Fracture Mechanics

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Authors: Aaron Alejandro Aguilar Espinosa, Neil Fellows, Oscar Portillo
Abstract: The numerical simulation of crack closure is employed to assist on the prediction of crack growth rate. Under fatigue load, the stress-strain response of metals is altered due to cyclic loading. For this reason, the material properties characterization is of prime concern as an input parameter to obtain reliable results. From numerical simulations, it was observed that simple material models do not provide accurate data for long crack lengths. In this paper, the effect that different hardening models have on the opening response of a cracked component when it is subject to variable amplitude loading is analyzed. The interaction effects (crack arrest/acceleration) for long crack length simulation are specially highlighted. For this purpose, a 6082-T6 aluminium alloy was analyzed experimentally and numerically in order to measure crack closure, and then, those data were used to predict fatigue crack growth rate under different patterns of overload. The Paris equation and the Elber crack closure concept were employed. The results showed that small variations in the opening stresses obtained from different material models produce high overestimated simulations of crack growth rate. Also, it was proved that the crack closure mechanism is able to take into account interaction effects due to variable amplitude loading.
Authors: Ivan Campos-Silva, M. Ortíz-Domínguez, E. Hernández-Sánchez, D. Bravo-Bárcenas, O. Bravo-Bárcenas, Marco Antonio Doñu-Ruíz, J. Martínez-Trinidad, J.A. Jiménez-Bernal, Y. Domínguez-Galicia
Abstract: Fracture indentation was applied to estimate the fracture toughness of AISI 1018 borided steels. The Fe2B hard layers were formed using the powder-pack boriding process for two temperatures with 4 and 8 h of exposure times. The fracture toughness of the iron boride layer of the AISI 1018 borided steels was estimated using a Vickers microindentation induced-fracture testing at distances of 15 and 30 m from the surface, applying four loads (0.49, 0.98, 1.96 and 2.9 N). The microcracks generated at the corners of the Vickers microindentation were considered as experimental parameters, which are introduced in a Palmqvist crack model to determine their corresponding fracture toughness KC. As a result, the experimental parameters, such as exposure time and boriding temperature are compared with the resulting fracture toughness of the borided phase.
Authors: Martin Castillo-Morales, A. Salas-Zamarripa
Abstract: The Ultrasonic Impact Treatment (UIT) has been used in different materials to reduce residual welding tensile stresses and improve the fatigue life of welded joints, and also to increase the fatigue resistance at low temperatures. The main aim of this research was to explore the effects of UIT in the fatigue life of a 2024-T3 aluminium alloy. Load controlled fatigue tests were carried out at high and low cycle fatigue, and three UIT parameters at a carrier frequency of 36 kHz were evaluated. These parameters were feed rate, amplitude under load and impact frequency. The results revealed an increase in compressive residual stress and microhardness, as well as some evidence of porosity. However, the fatigue life was reduced drastically. The possible causes of this decrease are still under discussion.
Authors: C. Alberto Mora Santos, M. del R. Rivera Martínez, Orlando Susarrey Huerta, A. Balankin, Maribel Mendoza Nuñez
Abstract: In this work the self-affine crack pattern in Filter Paper sheets is studied. This paper has a well-defined anisotropy of mechanical properties associated with visible preferable orientation of fibers in the machine direction. Fracture behavior is in essence brittle, the rupture lines have self-affine invariance, and the stresses ahead of the straight notch follow a power-law behavior. The roughness exponent value is of H = 0.50 0.01, different from the suggested universal value H = 0.8. The classical theory has demonstrated that, in materials such as metal, there is a relationship between the size and the starting crack stress, which does not happen in this material. The tests show that the starting crack stress from stress-strain behavior curves remains stable for the different specimen sizes w and crack length size. Moreover, different types of geometric groove, circular and linear, and without a crack, were tested and show almost the same behavior.
Authors: V.R. Nossov, J.C. Gómez-Mancilla, I. Ramirez-Vargas, D.E. Rivera-Arreola
Abstract: This paper deals with the modeling and identification of a stationary eccentricity and attitude angle for a short journal bearing with horizontal point injection port. In modeling we use the Dirac spatial delta function which allows to obtain a pressure field in a close analytical form. After using the pressure force components in the oil film, which is the addition of the Ocvirck force in the classic case and pressure force we obtain a non linear equations system with respect to stationary eccentricity and attitude angle. We found the solution equation system by obtaining the roots using the program MATHEMATICA 6 and by iteration. We developed and analyzed the work by both point pressurizing ports π/2 and 3π/2 in the journal bearing. The results are presented in tables that show the relation between eccentricity, attitude angle and the Sommerfeld´s number S.
Authors: E. Pineda, Ferri M.H.Aliabadi, Janis Zapata
Abstract: This paper presents a new formulation of the Dual Boundary Element Method to visco-plastic problems in a two-dimensional analysis. Visco-plastic stresses and strains around the crack tip are obtained until the visco-plastic strain rate reaches the steady state condition. A perfect visco-plastic analysis is also carried out in linear strain hardening (H’=0) materials. Part of the domain, the part that is susceptible to yield is discretized into quadratic, quadrilateral continuous cells. The loads are used to demonstrate time effects in the analysis carried out. Numerical results are compared with solution obtained from the Finite Element Method (FEM).
Authors: J.A. Quintana-Rodríguez, J.F. Doyle, F.J. Carrión-Viramontes, Didier Samayoa-Ochoa, J. Alfredo López-López
Abstract: Generally, simulation of non-homogeneous materials requires a homogeneous representation with equivalent properties different from the constitutive elements. Determination of the equivalent properties for dynamic simulation is not always a direct and straightforward calculation, as they have to represent, not only the static reactions, but also the dynamic behavior, which depends on a more complex relation of the geometrical (area, inertia moment), mechanical (elastic modulus) and physical (density) properties. In this context, the Direct Sensitivity Method (DSM) is developed to calibrate structural parameters of a finite element model using a priori information with an inverse parameter identification scheme, where parameters are optimized through an error sensitivity function using experimental data with the dynamic responses of the model. Results demonstrate that parameters of materials can be calibrated efficiently from the DSM and that key aspects for this calibration are noise, sensitivity (structural and sensor), and the finite element model representation.
Authors: Alejandro Rodríguez-Castellanos, Esteban Flores Mendez, Francisco José Sánchez-Sesma, José Efraín Rodríguez-Sánchez
Abstract: In this paper, the scattering of elastic waves in a fluid-solid interface is researched. The Indirect Boundary Element Method (IBEM) was used to study this wave propagation phenomenon in a 2D fluid-solid model. The source, represented by a Hankel´s function of the second kind, is always applied in the fluid. This approximate boundary integral technique is based upon the integral representation for scattered elastic waves using single-layer boundary sources. The approach presented is usually called IBEM as the sources’ strengths should be obtained as an intermediate step. This indirect formulation can give a deep physical insight to the analyst on the generated diffracted waves, because it is closer to the physical reality and can be regarded as a realization of Huygens’ Principle, which mathematically is fully equivalent to the classical Somigliana’s representation theorem. In order to gauge accuracy, the method was tested by comparing it to an analytical solution. A near interface pulse generates scattered waves that can be registered by sensors located in the fluid. Results are presented in time domain, where several aspects related to the different wave types that emerge from this kind of problems are pointed out.

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