Results Of Low-cycle Fatigue Tests Research Articles

A technique for estimation of cyclic stress-strain curves using an optical strain measurement and its application to additively manufactured AlSi10Mg

The article describes a new accelerated technique for estimation of cyclic stress-strain curves of cyclically stable materials based on a full-field measurement of strain during low-cycle testing on a large amplitude of loading. Any optical method suitable for capturing the strain contours on the curved part of the specimen during cyclic loading is applicable in the approach developed. In this study, the digital image correlation method is used to measure the deformation on the curved part of the specimen. The technique is presented on chosen results of low-cycle fatigue tests performed on AlSi10Mg printed with selective laser melting technology. Proportional loading cases were performed in tension-compression and pure torsion. The case of non-proportional loading was realized in the form of 90 degree out-of-phase tests. The stress-strain behavior is evaluated using a standard procedure and an accelerated technique with a good correlation for the three loading cases.

Investigation of Low Cycle Fatigue Behaviors of Inertia-Friction-Welded Joints of the TC21 Titanium Alloy

As a new highly damage-tolerant structural material, the TC21 titanium alloy has been widely used in aerospace applications. Inertial friction welding (IFW) is a form of pressure welding technology with less welding parameters and high welding joint performance, which is especially suitable for the connection of rotors of aero-compressors and engines. In this paper, inertia friction welding of TC21 titanium alloys was successfully carried out, and the microhardness, tensile properties and low cycle fatigue (LCF) behaviors of IFW joints were studied. Based on the mechanical parametric results of the tensile test, the true stress–strain curves of the IFW joint of TC21 titanium alloys are obtained by further calculation. Based on the LCF test results under different strain amplitudes, life prediction of IFW joints was investigated. The results of the LCF test show that there is no obvious cyclic hardening and cyclic softening of the IFW joints. Moreover, the fracture morphology of LCF samples under high strain amplitude (0.9%) and low strain amplitude (0.6%) was observed. The results show that the fatigue cracks initiate and propagate at multiple points in the LCF samples, and the transient fracture zone is larger under high strain amplitude. However, under low strain amplitude, a fatigue crack nucleates and propagates at a single point, and the crack propagation zone is larger.

The features of the evaluation of the results of experimental data in interlaboratory comparison tests for low-cycle fatigue

Improving the efficiency of assessing the results of low-cycle fatigue (LCF) test in the course of interlaboratory comparison tests (ICT) requires a reduction of the uncertainty associated with a number of random factors independent on the quality of testing and qualification of testing laboratories. Those factors are attributed to a random nature of the initiation and development of the fatigue fracture due to the inhomogeneity of the macro- and microstructure of the material, the state of the sample surface, stress concentrator, etc. A method for evaluating the results of LCF tests at ICT is presented. The method is implemented on the basis of linear regression analysis of experimental values of the mean and standard deviation of the logarithm of the durability at a given stress level. The features of evaluating the results of experimental data of interlaboratory comparison tests for low-cycle fatigue are presented. We propose to use the median estimate of the logarithm of fatigue life (corresponding to a given level (amplitude) of stresses obtained from the median fatigue curve constructed from the results of statistical analysis including regression analysis of the variances of experimental data) as an assigned value for assessing the characteristics of functioning of the laboratories participating in the ICT. The assigned value and estimates of the performance of laboratories is obtained using the median fatigue curve, which is plotted depending on the number of participants in the ICT. With a small number of participants, the median low-cycle fatigue curve is plotted according to the test results obtained by the expert laboratory, whereas for a sufficiently large number of participants according to the test results combined for all laboratories participating in the ICT.

A Study of Fatigue Crack Growth Rate in Steels in Relation to Crack-Tip Plastic Deformation and Fracture. Part 2. Parametric Relation between Fatigue Crack Growth and Plastic Strains

The first part of this study presented low-cycle fatigue and cyclic fracture toughness test methods for 10GN2MFA steel at 20 and 270°C, experimental results to construct kinetic fatigue fracture diagrams and to determine low-cycle plastic strain and fracture characteristics. Here, those results are estimated as to the application of cyclic plasticity to describe the fatigue crack growth rate. The cyclic strain-hardening coefficient and the Paris–Erdogan equation exponent for ferrite-pearlite steels are shown, to be closely related as well as the intersection point of the diagram in the range of high crack growth rates was defined. The parameters of the above relation and coordinates of the intersection point were determined for the steels of this class. In the latter case, the optimization methods were used to find the minimum sum of squares of residual functions. Based on the correlation between the cyclic strain-hardening coefficient and Paris–Erdogan equation exponent, the conclusion was drawn of the lower boundary value of this characteristic predicted for an ideal elastoplastic material (zero cyclic strain-hardening coefficient), to be equal to 1.0. As also follows, other classes of the materials (and other cycle asymmetries) can exhibit similar relations in the form of rays coming from the point (0; 1.0) at a corresponding experimental slope to the coordinate axes. From the common intersection point of the diagram (Gurney point) and their slopes to the axes for ferrite-pearlite steels, the procedure was advanced to construct the linear section of the diagram based on the results of low-cycle fatigue tests with plotting the stress amplitude vs. plastic strain amplitude curve. The procedure was approved on 10GN2MFA steel at 20 and 270°C and SAE-1020 and API5L X60 steels taken from the literature. In view of data scattering for cyclic fracture toughness test results, the estimation of the linear diagram sections is quite adequate, and the procedure of their construction is appropriate and can be extended to other classes of the materials and stress ratios.

The effects of strain amplitude and low cycle fatigue (LCF) behavior on nodular cast iron with two-step austempering (TSA) process

The strain amplitude analysis of low cycle fatigue (LCF) on nodular cast iron processed with TSA aimed to observe the fatigue behavior and fatigue cracking phenomenon until the failure. The LCF test followed the ASTM A606 standard and it was performed at room temperature with strain ratio = −0.1, strain rate 2.5 × 10–3s−1, various strain amplitudes of 0.002, 0.0025, 0.00275 and 0.003 mm mm−1, and TSA holding time of 5, 10, and 15 min. LCF test results were shown in the hysteresis loops curve (s versus e) and peak stress versus number of cycle curve. LCF test showed that the 0.0025 mm mm−1 strain amplitude at 5- and 10-minutes TSA generated the highest cycles compared to 0.002, 0.00275 and 0.003 strain amplitudes. The 0.0025 strain amplitude and 5 min TSA had 10845 cycles and 10 min TSA had 7414 cycles. Therefore, 0.002, 0.00275 and 0.003 strain amplitudes had total cycles which were below the above cycles.

Low-Cycle-Fatigue (LCF) behavior and cyclic plasticity modeling of E250A mild steel

A series of strain-controlled fully-reversed uni-axial tension-compression Low-Cycle-Fatigue (LCF) tests are carried out on IS 2062:E250A (Fe410W-A) mild steel specimens with strain amplitudes upto ±1.2%, to study the cyclic in-elastic material behavior. Accordingly, parameters of Cyclic-Stress-Strain (CSS) and Basquin-Coffin-Manson relationships are derived using the LCF test results. Further, the calibration of Non-Linear Combined (isotropic-kinematic) Hardening Material (NLCHM) model of cyclic plasticity is illustrated based on the experimental hysteresis response data. Finally, the efficiency of the calibrated NLCHM model parameters is substantiated by accurately simulating the material response from the tests conducted, using a non-linear FE software, Abaqus.

Experimental Study of Low Cycle Fatigue Properties for Epoxy Resins with Dibutyl Phthalate (Dbp)

AbstractIn order to improve the toughness of traditional epoxy resin, dibutyl phthalate (DBP) was introduced into the epoxy resin. The static mechanical performance of plasticized and unplasticized epoxy resin was evaluated. The test results showed that the DBP modified epoxy resin can obtain a higher toughness than conventional epoxy resin, but the elastic modulus and the tensile strength were slightly reduced. The low cycle fatigue test results indicated that the stress ratio and the stress level were two critical factors of fatigue life, which was increased with the growth of stress ratio. It was also found that the fatigue life of plasticized specimen was much less than that of the unplasticized specimen because of the plastic deformation. A logarithmic linear relationship was then established to predict the fatigue life for plasticized epoxy resin. The strain energy density was also applied to demonstrate the accumulation of energy loss. In addition, the fatigue toughness can be obtained by the hysteresis loop area method.

Energy-based method of fatigue damage cumulation

The study presents the results of low-cycle fatigue tests on AW-2024 aluminium alloy specimens under constant amplitude and multistage loading conditions. As expected, significant hardening of the material was observed in both cases. The tests confirmed that cyclic properties of AW-2024 alloy under multistage loading conditions can be represented based on the results of tests under constant amplitude conditions for the same alloy. The basis for a new fatigue damage cumulation method was formulated based on a comparative analysis of the test results. The main feature of the proposed method consists in including into the analysis the cyclic hardening of AW-2024 alloy during fatigue life calculation. An experimental verification of the method proved its practical applicability. A comparative analysis of the results for a standard Palmgren-Miner hypothesis and the new approach demonstrated the advantages of using the new method.

Effects of fatigue testing on low-cycle properties of P91 steel

The study presents the results of low-cycle fatigue tests on P91 steel specimens. The tests were carried out under constant strain (εac = const) and constant stress (σa = const) conditions, at two temperatures (T = 20 °C and T = 600 °C) and five εac and σa levels. A comparative study of the test results was performed using analytical models to characterize fatigue in relation to the strain and energy. The analysis of the experimental results shows the effect of the fatigue testing conditions on the fatigue life predictions. Simplifications applied in the theoretical models which affect the final fatigue characteristics are discussed in detail. It is demonstrated that the analytical models using strain-based characteristics of fatigue do not reflect the actual test conditions at σa = const. The conclusions are confirmed by the analysis of the test results with the use of energy parameters of fatigue.

The Influence of Plastic Deformation on the Low-Cycle Fatigue During the Burnishing of Holes in Flat Specimens of D16chT Steel

The paper deals with one of the most important problems of such closely related industries as mechanical engineering and aircraft manufacturing, involving the determination of the service life of components and structures operating under cyclic loading conditions. The presence of local structural and manufacturing stress concentrators greatly complicates the determination of the predicted life of components and structures at their design stage. The burnishing technique, namely, the method of plastic deformation of walls of the hole, aimed at achieving the amount of the residual plastic strain on their surface, is used to strengthen components with holes. The results of low-cycle fatigue tests conducted in repeating tension are presented for flat laboratory specimens of an aircraft aluminum alloy D16chT with a central cylindrical manufacturing hole hardened by burnishing. The burnisher geometry that ensures the required value of the residual plastic strain level (1, 2, and 3%) on the hole surface is calculated. The cyclic tensile loading of specimens at the stresses in a pulsating load cycle was performed in the range of 150 to 270 MPa at a frequency of 3 Hz using the equipment Bi-02-112. The influence of the amount of plastic strain in the burnished hole of the specimen on its cyclic life and fracture as a function of the load value is observed. It is found that due to plastic strain hardening of the surface of manufacturing holes, a local area of compressive residual stresses occur, causing the stress concentration around the manufacturing hole to decrease and the level of limit loads to increase. It is shown that the specimen, having a manufacturing hole hardened by burnishing (up to a plastic strain of 3%) and being cyclically loaded by a tensile stress to 170 MPa, failed not at the hole (the stress concentrator). Both the onset of the microcrack initiation and its further propagation take place in a solid region of the specimen. This is indicative that with a certain amount of residual stresses in the burnished hole, the specimen fracture under low-cycle fatigue becomes insensitive to the presence of this concentrator.

The impact of corrosion and inelastic buckling on low cycle fatigue life of steel bars

The current experimental study examines the effect of corrosion damage and inelastic buckling on low cycle fatigue (LCF) life of reinforcing steel bars grade B500A and B500B, with nominal diameter of 12 mm. In a total of 110 specimens, experimental tests were conducted before and after exposure in laboratory salt spray environment. The results of LCF tests on corroded reinforcing bars varied in mass loss percentage, strain amplitudes and buckling lengths. The results of mechanical tensile tests confirmed that corrosion is a significant factor of degradation in mechanical properties of steel bars. The percentage mass loss, the pitting corrosion and the inelastic buckling constitute the main parameters of affecting negatively the (seismic behavior) low-cycle fatigue life of the steel bars. Additionally, a reduction to the number of cycles to failure under conditions of low cycle fatigue was recorded. The findings also suggest that the degradation of the mechanical performance of steel on seismic loads can be attributed to mechanism of inelastic buckling influenced by history of loads combined with the presence of extensive porosity close to the surface of the steel bar.

Thermo mechanical fatigue life prediction of Ni-based superalloy IN738LC

The thermo mechanical fatigue (TMF) test is known to most accurately simulate the operational environment of a gas turbine’s substate. However, because the TMF test equipment and methodology are very complex, these tests are difficult to conduct. Some researchers have recently attempted to examine methods for evaluating the TMF lifetime of superalloy materials used in turbine blades by using the results of a low cycle fatigue (LCF) test in which the test equipment and method are relatively simple. However, this research was mainly conducted on specific materials such as M963 and GTD-111, and only a few studies have been conducted on IN738LC, which is a widely used material for commercial gas turbine blades. Therefore, this study examined a method for evaluating the TMF lifetime of an IN738LC material by using LCF rtest results. For that purpose, LCF and TMF tests were conducted, and the TMF lifetime was predicted with the LCF test results, using the Ostergren and Zamrik models. Lifetime prediction of IN738LC using the LCF test results from this study was compared with previous lifetime prediction results on other superalloys such as M963 and GTD-111 to review the cause of the difference in lifetime prediction results.

Low Cycle Fatigue Life Assessment of Alloy 617 Weldments at 900℃ by Coffin-Manson and Strain Energy Density-Based Models

This work aims to investigate on the low cycle fatigue life assessment, which is adopted on the strain-life relationship, or better known as the Coffin-Manson relationship, and also the strain energy density-based model. The low cycle fatigue test results of Alloy 617 weldments under <TEX>$900^{\circ}C$</TEX> have been statistically estimated through the Coffin-Manson relationship according to the provided strain profile. In addition, the strain energy density-based model is proposed to represent the energy dissipated per cycle as fatigue damage parameter. Based on the results, Alloy 617 weldments followed the Coffin-Manson relationship and strain energy density-based model well, and they were compatible with the experimental data. The predicted lives based on these two proposed models were examined with the experimental data to select a proper life prediction parameter.

Low-Cycle Fatigue Behavior and Fracture Mechanism of HS80H Steel at Different Strain Amplitudes and Mean Strains

This work studied the low-cycle fatigue (LCF) behavior of HS80H steel at room temperature. LCF tests at strain amplitudes of 0.4, 0.6, 0.8, 1.0, 1.2, 1.6, and 2.0% were conducted under constant mean strain. In addition, tests at mean strains of −0.8, −0.3, 0, 0.5, and 1.2% were conducted under constant strain amplitude. The microstructure and fracture surface of the material after the tests were characterized using scanning electron microscopy and transmission electron microscopy, respectively. Results of LCF test demonstrate the hardening-softening transition of HS80H steel. And it is associated with strain amplitude and mean strain. In addition, LCF life is affected by strain amplitude and mean strain. In asymmetric fatigue, the maximum absolute value of strain and fatigue life displays a linear relationship in double logarithmic coordinates. Microscopic observation showed that fatigue crack propagates through transgranular propagation resulting from the interaction between dislocation pileup and precipitates.

Ni基鋳造材246合金の片振り疲労強度評価および微小き裂発生と進展挙動に及ぼす微視組織の影響

The purpose of this study is to investigate low cycle fatigue (LCF) and high cycle fatigue (HCF) strengths, crack initiation and growth behaviors of Ni based castalloy 246. LCF and HCF tests are conducted under load controlled conditions. As the result of the observation of etched specimen, the grain size is from 3 mm to 5 mm. And the microstructure is mainly composed of dendritic structure. Growth orientation of dendritic structure varies in every grain. The result of elemental analysis, Si and Ti are detected at dendritic boundary. The results of LCF test show that small cracks initiate at the first cycle, followed by the crack growth along slip direction. Observation of fracture surface reveals that small cracks originate at casting defect or dendritic boundary. The results of HCF, on the other hand, show that small cracks initiate from subsurface casting defect under Δσ ≦ 600 MPa, while fracture origin becomes surface under Δσ > 600 MPa. The result of HCF test after introducing small crack on the surface shows that small crack on the surface does not affect on the fatigue life and the location of crack initiation. As the result of EBSD, the crack initiation site is near grain boundary and the crack grows along slip plane.

Estimation of residual stress influence on the low-cycle fatigue of threaded parts

The possibility of using the criterion of average integral residual stresses to estimate the cyclic life of threaded parts in the area of low-cycle fatigue is examined in the article. The peculiarities of fracture under high-cycle and low-cycle stresses are pointed out. The problems of using equations and criteria of linear mechanics in the low-cycle fatigue area are specified. These problems are mostly related to the origination of plastic deformation regions; the sizes of the latter may be significantly greater than those of the cracks. Technological factors affecting the origination and distribution of residual stresses are considered. It is noted that the diameter of the workpiece for thread rolling, heat treatment, the force and time of rolling are of the greatest importance. The connection between the number of cycles prior to fracture and the residual stress value, expressed in terms of the average integral residual stress criterion under the influence of different technological factors is illustrated. According to the results of low-cycle fatigue tests of threaded parts it has been shown that the dependence of the number of cycles prior to fracture on the value of the average integral residual stress criterion is approximated closely by equations of the quadratic form. The appropriate magnitudes of approximation reliability are not lower than 0.9163.

Evaluation of Combined Hardening Parameters for Type 304LN Stainless Steel Under Strain-Controlled Cyclic Loading

Low cycle fatigue (LCF) is the primary degradation mechanism affecting coolant piping of pressurized water reactor caused by combination of pressure and transient mechanical or thermal loads. In the case of LCF, stresses are high enough for plastic deformation to occur and the fatigue life is correlated with the cyclic plastic strain. Modelling cyclic plastic deformation of a material requires hardening parameters, which have to be obtained from LCF test results. It is customary in low cycle fatigue tests that the strain ranges are kept constant and the stresses are allowed to vary which typically leads to a hysteresis loop that consists of linear and nonlinear parts. In this paper, numerical studies on mechanical behaviour of Type 304LN stainless steel under fully reversed strain-controlled cyclic loading have been carried out. A linear combination of the two hardening types, isotropic and kinematic, governed by a scalar parameter, β (0 ≤ β ≤ 1) is used. A value of β = 1 indicates a pure isotropic hardening while a value of β = 0 indicates pure kinematic hardening. The details of the combined isotropic-kinematic hardening model are also presented. Constitutive relations for the classical von Mises theory along with a bilinear hardening theory have been used. The model is implemented in finite element software ABAQUS using a user subroutine written in FORTRAN, UMAT. An iterative method is adopted to arrive at the model’s hardening parameters and the value of β.

Indicators of HSLA steel behavior under low cycle fatigue loading

In present paper, the experimental research of the behaviour of high-strength low-alloy steel (HSLA) exposed to low-cycle fatigue (LCF) with controlled and fully reversible strain (Δɛ/2 = const, Rɛ = ɛmin/ɛmax = -1) has been analysed. Low-cycle fatigue tests were performed on a series of smooth specimens made of steel Nionikral 70 (NN-70), with semi-amplitude of controlled strain, Δɛ/2=0.35, 0.45, 0.50, 0.60, 0.70 and 0.80. Characterization and description of the low-cycle elastoplastic behaviour of the material by construction of characteristic curves of low-cycle fatigue are made using the characteristic hysteresis, load -strain force for each level of controlled strain, selected from the areas of stabilization. For the selection of stabilized hysteresis and processing of the results of low-cycle fatigue tests the recommendations of the ISO 12106:2003 (E) and ASTM E 606-04 standards were used, which in this work means to compare the two results. For each strain level characteristic stabilized hysteresis were selected, determined in accordance with recommendations of the ISO 12106:2003 (E) and ASTM E 606-04 standards, from which the data necessary for further processing of the results and finally for the construction of characteristic curves of low-cycle fatigue were read. Constructed curves are compared, and the effect of selection of the recommendations from the standards on characterization of the behaviour of steel NN-70 estimated. The results of experimental investigation have given us important information on the understanding of fatigue behaviour of steel NN-70. In this work the results of static and dynamic, that is monotonous and fatigue behaviour of the material, were compared, which is a practical contribution to the assessment of the behaviour of steel NN-70 exposed to the effects of low- cycle fatigue, i.e. monotonous and fatigue behaviour of the material.

Effect of the Fine-Grained Structure on the Fatigue Properties of the Heat-Resistant Nickel–Iron Alloy Inconel 718

It is well known that ultrafine-grained nickel alloys with average grain sizes d = 0.1–1 μm possess improved hot workability and can be used for superplastic forming or rolling. However, microstructure refinement can worsen some performance characteristics of the alloy, for example, heat-resistant or fatigue properties. In the present work, fatigue characteristics of the fine-grained alloy Inconel 718 are investigated. Ultrafine-grained alloys with average grain sizes d = 0.1–1 μm were manufactured by multiple forging with stage-by-stage deformation temperature decrease. During standard heat treatment of the alloy performed to obtain the desired properties, the γ-grain size was controlled by precipitations of δ-phase particles along the boundaries. Results of low-cycle fatigue tests of the fine-grained alloy at room and elevated temperatures are compared with the properties of the coarse-grained alloy.

Analyzing the mechanisms of thermal fatigue and phase change of steel used in brake discs

Thermal cracks on the friction surface of railway brake discs can develop during their lifespan. Cracks often initiate after severe braking conditions along with the occurrence of hot spots. The cyclic thermal and mechanical loads causes high temperature, plastic strain and even phase change of the brake disc steel. In this paper, full scale emergency braking tests were conducted and the peak temperature of localized area was found exceeding the austenitizing temperature of the steel. Thermal cyclic tests was performed to simulate the temperature variation during braking. Volume change of the steel caused by microstructure transformation was taken into consideration in numerical simulation. Combining with the fracture behavior of brake disc steel in low cycle fatigue (LCF) tests in different temperature level, the simulation results show a good consistency with the results of microstructure observation and crack initiation. The occurrence of embedded crack could be well explained according to the simulation results and low cycle fatigue test results.