Zheng Zhao Liang

3-D Micromechanics Model for Progressive Failure Analysis of Laminated Cylindrical Composite Shell

A newly developed numerical code MFPA3D is applied to simulate the progressive damage and failure process of laminated cylindrical composite shell. Heterogeneities in meso-scale are taken into account by randomly distributing the material properties throughout the model by following a Weibull statistical distribution. The cylindrical composite shell is discretized into 3-D block elements with the fixed size and is subjected to a lateral compressive loading, applied with a constant displacement control manner. The numerical simulation results show that not only the process of crack initiation, propagation and coalescence but also the failure process can be numerically obtained in three dimensional. The MFPA3D modeling demonstrates that the code can simulate non-linear behavior of brittle materials with a simple mesoscopic constitutive law with a strength and elastic modulus reduction of the weaken elements.

Three-Dimensional Material Failure Process Analysis

This paper introduces a newly developed three-dimensional Material Failure Process Analysis code, MFPA3D to model the failure processes of brittle materials, such as concrete, ceramics, fibrous materials, and rocks. This numerical code, based on a stress analysis method (finite element method) and a material failure constitutive law, can be taken as a tool in numerical modeling analysis to enhance our understanding of the failure mechanisms of brittle materials. Properties of material heterogeneity are taken into account. The material is discretized into numerous small elements with fixed size. Fracture behavior can be modeled by reducing the material stiffness and strength after the peak strength of the material has been reached. The evolution of the cracking process down to full fracture implies strain softening, which describes the post-peak gradual decline of stress at increasing strain. In the present study, a Mohr-Coulomb criterion envelop with a tension cut-off is used so that the element may fail either in shear or in tension. Simulated fracture or crack patterns of two examples are found quite realistic, and the results strongly depend on the heterogeneity level.

Effect of Element Size on Rock Shear Strength and Failure Pattern by Rock Failure Progress Analysis (RFPA2D)

Six types of numerical specimens containing two notches are set up to numerically investigate the effect of element size on rock shear strength and failure pattern using RFPA2D (rock failure process analysis) code. These specimens are of the same geometrical dimension 180 mm×180 mm and have been discretized into 61×61, 122×122, 183×183, 244×244, 305×305, and 366×366 elements.The width of notches is about 2.95 (180/61) mm and the length is 45mm. The specimens are placed in a direct shear box. A lateral confining pressure with a value of 0.15MPa is invariably loaded in the vertical direction and an increasing horizontal displacement with 0.002mm/step is applied in the horizontal direction. The whole shear failure progress and associated stress field for the specimens are visually represented. Results show that the crack propagation is mostly influenced by the stress field in the vicinity of the notch tip, the required element size would be necessary in order to obtain good results. In general, for a coarse mesh, the stress field close to the notch tip can’t be represented accurately and shear strength obtained by such discretisation is slightly higher than the accurate value. For a fine mesh, the notch tip spreads through a relatively large number of elements and the stress field in vicinity of notch tip is well represented by the finite element approximation, therefore the failure pattern is consistent with real physical fracture mode.

Numerical Model for Thermal Cracking of Concrete

Thermal stresses are identified as one of the major causes of concrete failure. In order to consider the heterogeneity of concrete at mesoscopic level, and to simulate its failure processes during temperature change, a coupled thermo-mechanical model, which is on the basis of statistical damage model, is proposed. The model revealed the effect of the heterogeneity on concrete, and by analysis one of the important thermal stresses, i.e. thermal mismatch stresses, which are caused by thermal mismatch between the aggregate and mortar due to uniform change in temperature, it indicate that the presence of thermal mismatch causes stress concentration along the interface between aggregate and mortar, and the superpose of those stresses cause the crack propagation in the line of the two aggregate. The crack patterns, simulated by the proposed model, show a good agreement with the experimental results.

Numerical Simulation of 3-D Fracture Behaviors on the FRP-Strengthened Concrete Structures

A numerical code RFPA3D (Realistic Failure Process Analysis) is used to simulate the crack initiation and propagation in FRP-strengthened concrete beam under external loading. In our model, the FRP-strengthened concrete is assumed to be a three-phase composite composed of concrete, FRP, and interface between them. The displacement-controlled loading scheme is used to simulate the complete failure process of FRP-strengthened concrete the numerical simulation of failure process of the specimens. It is found that the main failure mode is the interfacial debonding and the interfacial debonding may propagate either within the adhesive layer or through concrete layer in the vicinity of bond interface. The simulation results agree well with the experiment observations. The width of the FRP sheet is considered an important factor not only to significantly influence the debonding propagation type and crack distribution but also to control the ultimate load-capacity and ultimate strain. This study is focused on the failure process of the FRP-strengthened concrete beam and the effects of the width of FRP sheet on the failure mode and on the structural load-carrying capacity of concrete structures.

Avalanche Behaviour in Microfracturing Process of 3-D Brittle Disordered Material

Using a newly-developed Material Failure Process Analysis code (MFPA3D), the micro-fracturing process and the avalanche behavior characterization of brittle disordered materials such as rock or concrete is numerically studied under uniaxial compression and tension. It is found that, due to the heterogeneity of the disordered material, there is an avalanche behavior in the microcrack coalescence process. Meanwhile, a hierarchy of avalanche events also numerically observed though a study of numerically obtained acoustic emissions or seismic events. Numerical simulations indicate that macro-crack nucleation starts well before the peak stress is reached and the crack propagation and coalescence can be traced, which can be taken as a precursory to predict the macro-fracture of the brittle disordered materials. In addition, the numerically obtained results also reveal the presence of residual strength in the post-peak region and the resemblance in the stress-strain curves between uniaxial compression and tension.

Digital Image Based Modeling of Rock Failure at Meso-Scale

This paper presents a new meso-mechanical analysis method of rock failure. The actual inhomogeneity of rock at meso-scale level is represented by processing the image of rock section and incorporated into Realistic Failure Process Analysis code (abbreviated as RFPA2D). Here, this numerical tool is employed to study the fracture phenomena of granite sample considering the interface strength between mineral grains. Numerical results show that interface strength has significant influence on the strength of sample and its failure mode. The larger the interface strength is, the more brittle rock samples become and the strength is bigger. With the interface strength increasing, failure mode gradually varies from intergranular frature to transgranular fracture.

Numerical Simulation of 3-D Failure Process of Reinforced Concrete Specimen under Uniaxial Tension

The periodically distributed fracture spacing phenomenon exists in the failure process of the reinforced concrete prism under uniaxial tension. In this paper, A numerical code RFPA3D (3D Realistic Failure Process Analysis) is used to simulate the three-dimensional failure process of plain concrete prism specimen and reinforced concrete prism specimen under uniaxial tension. The reinforced concrete is represented by a set of elements with same size and different mechanical properties. They are uniform cubic elements and their mechanical properties, including elastic modulus and peak strength, are distributed through the specimens according to a certain statistical distribution. The elastic modulus and other mechanical properties are weakened gradually when the stresses in the elements meet the specific failure criterion. The displacement-controlled loading scheme is used to simulate the complete failure process of reinforced concrete. The analyses focus on the failure mechanisms of the concrete and reinforcement. The complete process of the fracture for the plain concrete prism and the fracture initiation, infilling and saturation of the reinforced concrete prism is reproduced. It agrees well with the theoretical analysis. Through 3D numerical tests for the specimen, it can be investigated the interaction between the reinforcement and concrete mechanical properties in meso-level and the numerical code is proved to be an effective way to help thoroughly understand the rule of the reinforcement and concrete and also help the design of the structural concrete components and systems.

Influence of Heterogeneity on Direct Tensile Failure Process of Rocks and Associated Fractal Characteristic of AE

The investigation on the behavior of a specimen under uniaxial tension and the process of microfracture attracts considerable interest with a view to understanding strength characterization of brittle materials. Little attention has been given to the detailed investigation of influence of heterogeneity of rock on the progressive failure leading to collapse in uniaxial tension. In this paper, a numerical code RFPA3D (Realistic Failure Process Analysis), newly developed based on a three-dimensional model, to simulate the fracture process and associated fractal characteristic of heterogeneous rock specimen subjected to direct uniaxial tension. Specimens with different heterogeneity are prepared to study tension failure. In a relatively homogeneous specimen, the macrocrack nucleates abruptly at a point in the specimen soon after reaching peak stress. In more heterogeneous specimens, microfractures are found to appear diffusely throughout the specimen, and the specimens show more ductile failure behavior and a higher residual strength. Development of fractal theory may provide more realistic representations of rock fracture. The fractal dimension of distributed AE is computed during the fracture process. For all specimens, the fractal dimension increases as the loading proceeds, and it reaches the peak value when macrocrack nucleates abruptly. It is also found that fractures scatter more diffusely in relatively heterogeneous specimens, and the fractal dimension has a smaller value. The homogenous rock specimens have flat and smooth rupture faces which are consistent with the fractal results.

Three Dimensional Numerical Approach to Splitting Failure of Rock Discs

The Brazilian splitting tests have been commonly and widely used as a standardized test method on disc or cylinder specimens to measure the indirect tensile strength of rocks in mining engineering and other rock engineering. In this paper, a novel numerical code, 3D Rock Failure Process Analysis code, was applied to implement the splitting tensile failure tests on rock discs. The influences of the heterogeneity on stress distribution in rock are also discussed and the splitting failure patterns of specimens subjected to Brazilian tests are simulated. The simulated splitting results of rock discs were found quite realistic, which indicate that the rock failure analysis method is applicable and practical for the study of rock disc splitting failure.