The Effect of Post Fatigue Tensile Loading on Mechanical Properties and Failure Behaviour of Steel (original) (raw)
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Influence of microstructure on fatigue process in a low carbon steel. Analysis and modelling
Engineering Failure Analysis, 2017
Fatigue in a low-carbon steel is investigated through observation on surface crack propagation and on growth of cracks in preliminary notched specimens. Testing uses three groups of specimens. For surface crack observation there are two groups of samples consisting of cylindrical specimens subjected to tension-tension and rotating-bending fatigue; in this case surface microstructurally-short crack propagation is monitored by acetate-foil replica technique. For crack growth observation (in situ) in notched specimens there is a third group of samples including flat specimens preliminary notched by FIB-technique and then subjected to pure-bending fatigue. Here microstructurally-short crack propagation is examined at interruptions of each test at a given equal number of cycles for detailed observation of specimen surface by optical-and SEM-microscopy. The study is focused on examining of crack paths in terms of interaction between the propagating short cracks and the microstructure, and on a suitable mathematical description of crack growth in the investigated microstructure. The obtained data for pure-bending fatigue show higher crack growth rates (dominated by the interaction with ferrite and pearlite grain boundaries and interfaces, ferrite grains, pearlite colonies and non-metal inclusions) and shorter fatigue lifetimes than those found for rotating-bending fatigue. In comparison, the registered tension-tension fatigue data present the lowest crack growth rates, due to much lesser loading than that applied at rotating-bending and pure-bending fatigue. Based on data obtained, a Parabolic-linear model "Crack growth rate-Crack length" is used for describing and predicting adequately short crack propagation under the specified three types of fatigue. The model is supported by a comparison between the predicted and the actual fatigue lifetimes.
Fatigue initiation in C35 steel: Influence of loading and defect
International Journal of Fatigue, 2010
The aim of this work is to study the influence of both defect and tension and torsion loading on stress number of cycles (S-N) 25 curves for C35 steel. A spherical artificial defect has been machined at the surface of gauge length of fatigue samples. The crack initiation mechanisms have been identified based on several observations on Scanning Electron Microscope (SEM) at different stage of fatigue life. The initiation crack length definition is proposed for defect free and defective material. For defect free material, torsion loading allows relatively earlier initiation compared to tension loading. In the case of defective material, it is observed that, for the both types of loading, initiation fatigue life is not negligible by comparison to total fatigue life. It is also concluded that defects are much more deleterious to fatigue life in the range of high cycle fatigue regime. However, for the limited fatigue lives (between 10 4 and 10 5 cycles), the defect free and defective material S-N curves are relatively comparable.
The Influence of Combined Loading on Fatigue Crack Growth from Small Defects
ICMFF9, 2013
A basic equation of crack growth: da/dN = A(∆K eff-∆K effth) 2 , proposed by McEvily et al., was used for the evaluation of growth and threshold behaviors of small cracks initiated from small defects in combined loading fatigue. Here A is a material constant, a is the crack length, N is the number of cycles, ∆K eff is the effective stress intensity factor range and ∆K effth is its threshold value. In the detailed evaluation of the behavior of small fatigue cracks, the Kitagawa effect, the elastic-plastic behavior of cracks in biaxial stress fields and crack closure effects were taken into account. In-phase and out-of-phase combined tension and torsion fatigue tests were conducted using 0.37 % carbon steel (JIS S35C) specimens containing holes whose diameters were 100 µm, 200 µm and 500 µm. The direction of crack propagation, S-N curves and fatigue limits were found to be in agreement with the analyzed behavior.
Microstructural study of multiaxial low cycle fatigue.PDF
This paper discusses the relationship between the stress response and the microstructure under tension-torsion multiaxial proportional and nonproportional loadings. Firstly, this paper discusses the material dependency of additional hardening of FCC materials in relation with the stacking fault energy of the materials. The FCC materials studied were Type 304 stainless steel, pure copper, pure nickel, pure aluminum and 6061 aluminum alloy. The material with lower stacking fault energy showed stronger additional hardening, which was discussed in relation with slip morphology and dislocation structures. This paper, next, discusses dislocation structures of Type 304 stainless steel under proportional and nonproportional loadings at high temperature. The relationship between the microstructure and the hardening behavior whether isotropic or anisotropic was discussed. The re-arrangeability of dislocation structure was discussed in loading mode change tests. Microstructures of the steel was discussed in more extensively programmed multiaxial low cycle fatigue tests at room temperature, where three microstructures, dislocation bundle, stacking fault and cells, which were discussed in relation with the stress response. Finally, temperature dependence of the microstructure was discussed under proportional and nonproportional loadings, by comparing the microstructures observed at room and high temperatures.
Effect of Microstructure on Torsional Fatigue Endurance of Martensitic Carbon Steel
Journal of Solid Mechanics and Materials Engineering, 2009
The microstructural influence of martensitic carbon steel on torsional fatigue endurance was investigated, taking into consideration the application of high strength steel electric resistance welded (ERW) tubes to automotive structural parts. The chemical composition of the base steel alloy was 0.1-0.2%C-0.2-1.5%Si-1.3-1.9%Mn-0.01%P-0.001%S-(Cr,Mo,Ti,Nb,B). Laboratory vacuum-fused ingots were hot-rolled, heated to 1023 or 1223 K in a salt bath, and then waterquenched and tempered at 473 K. Consequently, three types of microstructure, martensite (M), martensite and ferrite (M+F), and ferrite and pearlite (F+P), were prepared. Fully reversed torsional fatigue testing was conducted with 6 mm diameter round bar specimens. Torsional fatigue endurance was found to monotonously increase with increases in the tensile strength of the specimen from 540 to 1380 MPa. The martensitic single structure and the M+F dual-phase structure showed a similar level of fatigue endurance at a tensile strength of approximately 950 MPa. However, fatigue micro-crack morphology varied slightly between them. At the surface of the M+F specimen, many small cracks were observed in addition to the main crack. Conversely, in the martensitic specimen, these small cracks were rarely observed. ∆K decreasing/increasing crack growth testing with compact tension (CT)-type specimens was also conducted. Based on these experimental results, the effect of microstructure and stress level on the initiation/propagation cycle ratio is discussed. In addition to fatigue properties, some practical properties, such as low-temperature toughness and hydrogen embrittlement resistance, were also evaluated in view of actual applications for automotive structural parts.
Competition between microstructure and defect in multiaxial high cycle fatigue.PDF
This study aims at providing a better understanding of the effects of both microstructure and defect on the high cycle fatigue behavior of metallic alloys using finite element simulations of polycrystalline aggregates. It is well known that the microstructure strongly affects the average fatigue strength and when the cyclic stress level is close to the fatigue limit, it is often seen as the main source of the huge scatter generally observed in this fatigue regime. The presence of geometrical defects in a material can also strongly alter the fatigue behavior. Nonetheless, when the defect size is small enough, i.e. under a critical value, the fatigue strength is no more affected by the defect. The so-called Kitagawa effect can be interpreted as a competition between the crack initiation mechanisms governed either by the microstructure or by the defect. Surprisingly, only few studies have been done to date to explain the Kitagawa effect from the point of view of this competition, even though this effect has been extensively investigated in the literature. The primary focus of this paper is hence on the use of both FE simulations and explicit descriptions of the microstructure to get insight into how the competition between defect and microstructure operates in HCF. In order to account for the variability of the microstructure in the predictions of the macroscopic fatigue limits, several configurations of crystalline orientations, crystal aggregates and defects are studied. The results of each individual FE simulation are used to assess the response at the macroscopic scale thanks to a probabilistic fatigue criterion proposed by the authors in previous works. The ability of this criterion to predict the influence of defects on the average and the scatter of macroscopic fatigue limits is evaluated. In this paper, particular emphasis is also placed on the effect of different loading modes (pure tension, pure torsion and combined tension and torsion) on the experimental and predicted fatigue strength of a 316 stainless steel containing artificial defect.
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.
Materials Science and Engineering: A, 2003
In this study, the fatigue crack propagation from the surface and from internal inclusions is analyzed and modeled in high strength steels that show both types of crack initiation. Fatigue crack propagation lives of a crack initiated from the surface and from internal inclusions are estimated and analyzed. The high cycle fatigue life for a given crack range is estimated by: (a) defining material resistance to crack propagation as a function of crack length, and (b) assuming that the difference between the applied driving force and material resistance for crack propagation defines the effective driving force applied to the crack. If the crack growth rate as a function of this effective driving force is known for a given material, the high cycle fatigue life for a given crack length range can be estimated. The present model estimates reasonably well the fatigue life associated with crack initiated from the surface. On the other hand, the fatigue life of crack initiated from internal inclusions has an associated initiation life defined by a hydrogen assisted fatigue mechanism that represents a great part of the total fatigue life. In this case, the fatigue crack propagation life predicted by the model is a small part of the total fatigue life. The estimations and analysis made in this study, in accordance with previous observations reported in the literature, reveal that the total fatigue life associated with cracks initiated at internal inclusions is valid only if the number of cycles necessary to develop the optical dark area (ODA) by hydrogen assisted fatigue can be properly estimated. #
Metallurgy Fe-1.5Cr-0.2Mo-0.7C steel specimen fatigued in bending with R = −1 at 24 Hz and a stress amplitude of 312 MPa. The fatigue limit was ∼240 MPa, at which stress level no microcracks were detected in static loading. Testing was interrupted at 100 cycles and at further 29 intervals until failure after 49 900 cycles. For each arrest, surface replicas were made in the two regions where maximum stress was applied. Microcracks could nucleate below 100 cycles, when their sizes ranged from <5 to ∼20 μm. Fractographic examination identified the failure-originating site, which was then associated with the crack system observed on the 'last' pre-failure micrograph. Detailed examination of the eventual failure region showed nucleation, at various cycle intervals, of 18 microcracks, their subcritical growths, arrests and coalescences with continuing cycling to form a critical crack 2.25 mm deep. Stepwise microcrack growth was probably rapid -to the next arrest or coalescence. For each (micro)crack size stress intensity factors, K a s, were estimated and, at the end of Stage II, for the coalesced crack, K a reached K 1C , independently estimated to be ∼36 MPa m 1/2 .