Pit to crack transition and corrosion fatigue lifetime reduction estimations by means of a short crack microstructural model

Article number 109171A microstructural model is presented to assess pit-to-crack transition and corrosion fatigue strength in pitted components in different environments. The model is first validated using available experimental data in the literature for pitting corrosion fatigue strength and S-N curves for both carbon and stainless steels. The value of the method proposed and its applicability is then shown by the development of fatigue knock down factor maps to the in-air S-N curve. Finally, the influence of pit local topology on pit-to-crack transition damage tolerance and the links to the NDE methods quantitative resolution necessary to account for defect shape or acuity in structural integrity assessments are discussedMinisterio de Educación DPI2014-56904-PMinisterio de Educación DPI2017-84788-PConsejo de Investigación de Ingeniería y Ciencias Físicas del Reino Unido EP / S012362 /

An iterative technique to assess the fatigue strength of notched components

Incluído en: Vol. 28 Procedia Structural IntegrityThe present work provides an efficient formulation to assess the growth of short fatigue cracks in metallic components. The proposed technique consists on the iterative combination of a micromechanical short-crack growth model and the Finite Elements Method. The interaction of the crack with the microstructure of the material is evaluated through the dislocations distribution technique. The finite elements analysis of the problem is needed to obtain the stress gradient ahead of the notch. The division of the main problem into simpler scenarios makes the resolution of the method easier since cases with known solutions are required exclusively. The iterative method formulation is properly described and application examples are given in order to show its usefulness.Ministerio de Educación y Ciencia (España) y Junta de Andalucía DPI2014-56904-PMinisterio de Educación y Ciencia (España) y Junta de Andalucía DPI2017-84788-PMinisterio de Educación y Ciencia (España) y Junta de Andalucía P18-FR-430

Crack paths for mild steel specimens with circular holes in high cycle fatigue

The most common current models for predicting the fatigue limit in notched solids use the stresses along a straight line, beginning at the notch root, to make the prediction. This line represents a simplification of the path of a real crack, which usually has a first part, known as stage I, in the direction of the maximum tangential stress, and a second part, known as stage II, in the direction perpendicular to the maximum normal stress. In this work, experimental crack paths for notched solids are analysed, with the objective of establishing the directions and lengths of stages I and II of fatigue crack growth from notches. The material was a mild steel, the geometry of the specimen was a thin-walled tube with a passing-through hole and the tests were axial, with R = -1. From the tests, the S-N curves were constructed and the fatigue limits were calculated. For the high cycle fatigue tests, the cracks paths were studied, with special attention to the crack initiation point and the crack direction along the first grains. The cracks paths on the specimen outer surface were studied with an optical microscope. In this surface, the crack initiation point was close to the maximum principal stress point at the hole contour. The direction of the crack in the first and second grain showed great variability. This variability noticeably decreased as the crack reached a length of 10-20 grains, approaching the direction of Mode I. However, the crack might actually start at an interior point on the surface of the hole, which has a depth of 1500 μm. In fact, the point of maximum principal stress of the entire specimen is not at the specimen outer surface but on the internal surface of the hole at 750 μm from the outer surface, that is, half the thickness of the specimen. The crack path in the plane transverse to the hole containing this point of maximum principal stress was analysed. For this, the fracture surfaces, at both sides of the hole, were analysed with a non-contact 3D optical profiler. The crack path in this internal transverse plane followed the trend described for the crack path on the specimen outer surface: the initiation point close to the maximum principal stress point at the hole contour, great variability in the direction of the crack along the first grains and tendency to Mode I direction when the crack gets longer.Comisión Europea. Fondo Europeo de Desarrollo Regional (FEDER), Ministerio de Ciencia e Innovación Agencia Estatal de Investigación DPI2017-84788-PJunta de Andalucía, Fondo Europeo de Desarrollo Regional (FEDER), Consejería de Economía, Conocimiento, Empresas P18-FR-430

Calculations of biaxial fatigue limits with models using the experimental crack direction

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).Several models from the literature were used to predict the fatigue limit in notched components subjected to biaxial cyclic loading. The predictions of these models are based on the elastic stresses along a line which is considered to be representative of the crack direction in its initial part. The line used in the models changes considerably. For one of the studied models, the line direction corresponds to Mode I, while for another it is Mode II, and for the other two models considered the direction is between Mode I and Mode II. However, quite naturally, the experimental crack direction is unique. In recent years, a study of experimental fatigue limits and crack directions in its initial part for three materials was carried out in hollow cylindrical specimens with a circular hole subjected to cyclic axial, torsional and in-phase biaxial loading. The directions of the cracks that were measured experimentally are on average similar for the three materials and close to Mode I. The analysed models give, in general, good predictions of the experimental fatigue limits, although they use directions that are completely different and that they too differ markedly from the experimentally found ones. The predictions of the models using, in a forced way, the measured experimental directions are good in most cases, which reveals a surprising insensitivity of these models to the main hypotheses on which their own formulations are based