Papers by Keyword: Failure Process

Abstract: The failure process of heterogeneous rock specimen with initially random material imperfections in uniaxial plane strain compression and the macroscopically mechanical response are numerically modeled by using FLAC (Fast Lagrangian Analysis of Continua). A FISH function is generated to prescribe the initial imperfections within the heterogeneous specimen by using Matlab. The imperfection is weaker than the intact rock. Beyond the failure of the imperfection, it undergoes ideal plastic behavior, while intact rock exhibits linear strain-softening behavior and then ideal plastic behavior once failure occurs. The specimen with smooth ends is loaded at a constant strain rate and is divided into 3200 elements. The maximum numbers of the initial imperfections in five schemes are 100, 300, 500, 700 and 900. The effects of the number of the imperfections on the fracture process, the final fracture pattern and the complete stress-strain curve are investigated. Prior to the peak stress, some imperfections extend in the axial direction and then a part of them coalesce to form inclined shear bands. Beyond the peak stress, shear bands progressively intersect the specimen; in the process the number of the yielded elements approximately remains a constant. With an increase of the number of the initial imperfections, the spacing of shear fractures decreases, the peak stress and corresponding axial strain decrease; the post-peak branch of stress-strain curve becomes steeper; much more elements fail in tension; the number of the yielded elements in tension in the vicinity of the two lateral edges of the specimen remarkably increases.

Abstract: Based on physical model of three-point-bending test, the AE characteristics of three-point-bending beams with different relative notch depth during the entire loading period was simulated by using RFPA3D(realistic failure process analysis) code. Simulation results show that the relative notch depth affects the AE characteristics significantly. With increasing relative notch depth, the occurrence of AE events decreases remarkably. The stress distribution figures, elastic modulus photo and AE relative energy time-space distribution figures as well as an analysis on the failure process are also provided. Based on the analysis of simulation results, it is concluded that the heterogeneity of rock and concrete has great influence on the crack propagation path, which leads the crack propagation path becoming curvilinear.

Abstract: 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.

Abstract: 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.

Abstract: With the knowledge of heterogeneous characteristics of thermal barrier coating materials at mesoscopic level, a coupled thermo-mechanical-damage (TMD Model) model was introduced and used to numerically quantify the thermal stresses and crack development of in thermal barrier coatings (TBCs) composite subjected to decreased temperatures. The effect of different surface precrack morphologies, such as precrack length and precrack density, on an interface crack subjected to thermal loading caused by a temperature change is presented. It provides us with a more sensible physical intuition and a more accurate mathematical for optimizing the design and the processing of ceramic coatings subjected to the coupled thermal-mechanical loading.

Abstract: In this paper, a numerical code, Realistic Failure Process Analysis code (RFPA), was used to perform a microscopic analysis of a crack in a fiber-reinforced ceramic, when the crack length is the same order of magnitude as the fiber spacing. The numerical results performed in the paper shown the failure process of fiber-reinforced ceramic subjected to tension loading, which indicate that the reinforcing fibers in a ceramic composite have a significant effect in inhibiting crack propagation even during the stages of the development of crack. Moreover, the fiber evidently increased the load-carrying capacity.

Abstract: Rock and concrete are typical heterogeneous material that the meso-scale heterogeneity may have a significant effect on their macro-scale mechanical responses. In this work, a digital image-based (DIB) technique is employed to characterize and quantify the heterogeneity of concrete, and the obtained data is directly imported into a numerical code named RFPA (Rock Failure Process Analysis) to study the effect of heterogeneity on the failure process of concrete. The upgraded RFPA is capable to simulate the progressive failure of brittle materials such as rock and concrete, representing both the growth of existing fractures and the formation of new fractures, obviating the need to identify crack tips and their interaction explicitly. The simulated results are in reasonable agreement with experimental measurements and phenomenological observations reported in previous studies.

Abstract: Many stiff clays forming part of natural slopes and earth dams exist in the fissured state. When these cracks are subjected to gravity induced normal and shear stresses they may propagate. The present discussion presents a numerical method to study the propagation direction of cracks under stress fields similar to those found in the field. Not only did the results on one crack propagation direction obtained from the numerical method and the analytical results agree well, but numerical results have been used to investigate the mechanisms of the whole process of two horizontal cracks initiation and propagation and coalescence in stiff soils.

Abstract: Using RFPA code, analyses have been carried out to investigate the stability of a rock pillar in a experiment for nuclear waste repositories, the numerically obtained stress field, temperature distribution, failure pattern of the pillar rock and associated AE events are all agree well with the in-situ data. Minor fracture initiation may take place in the vicinity of the boreholes after heating. Heating induces minor spalling at central pillar wall for 0.5 m sections below the tunnel floor, but the area of spalling is found to be limited. The core of the pillar remains intact for stress conditions corresponding to 120 days of heating which not only prove that the proposed technique provides a powerfully alternative and effective approach for the study on thermal-mechanical-damage coupling mechanism but also provide meaningful guides for the experiment design and associated applications.

Abstract: The phenomenon creep fracture is well-known for concrete. In the present paper, the Material Failure Process Analysis (MFPA2D) model for concrete in the failure process is coupled in series with the time-dependence of the concrete damage and deformation. Further, the progressive creep failure of concrete specimens under constant tensile loading was numerically simulated and the typical time-dependent deformations: the transient creep, the steady-state creep and the accelerating creep were also represented. The numerical simulations indicate that the macroscopic creep failure is induced by clusters of micro-fractures on a mesocopic scale. The above numerical results offer us some theoretical indications and instructions to further investigate the instability failure mechanisms of engineering concrete structures in civil and hydraulic engineering.