Papers by Keyword: Crack Growth

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Authors: Hans A. Richard, Friedrich G. Buchholz, G. Kullmer, M. Schöllmann
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: L. Qian, Hiroyuki Toda, Kentaro Uesugi, Masakazu Kobayashi, Toshiro Kobayashi
Abstract: Traditional computational models always assume idealized crack geometry. However, actual crack geometry is very complex in real materials and thus, those simulations do not realistically represent the actual loading conditions of a real crack. In this paper, three-dimensional (3D) image-based simulation was performed to investigate the fracture behavior of an aluminum alloy, and the model takes into account the real crack geometry based on the 3D images of the crack. Accordingly, many essential features of fracture can be identified and interpreted, and some new insight into fracture behavior in real materials can be offered.
Authors: Wen Guang Liu, Hong Lin He
Abstract: There are different modes of damage in any engineering structures, and most of them are cracks. In order to study the influence of coupling effect on the fatigue life, a calculation method of structure vibration fatigue life with crack propagation is proposed. In analysis, a series of finite element model with crack of different length is built to simulate the crack propagation, and Paris equation is employed to calculate the vibration fatigue life by stepwise method. The crack initiation life is got based on the change law of natural frequency from test results, and the total life is calculated in the end. Results indicate that the simulation results identical with the experimental results well.
Authors: Roberto G. Citarella, Pierpaolo Carlone, Marcello Lepore, Gaetano S. Palazzo
Abstract: Medium to high strength aluminum alloys, such as 2xxx, 6xxx, and 7xxx series, are actually considered of great interest in the transport industries. For aeronautical applications, the precipitation hardenable AA2024 (Al-Cu) alloy is gaining considerable attention, in particular for the realization of nose barrier beam or fuselage panels. In this context, remarkable research effort is currently focused on the application of the Friction Stir Welding (FSW) process, as a suitable alternative to fusion welding processes. The interest in aeronautical application of FSW process is also justified by the reduction of production costs and weight and by the increase of strength and damage tolerance with respect to riveted lap joints. The implementation of the technique in safety-critical components, however, requires a deeper understanding of static strength as well as of fatigue behavior of FSWed assemblies. In this sense some experimental results have already been presented in the inherent literature, relatively, for instance, to AA6082-T6 and AA6061-T6, AA6063-T6, AA2024-T351, AA2024-T8 alloys processed by FSW. Despite the unavoidable relevance of experimental testing, a numerical approach able to predict the mechanical behavior of FSWed assemblies is very desirable, in order to achieve time and cost compression and to implement computational optimization procedures. This paper deals with a numerical investigation on the influence of FSW process parameters, namely the rotating speed and the welding speed, on fatigue crack growth in AA2024-T3 butt joints. The computational approach is based on a combined Finite Element Method (FEM) and Dual Boundary Element Method (DBEM) procedure, in order to take advantage of the main capabilities of the two methods. In particular, linear elastic FE simulations have been performed to evaluate the process induced residual stresses, by means of a recently developed technique named contour method. The computed residual stress field has then been superimposed to the stress field produced by an applied fatigue traction load in a Dual Boundary Element Method (DBEM) environment, where the simulation of a crack, initiated and propagating along the previously mentioned cutting line, can be performed in an automatic way. A two-parameters crack growth law is used for the crack propagation rate assessment. The DBEM code BEASY and the FEM code ANSYS have been sequentially coupled in the aforementioned numerical approach by using a BEASY interface module and in house developed routines. Computational results have been compared with experimental data, showing a satisfactory agreement. The influence of process parameters on the residual stresses distribution has also been highlighted.
Authors: Gernot Trattnig, Christof Sommitsch, Reinhard Pippan
Abstract: To understand the crack growth in massive forming and to consequentially avoid crack growth in workpieces, it is necessary to investigate its dependence on the crack depth and thus on the state of hydrostatic stress. Prior work shows that the crack opening displacement (COD) for shallow cracked tension specimens with low stress triaxiality is twice as high as for deep cracked specimens with high stress triaxiality. This work examines the crack growth in compression specimens with pre-cracked cylindrical upsetting samples. The compression samples were cut in the stress symmetry plane in order to observe crack initiation and crack growth by a single specimen technique. In this way it is possible to observe blunting, crack initiation and crack growth inside the upsetting specimens. The resulting COD does not differ significantly from the values achieved in tension samples with short surface cracks.
Authors: Bing Yang, Yong Xiang Zhao
Abstract: A new method is proposed to estimate the parameters of probabilistic fatigue crack growth rate models, including the Paris equation and its’ improved type. To take the statistical characteristics of whole test data into account, the method inherits the idea from the general maximum likelihood method which is widely used in parameters estimation of fatigue S-N curves, ε-N curves, and da/dN-"K curves, and extends the conventional correlation coefficient optimization method into parameters evaluation not only for mean curve, but also for standard deviation curve and probabilistic curve. Analysis on the test data of 16MnR steel indicates that present method is available and feasible. Comparing to the general maximum likelihood method, present method has simpler algorithm, and can avoid constructing and solving the likelihood function, so it is speedier in calculation.
Authors: Guo Liang Peng, Yu Heng Wang
Abstract: A node split method based on Partition-of-unity method and mesh modified has been developed for numerical simulation of crack growth. The crack tip could be represented by the enrichment function based on Partition-of-unity method. The crack besides the tip always locates on the edge of element by moving the nodes around crack, and the continuity of crack could be kept on. For an example of 2D quadrilateral mesh, mesh processing has been presented on different condition. The simulation results indicate that applying the node split method to simulate crack growth problem can achieve relatively good results even for sparse grid.
Authors: Alessandro Soprano, Francesco Caputo, Alfonso Grimaldi
Abstract: A new interest has been increasingly directed in recent years to study the behaviour of joints provided with various kinds of fasteners and the mechanism of load transfer; profiting of the present capabilities of computing systems and analysis codes, which appear such as to deal with models built with more than one million elements, thanks to new numerical techniques which can solve huge systems of equations. However, the main interest has been limited to the case of bolted joints, for which several models are now available, while a few results are known for the case of riveted joints, where more complex mechanisms are involved. In the present paper a full set of analyses has been reported, with reference to both normal and blind rivets, where the manufacturing operation has been simulated through Ls-Dyna to evaluate the stress and strain states induced into the jointed sheets, varying the dimensional and riveting load characteristics of the joint. A subsequent investigation is presented, which aims to evaluate the effects of the compressed zone over the behaviour of an approaching crack.
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