A numerical analysis of the relation between CTOD and fatigue crack growth

El cálculo de la vida a fatiga durante la etapa de propagación de la grieta se suele realizar relacionando da/dN con ΔK. Sin embargo, la influencia que en estos análisis tiene el parámetro ΔK debe recaer en algún otro parámetro no lineal cercano al frente de la grieta, ya que estos son los que realmente controlan la velocidad de crecimiento de la grieta. El principal objetivo del presente artículo es intentar mejorar la comprensión del crecimiento de grieta en fatiga empleando el desplazamiento de apertura de la punta de la grieta (CTOD). Este parámetro no ha sido muy empleado en el problema del cierre de grieta y su propagación en fatiga, teniendo un gran potencial. Por este motivo, se ha realizado un análisis numérico para un amplio rango de cargas de amplitud constante en dos aleaciones de aluminio (6016-T4 y 6082-T6). Cuando no se considera el contacto entre los flancos de la grieta se puede observar una relación bien definida entre el valor máximo de CTOD y ΔK, lo cual indica que no existe influencia de la relación de cargas y valida la mecánica de la fractura elástica lineal. Se ha encontrado una relación lineal entre los valores de CTOD y ΔK al considerar una doble escala logarítmica. Los valores de CTOD cuando se considera el contacto se superpone a los resultados sin contacto, únicamente cuando se emplea el ΔK efectivo, lo cual valida el concepto del cierre de grieta. Se ha encontrado una relación lineal entre da/dN y CTOD cuando se considera una doble escala logarítmica para la aleación de aluminio 6082-T6.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

A numerical analysis of the plastic wake influence on plasticity induced crack closure

Fatigue crack closure has been studied by means of finite element method since long time ago. Most work has been performed considering bi-dimensional models. Lately, the use of threedimensional models has been extended. Nevertheless, the methodology employed has been taken from that developed for bi-dimensional cases. There are a great number of previous bi-dimensional studies which analyse different numerical parameters and optimise them. The current computational capabilities allow a comprehensive study of the influence of the different modelling parameters in a similar way to those studies carried out with bidimensional models, with the advantage, that the evolution along the thickness of the analysed parameters can be taken into consideration. In particular, one of the key issues is related to the plastic wake length which is developed during the previous loading cycles. This residual stresses have a great influence on the crack opening and closure values. As the numerical analysis are complex and computationally expensive, the length of the simulated wake is a critical parameter. In this work, a comprehensive study of the effect of the plastic wake in fatigue crack closure is made. On this purpose, a CT aluminium specimen has been modelled three-dimensionally and several calculations have been made in order to evaluate the influence of the simulated plastic wake length. The numerical analysis is made in terms of crack closure and opening values as in terms of the stress and strain fields near the crack front.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

A parameter for quantitative analysis of plasticity induced crack closure

Numerical models have been successfully developed to predict plasticity induced crack closure (PICC). However, despite the large research effort a full understanding of the links between physical parameters, residual plastic wake and PICC has not been achieved yet. The plastic extension of material behind crack tip, Dyp, obtained by the integration of vertical plastic deformation perpendicularly to crack flank, is proposed here to quantify the residual plastic field. The values of Dyp and PICC were obtained numerically in a M(T) specimen using the finite element method. An excellent correlation was found between PICC and Dyp which indicates that this parameter controls the phenomenon, and can be used to quantify the effect of physical parameters. An empirical model was developed to predict PICC assuming that the residual plastic field is a set of vertical plastic wedges, that the linear superposition principle applies and that the influence of a particular wedge exponentially decreases with distance to crack tip. The model was applied successfully to predict PICC for different residual plastic fields which provided an additional validation of Dyp as the parameter controlling PICC.info:eu-repo/semantics/publishedVersio

Plasticity induced crack closure: a sensitivity analysis

Plasticity induced crack closure (PICC) is closely linked to the monotonic and reversed plastic deformation occurring at the crack tip. The objective of the paper is to identify the different physical and numerical parameters affecting PICC, and develop a sensitivity analysis to quantify their relative importance. The main parameters affecting PICC are the load parameters, the yield stress, the size of finite elements and the numerical parameter considered to quantify PICC. The numerical predictions should be independent of numerical parameters, therefore further work is required to optimize the numerical modelsFCT e FEDE

Retrofitting of Welded Structures by TIG and Plasma Dressing

Due to economic and environmental constrains, the currently trend is to use the welded structures beyond their design lives. The predominant cause of in service failure of these aged structures is the fatigue of the welded joints. The use of improvement techniques in welded joints, as a repair technique, has been suggested by several authors. TIG dressing is one of the most promising of these repair techniques. However, the effectiveness of TIG remelting is closely linked to the depth of the repaired crack. The use of strain gauges can be effective to detect the presence of fatigue cracks in their initial phase of propagation, however their effectiveness in inspection programs on jobsite needs to be proven. Some TIG variants associated to recent technological innovations of fusion arc welding, are appointed to improve the penetration and the sustainability of the remelting process. In this article are presented some results of the work developed by the authors in the last years, relevant to assess the efficiency of sustainable repair, by TIG and plasma dressing, of welded structures. Relevance is given to the repair, complete or defective, of deep cracks, as well as the possible advantage of using TIG variants.info:eu-repo/semantics/publishedVersio

Simulation of crack growth in T-welded joints: residual stress field effect

In this article, a three-dimensional finite element model is used to predict the growth of cracks at the weld toe of a T-joint. The model is developed using the MSC Marc software. Fatigue life is estimated by integrating the Paris-Erdogan law and the stress intensity factors are obtained by the virtual crack closure technique. The influence of residual stresses generated by plastic deformation at the weld toe on the crack propagation speed is analyzed. The existence of residual compression stress fields causes a delay in crack growth. The obtained results are compared with the integration solutions of the Paris-Erdogan law using the stress intensity factor computed through the Mk factor proposed by Bowness and Lee, included in BS 7910 standard.info:eu-repo/semantics/publishedVersio

A numerical study of the effect of single overloads on plasticity induced crack closure

In this work, the effect of single overloads on plasticity induced crack closure is studied. An elastic-plastic finite element model was developed and the crack opening level was calculated from the contact forces along the crack flank. The effects of the loading parameters and stress state are analysed, and the mechanisms behind crack closure variations are identified. An overload is a traumatic event that eliminates material’s memory relative to the load history. Crack tip blunting is the mechanism behind this memory loss, since it eliminates crack closure. Material hardening has a major relevance on the evolution of plastic blunting, which was evident in the variation of the CTOD parameter. On the other hand, the overload produces strong plastic deformation ahead of the crack tip, giving rise to conditions for the rapid generation of crack closure higher than before the event. The peak of crack closure was found to increase linearly with the load increase above the maximum baseline value. The crack is totally closed for overload ratios of about 2.5. Empirical models were developed for the peak of crack closure, for the delay of this peak and for the stabilization distance after the overload. Finally, the stress state was found to have a major effect on crack closure level after an overload.info:eu-repo/semantics/publishedVersio

Effect of crack propagation on crack tip fields

Crack closure influences fatigue crack growth rate and must be included in the design ofcomponents. Plasticity induced crack closure is intimately linked with the crack tip plastic deformation, whichbecomes residual as the crack propagates. The objective here is to study numerically the effect of crackpropagation on crack tip fields. The transient effect observed at the beginning of crack propagation is linked tothe hardening behavior of material. The effect of mesh refinement is studied, and a singular behavior is evident,which is explained by the sharp crack associated with mesh topology, composed of a regular pattern of squareelements. The plastic zone size measured perpendicularly to crack flank in the residual plastic wake is quantifiedand compared with literature models. Finally, the removal of material at the first node behind crack tip withload cycling was observed for plane strain state and some hardening models in plane stress state

Through thickness evolution of crack tip plasticity

Experimental methods to measure fracture mechanics parameters tend to provide information from or about the surface of cracked components. However, information about the interior of the component is key to understanding the mechanisms governing the damage processes at a crack tip for both fatigue and fracture events. In this work we present a detailed numerical analysis of the evolution of the plastic zone through the thickness of an aluminium alloy specimen. This is done by means of ultra-fine non- linear finite element models. The simulated results are compared with experimental displacement data measured optically from the surface of the specimen.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Embedded Topological Defects in Hot Electroweak Theory: a Lattice Study

We study the properties of Nambu monopoles and Z-vortices in the 3D lattice SU(2) Higgs theory which represents the Standard Model at high temperature. We show that the densities of the Nambu monopoles and the Z-vortices are O(1) in the symmetric phase and generically small in the Higgs phase. Near to the critical Higgs mass and in the vicinity of the phase transition the densities are no more negligible in the broken phase. The percolation probability of the Z-vortex lines is found as a new disorder parameter for this phase transition. We conclude that the transition to the symmetric phase is accompanied by Z-vortex condensation. Simulations comparing elementary and extended vortices and monopoles at different \beta_G values, aiming to show that the density of vortices and monopoles of fixed physical size might have a well-defined continuum limit, gives encouraging but so far inconclusive results.Comment: 13 pages, LaTeX, 8 figures, epsf.sty needed; revision: minor changes and reference adde