Key Engineering Materials Vols. 353-358

Abstract: The failure of rock mass under loading is resulting from preexisting flaws, such as cracks, pores and other defects. However, the propagation and coalescence mechanism among multi-group cracks is still a puzzle, especially to the engineering rocks in site. In this study, the failure of rock samples with two groups of preexisting parallel cracks under the axial load were numerically investigated by the Rock Failure Process Analysis code (RFPA) from a mechanics point of view. The simulated results reproduce the rock failure process: at the first loading stage, the particle is stressed and energy is stored as elastic strain energy with a few randomly isolated fractures. As the load increases, the isolated fractures are localized to form a macroscopic crack. At the peak load, the isolated fractures unstably propagate in a direction parallel to the loading direction following tortuous paths and with numerous crack branches. Finally, the major crack passes through the rock sample and several coarse progeny cracks are formed. Moreover, in the vicinity of the contacting zone the local crushing is always induced to cause fines. On the base of the simulated results, it is found that the dominant breakage mechanisms are catastrophic splitting and progressive crushing. It is pointed out that the particle breakage behavior strongly depends on the heterogeneous material property, the irregular shape and size, and the various loading conditions. Because of heterogeneity, the crack propagates in tortuous path and crack branching becomes a usual phenomenon. The failure process of rock sample demonstrated that due to the high stress concentration at the cracks tip or some weaker strength elements which are not on the cracks surface initiate some micro-fractures, those cracks and fractures may gradually become larger and larger, more and more with the progress of loading so that join into the branch cracks leading to the rock failure in the end. Not only did the output of the numerical simulation study compare well with the experiment results, but also the further insights of the mechanism of cracks propagation and coalescence process in rock mass were obtained.

Abstract: The fracture behavior of brittle materials under different stress ratio has been investigated by means of numerical simulation method with software RFPA2D (Realistic Failure Process Analysis). The fracture dependence of brittle material on biaxial plane stress state was confirmed. The results show that the critical stress intensity factor under biaxial stress increases with the increase of biaxial stress ratio. The simulation tests reveal that the biaxial stresses have strong influence on the fracture properties of glass. The results confirmed that the strain criterion of fracture is feasible while brittle materials under complex stress state.

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: 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: 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.

Abstract: In numerical simulation of engineering problems, it is important to properly simulate the interface between two adjacent parts of the model. In finite element method, there are generally three methods for simulating interface problems: interface element method, surface based contact method and the method by using a thin layer of continuum elements. In this paper, simulation of interface problems is conducted using continuum elements and surface based contact methods. The results from each method are presented and compared with each other.

Abstract: Accurate stress intensity solutions for multiple site damage (MSD) cracks in riveted stiffened panels are difficult to determine due to geometric complexity along with variations in rivet load transfer and corrosion, especially the interaction of MSD cracks. A methodology was proposed for efficiently depicting rivet in stiffened panel using finite elements. Rivet material properties were determined based on an empirical force-displacement relationship, for highly refined rivet model as well as for idealized spring element representations of rivets. Parametric studies of panels with a middle crack and a central stiffener indicate that rivets can provide comparable load transfer and relative displacement if the rivets closed to the crack are explicit modeled. Using idealized combination of explicitly and spring element representations of rivets, the stress intensity factors (SIFs) for uncorroded and corroded one-bay stiffened panels were predicted. The results show that the effect of MSD and thinning of the sheet is to increase substantially SIFs values compare to that of a single crack without corrosion. But the SIF is insensitive to corrosion of stiffener. The particular rivet material has relatively little effect on SIF values, while the rivet diameter has a significant effect.

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: The influences of the polyhedral oligomeric silsesquioxane (POSS) on the mechanical properties of the polystyrene (PS) under tensile strain loading were studied using the atomistic finite element simulations. Firstly, the nano scale pole-like atomistic models and the micro scale pole-like finite element models of two kinds of homopolymers, the pure PS and the polystyrene attached with 5 mol% propyl-POSS (P-POSS-PS), were built, after which the atomistic finite element models were built by connecting the carbon and silicon atoms on the boundaries of the atomistic models to the corresponding nodes in the finite element ones. Then the mechanical behaviors of the two kinds of homopolymers under different tensile strains were investigated. The results show that, when the tensile strain of PS reaches 0.05 the micro voids appear. The tensile stresses of PS approximatively keep increasing with the increasing strain before the tensile strain of 0.07, after which the stresses declines. And the corresponding tensile modulus of PS is 4.52GPa. By contrast with PS, when the tensile strain of P-POSS-PS reaches 0.06 the micro voids appear. The tensile stresses of P-POSS-PS are almost keeping increasing before the tensile strain of 0.08, and the corresponding tensile modulus is 5.32GPa. Conclusions can be made from this work that the tensile strength of homopolymers can be observably improved by POSS. A small quantity of POSS can even improve the toughness of homopolymers slightly. This work has realistic theoretical significance to understand the reinforcement mechanism of POSS and provides important referential message to the applications of POSS.

Abstract: Atomistic simulations using molecular dynamics (MD) method are conducted to check the conditions of the onset of fracture at the interface edges with a variety of angles. The simulations are facilitated with model bi-material systems interacting with Morse pair potentials. Three simulation models are considered, i.e. the interface edges with angles 45°, 90° and 135°, respectively. The simulation results show that, at the instant of crack initiation, the maximum stresses along the interfaces reach the ideal strength of the interface; also, the interface energies just decrease to below the value of the intrinsic cohesive energy of the interface. And the onset of fracture at the interface edges with different geometries is controlled by the maximum stresses or the cohesive interfacial energy.