Fatigue Strength Of High-strength Steel Research Articles

A new model for predicting the gigacycle fatigue strength of high-strength steels

Three models for predicting the gigacycle fatigue strength of high-strength steels were investigated in this study: the Tanaka-Akiniwa model, the Murakami equation and a newly-formulated model. The Tanaka-Akiniwa model proved to have the drawback of overestimating the inclusion size effects. In the Murakami equation, the calculated ODA growth curve was imaginary. Our results demonstrated that the new model generated the most accurate predictions. Predictions were then derived, with each constant calculated by fitting the experimental data to each steel grade. The derived predictions were therefore not universal but different for each steel grade. The accuracy of the derived predictions varied according to the grade of steel. The worst accuracy was for JIS-S40C, the second worst was for JIS-SUJ2 and the third worst was for JIS-SCM440 under R = –1. Other than these three predictions, though, the accuracy was excellent. During the fitting of the experimental data, the existence of new fatigue limits was suggested in the gigacycle regions: these were revealed by fatigue testing up to 1011 cycles. The derived predictions indicated only minor differences between steel grades for gigacycle fatigue strength. With increasing inclusion size, the drop in gigacycle fatigue strength became less steep, and looked at in terms of the mean stress effect, a modified Goodman line could safely be used for high-strength steel.

Fatigue strength of high-strength steel after shipyard production process of plasma cutting, grinding, and sandblasting

This paper investigates experimentally the fatigue strength of high-strength steel, which has undergone the normal shipyard production process of plasma cutting, grinding, and sandblasting. The study includes steels with the yield strength of 355 and 690 MPa. The tested specimens are of dog-bone shape and represent the large-scale situation of a cruise ship balcony opening corner, loaded in shear or tension. The influence of surface roughness, internal inclusions, hardness, and residual stress on the fatigue strength are studied and discussed. Compared to the design curve as well as to the untreated surfaces, the results show significantly improved fatigue strength under constant amplitude loading at a load ratio of R = 0.1. However, very flat or even rising slope of the S-N curve indicates variations in the material and surface quality as well as in the residual stress. Surprisingly, internal defects even up to 100 μm in size did not decrease the fatigue strength.

Effects of microstructure and surface roughness on the fatigue strength of high-strength steels

Rotating bending fatigue (RBF) tests were conducted on a high-carbon steel 100CrMnMoSi8 (martensite and bainite), and a tough-tempered medium-carbon steel 50CrMo4. The specimens were surface-finished to different conditions: polished surface and ground surface with a range of roughness levels. The experimental results indicate that the hardened high-carbon steel specimens with rough surface failed predominantly by surface crack initiation, whereas the fatigue fracture of the specimens with smoother surface tend to fail by subsurface crack initiation from non-metallic inclusions. Moreover, the fatigue strength of the martensitic specimen is lower than that of the bainitic specimen in the low stress-cycle range in which failure is dominated by surface initiated fatigue fracture, whereas this difference diminishes in the high-cycle fatigue regime where subsurface initiated fatigue prevails. The non-hardened medium-carbon steel samples, however, fail only by surface crack initiation, and the fatigue strength is much less sensitive to surface roughness. A unified model is developed to predict the fatigue strength of the tested samples with different microstructures and surface finish.

Influence of surface integrity on the fatigue strength of high-strength steels

This paper investigates experimentally the influence of surface integrity on the fatigue strength of high-strength steel used in large structures. The investigation utilises large-scale specimens of the balcony openings of a cruise ship. The test specimens, which have a dog-bone shape and yield strength of 355MPa, 460MPa, or 690MPa, are cut by plasma. After the cutting, the specimens are treated by grinding or by grinding followed by sandblasting, i.e. using post-cutting treatments that are suitable for shipyard conditions. The resulting surface roughness, hardness profile, and residual stress are measured. Fatigue tests with a load ratio of R=0.1 are carried out until the final failure of the specimens. The investigation shows that post-cutting treatments suitable for shipyard conditions can considerably increase the fatigue strength of high-strength steel used in opening corners of a large-scale structure. Sandblasting after grinding increases the surface roughness, but reduces the fatigue strength only slightly.

Effects of inclusion size and stress ratio on fatigue strength for high-strength steels with fish-eye mode failure

Experimental results indicate that the fatigue life reduces by about two orders of magnitude when inclusion size doubles. Then, a model is proposed for predicting the fatigue strength of high-strength steels with fish-eye mode failure based on the experimental results for the effect of inclusion size and stress ratio. In the model, the effect of inclusion size a0 and stress ratio R on fatigue strength σa is expressed as σa∝a0m[(1−R)/2]α, where m and α are material parameters. The predicted results are in good agreement with our experimental results and the ones reported in literature.

INFLUENCE OF DEPOSITION CONDITIONS ON FATIGUE PROPERTIES OF MARTENSITIC STAINLESS STEEL WITH TIN FILM COATED BY ARC ION PLATING METHOD

The surface properties like roughness etc. strongly influence the fatigue strength of high-tensile steel. To investigate the effect of surface condition and TiN coating on the fatigue strength of high-strength steel, four-point bending fatigue tests were carried out for martensitic stainless steel with TiN film coated using arc ion plating (AIP) method. This study, using samples that had been polished under several size of grind particle, examines the influence of pre-coating treatment on fatigue properties. A 2-µm-thick TiN film was deposited onto the substrate under three kinds of polishing condition. The difference of the hardness originated in the residual stress or thin deformation layer where the difference of the size of grinding particle of the surface polishing. And it leads the transformation of the interface of the substrate and the TiN film and improves fatigue limit.

Effect of Hydrogen on Fatigue Strength of High‐Strength Steels in the VHCF Regime

Diffusible hydrogen or non-diffusible hydrogen can decrease the fatigue strength of high-strength steels. The hydrogen influence factor f(C) describes the hydrogen damage level of fatigue strength. Fatigue strength decreases with increasing non-diffusible hydrogen content in the range 1 ppm < Ci < 3.0 ppm. Fatigue strength decreases significantly with increasing diffusible hydrogen content in the range 1 ppm < Cr < 3.0 ppm, but shows almost no obvious change in the range 3.0 ppm < Cr < 10.0 ppm.

EFFECT OF BIAS VOLTAGE ON THE FATIGUE LIFE OF MARTENSITIC STAINLESS STEEL WITH TiN FILM COATED USING ARC ION PLATING METHOD

To investigate the effect of TiN coating on the fatigue strength of high-strength steel, four-point bending fatigue tests were carried out for martensitic stainless steel with TiN film coated using arc ion plating (AIP) method. A 2-μm-thick TiN film was deposited onto the substrate surface under bias voltage of four kinds: VB = 0, -60, -160 and -260 V. For VB = 0, -160 V and -260 V, the fatigue limit increased. The highest fatigue limit of σ max = 900 MPa was obtained for VB = -160 V. But some samples for VB = -260 V showed the decrease of fatigue limit due to film delamination during the fatigue test. For VB = -60 V, the fatigue limit was unchanged by coating. As a result of a coating property analysis, the following conclusions were obtained. Fatigue crack propagation was almost independent of the bias voltage. Fatigue crack initiated from the subsurface in the substrate and the crack initiation behavior depended on the film property of the adhesion, residual stress, elastic modulus, and the film's hardness depended on the bias voltage especially for low fatigue stress level.

Dependence of fatigue strength on inclusion size for high-strength steels in very high cycle fatigue regime

The fatigue strengths of eighteen kinds of high-strength steel specimens with different inclusion sizes, ultimate tensile strength ranging from 1680 MPa to 2180 MPa, were investigated by using 20 kHz ultrasonic fatigue testing machine. The relation between fatigue strength and inclusion size was examined according to the experimental results. Based on the effect of hydrogen during formation of granular-bright-facet (GBF), a model to predict the fatigue strength of high-strength steels in very high cycle fatigue (VHCF) regime was developed. The predictions are in good agreement with the experimental results.

The maximum size of the planar sections of random spheres and its application to metallurgy

A theorem of this paper proves that if the size distribution of random spheres is generalized gamma, its Wicksell transform and other related distributions belong to the domain of attraction of the Gumbel distribution. The theorem also shows the attraction coefficients of the distributions. The fatigue strength of high-strength steel is closely related to the maximum size of nonmetallic inclusions in the region of maximum stress of the steel. Murakami and others developed a method, making use of the Gumbel QQ-plot, for predicting the maximum size from the size distribution of inclusion circles in microscopic view-fields. Based on the Gumbel approximation of the maximum of wicksell transforms, a modified and extended version of Murakami's method is justified, and its performance is evaluated by simulation.

An experimental and theoretical study of the fatigue properties of hot- dip-galvanized high-strength sheet steel

The mechanisms behind the reduction of the fatigue strength of high-strength steel owing to hot-dip galvanizing have been studied. Three coating thicknesses were considered, ranging from 80 to 220 μm. The steel has a proof stress of 680 MPa. Fatigue loading was performed at an amplitude below the fatigue limit of the uncoated steel. The coated steels gave lives of between 5×10 4 and 8×10 4 cycles. The thickest coating gave the shortest life. Metallographic observation of material not exposed to fatigue loading showed there was a crack network in the δ- and ζ- layers of the coatings. Some initial cracks penetrated through half or more of the coating depending on the coating thickness. Short cracks were found in the steel at the interface to the coating. The crack pattern after fatigue loading followed the original pattern in the coating. Additional cracks were created in the thickest coating. A higher frequency of cracks inside the steel was observed for the steels with thicker coatings. These cracks started at cracks in the coating. Finite-element calculations for the driving forces of the original cracks showed that the material with the thinnest coating was fatigue loaded close to the threshold for crack propagation. The two materials with thicker coatings had driving forces for their original cracks that exceeded the threshold. These results were in good agreement with the lives observed experimentally.

Quantitative evaluation of effects of shape and size of artificially introduced alumina particles on the fatigue strength of 1.5NiCrMo (En24) steel

In order to investigate the effects of non-metallic inclusion on the fatigue strength of high-strength steels, in 1963 W.E. Duckworth and E. Ineson conducted fatigue tests using specimens that contained artificially added spherical and angular alumina particles of various controlled sizes. Although the fatigue tests were carried out under the same nominal stresses in rotating-bending and tension-compression tests the fatigue lives of specimens showed a large scatter. They reported in some detail typical complicated aspects of the effects of non-metallic inclusions on the fatigue strength. In the present study the authors have reanalysed these complicated fatigue data using the prediction equation that was proposed by Murakami et al for the quantitative evaluation of the effects of small defects on fatigue strength. The geometrical parameter that controls the scatter of the fatigue strength is the square root of the projection area √ area and not the shape of the inclusions, whether they are spherical or angular. It is shown from the data from the failed specimens that the fatigue strength of materials containing inclusions larger than a critical size can be predicted by the Vickers hardness (Hv) of the matrix and √ area of the inclusion regardless of the shape.

Effect of mean stress on the fatigue strength of high-strength steels containing small defects or nonmetallic inclusions.

In order to investigate the effects of small defects and nonmetallic inclusions on the fatigue strength of high-strength steels under tensile or compressive mean stress, fatigue tests for a maraging steel with the static strength level of 2500MPa (Hv=740), were carried out. The experimental data on a 0.13 % C steel (Hv=105) as well as the maraging steel are analyzed from the viewpoint of the previous study in which the prediction equation for the fatigue strength under zero mean stress (R=-1) was proposed on the basis of the Vickers hardness Hv and the square root of the projection area (√(area)) of a defect or nonmetallic inclusion. The new equation is proposed to predict the fatigue strength under tensile or compressive mean stress. The proposed equation gives not only accurate prediction of the fatigue strength of a high-speed cutting steel SKH-51 (Hv=615, 654) which fractured from nonmetallic inclusions, but also can be used to predict the lower limit of the wide scatter of the fatigue strength of many specimens.

A numerical simulation of evaluating the maximum size of inclusions to examine the validity of the metallographic determination of the maximum size of inclusions.

Nonmetallic inclusions are the cause of a marked reduction and a large scatter of fatigue strength of high-strength steels. The sizes and positions of inclusions influence the fatigue strength. Murakami et al.'s equation has successfully shown the correlation betwween the fatigue strength and the size of the inclusion at the fracture origin, and has enabled us to predict the fatigue strength considering the inclusion size. The maximum size of inclusions contained in a specimen or a machine component must be determined in order to predict the reduction of the fatigue strength. The statistics of the extreme values for the determination of the maximum size have been applied to the experimental microscopic inspection data, though those data give essentially two-dimensional information, because the metallographic inspections were conducted only on a section of a specimen. In order to examine the validity of two-dimensional metallographic inspection data of inclusions, the numerical simulations considering the spatial and size distributions of inclusions are carried out. The prediction errors resulting from conventional metallographic inspections are estimated.

Prediction of fatigue strength of high-strength steels based on statistical evaluation of inclusion size.

The reasons for relatively low fatigue strength and large scatter of fatigue strength were quantitatively elucidated considering the combined effects of hardness and nonmetallic inclusions on fatigue strength. Nonmetallic inclusions are equivalent to small defects or cracks from the viewpoint of fracture mechanics. The effects of nonmetallic inclusions on fatigue strength depend on their size, shape and location where they exist, and this becomes the cause of large scatter of the fatigue limit. However, it is possible to predict the upper (σwu) and lower limits (σwl) of the fatigue strength from the hardness (Hv) of a matrix and the maximum size of inclusions (√(area)max) defined by the square root of the projected area of an inclusion. The expected value of √(area)max in a definite number of specimens or machine components can be estimated by statistics of extreme values.

The effect of surface micropits upon the fatigue strength of high-strength steel.

A study has been made of the influence of artificial semispherical surface flaws with diameters of 25 to 175 μm upon the fatigue limit of high-strength steel. This has been done to better understand the origin of the harmful effect of hard inclusions on the fatigue resistance of high-strength steel in which hard inclusions are poorly bonded to the matrix microstructure. An attempt has been made to estimate the true fatigue limit of the ideal inclusion-free microstructure of the steel, and the effect of inclusions on the fatigue strength of the steel has also been discussed quantitatively on the basis of the results of such an estimation of the true fatigue limit.

Fatigue strength of high-strength structural steels

1. The fatigue strength of high-strength steels 30KhN2MFA and 35KhN2MFA-Sh after quenching and tempering was studied in symmetrical cyclic bending with and without a stress raiser. 2. The fatigue strength of the steels was evaluated according to the appearance of a crack of standard size and in accordance with fatigue fracture. 3. The presence of a near-functional correlation between cyclic stress and the logarithm of fatigue strength was established for steels 30KhN2MFA and 35KhN2MFA-Sh. 4. It was found that the fatigue curves of these steels contain discontinuities defining characteristic stress levels and fatigue strength. 5. Estimates of total lifetime, fracture toughness, and residual life with a crack were obtained for these steels within the fatigue-strength region.

Effect of stress concentrations on the fatigue strength of high-strength steels and heat-resistant alloys at elevated temperatures

Effect of stress concentrations on the fatigue strength of high-strength steels and heat-resistant alloys at elevated temperatures

Effect of anodic and cathodic processes on the low-cycle fatigue strength of high-strength steel subjected to a corrosive solution

Effect of anodic and cathodic processes on the low-cycle fatigue strength of high-strength steel subjected to a corrosive solution