical aspects (DECOVALEX-THMC) phase on coupled modelling extended to include chemical effects and with application to the excavation damaged zone (EDZ) in crystalline rock.     

Micromechanical modelling of the mechanical properties of a granite under dynamic uniaxial compressive loads

A micromechanics-based model is used to study the mechanical properties of a granite under dynamic uniaxial compressive loads. The model is based on interacting sliding cracks uniformly distributed in the rock material. A dynamic crack growth criterion, which is related to crack growth velocity and dynamic fracture toughness of the rock material, is employed in the analysis. Under dynamic loads at strain rates from 10⁻⁴ to 10⁰ s⁻¹, the crack growth velocity is small compared to the critical crack growth velocity and the effect can be ignored. The strengths of the granite obtained by micromechanical modelling agree with the experimental results. A constitutive relation of the granite is derived from the energy equilibrium equation and correlated with the experimental results.     

钢管混凝土短柱在轴向冲击荷载下的动力分析

通过有限元计算软件,分析了在冲击荷载作用下钢材强度、混凝土强度以及含钢率对钢管混凝土短柱承载力强度的影响,对沿试件长度方向试件的径向位移进行了分析,并对试件进行了变质量分析,得出钢管混凝土试件在材料、尺寸确定的情况下承受轴向荷载时,位移主要由冲击能量决定。     

砌体局部受压有限元分析

采用有限元方法,分析了典型砌体局部受压时主应力和Mises应力分别沿截面宽度和深度的变化规律。结果表明,最大主拉应力位置与已有试验结果大致吻合;应力云图可清晰地反映砌体局压时的应力扩散作用,主要应力扩散范围约为一倍的局压范围的边长;最大Mises应力值可反映不同局压位置对局压强度的影响,研究成果有助于进一步了解砌体局压的受力机理,为下一步修订规范提供参考。     

高速列车运行下WJ-7型扣件动力分析

采用大型有限元软件分析WJ-7型扣件弹条在列车运行下所引起的铁轨对弹条的冲击,建立详细的扣件系统有限元模型,基于车辆轨道耦合动力学理论下分析WJ-7型扣件弹条在安装过程中的受力,以及列车冲击荷载作用下的受力特性。分析静力与冲击力下的应力大小与位置,在不同冲击力作用下发现弹条的最大应力发生在与铁垫板接触的部位,最大应力为1 853 MPa。10kN的扣压力下的静力分析,弹条的最大应力同样发生在与铁垫板接触的部位,最大应力为1 765MPa。弹条与绝缘块接触部位为了实现固定钢轨的作用受到的冲击随着冲击力的增大位移也不断增加,同时两臂向上翘起且变形越来越明显,弹条的弹程随扣压力的增大呈线性增加趋势。随着列车的反复通过,弹条容易在这区域萌生裂纹最终裂纹扩展导致弹条发生疲劳断裂。数值分析结果与现场多处弹条断裂破坏位置吻合,分析结果可为以后弹条的设计、优化、安全提供参考依据。     

片岩和纤维混凝土动态压缩性能比较

为了研究冲击压缩荷载作用下绢云母石英片岩和玄武岩纤维混凝土的动态抗压强度、破坏情况、能量吸收的应变率效应问题,采用波形整形器改进后的分离式Hopkinson压杆装置,以不同的速度分别对2种材料进行单轴冲击压缩试验.试验结果表明:绢云母石英片岩和玄武岩纤维混凝土的动态抗压强度、破坏情况、能量吸收能力均有显著的应变率相关性...     

冲击荷载作用下花岗岩残积土的动力损伤与破坏机理

为调查冲击荷载作用下花岗岩残积土的力学行为,开展了不同冲击频率(3～15 Hz)和振幅(100～400 kPa)影响的循环冲击试验,分析冲击荷载引起的超静孔隙水压力和变形的发展规律。结果表明:振幅和频率的影响均存在临界值,振幅与频率超过临界值时,土体损伤强烈会引起强度迅速衰减。低频与超高频冲击下更易产生较高孔压,从而导致有效应力降低进而引起强度下降。根据冲击应力与应变的滞回曲线的形态特征提出了花岗岩残积土冲击动力损伤的3个定量评价参数,并据此提出了3种冲击破坏类型与辨识方法,指出冲击能量耗散引起的结构损伤及塑形变形累积是花岗岩残积土产生冲击破坏的根本原因,其影响程度取决于土的原始结构强度与微观裂隙发育程度,也与冲击模式和应力水平导致的裂隙扩展规律和塑性累积变形大小有关。工程实践中应查明土体在冲击荷载下的临界振幅与临界频率,尽可能避免采用高振幅与低频率及超高频率荷载冲击土体。研究有助于了解冲击荷载的作用规律和土体力学响应,为中国花岗岩风化地层的施工与设计提供科学理论指导。     

Behaviour of low-temperature fired laterite bricks under uniaxial compressive loading

It has been shown previously that the properties of some laterite building bricks abundant in tropical areas can be improved by stabilization through heat at low temperatures. Further investigations based on a series of laboratory tests were carried out on these brick specimens subjected to uniaxial compressive loading at room temperature. The characteristics of the stress–strain relationship are presented for predicting brick performance. When subjected to deformation, they behaved nonlinear plastic–elastic–plastic. They were found to exhibit linear elasticity in a domain where strain varied between 2 and 5% according to the type. These values of strain were found to be higher than the maximum elastic strain (0.5%) observed for many ceramics.     

单轴加载条件下花岗岩声发射及波传播特性研究

 采用声波、声发射一体化装置,研究单轴压缩下花岗岩波速与声发射演化规律,通过宏细观方法确定各应力门槛值,研究裂纹扩展不同阶段声发射演化及波传播规律。结果表明：细观裂纹的演化与宏观变形直接对应,由于微裂纹主要沿轴向扩展,导致轴向刚度对裂纹起裂及贯通的敏感度弱于非线性增长的侧向变形,瞬时泊松比曲线斜率变化点与应力门槛值对应,声发射测试确定的起裂应力比宏观应变法偏小,但反映了微裂纹的初始萌生;采用实测波速变化分析声发射震源的时空及幅值演化分布,较好地描绘了裂纹的扩展过程,由于不同阶段声发射信号的幅值及能量存在差异,导致声发射特征参数演化规律差异较大(尤其在损伤应力之后),AE能量在破坏前呈突发性增长,可作为灾害性破坏的前兆;加载初始阶段,由于微裂隙的闭合,波速及波幅均随应力逐渐增大,但增加速率逐渐下降,侧向波速在闭合应力附近基本达到峰值,此后一定阶段基本保持不变,但其他方向波速则继续增大,随着波传播方向与径向夹角的增大,波速增加幅度及波速下降点对应的应力(损伤应力前、后)逐渐增大,峰值应力附近对应波速下降幅度减小;波速受损伤演化的影响要滞后于声发射事件。     

The effect of rock classes on the relation between uniaxial compressive strength and point load index

Uniaxial compressive strength and point load tests were carried out on 17 igneous, 16 metamorphic and 19 sedimentary rocks and the values correlated with their I _s values. The influence of the different rock type was investigated using regression analysis and the derived equations were statistically tested. Although the derived equation for all data is significant, the data points are scattered and the coefficient of correlation is not strong. However, when the regression analysis was repeated for igneous, metamorphic and sedimentary rocks respectively, the data were less scattered and stronger correlation coefficients were obtained.      

Experimental research on creep behaviors of sandstone under uniaxial compressive and tensile stresses

The consideration of time dependence is essential for the study of deformation and fracturing processes of rock materials, especially for those subjected to strong compressive and tensile stresses. In this paper, the self-developed direct tension device and creep testing machine RLW-2000M are used to conduct the creep tests on red sandstone under uniaxial compressive and tensile stresses. The short-term and long-term creep behaviors of rocks under compressive and tensile stresses are investigated, as well as the long-term strength of rocks. It is shown that, under low-stress levels, the creep curve of sandstone consists of decay and steady creep stages; while under high-stress levels, it presents the accelerated creep stage and creep fracture presents characteristics of brittle materials. The relationship between tensile stress and time under uniaxial tension is also put forward. Finally, a nonlinear viscoelastoplastic creep model is used to describe the creep behaviors of rocks under uniaxial compressive and tensile stresses.