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COB154 Crack GROWTH RESISTANCE curveS in wood MODELLED AS AN ORTHOTROPIC strain-softening MATERIAL
Theophile Mehinto, Gerard Chalant, Sylvie Idrissi-Ronel
Université des Antilles et de la Guyane (UAG), Centre de Recherche Scientifique et Technologique de Guyane (CRSTG)
Laboratoire "Mécanique, Matériaux, Environnement" (LMME), B.P. 792, F-97337 CAYENNE Cedex, Guiana Francesa
This paper depicting resistance curves in wood is based on two concepts. Firstly, the fictitious crack model which takes into account the strain-softening behaviour of the material. Since wood is modelled as an orthotropic (radial R, longitudinal L and tangential T axes) medium, crack propagation along the grain in poplar had been studied using a nonlinear finite element method. Thus load (P)-load displacement (d ) curves have been deduced for three specimen geometries submitted to opening mode : the double cantilever beam (DCB), the compact tension (CT) and the single-edge notched in bending (SENB). Three crack configurations had been studied : TL, RL and TR. Secondly, the equivalent crack length concept is used, taking into account the effective length of the fracture process zone. From P-d curves, the estimation of the equivalent crack length makes it possible to plot R-curves. The effects of initial crack lengths and specimen geometries are thus analyzed. The existence of a single - resistance versus crack extension - curve for each crack configuration makes it possible to predict fracture loads confirmed by experimental results. This successful comparison allows future research taking into account the behaviour of other wood species particularly tropical ones.
Keywords: Equivalent length, Finite elements, Fracture process zone, R-curves, Wood.
COB184 Continuum Damage Problems Solved by the Splitting technique in connection with Finite element method
Stella Maris Pires-Domingues1, Heraldo Costa-Mattos1 & Fernando Alves Rochinha2
1Mechanical Engineering Department, Universidade Federal Fluminense - UFF
CEP 24210-240, Niterói, RJ, Brazil - E-mail: maris@caa.uff.br
2Mechanical Engineering Program, EE-COPPE/UFRJ, Universidade Federal do Rio de Janeiro
CEP 21945-970, Rio de Janeiro, RJ, Brazil - E-mail: faro@labmf1.com.ufrj.br
Different continuum damage theories for elastic materials have been proposed in the last years. In order to avoid the loss of well-posedness in the post-localization range, some continuum damage theories for elastic materials introduce higher order gradients of the damage variable in the constitutive model. Although such theories allow a mathematically correct modeling of the strain localization phenomena, they are usually considered very complex to handle from the numerical point of view. The present work is concerned with the numerical implementation of a gradient-enhanced damage theory for elastic materials. A simple numerical technique, based on the finite element method, is proposed to approximate the solution of the resulting nonlinear mathematical problems. The coupling between damage and strain variables is circumvented by means of a splitting technique.
Keywords: Damage Mechanics; Finite Elements; Splitting Technique.
Roberto Suarez Antola
Facultad de Ingeniería, Universidad Católica del Uruguay
8 de Octubre 2738, C.P. 1.600, Montevideo, Uruguay - E-mail: rsuarez@ucu.edu.uy
Filtration was produced in a small scale physical model of a granular porous medium of cylindrical shape. The same volume flow was obtained either applying a difference in hydrostatic pressure or in osmotic pressure. In the first case a process of sustained erosion ending in an hydraulic short-circuit was observed, while in the second case the material remained stable. This paradoxical strength behaviour is explained using some results from differential geometry, classical field theory and thermo-kinetic theory. The fracture process of a continuous matrix in a porous medium under the combined effect of filtration and external mechanical loads is then considered. The obtained results can be applied to the flexural and compressive strength of wet concrete.
Keywords: Filtration pressure, osmosis, mechanical failure.
COB391 ANALYSIS OF FRACTURE IN THIN COATINGS SUBJECTED TO CONTACT LOADING
Sonia A.G. Oliveira1 & Allan Bower2
1-Dep. Engenharia Mecânica , Univ. Federal de Uberlândia -UFU, CEP. 38400-900 Cx.P. 593 Uberlândia, MG E-Mail sgoulart@ufu.br
2- Brown University, Providence, RI - USA
Hard coatings, which are used to protect surfaces that are subjected to contact loading, may fracture or delaminate from their substrates. A simple model of the fracture process is presented. The thin coating is idealized as a brittle elastic layer of uniform thickness, which is bonded to the surface of a half-space. The coating and substrate are assumed to contain microcracks. The solid is loaded by a rigid cylindrical indenter, which slides over the surface of the coating. Methods of linear elastic fracture mechanics are then used to calculate the loads required to initiate fracture in the coating. It is shown that the fracture loads are strongly influenced by the mismatch in elastic properties between the layer and its substrate, the thickness of the layer, the initial crack size and the friction coefficient.
Keywords: Fracture; Rolling Contact; Coating; Wear / Fratura, Contato, Revestimento, Desgaste.
COB1003 DETERMINAÇÃO DA CURVA J-R UTILIZANDO UM ÚNICO CORPO DE PROVA BASEADO NO MÉTODO DA NORMALIZAÇÃO DE HERRERA E LANDES/ DETERMINATION OF J-R CURVE USING A SINGLE SPECIMEN BASED ON HERRERA AND LANDES’ NORMALIZATION METHOD
Neilor Cesar dos Santos
Coordenação de Mecânica, Escola Técnica Federal da Paraíba - COMEC/DDE/ETFPB, Av. Primeiro de Maio, 720 - Jaguaribe - CEP 58059 900, João Pessoa - PB - Brasil
E-mail: neilor@jpa.etfpb.br.
Marco Antonio dos Santos
Departamento de Engenharia Mecânica, DEM/CCT/UFPB-Campus II, Caixa Postal 10023 - Campina Grande - PB - Brasil.
E-mail: santos@lctf.ufpb.br
Herrera and Landes normalization method (Key Curve) to determine the J-R curve was studied. Its aplicability was analised for two HSLA steels: a controlled rolled API 5L-X70 pipelines steel plate, produced with inclusions morphology control, and an ASTM A516-GR65 pressure vessels steel plate. The tests were made at room temperature and three point bend specimens were used. The normalization curve(Key Curve) was analytically determined and the points fit was executed by a power law. The Ramberg-Osgood’s functional form relationship was used. The results were confronted with other obtained by the multiple specimens test method. Such procedure brought in to evidence that Herrera and Landes normalization method is suitable to determinate J-R curve for the studied steels.
Keywords: Integral J, Curva J-R, Método da Normalização, Método de Um Único Corpo de Prova, "Key Curve".
J Integral, J-R Curve, Normalization Method, Single Specimen Method, "Key Curve".
COB1078 O EFEITO DA ESPESSURA DO CORPO DE PROVA SOBRE OS VALORES DE TENACIDADE À FRATURA DO AÇO ASTM A516-GR65/ EFFECT OF SPECIMEN THICKNESS ON THE VALUES OF THE FRACTURE TOUGHNESS OF THE ASTM A516-GR65 STEEL
José Jefferson da Silva Nascimento & Marco Antônio dos Santos
Departamento de Engenharia Mecânica, Universidade Federal da Paraíba - Brasil
CCT/UFPB - Brasil, E-mail: santos@lctf.ufpb.br
This work is concerned with the thickness effect of the specimen in values of toughness to initial fracture (d i) and to maximum load (d máx, Jmáx). Specimens of the type flexure at three points were tested at the room temperature in a universal machine for testings, hydraulic-servo (MTS 810), under control of displacement with displacement speed of the table equal to 3,33x10-3 m/s (0,2 mm/min). The thicknesses of the tested specimens changed between 7x10-3 m and 20x10-3 m. The investigated steel was the ASTM A516-GR65 class. The values of toughness to fracture in the beginning of propagation of crack (d i) as well as the ones of maximum load (d máx, Jmáx), decreased with the reduction of the thickness of the specimen. The behaviour that the steel presents was investigated and discussed.
Keywords: Pressure vessel steel, d -R Stress curve, Fracture toughness initial, Maximun load toughness.
COB1270 ADAPTIVE FINITE ELEMENT MODELING OF THREE-DIMENSIONAL ELASTIC-PLASTIC CRACK PROPAGATION
Fernando César Meira Menandro
Departamento de Engenharia Mecânica, Universidade Federal do Espírito Santo, Brasil.
James D Lee & Harold Liebowitz
Civil, Mechanical, and Environmental Engineering Department, The George Washington University, U.S.A.
A three-dimensional adaptive elastic-plastic finite element algorithm for crack propagation is developed. The crack propagation algorithm uses theoretical parameters from fracture mechanics to model crack growth. A maximum energy release rate criterion is adapted to use in three dimensions, combined with a maximum crack tip opening angle criterion. The error estimator adopted is of the flux projection type, which uses the difference between the calculated strains/stresses and a continuous ‘smooth’ strain/stress distribution calculated also from the obtained solution. This difference is, in general, integrated over the element to give an energy error value. The mesh is then regenerated, targeting an equidistribution of this error value, using a mesh generator which takes the previous mesh to serve as a background grid. New procedures for strain recovery and error estimation, adequate for elastic-plastic problems and for crack propagation problems, are proposed and tested. The proposed algorithm is analyzed qualitatively, and found to perform adequately for the problems of interest.
Keywords: adaptive finite elements, crack propagation, error estimation, fracture mechanics, plasticity.
COB1271 InfluÊncia da emissão de discordâncias sobre a Transição Frágil-Dúctil
Maria Angela Loyola de Oliveira * & Gerard Michot**
*Departamento de Engenharia Mecânica -Centro Tecnológico da Universidade Federal do Espírito Santo - Brasil.
**Laboratoire de Physique du Solide et Sciences des Matériaux - Ecole des Mines de Nancy - INPL – France
In-situ syncrotron observations of loaded pre-cracked silicon single crystals at high temperature allowed for a better understanding of the conditions for dislocation nucleation and development and, consequently, a better comprehension of the origins of the brittle to ductile transition. For a given load rate it can be considered that when the test is accomplished at transition temperature there is a balance between increase of applied stress and increase in the number of dislocations emitted generating a stress field opposed to the applied and causing a retardation of the critical rupture condition given by Kef = KIC. Such phenomenon occurs due to the interaction between the stress fields from the crack and the dislocations (shielding). It has been observed that the dislocations are emitted preferably from defects on the crack tip, heterogeneously, which involves the introduction of the crack tip quality as a supplementary parameter. Furthermore, it is shown that in the absence of dislocation emission a variation of the notch curvature radius can occur, which recalls the interest for the crack tip blunting based models. This geometric variation of the crack tip can occur either by plastic deformation, caused by the emergence of dislocations on the crack tip when these are located in preferred planes, or by superficial atomic diffusion which occurs preferably on the highly stressed surface. Experimental results show that, when the crystalline perfection is sufficient, the dislocation nucleation can be inhibited. This supposes that the brittle to ductile transition temperature measurements taken to this date are approximate and that this is not an intrinsic characteristic of the material. Pre-cracked specimens were loaded at high temperature (650 to 800o C) during a period that varied from a few minutes to a few hours and then ruptured at room temperature: the observed toughness increase D K is quantified and depends on the applied load and on the time of heat treatment..
Keywords: Trincas, transição frágil - dúctil, discordâncias, zona plástica, fator de intensidade de tensões.