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Author: Kristine B. Cochran Publisher: ProQuest ISBN: 9781109218688 Category : Languages : en Pages : 195
Book Description
Plasticity induced crack closure (PICC) has a potentially strong effect on the growth rate of fatigue cracks in metal structures. As a crack grows through previously yielded material, the permanent deformation in the crack wake Shields the crack tip from the full impact of the remote load. This work sought to add to the large body of PICC finite element studies by introducing two nonlinear plasticity models---nonlinear kinematic hardening (Armstrong and Frederick's model) and nonlinear plastic strain evolution (Lubliner's generalized plasticity)---into a 3D small scale yielding boundary layer approximation of a mode I crack. With the traditional method of growing the crack by one element per load cycle, the results display strong sensitivity to the element size. This work and the PICC literature demonstrate that linear plasticity models may also produced mesh dependent results in certain conditions. The manifestation of mesh sensitivity depends on not only the material model, but also the material parameters, whether the model is 3D, 2D plane strain, or 2D plane stress, and if the T-stress is compressive or tensile. This work reveals that cyclic accumulation of permanent deformation (strain ratcheting) is responsible for mesh dependence when the rate of growth depends on the element size. Mesh refinement may commute with the use of multiple load cycles between crack growth increments to provide the same effective growth rate. Sufficient cycles between crack growth increments can reduce mesh dependence while approaching real crack growth rates. For conditions that create severe ratcheting, the number of cycles needed to obtain fully mesh independent results may be impractical. Spontaneous crack growth shows some promise for reducing mesh dependence; a critical COD criterion for crack extension produced similar crack growth rate and opening loads for different mesh sites. Nevertheless, strong gradients in crack closure near the crack tip imply that there will always be an element size effect with respect to measuring 3D variations in near-tip behavior.
Author: Kristine B. Cochran Publisher: ProQuest ISBN: 9781109218688 Category : Languages : en Pages : 195
Book Description
Plasticity induced crack closure (PICC) has a potentially strong effect on the growth rate of fatigue cracks in metal structures. As a crack grows through previously yielded material, the permanent deformation in the crack wake Shields the crack tip from the full impact of the remote load. This work sought to add to the large body of PICC finite element studies by introducing two nonlinear plasticity models---nonlinear kinematic hardening (Armstrong and Frederick's model) and nonlinear plastic strain evolution (Lubliner's generalized plasticity)---into a 3D small scale yielding boundary layer approximation of a mode I crack. With the traditional method of growing the crack by one element per load cycle, the results display strong sensitivity to the element size. This work and the PICC literature demonstrate that linear plasticity models may also produced mesh dependent results in certain conditions. The manifestation of mesh sensitivity depends on not only the material model, but also the material parameters, whether the model is 3D, 2D plane strain, or 2D plane stress, and if the T-stress is compressive or tensile. This work reveals that cyclic accumulation of permanent deformation (strain ratcheting) is responsible for mesh dependence when the rate of growth depends on the element size. Mesh refinement may commute with the use of multiple load cycles between crack growth increments to provide the same effective growth rate. Sufficient cycles between crack growth increments can reduce mesh dependence while approaching real crack growth rates. For conditions that create severe ratcheting, the number of cycles needed to obtain fully mesh independent results may be impractical. Spontaneous crack growth shows some promise for reducing mesh dependence; a critical COD criterion for crack extension produced similar crack growth rate and opening loads for different mesh sites. Nevertheless, strong gradients in crack closure near the crack tip imply that there will always be an element size effect with respect to measuring 3D variations in near-tip behavior.
Author: Marcos Lugo Publisher: ISBN: Category : Aluminum alloys Languages : en Pages :
Book Description
From its discovery, crack closure was recognized as a key aspect in understanding the fatigue crack growth process. Considering the condition of plane stress, a vast amount of research has been conducted experimentally, analytically, and numerically to understand the complex process of fatigue crack growth and crack closure. Nonzero crack opening stress values are routinely observed, and it seems that there is a general agreement regarding the incidence of the phenomenon under plane stress. However, investigations regarding crack closure under plane strain conditions are less abundant. Moreover, the existence of crack closure under the plane strain state of the stress has been questioned. The importance of accurate measurements of closure to predict adequately fatigue crack growth rates should not be underestimated. Models employed to predict fatigue crack growth rates rely on plasticity-induced crack closure concepts, and the validity of plasticity-induced crack closure depends on crack closure measurements. Crack closure measurements can be performed with Elber's Method, the ASTM standard (Compliance offset method), or it may be done alternatively by the compliance ratio (CR) or the adjusted compliance ratio method (ACR). In this research, a small scale yielding two-parameter modified boundary layer analysis is performed to study the occurrence of plasticity-induced fatigue crack closure under constant amplitude loading and plane strain conditions. A wide range of T-stresses and KI levels are considered in the finite element analysis with the purpose of exploring the behavior of the crack opening stress. Crack closure was observed for some values of T-stress. Other values of T-stress resulted in an absence of closure under steady state conditions. In addition, an elastic-plastic finite element model was used to simulate a growing fatigue crack with WARP3D software. The computed displacements were used to determine the effective stress intensity factor range [delta]Keff with the ASTM standard compliance offset approach, the (CR) method, and the (ACR) method. Finally, measurement location effects on ACR and the ability of ACR method to remove residuals stresses were investigated.
Author: Mehmet Gkhan Gken Publisher: LAP Lambert Academic Publishing ISBN: 9783838357911 Category : Languages : en Pages : 84
Book Description
In spite of the fact that the use of contact elements in modeling crack surface contact, and thus in modeling of crack closure is inherently natural, almost no effort to incorporate it in the finite element analysis of fatigue crack growth has been reported in the literature. The traditional method is based on placing truss elements on the crack surface nodes In this research contact elements are used to model crack surface contact and similar to mentioned method the determination of crack opening stress is accomplished by monitoring the state of contact elements during incrementally applied loading. Instead of determining crack opening stress of every load cycle, an algorithm that makes it possible to find crack opening stress at predetermined load cycle intervals is developed. With the developed algorithm it was possible to analyze crack closure behavior during a larger number of load cycles with less execution time. Due to simulation of relatively high number of load cycles, final stabilization of crack opening stress values after a subsequent decay in the initially "stabilized" opening stress values is observed.
Author: Eugene J. Bednarz Publisher: ISBN: Category : Languages : en Pages : 186
Book Description
Plasticity induced closure and its effect on the fatigue growth of short cracks were investigated analytically in a high strength titanium alloy at room temperature as well as in a nickel base alloy at elevated temperature. The analysis consisted of subjecting a single-edge cracked specimen with an initial crack length of .001 inch to cyclic loading and allowing the crack to propagate. This was accomplished using a two-dimensional finite element code and a theoretical finite element model of a typical test geometry. Visco II, the two-dimensional plane stress/plane strain finite element code, uses constant strain triangular elements and incorporates the Bodner-Partom Viscoplastic Flow Law to handle non-linear material behavior. The numerical simulations involved subjecting specimens of TI-6246 at room temperature (time-independent behavior) to cyclic loads with maximum nominal stress values of approximately 60 and 90 percent of the material yield strength and load ratios of -1.0 and 0.1 with a frequency of 1.0 Hz. The numerical simulations of specimens consisting of Inconel 718 at 1200 F (time-dependent behavior) consisted of cyclic loads where the maximum nominal stress was .90 yield strength, the applied load ratio was 0.1, and the cyclic frequencies were .01 and 1.0 Hz. The formation of a plastic wake and the affects of plasticity induced closure were observed in these specimens. (AW).
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Elastic-plastic finite element analyses were performed to predict the crack opening level profiles in semi-elliptical surface cracks. A script was written to use the commercial finite element code ANSYS to predict opening levels in cracked geometries. The functionality of the scripts was verified by comparing predicted opening levels in two and three-dimensional center-cracked geometries to experimental results. In addition, a parameter study was performed in which various aspects of the modeling routine were modified. This included a mesh refinement study as well as a study into the effect of a strain hardening material. The main focus of the current research, however, is to compare finite element predicted opening levels with published opening levels determined experimentally. Due to the complexities and long run-times involved with these models, no attempt was made at growing the cracks from initial length to final length. Instead, discrete crack lengths at which experimental opening levels were published were instead used. Also, no attempt was made to predict the crack aspect ratio evolution. The finite element predicted opening levels were in all cases significantly lower than those reported experimentally, however, similar trends in both crack opening level profile along the crack front, and opening level variations with crack growth were shown.
Author: Kristine B. Cochran
Author: Kristine B. Cochran
Author: Marcos Lugo
Author: Mehmet Gkhan Gken
Author: Wolf Elber
Author: Eugene J. Bednarz
Author: Gary R. Halford
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Author: 
Author: Kenneth L. Jerina
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