The cyclic deformation and damage behaviors of the Fe–Mn and Fe–Mn–C TRIP/TWIP steels are comprehensively studied in a wide range of strain amplitude (from 0.3% to 8.0%). It is found that with ...increasing C content, the dislocation structures change from wavy slip to planar slip after cyclic deformation. In order to evaluate the low-cycle and extremely-low-cycle fatigue (LCF and ELCF) properties, a fatigue life prediction model, Nf = (Wa/W0)β, with a hysteresis energy-based criterion is used and developed. The model reveals that the LCF and ELCF damage mechanisms can be controlled by the material's damage capacity (the intrinsic fatigue toughness W0) and its ability of transforming mechanical work into effective damage (the damage transition exponent β). From a macroscopic point of view, W0 is related to the match of strength and ductility (approximately the static toughness U), and β mainly has a negative correlation with the cyclic strain hardening exponent n′. On the micro-scale level, W0 represents the defect-accommodated ability of the materials, and β is determined by the uniformity and reversibility of plastic deformation. For the current Fe–Mn(–C) TRIP/TWIP steels with increasing C content, the cooperation between an increasing damage capacity and an incremental damage accumulation rate leads to a higher ELCF property and a lower LCF property.
The low-cycle fatigue (LCF) damage mechanisms may be controlled by the material's damage capacity and damage accumulation rate. The influences of planar slip caused by short-range order (SRO) on fatigue damage and cracking are quite different from those caused by lowering the stacking fault energy (SFE), which has its origin from the various effects on damage accumulation rate. Display omitted
Artificial neural networks (ANNs) are a widely used machine learning approach for estimating low‐cycle fatigue parameters. ANN structure has its parameters such as hidden layers, hidden neurons, ...activation functions, training functions, and so forth, and these parameters have a significant influence over the results. Three hidden layer combinations, the hidden neurons ranging from 1 to 25, and different activation functions like hyperbolic tangent sigmoid (tansig), logistic sigmoid (logsig), and linear (purelin) were used, and their effects on the low‐cycle fatigue parameter estimation were investigated to determine optimal ANN structure. Based on the results, suggestions regarding ANN structure for the estimation of the low‐cycle fatigue parameters and transition fatigue life were presented. For the output layer and hidden layers, the most suitable activation function was tansig. The optimal hidden neuron range has been found between 4 and 9. The neural network structure with one hidden layer was determined to be most suitable in terms of less knowledge, structural complexity, and computational time and power.
Total strain-controlled low cycle fatigue (LCF) tests of a nickel based superalloy were performed at 650°C. Various hold times were introduced at the peak tensile strain to investigate the ...high-temperature creep-fatigue interaction (CFI) effects under the same temperature. A substantial decrease in fatigue life occurred as the total strain amplitude increased. Moreover, tensile strain holding further reduced fatigue life. The saturation phenomenon of holding effect was found when the holding period reached 120s. Cyclic softening occurred during the LCF and CFI process and it was related to the total strain amplitude and the holding period. The relationship between life-time and total strain amplitude was obtained by combining Basquin equation and Coffin-Manson equation. The surface and fracture section of the fatigued specimens were observed via scanning electronic microscope (SEM) to determine the failure mechanism.
•Hysteretic behaviour of corroded bars.•Analytical modelling of post-yield buckling of corroded bars.•Influence of corrosion on low-cyclic fatigue behaviour of reinforcing bars.•Modelling the impact ...of corrosion on cyclic degradation of corroded bars.
A new phenomenological hysteretic model for reinforcing bars with and without corrosion damage is presented. The model simulates buckling of reinforcement, deterioration in post-buckling compressive strength due to strain history and the impact of low-cycle fatigue on tension response. The model, for uncorroded reinforcing bars, is calibrated using data from numerical simulations and corrosion damage parameters are calibrated using experimental data. The model is evaluated using a comprehensive experimental data set, and the results show that the model is in a good agreement with the data.
The low-cycle fatigue (including extremely-low-cycle fatigue) behaviors of the FeMnC TWIP steels are comprehensively investigated focusing on the effects of the imposed strain amplitude and Mn ...content on the deformation and damage characteristics. It is found that fatigue performance varies obviously with increasing Mn content, though their tensile properties do not change much. With the increase of strain amplitude or Mn content, the dislocation slip mode changes from planar slip to wavy slip. In order to evaluate the low-cycle fatigue properties, more impartial and reasonable, a hysteresis energy-based fatigue life prediction model is applied and developed. According to the experimental results and model analysis, unlike tensile properties, the low-cycle fatigue performance of the TWIP steel is extremely sensitive to their slip mode. A remarkable improvement of fatigue resistance is found to be related to the increase of Mn content, which may originate from the relatively low damage accumulation rate of planar slip. Specifically, planar slip caused by lowering the stacking fault energy is much more uniformly distributed than which caused by disordering short-range order. Such a variation in fatigue damage micromechanisms is believed to be the major reason for the occurrence of a special bilinear relationship for the Coffin-Manson curve.
Fe-Mn-C TWIP steels manifest a similar tensile deformation behavior with increasing Mn content, while their low-cycle fatigue performances vary significantly. Unlike tensile property, the fatigue performance is extremely sensitive to the slip mode, which has its origin from material’s inherent attributions, e.g. stacking fault energy and short range ordering. Display omitted
•Proposal of a novel buckling-restrained damper with friction energy dissipation.•Low-cycle fatigue tests on nine BFD specimens considering four parameters.•Numerical analyses on the seismic ...effectiveness of the BFD in a SDOF system.
Aiming at solving the issue of the conventional buckling-restrained brace, i.e., the fixed yield strength and residual deformation, a novel buckling-restrained damper with additional friction energy dissipation (BFD) was proposed and tested in the present study. By fabricating an inner core with a small yield segment (SYS) and a large yield segment (LYS) connected in series and setting a friction device in parallel with the SYS, the dual stiffness and stable energy dissipation capacity of the BFD were achieved. Low-cycle fatigue tests were conducted on nine BFD specimens to investigate the influences of the length of the LYS, the length of the slotted holes, the bolt torque and the loading protocol on the hysteretic behavior of the BFD. The experimental results demonstrated that the strength of the BFD increased with the maximum strain of the LYS, whereas the low-cycle fatigue life of the BFD was negatively affected by the maximum strain of the LYS. The friction energy dissipation device could enhance the energy dissipation capability of the BFD throughout the working process. Moreover, the results from the numerical case study determined that the BFD could effectively control the displacement response of the SDOF system under different earthquake magnitudes.
Low‐cycle fatigue behavior of rolled WE43‐T5 magnesium alloy Ghorbanpour, Saeede; McWilliams, Brandon A.; Knezevic, Marko
Fatigue & fracture of engineering materials & structures,
June 2019, 2019-06-00, 20190601, Volume:
42, Issue:
6
Journal Article
Peer reviewed
Open access
This paper describes the main results from an investigation into the strength and low‐cycle fatigue (LCF) behavior of a rolled plate of WE43 Mg alloy in its T5 condition at room temperature. The ...alloy was found to exhibit small tension/compression yield asymmetry and small anisotropy being stronger in transverse direction (TD) than in rolling direction (RD) along with some anisotropy in strain hardening. The LCF tests were conducted under strain‐controlled conditions with the strain amplitudes ranging from 0.6% to 1.4% without the mean strain component. While the stress amplitudes during the LCF were higher for tests along TD than RD, the LCF life was similar for both directions. As revealed by electron microscopy, the fractured surfaces under tension consisted mainly of microvoid coalescence with some transgranular facets, while those fractured in LCF showed a combination of intergranular fracture and transgranular facets with minor content of microvoid coalescence.
In the present study, the stress gradient modified critical distance method is proposed to predict the low‐cycle fatigue lifetime of notched TA19 specimens by introducing a weight function of ...relative stress gradient into conventional critical distance theories. The accuracy and reliability of the proposed method were validated based on the experimental data by performing low‐cycle fatigue tests on smooth bar TA19 specimens as well as geometrically similar plate specimens containing a bore hole (BHP). Predicted results demonstrate that the mean absolute errors for entire tested specimens using the stress gradient modified critical distance method on the basis of the point method and the line method are 7.49% and 7.41% respectively. Furthermore, the fatigue striation width at the fracture surface was determined by scanning electronic microscope observations to examine the stress gradient effect on the crack growth rate.
Highlights
The effect of circular hole size on the LCF life is investigated experimentally.
SEM observations reveal that stress gradient inhibits the fatigue crack growth.
The conventional CDTs extended to the LCF regime fail to evaluate the notch size effect.
A novel method is proposed by combining weight function of relative stress gradient and CDTs.
•The classical Markl’s method for low cycle fatigue (LCF) evaluation is reexamined.•The applicability and limitations of Markl’s method are pointed out.•The structural strain method is applied to ...improve the Markl’s method.•Simplified structural strain method is proposed for pipe structure LCF evaluation.
Markl’s pseudo-elastic stress method has been the basis for low cycle fatigue (LCF) evaluation of girth-welded piping connections for decades, which underpins several international codes and standards. Here, we reexamine the applicability of Markl’s method in correlating some recent LCF fatigue data on girth-welded pipes. To mitigate its deficiencies, a simplified structural strain method is proposed for achieving an effective correlation of the same test data. The proposed method can be conveniently used for LCF evaluation of girth-welded pipe components either as an experimental measurement technique or a post-processing procedure of elastic-plastic finite element results.
•Discover competitive cracking behavior in accelerated CCF failure for turbine blades.•Find crack initiation change from slip planes to metallic pores and carbides, to oxides.•Reveal damage ...mechanisms in acceleration states by microstructural dominant features.•The source of cracking transition from transgranular to intergranular mode is the detected behavior.•Provide a promising mechanism insight for CCF estimation and acceleration test.
The cracking behavior and microscopic mechanism of K403 superalloy turbine blade are investigated respecting the Combined high and low Cycle Fatigue (CCF) with four acceleration states. It concludes that: (1) the crack initiation sites transform from slip planes inside alloy matrix to subsurface pores and carbides, then to oxides outside surface with increasing loads; (2) the behavior in (1) is attributed to the competition and alliance of different microstructural factors and the interaction of the factors with grain boundaries; (3) hereinto, the role shift of high cycle fatigue in CCF causes the transformation of transgranular to intergranular cracking mode.
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