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

Thermoelastic investigations for fatigue life assessment

An investigation is presented on the suitability and accuracy of a thermoelastic technique for the analysis of fatigue cracks. The stress intensity factor ranges ΔKI and ΔKII are determined from thermoelastic data recorded from around the tip of a sharp slot in a steel specimen under biaxial load, in order to assess the accuracy of the technique. ΔKI and ΔK II are determined to within 4% and 9% of a theoretical prediction, respectively. The results from a similar test on a fatigue crack under biaxial load are also presented. These show that thermoelastic stress analysis is a rapid and accurate way of analyzing mixed-mode fatigue cracks. A discussion is given on the potential of thermoelastic stress analysis of propagating cracks

A new displacement-based approach to calculate stress intensity factors with the boundary element method

The analysis of cracked brittle mechanical components considering linear elastic fracture mechanics is usually reduced to the evaluation of stress intensity factors (SIFs). The SIF calculation can be carried out experimentally, theoretically or numerically. Each methodology has its own advantages but the use of numerical methods has be-come very popular. Several schemes for numerical SIF calculations have been developed, the J-integral method being one of the most widely used because of its energy-like formulation. Additionally, some variations of the J-integral method, such as displacement-based methods, are also becoming popular due to their simplicity. In this work, a simple displacement-based scheme is proposed to calculate SIFs, and its performance is compared with contour integrals. These schemes are all implemented with the Boundary Element Method (BEM) in order to exploit its advantages in crack growth modelling. Some simple examples are solved with the BEM and the calculated SIF values are compared against available solutions, showing good agreement between the different schemes

Review of fatigue and fracture research at NASA Langley Research Center

Most dynamic components in helicopters are designed with a safe-life constant-amplitude testing approach that has not changed in many years. In contrast, the fatigue methodology in other industries has advanced significantly in the last two decades. Recent research at the NASA Langley Research Center and the U.S. Army Aerostructures Directorate at Langley are reviewed relative to fatigue and fracture design methodology for metallic components. Most of the Langley research was directed towards the damage tolerance design approach, but some work was done that is applicable to the safe-life approach. In the areas of testing, damage tolerance concepts are concentrating on the small-crack effect in crack growth and measurement of crack opening stresses. Tests were conducted to determine the effects of a machining scratch on the fatigue life of a high strength steel. In the area of analysis, work was concentrated on developing a crack closure model that will predict fatigue life under spectrum loading for several different metal alloys including a high strength steel that is often used in the dynamic components of helicopters. Work is also continuing in developing a three-dimensional, finite-element stress analysis for cracked and uncracked isotropic and anisotropic structures. A numerical technique for solving simultaneous equations called the multigrid method is being pursued to enhance the solution schemes in both the finite-element analysis and the boundary element analysis. Finally, a fracture mechanics project involving an elastic-plastic finite element analysis of J-resistance curve is also being pursued

Simulation of Near-Tip Crack Behaviour and Its Correlation to Fatigue Crack Growth with a Modified Strip-Yield Model

A modified strip-yield model has been developed to simulate the plasticity-induced crack closure under the constant amplitude (CA) and a single overload loading conditions. The paper focuses on the simulation of the near tip crack profiles and stress distributions during the fatigue process. Detailed information on near-tip stress and displacement fields at the maximum load (Pmax), the minimum load (Pmin), and the crack opening load (Pop) of a fatigue load cycle have been presented. The correlation of the crack closure to the near-tip material fatigue damage has been investigated and used to rationalize the crack growth behaviour under the CA and a single overload loading conditions.Peer reviewedSubmitted Versio

Crack-closure and crack-growth measurements in surface-flawed titanium alloy Ti6Al-4V

The crack-closure and crack-growth characteristics of the titanium alloy Ti-6Al-4V were determined experimentally on surface-flawed plate specimens. Under cyclic loading from zero to tension, cracks deeper than 1 mm opened at approximately 50 percent of the maximum load. Cracks shallower than 1 mm opened at higher loads. The correlation between crack-growth rate and the total stress-intensity range showed a lower threshold behavior. This behavior was attributed to the high crack-opening loads at short cracks because the lower threshold was much less evident in correlations between the crack-growth rates and the effective stress-intensity range

T-stress determination using thermoelastic stress analysis

T-stress and mixed-mode stress intensity factors have been determined experimentally using thermoelastic stress analysis and using a finite element method. Pure mode I, strong mixed-mode I and II, and interacting cracks have been used as the case studies. A new technique has been proposed to identify the crack tip from thermoelastic images. It has also been shown that using three terms of Williams's stress field formulation to determine the T-stress, yields a more accurate solution than using only the first two terms of the expansion

Predicting fatigue crack growth rate in a welded butt joint: The role of effective R ratio in accounting for residual stress effect

A simple and efficient method is presented in this paper for predicting fatigue crack growth rate in welded butt joints. Three well-known empirical crack growth laws are employed using the material constants that were obtained from the base material coupon tests. Based on the superposition rule of the linear elastic fracture mechanics, welding residual stress effect is accounted for by replacing the nominal stress ratio (R) in the empirical laws by the effective stress intensity factor ratio (Reff). The key part of the analysis process is to calculate the stress intensity factor due to the initial residual stress field and also the stress relaxation and redistribution due to crack growth. The finite element method in conjunction with the modified virtual crack closure technique was used for this analysis. Fatigue crack growth rates were then calculated by the empirical laws and comparisons were made among these predictions as well as against published experimental tests, which were conducted under either constant amplitude load or constant stress intensity factor range. Test samples were M(T) geometry made of aluminium alloy 2024-T351 with a longitudinal weld by the variable polarity plasma arc welding process. Good agreement was achieved