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Fracture and Structural Integrity, 2017
This article deals with the locally multiaxial fatigue behaviour of high strength steel. To this end, the influence of the cracking path deflections (at the micro level) on the plasticity-induced fatigue crack growth is analyzed. With regard to this, a modelling by means of the finite element method was performed for a given stress intensity factor in the Paris regime, considering the existence of micro-roughness in the crack path (local micro-deflections with distinct micro-angles as a function of the microstructure of the material). The numerical results allow one to obtain the fatigue crack propagation rate and compare it with that for the same material in the absence of micro-roughness (with no micro-crack deflections, i.e., uniaxial fatigue behaviour).
Materials Science Forum, 2007
In the present paper examples for propagating and non-propagating conditions of slip bands and short fatigue cracks in a ferritic-austenitic duplex steel are given, which were quantified by means of SEM in combination with automated EBSD. To classify the results within the scope of predicting the service life under HCF- and VHCF-loading conditions a numerical model based on the boundary-element method has been developed, where crack propagation is described by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). This concept is capable to account for the strong scattering in fatigue life for very small strain amplitudes and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.
Influence of Micro-Structure on the Fatigue Crack Propagation in Bridge Steel
Proceedings
The use of high strength steels (HSS) allows designing lighter, slenderer and simpler structures with high structural performance. In general, the use of HSS leads to weight reduction of the whole structure, which compensates the higher cost of such a material comparing to the conventional construction steels. Knowledge of the fatigue resistance of material plays the key role during design and maintenance of the bridge structures. This contribution brings a comparison of the fatigue crack growth resistance of S355 J0 steel. Differences in microstructure and the texture of material structure could generally play a role in the fatigue crack growth. This study shows that in the case of studied steel texture of material structure has an influence on material fatigue behavior in Paris' law regime.
Ecf19, 2013
A new approach is proposed, considering the tensor character of the two first terms of the generalized Williams expansion, for the analysis of the 3-D stress field near the crack front of cracked plates under mode-I loading. The attention is focused on the constraint tensor t ij at the midplane, identified as the second order constant term of the Williams expansion, the out-of-plane component t 33 of which seems to play a significant role in characterizing the in-plane and out-ofplane loss of the constraint. This justifies a detailed study of t 33 using finite element analyses for through-thickness cracked plates under mode I loading with three different geometries and loading configurations: a cracked plate loaded in tension. Similar dependency with respect to thickness and crack length ratio is also observed for the in-plane component t 11 (the so-called T-stress). Mutual dependencies are observed, pointing out that a unified approach to the problem is the proper way to address loss-of-constraint effects.
Metals, 2021
This article studies the retardation effect in plasticity-induced fatigue crack growth rate for a low-medium strength steel, due to the appearance of microdeflections in the crack path. To this end, the finite element method was used to model the crack with its kinked tip under several stress intensity factor (SIF) ranges. The results allowed a calculation (after a small number of cycles) of the fatigue crack propagation rate for the multiaxial and uniaxial fatigue configurations at the microscopic level. It was observed that the retardation effect rose with an increase in the initial kinked crack tip angle, an increase in the initial projected kinked crack tip length, and with a decrease in the SIF range.
Influence of High Strength Steel Microstructure on Fatigue Crack Growth Rate
This study examines the effect of high strength steel microstructure morphology on fatigue crack growth rate (FCGR). To achieve this aim, three different heat treatment methods (normalizing, austempering quenching and tempering) were considered and all the steel specimens were initially heated to 950 0 C austenization temperature for ninety minutes and then processed via the different heat treatment methods before viewing the resultant microstructures under light optical microscope (LOM). Fatigue crack growth rate tests were conducted on the resultant microstructures with compact tension specimens at room temperature as prescribed by American standard testing method E647. Results of FCGR tests showed normalized microstructure has the lowest FCGR (6.2698E-06), followed by quenched and tempered (7.9519E-06), as-received (8.15E-06) and austempered (9.6667E-06) microstructure considering a low stress intensity factor range. The trend of results showed insignificant effect of microstructure over the Paris regime growth indicating fatigue crack growth rate is not a reliable parameter for correlating rate of crack propagation to microstructure.
Microstructural effects on fatigue crack growth behavior of a microalloyed steel
Procedia Engineering, 2010
Thermal transformations on microalloyed steels can produce multiphase microstructures with different amounts of ferrite, martensite, bainite and retained austenite. These different phases, with distinct morphologies, are determinant of the mechanical behavior of the steel and can, for instance, affect the crack path or promote crack shielding, thus resulting in changes on its propagation rate under cyclic loading. The aim of the present work is to evaluate the effects of microstructure on the tensile strength and fatigue crack growth (FCG) behaviour of a 0.08%C-1,5%Mn (wt. pct.) microalloyed steel, recently developed by a Brazilian steel maker under the designation of RD480. This steel is being considered as a promising alternative to replace low carbon steel in wheel components for the automotive industry. Various microstructural conditions were obtained by means of heat treatments followed by water quench, in which the material samples were kept at the temperatures of 800, 950 and 1200 C. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The results allowed correlating the tensile properties and crack growth resistance to the microstructural features. It is also shown that the Region II FCG curves of the dual and multiphase microstructural conditions present crack growth transitions that are better modeled by dividing them in two parts. The fracture surfaces of the fatigued samples were observed via scanning electron microscopy in order to reveal the fracture mechanisms presented by the various material conditions.
On a kinked crack model to describe the influence of material microstructure on fatigue crack growth
Threshold condition and rate of fatigue crack growth in both short and long crack regime appear to be significantly affected by the degree of crack deflection. In the present paper, a theoretical model of a periodically-kinked crack is presented to describe the influence of the degree of crack deflection on the fatigue behavior. The kinking of the crack is due to a periodic self-balanced microstress field having a length scale, d. By correlating the parameter d with a characteristic material length (e.g. average grain size in metals, maximum aggregate dimension in concrete), the possibility of using the present model to describe some experimental findings related to crack size effects in fatigue of materials is explored. Well-known experimental results concerning two different situations (fatigue threshold and fatigue crack growth in the Paris regime) are briefly analysed.
On the estimation of microstructural effects in the near-threshold fatigue of small cracks
The Journal of Strain Analysis for Engineering Design, 2008
The growth of small fatigue cracks is strongly influenced by the microstructure of the material concerned. Particularly in the early stages, the crystallographic orientation of the grains through which the crack must propagate plays a fundamental role. Cracks are generally initiated in well-oriented grains, but afterwards they are forced to grow through less favourably oriented grains. This work examines the influence of this crystallographic resistance to small crack growth in micromechanical terms. It is argued that the crystallographic orientation alone cannot explain the difference between small-and long-crack growth thresholds found in metals. Other phenomena must be called upon to account for this difference. For instance, crack closure exerts a resistance to crack growth whose evolution with crack length is similar to that due to the crystallographic orientation. Finally, it is shown that, when microstructural and mechanical thresholds are interpreted within the context of micromechanical models, a number of classical parameters and expressions usually employed in engineering practice can be naturally obtained and understood.
On the theoretical modeling of fatigue crack growth
Although fatigue is by far the most common mode of failure of structural materials, mech-anistic understanding is still lacking. For example, the fundamental Paris law which relates the crack growth rate to stress-intensity factor range is still phenomenological and no reliable mechanistic model has been established for a given material or class of materials despite numerous investigations over a half a century. This work is an attempt to theoretically model fatigue crack propagation induced by alternating crack-tip plastic blunting and re-sharpening in the mid-range of growth rates on the basis of inputs from experiments that measure macroscopic material behavior, e.g ., response to uniaxial cycling loading. In particular, we attempt to predict Paris law behavior by accounting for the material consti-tutive behavior in response to cyclic loading by modeling crack advance solely in terms of the underlying plastic dissipation. We obtain the steady-state condition for crack growth based on plastic dissipation, numerically using finite element analysis, which involves a methodology to address plastic closure upon unloading. For a given stress-intensity range, we calculate the crack propagation rate from the steady-state condition through each cycle of loading and unloading of a cracked compact-tension specimen, without resorting to any specific criterion for crack advance. Published by Elsevier Ltd.