Search

An experimental campaign of stainless-steel specimen tests has been completed. Three different types of tests have been performed: UTS (Ultimate Tensile Strength), LCF (Low Cycle Fatigue) and HCF (High Cycle Fatigue). This experimental campaign has been conceived and performed to validate the fatigue damage initiation prediction capabilities of UMT (Unified Mechanics Theory). Two different formulations have been used for LCF and HCF. For the latter, a modification to the existing approach is proposed in this paper. Good agreement has been found between UMT predictions and experimental results.

A theoretical framework is developed for the evaluation of favorable residual stress profiles, suppressing fatigue damage initiation in rolling contact fatigue. Non-metallic inclusions at the microstructure of bearings are one of the most important reasons for fatigue damage initiation since they act as stress risers. In order to evaluate the stress state around such inclusions at the micro-scale, macroscopic stress histories are determined by Hertzian contact theory at different depths below the raceway for a typical roller bearing. These stress distributions are then used as far-field stresses for a micro-scale model accounting for single inclusions of different geometries and orientations. Eshelby’s method is used to relate far-field and local stresses in the vicinity of inclusions. The von Mises stress criterion is then used as a conservative estimator of crack initiation due to micro-scale plasticity. The effect of compressive residual stresses added to the axial and circumferential normal stress components at different depths is analyzed. The von Mises stress field around different inclusions at different depths is investigated in order to determine the most critical case in terms of micro-scale plastic deformation. Finally, an optimization process is carried out in order to determine the residual stresses that minimize the maximum observed von Mises stress as a function of depth.

[Display omitted] • Different shear direction results in high stress concentration at soft-soft GBs. • Activated TT boundaries and GBs constitute new triple junctions at soft-soft GBs, leading to micro fatigue cracks. • Strain quantity difference at soft-hard grains is insufficient to fatigue cracks. • The interrupt fatigue tests and EBSD-based CP models was developed. The intergranular stress was investigated at grain boundaries (GBs) of Ti-2Al-2.5Zr alloy under cyclic loading. The crystal plasticity (CP) simulations revealed that the strain incompatibility between soft-soft grains due to shear direction difference, which forced the slight activation of tension twins (TT) near GBs. Twin boundaries (TBs) and GBs constituted new triple junctions, leading to further stress concentration and fatigue damage initiation. While the strain quantity difference at soft-hard GBs induced the moderate stress jump and stable stress evolution, which was insufficient to generate fatigue cracks. The EBSD-based CP model successfully predicted the twin activation and fatigue cracks at GBs. [ABSTRACT FROM AUTHOR]

Abstract: The present work is the second part of an ongoing study of microstructural changes during fatigue damage initiation in pipe steels [B. Pinheiro et al., Mat. Sci. Eng., A 532 (2012) 158–166]. Microdeformations and residual stresses (macrostresses) are evaluated by X-ray diffraction during alternating bending fatigue tests on samples taken from an API 5L X60 grade steel pipe. Microdeformations are evaluated from the full width at half maximum (FWHM) of the diffraction peak and residual stresses are estimated from the peak shift. Here, to understand the role of the initial dislocation structure, annealed samples are considered. As previously found for as-machined samples, the evolution of microdeformations shows three regular successive stages, but now with an increase during the first stage. The amplitude of each stage is accentuated with increasing stress amplitude, while its duration is reduced. Residual stresses show a similar trend, with stages of the same durations than those observed for FWHM, respectively. Additionally, changes in density and distribution of dislocations are observed by transmission electron microscopy combined with the technique of focused ion beam. The results are very encouraging for the development of a future indicator of fatigue damage initiation for pipe steels based on microstructural changes measured by X-ray diffraction. [Copyright &y& Elsevier]

Abstract: Steel pipes used in the oil and gas industry undergo the action of cyclic loads that can cause their failure by fatigue. A consistent evaluation of the fatigue damage during the initiation phase should fundamentally be based on a nanoscale approach, i.e., at the scale of the dislocation network, in order to take into account the micromechanisms of fatigue damage that precede macrocrack initiation and propagation until the final fracture. In this work, microstructural changes related to fatigue damage initiation are investigated in the API 5L X60 grade steel, used in pipe manufacturing. Microdeformations and macro residual stress are evaluated using X-ray diffraction in real time during alternating bending fatigue tests performed on samples cut off from an X60 steel pipe. The aim of this ongoing work is to provide ground for further development of an indicator of fatigue damage initiation in X60 steel. This damage indicator could allow a good residual life prediction of steel pipes previously submitted to fatigue loading, before macroscopic cracking, and help to increase the reliability of these structures. [Copyright &y& Elsevier]