Influence of microstructure on fatigue process in a low carbon steel. Analysis and modelling (original) (raw)
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Materials Science and Engineering: A, 2004
A study has been made to develop a model predicting the low cycle fatigue life of a material in relation to its microstructural variables. To achieve this goal, the concept of damage accumulation by multiple surface cracks has been adopted. An equation for stage I crack growth suggested by Tomkins was modified to consider the effect of grain size on the crack growth rate at early stage, and statistical analysis was carried out to calculate the final crack length for fatal failure. A concept of equivalent crack length has been used to present the quantitative description of crack growth rate when multiple cracks grow at the same time. To verify the suggested model, low cycle fatigue tests were conducted for the polycrystalline single-phase steel with the various grain sizes. The results showed a good agreement between the experimental data and the predicted curve.
A microstructural model for the prediction of high cycle fatigue life
Scripta Materialia, 2002
In this study, a model predicting the high cycle fatigue life was developed in relation to the microstructural variable, especially, the grain size. The concept of small crack propagation theory was adopted to reflect the influence of the microstructural variables. Reasonable agreement was found between the experimental data and the predicted curve.
Relating fracture mechanics and fatigue lifetime prediction
Materials Science and Engineering: A, 2016
This article presents how to link together the results of fatigue crack growth tests, analytic fracture mechanics and experimental methods of fatigue lifetime predictions. The study at the beginning investigates the effect of mechanical load redistribution among failed and intact micro-structural bonds along fatigue crack growth to final crack at vulnerable locations in materials and structures under cyclic loads. The microstructural load redistribution model is analytically formulated as a mechanical interaction between fatigue crack growth and fatigue endurance on the macroscopic level. The article in continuation investigates how to link the parameters of fatigue crack growth in fracture mechanics to parameters of fatigue life directly from the work done on crack growth determined by testing. The article at the end illustrates the application of the analytic procedure for fatigue lifetime prediction that combines fracture mechanics and the load redistribution model for determination of S-N curve parameters important in structural analysis and design. In this research the fatigue life time parameters are derived from a single fatigue crack growth experiment instead from normally required sets of fatigue tests for different loading conditions.
A model for fatigue crack propagation
Engineering Fracture Mechanics, 1975
Ab&act-A model for fatigue crack propagation is presented which incorporates low cycle fatigue, mechanical proper&s and a microstructurally-associated process zone. Comparison of the model to publirhea data for 4340 (hard and soft), a s&es of TRIP steels, TiiAL4V, 20%T6 aed 300 ppdc mamgiag steel shows good agrccmcnt.
A fatigue crack propagation model
Engineering Fracture Mechanics, 1984
A model for fatigue crack propagation has been developed which incorporates mechanical, cyclic and fatigue properties as well as a length parameter. The latter can be associated with the microstructure of the material. The fatigue failure criterion is based on a measure of the dissipated plastic strain energy. This model predicts crack propagation at low and intermediate AK values, i.e. stage I crack growth rate as well as that of the stage II. A number of crack growth rate models proposed earlier, are shown to be particular cases of the one developed herein. Predictions of the model are in good agreement with the experimental data. The required data for predicting the crack growth rate, can be found in standard material handbooks where fatigue properties are listed.
In this note, we explore the possibility of simple extensions of the heuristic El Haddad formula for finite life, as an approximate expression valid for crack-like notches, and of the 'Lukáš and Klesnil' equation for blunt notches. The key starting point is to assume, in analogy to the Basquin power-law SN curve for the fatigue life of the uncracked (plain) specimen, a power law for the 'finite life' intrinsic El Haddad crack size. The approach has similarities with what recently proposed by Susmel and Taylor as a Critical Distance Method for Medium-Cycle Fatigue regime. Reasonable agreement is found with the fatigue data of Susmel and Taylor for notches, and in particular the error seems smaller in finite life than for infinite life, where these equations are already used. In these respects, the present proposal can be considered as a simple empirical unified approach for rapid assessment of the notch effect under finite life.
Modelling of Fatigue in Some Steels and Non-Ferrous Alloys
Ecf17 Brno 2008, 2013
For most engineering alloys the plot of short fatigue crack growth rate against crack length can be modeled by parabolic-linear system of equations. This model is illustrated on two low-carbon steels subjected to tension-tension loading. At elastic-plastic fracture mechanics conditions, the fatigue-data presentation including short fatigue crack growth rate against J-integral range takes place for some steels and non-ferrous alloys based on titanium and copper. An alternative method of short fatigue crack data presentation is proposed that involves a specific energy fatigue-function expressed by an almost straight line termed fatigue tendency of the material at a given stress-range. A comparative analysis between the standard and new presentations shows that at the same number of crack-size measurements, the precision of the latter is significantly higher. This result suggests a possible decrease of fatigue measurements and is approved for another 20 materials.
Fatigue crack development in a low-carbon steel. Microstructure influence. Modelling
Procedia Structural Integrity, 2016
During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Numerical and Experimental Verification of a New Model for Fatigue Life
Fatigue & Fracture of Engineering Materials and Structures, 1993
A new model for fatigue life prediction has been presented[l]. The model is based on Palmgren-Miner's rule in combination with a level crossing analysis. Here an effort is made to verify the model, numerically and experimentally. Data from the literature and experimental data generated by us have been compared with fatigue life predictions made with the new model. The data have also been compared with traditional fatigue life estimations based on the rain flow count method. The fatigue lives predicted by the new model often agree better with actual lives than predictions made with the RFC-method. This is especially pronounced when the loading sequence is very irregular, i.e. when the sequence contains many small cycles superimposed on large cycles. The new method is both fast and simple to use. NOMENCLATURE ub = ultimate strength u0.* = yield strength u = total stress, urn + ua 6, = mean stress u, = stress amplitude u,,, = upper stress limit u,,, = lower stress limit CL =constant in the Wohler equation / 3 = exponent in the Wohler equation