Fatigue Life Of Steel Research Articles

Fatigue performance of Q420C high-strength steel at room and low temperatures

Steel structures are frequently exposed to the environment with variable temperature, where the fatigue failure of steel is difficult to be predicted. In this paper, the standard Q420C high-strength steel specimens were tested to explore their fatigue performance at room (25°C) and low temperatures (0°C, −15°C and −30°C). Before fatigue tests, the tensile tests have been done, and the results indicate that the yield and ultimate strengths of Q420C steel significantly grew when the temperature decreased. The fatigue behavior of Q420C steel was discussed from different perspectives, including the crack initiation, crack growth, fracture mode and fatigue life. The fatigue failure was observed mainly near the arc transition section of specimen, with the fatigue crack initiated from the surface and internal discontinuity defects. Furthermore, the test fatigue life was compared with the recommended S-N curves provided by specifications from various countries. These comparison results suggest that recommended S-N curves were excessively conservative to evaluate the steel fatigue life at low temperature, leading to a potential economic cost for the steel structures at extremely cold conditions. To avoid needless loss, a calculation method of steel fatigue life considering the effect of low-temperature, stress range and yield strength was proposed.

High-temperature low-cycle fatigue characteristics of the 12Cr10Co3MoWVNbNB turbine rotor steel

Low-cycle fatigue (LCF) experiments of the 12Cr10Co3MoWVNbNB steel at 598 °C and a strain amplitude ranging from 0.27 % to 0.55 % were executed to reveal the fatigue characteristics, fatigue fracture mechanism, and further to predict the high-temperature fatigue life of the new-style turbine rotor steel. The results indicate that the steel exhibits a high-temperature fatigue softening characteristic. During the high-temperature LCF, the cracks are generated at surface or subsurface defects of the specimens, and the fatigue striations and dimples are apparent on the fracture surfaces of the crack propagation zone and the final fracture zone, respectively. The Smith-Watson-Topper (SWT) and Manson-Coffin models are suitable to predict the high temperature fatigue life of the steel at high/low strain amplitudes, respectively. In terms of finite element analysis of the turbine set rotor, it is determined that the steady state strain of the rotor steel is approximately 0.2 %. And thus, the fatigue life of the steel rotor is estimated as 172,500 cycles at the strain amplitude, based on the Eq. (6) in this work. After 100,000 cycles at the strain amplitude, the yield and tensile strengths of the steel specimens are only reduced by 1 % and 3 %, respectively, indicating a long residual life, which is consistent with the above predicting result.

A Multiscale Integrated Model for Predicting Fatigue Life of Steels

A Multiscale Integrated Model for Predicting Fatigue Life of Steels

The Role of the Distance between Fine Non-Metallic Oxide Inclusions on the Fatigue Strength of Low-Carbon Steel

The fatigue strength of steel is an important parameter determining the use of the alloy. Conducting material durability tests depending on the working conditions of the material requires a lot of work. Therefore, the industry knows methods to estimate the fatigue life of steel on the basis of other parameters or measurements of other mechanical properties. One of such parameters is the fatigue strength coefficient, which allows one to link the fatigue strength with the hardness results of a specific steel grade. Alloys produced in industrial conditions contain impurities that can affect the properties of steel, including fatigue strength. Impurities in steel depend mainly on the technology of its production. One of the technologies that allows one to obtain high-purity steel is by subjecting it to secondary metallurgy treatment consisting of desulfurization and refining with argon. The fatigue strength of steel depends, among other things, on the morphology of impurities. In the work, the influence of the distance between small non-metallic inclusions with a diameter of less than 2 µm on the fatigue strength of steel, expressed by the fatigue resistance factor, was assessed. The research was carried out in industrial conditions on seven independent melts of low-carbon steel capable of forming a martensite microstructure. Several dozen fatigue strength tests were carried out for each of the casts. The volume fraction, size, and distribution of pollutants were examined. It was found that the main impurity is Al2O3, with a diameter of about 1.8 µm occurring at a distance of about 12 µm. The distance between small non-metallic inclusions affects the fatigue resistance factor, and small non-metallic inclusions with a diameter of less than 2 µm hinder the destruction of high-ductility steel. The paper presents an example of the structure of non-metallic inclusions for heat, the relative volume of inclusions, the average impurity diameter and impurity spacing for impurity dimensional ranges, the impurity spacing λ for the total volume of impurities, and the bending fatigue strength coefficient tested in steel after hardening and tempering at different tempering temperatures.

Fatigue life prediction on the side hull structure of semi-submersible tourism

A semi-submersible tourism ship has been created to promote Indonesia’s marine tourism sector and make it easier for visitor to enjoy the underwater ecosystem. According to the actual data of domestic visitors has increased up to 23% between 2022 until 2018. Furthermore, the visitor’s convenience and safety must be a priority for the ship. The present study focuses on the fatigue life of steel on a semi-submersible ship using finite element analysis (FEA) and Paris Law approach. The analysis was carried out on the side hull of semi-submersible tourism ship with the stiffener and modelled with the solid element model 3CD8R. The boundary conditions of this model are fix on both sides to approach the actual condition. The results of the fatigue life that a semi-submersible tourism ship up to 9.53 years.

Fatigue damage development in 10CrMo9-10 steel for power plant pipes in as-received state and after 280,000 h of exploitation

This paper discusses an effect of 280,000 h of exploitation under internal pressure of 2.9 MPa and high temperature of 540 °C on the mechanical properties of 10CrMo9-10 (10H2M) power engineering steel. The mechanical response of the specimens obtained from two pipes: a new in the as-received state and exploited for a long period was assessed through the uniaxial tensile tests and subsequent fatigue tests. The long-term, high-temperature exposure of 10H2M steel resulted in the deterioration of its mechanical properties and was quantitatively described as a function of the fatigue damage measure, φ, and the fatigue damage parameter D. Finally, the proposed methodology of power exponent approximation with both damage sensitive parameters (φ, D) enabled for successful determination of the 10H2M steel fatigue life.

Effect of Impurities Spacing on Fatigue Strength Coefficient

The influence of impurities present in steel on fatigue strength has been the subject of research conducted for many years. Despite a large number of studies, there is no unambiguous explanation of the influence of impurities on the fatigue life of steel. The interpretation of the results becomes more complicated as the ductility of the steel increases. For this reason, most of the research concerns low-ductility hard steels. In addition, the studies presented in the literature mostly concern laboratory conditions, which the authors of the papers have tried (with varying degrees of success) to adapt to industrial research. There are a few studies on the influence of impurities in steel on the fatigue resistance factor. The coefficient k is the result of the fatigue strength zg divided by the hardness of the steel. With its help, it is possible to determine the fatigue strength depending on the hardness of the steel. In the presented work, an attempt was made to determine the impact of impurities of different sizes and located at different distances from each other on the fatigue strength coefficient. The analysis was carried out at seven heats made in industrial conditions. Melting was carried out in electric furnaces with a capacity of 140 tons. Steel from all melts was subjected to desulfurization. Samples with a diameter of 18 mm were taken. The samples were hardened from the austenitizing temperature of 880 °C. To diversify the microstructure and mechanical properties, the steel was tempered at temperatures from 200 to 600 °C. After heat treatment, the samples were subjected to rotational bending. Based on the tests, it was found that the fatigue strength coefficient k depends on the size of the impurities and the distance between the inclusions. A difference in the specific k-factors was noted depending on the microstructure of the steel.

Hydrogen Embrittlement and its Effect on Fatigue Life of Nitrided Steel in Gigacycle Fatigue: Analysis and Modelling

This paper deals with the effect of hydrogen embrittlement on the fatigue life of steels. This phenomenon is typical for the gigacycle fatigue region, but we also encounter it in surface-treated materials. in which a subsurface crack initiation has occurred. In samples in which the fracture is affected by the presence of hydrogen embrittlement, subsurface fatigue crack initiation occurs. In this work we observe the change in toughness and local elastic modulus depending on the hydrogen diffusion of hydrogen over time.

Influence of deep rolling on surface layer condition and fatigue life of steel welded joints

Deep rolling is an established mechanical surface treatment method to modify surface and subsurface properties and increase the wear- and fatigue strength of metallic parts and components. However, until now, deep rolling has not been considered for use as a post treatment method of welded joints to extend fatigue life of steel constructions. In this study, butt joints made of construction steels S355J2+N and S355G10+M were treated by means of hydrostatic deep rolling with tool diameter. The surface and subsurface properties after treatment were quantified by roughness-, hardness and residual stress measurements. Reduced roughness of 50–80%, increase of surface near hardness of up to 45% and compressive residual stresses of up to -1,000MPa were determined after deep rolling. Furthermore, the stress concentration factor of the weld toe was evaluated for each specimen based on surface scans. Due to the comparable low contact force of the deep rolling tool-process parameter combinations investigated, the stress concentration of the weld toe was mostly unaffected by deep rolling. However, a significant fatigue life improvement by deep rolling was determined for all test series, showing a maximum increase of FAT-class of approximately 60%. A tendency is shown that a higher tool diameter leads to higher fatigue life improvement.

Strain-ageing effects on the residual low-cycle fatigue life of low-carbon steel reinforcement

Low-cycle fatigue (LCF) failures can be expected when rebars are subjected to a small number of cycles at large stress/strain amplitudes. Major seismic events are often preceded or followed by other events of smaller/larger magnitude. During each event, the residual fatigue life of the steel reinforcement may reduce, eventually leading to failure of the bars due to the cumulative damage. Strain ageing may reduce even more the residual fatigue life of steel. This paper presents the results of an experimental testing campaign conducted on samples fabricated from New Zealand Grade 300E steel. A benchmark LCF life was obtained for 12-mm steel reinforcing bars subjected to axial-strain-controlled completely reversed cyclic tests at amplitudes ranging between 0.78 and 2.75%. The reduction in fatigue life attributed to strain ageing was obtained by comparing the benchmark fatigue life with that of steel reinforcing bars precycled up to 33% and 66% of the benchmark life and artificially strain aged for an equivalent period of 1 year at 15 °C. The Coffin-Manson and Koh-Stephens models were employed to evaluate the experimental results. The study demonstrates that, depending on the strain amplitude, strain ageing can cause a reduction in total and residual fatigue life ranging from 20 to 70%. Strain ageing should not be ignored during the assessment of earthquake-damaged steel reinforcing bars.

Construction of probabilistic model on interior crack initiation and growth behaviors for high strength steels in very high cycle fatigue

In very high cycle fatigue for high strength steels, one can usually observe the fish-eye on the fracture surface and an inclusion is observed at the center of the fish-eye. The fine granular area (FGA) is also found in the vicinity around the inclusion at the crack initiation site on the fracture surface. If the stress cycles required to form the FGA occupies the predominant part of the final fatigue life of these steels, the FGA formation mechanism is one of the most important subjects in the very high cycle fatigue. From this point of view, the authors have attempted to construct the FGA formation model based on the current experimental results obtained by the authors. Supposing four different probabilistic models, the FGA formation is interpreted as intermittent micro-debondings and their coalescence during the long sequence of the cyclic loadings. After the FGA was accomplished, the penny-shape crack tends to grow steadily following the well-known Paris’ law. Based on this analysis of the FGA formation, it was found that polygonization around the inclusion is more important behavior to govern the total fatigue life of the high strength steels.

Ultra Low-Cycle Fatigue Behavior Comparison between Additively Manufactured and Rolled 17-4 PH (AISI 630) Stainless Steels

This study investigates the mechanical behavior of additively manufactured (AM) 17-4 PH (AISI 630) stainless steels and compares their behavior to traditionally produced wrought counterparts. The goal of this study is to understand the key parameters influencing AM 17-4 PH steel fatigue life under ULCF conditions and to develop simple predictive models for fatigue-life estimation in AM 17-4 steel components. In this study, both AM and traditionally produced (wrought) material samples are fatigue tested under fully reversed (R = −1) strain controlled (2–4% strain) loading and characterized using micro-hardness, x-ray diffraction, and fractography methods. Results indicate decreased fatigue life for AM specimens as compared to wrought 17-4 PH specimens due to fabrication porosity and un-melted particle defect regions which provide a mechanism for internal fracture initiation. Heat treatment processes performed in this work, to both the AM and wrought specimens, had no observable effect on ULCF behavior. Result comparisons with an existing fatigue prediction model (the Coffin–Manson universal slopes equation) demonstrated consistent over-prediction of fatigue life at applied strain amplitudes greater than 3%, likely due to inherent AM fabrication defects. An alternative empirical ULCF capacity equation is proposed herein to aid future fatigue estimations in AM 17-4 PH stainless steel components.

The relationship between inclusions characteristic parameters and bending fatigue performance of 20Cr2Ni4A gear steel

Common heavy-duty 20Cr2Ni4A gear steel is widely used in industrial systems, but there are still problems with its use, such as materials processing issues and a short fatigue life during use. These drawbacks prevent 20Cr2Ni4A from meeting the quality requirements for heavy-duty gear steel in modern industrial systems. Therefore, in practical engineering application, the fatigue life of steel will be predicted to ensure that the fatigue failure of gear steel will not occur in the normal service process. By analysis of the bending fatigue process of structural steel, researchers found that almost every failure is closely related to the size, location and mechanical state of inclusions in the steel. Therefore, in order to accurately characterize the impact of inclusions on fatigue performance, we first used ASPEX inclusion field detection instrument and nanoindentation analyser to make statistics of all inclusions in the two steels, and the contents, distribution, average size and mechanical properties of inclusions in 20Cr2Ni4A steel were accurately counted. Subsequently, the bending fatigue life of two differently processed steels was tested, and a P-S-N curve was drawn. Then, the fatigue fracture was analysed and four fatigue fracture models were summarized to determine the influence of inclusions in the fatigue process. Based on the models by Nisha S, the effects of the number, area, and types of the inclusions were added as corrections, and a new set of life prediction models was proposed. The model was verified by experiments, and the results have a high accuracy. Thus, this approach can be applied to the prediction of the fatigue life of 20Cr2Ni4A steel.

Effect of microalloying on the stability of the endurance characteristics of hot-deformed powder steels

The stability of the characteristics of the contact and low-cycle fatigue life of steels obtained from iron powders with different contents of alloying and microalloying elements, as well as impurities, has been studied. The carbon content in powder steels was varied. It has been established that doping Na or Ca microadditives contributes to an increase in the stability and absolute value of the parameters of the contact and low-cycle fatigue life of hot-deformed powder steels due to the activation of cohesive interaction on interparticle surfaces and carbon diffusion, which contributes to the formation of a homogeneous structure. Doping microadditives of Al contributes to an increase in the contact and low-cycle fatigue life of the powder steels in comparison with the samples-witnesses. However, the formation of zones of structural heterogeneity in steels containing more than 0.4 wt. % C, causes a decrease in the stability of these characteristics.

Effects of the Ultrasonic Assisted Surface Rolling Process on the Fatigue Crack Initiation Position Distribution and Fatigue Life of 51CrV4 Spring Steel.

In this paper, the effects of the fatigue crack initiation position (FCIP) on fatigue life are discussed. Different modified gradient fields (MGFs) are prepared on the surface of 51CrV4 spring steel components by an ultrasonic assisted surface rolling process (USRP). Subsequently, the fatigue behaviour of steels with different FCIPs is systematically studied. The results show that the fatigue life of steels first exhibits an increasing tendency and then a decreasing tendency with increasing distance between an FCIP and the surface. When an FCIP shifts from the surface of the sample to the interior, the fatigue crack initiation resistance on the interior is greater than that on the surface, which leads to an increase in fatigue life. However, when the FCIP further shifts towards the centre of the specimen, the stress triaxiality experienced by the fatigue source gradually increases, which results in a peak in the curve of FCIP versus fatigue life. The magnitude of this peak fatigue life is related to the change in the stress triaxiality. Moreover, according to focused ion beam-high-resolution transmission electron microscopy (FIB-HRTEM) microstructural analysis near FCIPs, under a higher stress triaxiality, the crack tip area is subject to greater stress constraints, making the multiplication and movement of dislocations in this area more difficult, resulting in the decrease in movable dislocation density. This decrease in dislocation density leads to an increase in the stress concentration and accelerates the crack growth rate, decreasing the fatigue life. Therefore, the significant change in fatigue life is controlled by the MGF and stress triaxiality.

Increasing Fatigue Life of 09Mn2Si Steel by Helical Rolling: Theoretical-Experimental Study on Governing Role of Grain Boundaries.

The structure and mechanical properties of the 09Mn2Si high-strength low-alloyed steel after the five-stage helical rolling (HR) were studied. It was revealed that the fine-grained structure had been formed in the surface layer ≈ 1 mm deep as a result of severe plastic strains. In the lower layers, the “lamellar” structure had been formed, which consisted of thin elongated ferrite grains oriented in the HR direction. It was shown that the five-stage HR resulted in the increase in the steel fatigue life by more than 3.5 times under cyclic tension. The highest values of the number of cycles before failure were obtained for the samples cut from the bar core. It was demonstrated that the degree of the elastic energy dissipation in the steel samples under loading directly depended on the area of the grain boundaries as well as on the grain shapes. The fine-grained structure possessed the maximum value of the average torsional energy among all the studied samples, which caused the local material structure transformation and the decrease in the elastic energy level. This improved the crack resistance under the cyclic mechanical loading. The effect of the accumulation of the rotational strain modes at the grain boundaries was discovered, which caused the local structure transformation at the boundary zones. In the fine-grained structure, the formation of grain conglomerates was observed, which increased the values of the specific modulus of the moment of force. This could be mutually compensated due to the small sizes of grains. At the same time, the coarse-grained structures were characterized by the presence of the small number of grains with a high level of the moments of forces at their boundaries. They could result in trans-crystalline cracking.

Uniaxial Ratcheting Behavior of a Weather-Resistant Rail Steel: Effect of Mean Stress and Stress Amplitude

Uniaxial ratcheting behavior of a weather-resistant rail steel belonging to 60 kg/m 90-UTS variety was studied in true stress control mode with varying mean stress (σm) and stress amplitude (σa) at constant σa and σm, respectively. In all the loading conditions, the ratcheting strain (er) increased almost monotonically and accumulation of ratcheting strain was found to increase at higher magnitude of σm or σa, perhaps due to the increase in remnant dislocation density in the substructure of the investigated steel. After the initial sharp fall in ratcheting strain rate with elapsed cycles in all the cases, a constant ratcheting rate was attained very soon; thereafter, the ratcheting strain kept on accumulating in the specimen at this constant rate till the failure of the specimen occurred. It was quite interesting to see that on increasing the mean stress level at a fixed stress amplitude, after the initial decrease, fatigue life of the steel improved marginally owing to the effect of mean stress-induced hardening. In absolute contrast, under the influence of stress amplitude at a fixed mean stress, fatigue life of the rail steel deteriorated gradually but to a significant extent. Finally, stress amplitude was found to have more severe effect than that offered by the mean stress in terms of` ratcheting strain accumulation and fatigue life of the investigated rail steel.

Prediction of Fatigue Life of Steels in Consideration of Defect-induced Crack Initiation and Propagation

Prediction of Fatigue Life of Steels in Consideration of Defect-induced Crack Initiation and Propagation

Effect of Surface Condition on the Torsional Fatigue Behaviour of 20MnCr5 Steel

Torsional fatigue behaviour of steel is influenced by various significant factors on the surface of steel such as the stress state, the metallurgical conditions and the surface roughness etc. This research presents the results of the studies conducted on the torsional fatigue life of steel for different surface stresses, conditions and microstructures. Based on the results, inferences were derived out of the factors and conditions which are instrumental in increasing the fatigue performance. This research is done with the purpose of increasing the fatigue performance of power train shafts processed through vacuum carburizing followed by second tempering. The fatigue tests were conducted using a bi-directional torsional cyclic loading in a MTS make torque testing machine. Various researches has been conducted in this paper to find out the impact of surface roughness, second tempering, inter granular oxidation and carbon case depth on the transmission shafts. The existence of surface retained austenite has not yielded a notable enhancement in fatigue performance in the power train shaft whereas existence of martensite with minimum fraction of retained austenite enhanced the fatigue performance significantly from 12,000 to 35,000 cycles on ± 3100 N m torque load.

Relationship between S/N Ratios of Joint Stiffness and Fatigue Life of Steel Welded Joints

Relationship between S/N Ratios of Joint Stiffness and Fatigue Life of Steel Welded Joints