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]     

Time-dependent tests on intact rocks in uniaxial compression

The influence of strain history on rock specimen deformation during multi-level loading and unloading cyclic uniaxial compression creep tests is studied with a creep testing machine. An experimental data processing method for such creep tests is suggested. The correction formulas to determine the rheological model parameters are derived for the case when load relaxation is considered. Creep and relaxation tests under uniaxial compression on four types of rocks are conducted using an electronic hydraulic servo-controlled stiff testing machine. The creep and relaxation laws of the different rocks are compared. The complete stress–strain curves for red sandstone specimens are obtained at nine strain rates from 2.43×10⁻⁶ to 4.38×10⁻³/s. The effects of strain rates on rock strength and limit strain are discussed. Empirical equations to evaluate the strain rate dependence of rock mechanical properties are presented.     

中粒辉长岩单轴三轴压缩试验研究

研究了中粒辉长岩在饱和、风干两种状态下的单轴和三轴压缩强度变形特性。在单轴和三轴压缩试验中得到了应力与各种应变之间的关系曲线,采用回归分析得到了强度准则和强度参数c,φ值,分析了围压和含水量对中粒辉长岩的影响。     

Acoustic emission characteristics and energy mechanism in karst limestone failure under uniaxial and triaxial compression

Bulletin of Engineering Geology and the Environment - Acoustic emission (AE) tests were performed on saturated karst limestone under uniaxial and triaxial compression. Mechanical properties, AE...     

Study of failure and scale effects in rocks under uniaxial compression using 3D cellular automata

A numerical model for simulating the rock fracturing process has been developed and implemented in the three-dimensional code EPCA^3D, which is based on the earlier EPCA^2D (elasto-plastic cellular automaton). EPCA^3D has the ability to simulate the initiation, propagation and coalescence of cracks in the failure processes of rocks. Using this code, we studied the failure processes of simulated rock specimens with different sizes and shapes in three-dimensional space. It is concluded that the ‘scale effect’ is not directly related to the heterogeneity, if the rock specimens have the same homogeneity. If the platen-rock elastic mismatch effect in physical testing is not considered in the numerical simulations, there is almost no scale effect for rock specimens with different sizes and shapes. However, by considering the platen-rock interaction in the simulation of the rock failure process with the EPCA^3D code, the modeling results reproduced the well-known phenomenon of scale effects found in physical experiments.     

聚丙烯纤维混凝土单轴压缩性能研究

通过低速加载得到了不同条件下聚丙烯纤维混凝土的应力与应变关系曲线,通过对聚丙烯纤维与基体混凝土的界面力学分析,得到了聚丙烯纤维在混凝土中的适用范围,对单轴压缩下的聚丙烯纤维混凝土进行了细观机理研究,得到了单轴压缩混凝土的本构方程。     

Estimation of uniaxial compressive strength from point load strength,Schmidt hardness and P-wave velocity

Uniaxial compressive strength is considered one of the most important parameters in the characterization of rock material in rock engineering practice. The study investigated correlations between uniaxial compressive strength and point load index, P-wave velocity and Schmidt hardness rebound number together with the effects of core diameter size. A total of 150 core samples at five different diameters (54, 48, 42, 30 and 21 mm) were obtained from sandstone, limestone and cement mortar. Ten saturated samples at each diameter (length:diameter ratio 2:1) were prepared from each of the three materials. The best correlations were found between uniaxial compressive strength and point load or Schmidt hammer values. The closest relationship was observed for the 48 mm diameter cores.      

Investigation of mechanical properties of fractured marbles by uniaxial compression tests

Uniaxial compression tests(UCTs)on 34 naturally fractured marble samples taken from the transportation tunnels of Jinping II hydropower station were carried out using the MTS815 Flex test GT rock testing system.Rockburst proneness index WET is determined for the marble samples with the UCTs.According to the number,size and spatial structure characteristics of the internal natural fractures of the marble samples,fractures are basically divided into 4 types,namely,single fracture,parallel fracture,intersectant fracture and mixed fracture.The mechanical properties of naturally fractured rocks(4 types)are analyzed and compared with those of intact rock samples(without natural fractures).Experimental results indicate that failure characteristics of fractured rocks are appreciably controlled by fracture distribution or fracture patterns.In comparison with intact rocks,the failure of fractured marbles is a locally progressive failure process and finally rocks fail abruptly.Statistically,the uniaxial compressive strengths(UCSs)of rocks with single,parallel,intersectant and mixed fractures are 0.72,0.69,0.59 and 0.46 times those of the intact rocks,respectively.However,the elastic modulus of the fractured Yantang marbles is generally not different from that of intact rocks.But the elastic moduli of Baishan marble with single,intersectant and mixed fractures are 0.61,0.62 and 0.45 times those of intact rocks,respectively.Experimental results also indicate that WET of fractured marbles is generally smaller than that of intact marbles,which implies that rockburst intensity of fractured marble in field may be controlled to some extent.In addition,the bearing capacity of surrounding rocks is also reduced,thus the surrounding rocks should be supported or reinforced timely according to practical conditions.     

单轴压缩下混合片麻岩脆性破坏特征分析

岩石单轴压缩变形试验是地下工程勘察中的基本力学试验。本文根据混合片麻岩单轴压缩变形试验的峰前应力—应变曲线,把其破坏过程分为裂纹闭合阶段、线弹性变形阶段、裂纹稳定扩展阶段和裂纹非稳定扩展阶段,并采用滑点回归分析技术得到了混合片麻岩各阶段裂纹临界应力值,即:裂纹闭合应力为23MPa,裂纹起裂应力为45MPa,裂纹损伤应力为86MPa,最后进行了单轴压缩下混合片麻岩脆性破坏过程细观模拟分析,为工程支护设计和围岩破坏机理分析提供参考。     

碎石颗粒的压缩变形与破碎特性

基于颗粒流软件PFC2D,建立了颗粒接触本构模型,考虑不同孔隙率的大颗粒对压缩结果的影响,分别模拟了孔隙率为0.1,0.16,0.2,0.25四种颗粒在单轴压缩下的破碎与变形过程,为探究碎石颗粒的压缩变形和破碎特性提供了新的方法。     

Fracture coalescence in rock-type materials under uniaxial and biaxial compression

Fracture coalescence, which plays an important role in the behavior of brittle materials, is investigated by loading pre-fractured specimens of gypsum, used as a rock model material, in uniaxial and biaxial compression. Several new phenomena and their dependence on geometry and other conditions are observed. The specimens have two pre-existing fractures or flaws that are arranged in different geometries, and that can be either open or closed. Two different test series are performed with these flaw geometries, one under uniaxial loading and one with biaxial loading in which confining stresses of 2.5, 5.0, 7.5 and 10 MPa are applied. As the vertical (axial) load is increased, new cracks emanate from the flaws and eventually coalesce. Flaw slippage, wing crack initiation, secondary crack initiation, crack coalescence, and failure are observed. Two types of cracks occur: wing cracks, which are tensile cracks, and secondary cracks which initiate as shear cracks in a plane roughly co-planar with the flaw. The secondary cracks usually propagate as shear cracks in the same plane but, depending on the geometry, they also propagate out of plane as either tensile or shear cracks. The wing cracks initiate at the flaw tips for uniaxial or low confinement biaxial conditions but move to the middle of the flaw and disappear completely for higher confining stresses. Three types of coalescence, which depend on the geometry of the flaws and to some extent on stress conditions, occur; they can be distinguished by different combinations of wing cracks and secondary cracks. For closed flaw specimens, at least partial debonding and slippage of the flaws is required prior to initiation of a crack. In uniaxial compression coalescence and failure occur simultaneously, while failure in biaxial compression occurs after coalescence.     

Strengthening of perforated plates under uniaxial compression: Buckling analysis

This paper focuses on the cutout-strengthening of perforated steel plates subjected to uniaxial compressive loads. The square plates considered each has a centrally placed circular hole and four simply supported edges in the out-of-plane direction. Four types of stiffeners named ringed stiffener (RS), flat stiffener (FS), longitudinal stiffener (LS) and transverse stiffener (TS) are mainly discussed. The finite element method (FEM) has been employed to analyse the elastic and elasto-plastic buckling behaviors of strengthened and unstrengthened perforated plates. The results show that the strengthened perforated plates have higher buckling strengths than those of the unstrengthened ones, while the elevations in elastic buckling stress and elasto-plastic ultimate strength are closely related to stiffener types (i.e., RS, FS, LS and TS) as well as plate geometric parameters (i.e., a plate slenderness ratio and a hole diameter ratio). Furthermore, comparisons of strengthening efficiency considering the variations of buckling stress with stiffener weight are carried out, and recommendations on the most efficient cutout-strengthening methods for the uniaxially compressed perforated square plates with centric circular holes are proposed.     

A plastic collapse mechanism for cylinders under uniaxial end compression

Tests performed on ring stiffened steel cylinders, of geometries used in offshore structures, and subject to axial compressive loading have shown that failure can occur in the vicinity of the end supports or the stiffener with the formation of an axisymmetric plastic bulge. The axisymmetric post-failure responses is studied in this paper by modelling the collapse mode by a rigid plastic mechanism. The results of the plastic mechanism approach and the assumptions on which it is based are compared to the behaviour of the cylinder predicted by a large deflection elasto-plastic finite element program; good agreement is found between the two. Experimental observations also compare well with the rigid plastic mechanism theoretical results.     

单轴压缩下大理岩细观裂纹扩展试验研究

利用带有加载装置的扫描电镜（SEM）实时动态观测系统和数字图像处理技术,对大理岩试件在受单轴压缩时所产生的微裂纹的萌生、扩展过程进行了定量分析,得到微裂纹扩展的规律,对岩石力学研究有一定的理论意义。     

Optimal stiffener design of moderately thick plates under uniaxial and biaxial compression

In this paper, the optimal stiffener design of moderately thick plates under uniaxial and biaxial compression is investigated on the premise that the plate thickness and the required ultimate strength are given. As the theoretical basis of stiffener design, the ultimate strength formulations of weak stiffened thick panels under in-plane biaxial compression are first developed on the basis of large deflection orthotropic plate theory, in which the post-weld initial deflection is taken into account. The von Mises yield criterion is employed to determine the limit state of the panel, and the Nelder-Mead simplex algorithm is used to obtain the efficient solution of nonlinear differential equations. The optimization method presented is based on the stiffener design principles of the overall instability stress and of the working stress. In the optimization formulation, the numbers and geometric sizes of the stiffeners are defined as design variables; the weight ratio of stiffeners to plate is taken as a single objective function; requirements against overall buckling of the panel, local buckling of the plates between the stiffeners and local buckling of the stiffeners themselves are set as constraint functions. Results of both design examples and parameter studies show that, for moderately thick plates, the stiffener weight given by the proposed optimization method is much lower than the weight determined by the current stiffener design method on the premise of the same requirement of structural safety. Using the present optimization method to obtain the lightest and the most effective stiffener layout for moderately thick plates is proposed.     

混凝土轴压溃裂过程的细观数值试验研究

将混凝土视为由骨料、砂浆以及它们之间的界面过渡区(ITZ)组成的细观复合材料,建立了一种包含零厚度粘结单元在内的有限元模型,将粘结单元嵌入到ITZ中以及砂浆单元的边界上,以形成潜在的裂缝发展路径。利用该模型对已生成的二维混凝土试件进行了单轴压缩的数值试验,获得了试件的应力-应变关系曲线以及试件从微裂纹萌发、裂缝开展到最后压溃破坏过程的裂缝演变状态,得到的宏观裂缝及溃裂形态与轴压物理试验十分相近,得到的力学响应曲线与物理试验也呈现良好的一致性。     

Failure characteristics of rock-like materials with single flaws under uniaxial compression

Bulletin of Engineering Geology and the Environment - A uniaxial compression test was conducted with a servo loading apparatus to study the failure of a rock-like specimen with a pre-existing...