Papers by Keyword: Crack Initiation

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Authors: B. Michel, Thomas Helfer, I. Ramière, C. Esnoul
Abstract: This paper focuses on the numerical simulation of crack initiation and growth in ceramic materials. This work is devoted to nuclear fuel modelling under irradiation and more precisely to fuel pellet fragmentation assessment at macroscopic and microscopic scales. Simulation tools are developed in the framework of a cooperative program between the CEA, EDF and AREVA devoted to a unified fuel performance software environment called PLEIADES. A smeared crack model is proposed to have a continuous description of crack nucleation and growth at macroscopic scale. This unified description is based on crack extension process from the microscopic scale up to the macroscopic scale. In order to deal with unstable crack extension a specific algorithm is proposed to solve the quasi static nonlinear mechanical problem. A 3D application is presented to illustrate performances and robustness of the smeared crack approach to simulate crack extension in nuclear fuel ceramics. In this application with an internal pressure loading a new methodology is proposed in order to avoid convergence problem due to the indetermination of the quasi static formulation of a softening material equilibrium under Neumann boundary condition.
Authors: Olivier Plé, Ali Tourabi, Murad S. Abuaisha
Abstract: Landfills come with a cover barrier which includes a compacted clay liner essential to safety on site. Unfortunately differential settlement may cause stress in the clay layer leading to the development of cracks. This is why tensile stress damage and shearing are observed on the cap cover. Due to difficulties of strains determination in clayey soil during direct tensile tests a 3-dimensional digital image correlation is used. Two digital cameras are used at a constant distance from the sample tested. Pictures taken at preset intervals are post-processed to extract specimen strain and strain localization. The suitability of this method is discussed in regards of experimental results. This technique is particularly well adapted to the clayey soils when conventional measurements are impossible.
Authors: Xue Ren Zhang, Jian Hui Zhao, Keh Chih Hwang
Authors: Nam N. Huynh, Cheng Lu, Guillaume Michal, A. Kiet Tieu
Abstract: This paper proposes a criterion for crack opening in FCC single crystals based on analyses of lattice orientation and interface energy of two adjacent crystals in a crystal plasticity finite element model (CPFEM). It also demonstrates the implementation of the criterion in Abaqus/Standard to simulate crack initiation and propagation in single-edged notch single crystal aluminium samples. Elements in the FEM mesh that have crystalline structures satisfying the crack opening criterion are removed from the mesh at the end of every loading step and FEM analyses are restarted on the new mesh in the next loading step. Removed elements effectively act as voids in the material due to crack nucleation. Similarly, the coalescence of newly removed elements at the end of a loading step with the existent ones simulates crack growth in the material. Two advantages of this approach are noted. Firstly, crack nucleation and its subsequent growth in the material is simulated solely based on lattice evolution history in the material without any presumptions of crack paths or regions where cracks are likely to occur. Secondly, as the criterion for crack nucleation is evaluated based on, and thus changes with, the lattice evolution during loading, a predefined energy criterion for crack opening, which could be erroneous, is avoided. Preliminary results of void nucleation and void growth around the notch tip in Cube and Brass oriented samples using CPFEM modelling appear to agree with molecular dynamics simulations of void growth in FCC single crystals.
Authors: Chang Won Shul, Kee Bhum Lee, Jong Hwan Kim, Song Heo Koo
Abstract: AE technique was applied to the structural strength tests of the ablative composite liner bonded on steel nozzle to figure out the unexpected problem that happened during the flight test. Two different kinds of specimen, which were specimens with some initial crack and without initial crack each, were used for the test. AE test methodology and parameters were investigated to assess the damage of them. NDT using X-ray technique was performed to improve the test reliability before and after each test. The study revealed that AE technique is a good method to evaluate damage on this kind of specimen with better accuracy.
Authors: Adéla Podepřelová, Vratislav Mareš, Martin Kraus
Abstract: The aim of the article is piston damage evaluation of a highly exposed combustion engine. The analysed piston was made of an AlSi-based alloy. Atypical damage, which occurred relatively early in the lifetime of the component, was evaluated by metallographic and fractographic analyses. The analysis took into account influences of mechanical and thermal fatigue processes in relation to the microstructure of the material. The metallographic observations of the microstructure revealed the occurrence of cracks extending over the secondary phases and precipitates. Cracks were initiated on the coarser Si phase particles. The crack initiation site is located at the root of the bridge between the sealing piston rings. The damage of the piston was metallographically documented in wide range.
Authors: Radivoje Mitrovic, Dejan Momcilovic, Ivana Atanasovska
Abstract: Energy efficiency is a key issue worldwide, and not confined solely to the realm of engineers. Past failures of mechanical power system components must be examined carefully in order to minimise future occurrences and increase energy efficiencies. Improved design procedures have been highly sought by engineers and researchers over the past few decades. The latest verified method with strong application potential within the power industry is that of the Theory of Critical Distances (TCD). TCD is not one method, but a group of methods that have a common feature; the use of a characteristic material length parameter, the critical distance L, for calculating the influence of notch-like stress raisers under static and fatigue loading. A case study from a hydro power plant turbine shaft was chosen to illustrate the development of this methodology. The paper illustrates the application of TCD to the fatigue life assessment of a turbine shaft with stress concentrations due to pitting corrosion.
Authors: Fu Lin Shang, Takayuki Kitamura
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.
Authors: Yong Hak Huh, Philip Park, Dong Jin Kim
Abstract: Fatigue crack initiation and propagation behavior around a hole subjected to various biaxial fatigue loading modes has been investigated considering the effect of the biaxial static stress. Two different biaxial loading systems, i.e. cyclic tensile loading with static torsional load and cyclic torsional loading with static tensile load, were employed to thin-walled tubular specimens with a circular notch. Variation of the initiaiton and propagation directions of the crack around a hole under these loading ssytems was examined. The initiation directions were observed using replication technique and the crack propagation was measured by two crack gages mounted near the notch. It was found that the biaxial static stress superimposed on the cyclic tensile or torsional loading tests has no influence on the initiation and propagation directions of the cracks around a hole. The fatigue crack initiation and propagation direction under cyclic loading with biaxial static load can be very well explained by the maximum of tangential stress range, Dsqq,max. Furthermore, it was shown that the fatigue crack growth rates under biaixial faigue loading becomes higher with increment of biaxial static load.
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