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1.
Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10 6 cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size. 相似文献
2.
High cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of two 18Ni maraging steels with different cobalt and titanium content and similar static strength are investigated. Ultrasonic fatigue tests are performed with thin sheets with nitrided surfaces at load ratio R = 0.1. The specimens are mounted on a carrier and are forced to joint vibrations at approximately 20 kHz. The increase of Co content and the elimination of Ti improved the HCF and VHCF strength of 18Ni maraging steel. TiN inclusions if Ti is present and Al 2O 3 inclusions in the Ti free material with sizes (area INC) 1/2 smaller than 10 µm were preferential crack initiation locations. Considering inclusions as initial cracks, the minimum stress intensity range for VHCF failure is 1.2 MPam 1/2 for TiN inclusions and 1.8 MPam 1/2 for Al 2O 3 inclusions. Data scatter may be slightly reduced if lifetimes are presented versus stress amplitudes multiplied by (area INC) 1/12 rather than in an S–N diagram. 相似文献
3.
Thin sheets of nitrided 18Ni maraging steel are tested under cyclic tension (load ratio R = 0.1) in the very high cycle fatigue (VHCF) regime. The ultrasonic fatigue testing method with a cycling frequency of about 20 kHz has been further developed for these experiments. Sheet specimens with 0.35 mm thickness are mounted on a carrier specimen, they are pre-stressed and are forced to vibrate jointly. Between 10 7 and 10 9 cycles, fatigue cracks are initiated exclusively at internal TiN inclusions. The areas of the crack initiating inclusions projected perpendicular to the applied tensile stress are evaluated. The square root of inclusion areas, (area INC) 1/2 lies between 2.5 μm and 5.3 μm. Considering inclusions as cracks, their stress intensity range is between Δ KINC = 1.3 MPa m 1/2 and 2.4 MPa m 1/2. The sizes of crack initiating inclusions influence fatigue lifetimes. This is considered in a crack propagation model and by presenting lifetimes versus the stress amplitudes multiplied by (area INC) 1/12. A mean lifetime of 10 9 cycles is found at a stress amplitude of 22% of the tensile strength, which is comparable to other high strength steels tested under cyclic tension. 相似文献
4.
Fatigue properties of some steels are presented with the aim of highlighting both the need, and areas, for future work on these materials. Possible explanations for the lack of acceptance, by design engineers, of cast steels are given. Specific areas in which further research is required are indicated - the biggest problem is predicted as being the characterization and mathematical modelling of real defects. Discontinuities worthy of investigation are listed. 相似文献
5.
Fatigue tests up to 10 8 cycles were carried out for two spring steels (Heats A and D1) and one valve spring steel (Heat F) with tensile strength, σ B, of 1720, 1725 and 1764 MPa, respectively. The size and composition of inclusions in Heats Dl and F were controlled. The surface‐type fracture occurred at shorter lives below 10 6 cycles, while the fish‐eye‐type fracture occurred at longer lives. The fatigue limit, σ W, at 10 8 cycles was 640 MPa for Heats A and D1 and 700 MPa for Heat F. Al 2O 3 inclusions for Heat A and both TiN inclusions and matrix cracks, i.e. internal facets, for Heat F were observed at the fish‐eye‐type fracture sites, while only matrix cracks were observed for Heat Dl. ODA, i.e. optically dark area, which is considered to be related to hydrogen effects, were formed around Al 2O 3 and TiN inclusions. Fatigue tests were also conducted after specimens were heated up to 573 K in high vacuum of 2 × 10 –6 Pa. The heat treatment eliminated matrix cracks for Heat D1 and the fatigue limit at 10 8 cycles recovered to the estimated value of 920 MPa from the equation σ w= 0.53 σ B for the surface fracture. These results suggest that inclusions control and hydrogen influence the gigacycle fatigue properties for these high strength steels. In addition, it is expected that the creation of a martensite structure with a high resistance to hydrogen effects in the inclusion‐controlled steel could achieve the higher fatigue limit estimated for the surface‐type fracture. 相似文献
6.
为解决我国某国防用高精密弹性件材料强韧性不足的问题,研究了不同尺寸的对18Ni(1750MPa级)马氏体时效钢力学性能的影响。通过循环相应细化奥氏体晶粒到不同尺寸,在相同时效处理规程下进行时效处理,机械性能测试表明,晶粒细化是提高18Ni马氏体时效钢强度和塑性伯一种有效手段。 相似文献
7.
This paper presents a research dealing with the dependence of the fatigue strength of maraging steel parts, manufactured by direct selective laser sintering, on the production build orientation. Three sets of specimens have been manufactured according to the ISO 1143 Standard (2010) by EOSINT M280 additive manufacturing machine, with the following heat and mechanical treatments, in agreement with the recommendations by the material manufacturer and current literature. The expected outcomes are the Fatigue Limit values of the material and the maximum number of cycles observed at different stress levels for three different build orientations (three different angles, 0°, 45° and 90°, between the build direction and the longitudinal axis of the samples). The results have been processed and compared by statistical methods in order to determine the fatigue curves in the finite life domain and the fatigue limits, along with their confidence bands and intervals, and to investigate the significance of the build orientation factor. 相似文献
8.
通过滚动接触疲劳试验方法,研究了两种渗碳齿轮钢的接触疲劳性能.结果发现,渗碳齿轮钢接触疲劳试样失效方式主要为渗碳层的点蚀和剥落.氧含量较低的Nb微合金化齿轮钢(含0.04%的Nb)中氧化物夹杂数量少、尺寸小,接触疲劳裂纹起裂较难;同时,Nb微合金化齿轮钢渗碳层晶粒尺寸小、硬度高,提高了疲劳裂纹萌生及扩展阻力,导致Nb微合金化后,齿轮钢的接触疲劳寿命大幅度提高. 相似文献
9.
This study investigates the fatigue behaviour of additively manufactured 18Ni300 maraging steel. Specifically, the surface and material parameters impacting fatigue performance are analysed through various post‐treatment combinations. Vertically built miniaturised test samples produced by laser powder bed fusion are tested in as‐built and age‐hardening heat‐treated conditions. To utilise the potential of using additive manufacturing for complex‐shaped parts in which conventional machining tools could have limited access, vibratory finishing and sand blasting are employed. The fatigue results show that in as‐built microstructural condition, both the surface treatments significantly enhanced the fatigue performance, with vibratory finishing outperforming sand blasting owing to better surface finish. After heat treatment, sand‐blasted samples performed better than vibratory‐finished ones because of higher residual stresses. This competing interaction between post‐treatments sheds light on identifying the relative influence of various factors. With systematic postfracture and microstructural analyses highlighting the fatigue influencing factors, recommendations are drawn to select post‐treatments to achieve the desired fatigue performance. 相似文献
10.
MDN-250 maraging steel was electroslag-refined in a 350 kVA DC ESR unit using a slag of composition, CaF 2-65, CaO-9, Al 2O 3-2O, TiO 2-5, and MoO 3-1 (wt % basis) in straight as well as reverse polarity modes of melting. The chemical homogeneity of the ingots was established
by X-ray fluorescence, plasma scan and spectrovac methods. The dissolved gas content (O and N) in the ingots was determined
by a LECO gas analyser. Volume fraction, density and size distribution of inclusions in the ingots and the electrode were
determined in a Quantimet image analyser. These results were supplemented by EPMA and SEM studies on selected inclusions.
Mechanical properties of the ingots in the heat-treated and aged conditions were assessed and compared with those of the electrode
material.
The present results confirm that the reverse polarity mode of melting is best suited for producing quality maraging steels
for the ingots produced by this mode of melting were found to possess superior metallurgical characteristics in terms of cleanliness
and mechanical properties compared to those produced by the straight polarity ESR route and other conventional methods. 相似文献
11.
In this paper, two special aluminium cruciform specimens are designed and tested in an ultrasonic fatigue machine. They were designed based on Single‐Input‐Multiple‐Output (SIMO) modal analysis to induce in‐plane biaxial stress combinations (in‐phase tension‐tension [T‐T] and out‐of‐phase compression‐tension [C‐T]) when at resonance at 20 kHz. The geometries were subjected to both numerical analysis and experimental testing to understand if they can indeed create the intended biaxial state of stresses. Both numerical and experimental results showed an impact of nearby resonant modes of noninterest on the correct functioning of the specimens, especially regarding the T‐T specimen where a large deviation from the mode of interest was measured. This means that future work includes re‐designing T‐T specimens taking into account these mode shapes. Only out‐of‐phase specimens demonstrated to work properly and tests until failure were conducted. The first failure results showed to be consistent with literature when out‐of‐phase biaxial stress is applied cyclically. 相似文献
12.
This paper discusses the monitoring of the condition of the OREX ore export line rails in South Africa. Ultrasonic testing is widely used to monitor defect development and growth as well as to plan and manage programs to ensure a high level of rail integrity. An overview of the performance and history of the line is given in terms of rail defect levels and derailments due to rail failures in relation to traffic volumes. Some of the inherent characteristics of the rails are discussed and related to the defect development. From the available data it is clear that the rails are degrading in a ‘wear‐out mode’, with increasing defect rates. The use of high technology ultrasonic testing equipment has enhanced the level of knowledge of the condition of the rails tremendously and has led to programs that actually improved the overall condition of the line and eliminated derailments due to rail defects. 相似文献
13.
Influence of microstructural changes on the second harmonics of sinusoidal ultrasonic wave during low cycle fatigue (LCF) deformation in pearlitic steel was studied. Fatigue tests were interrupted and at every interruption, non-linear ultrasonic (NLU) parameter ( β) was determined. Microstructures of cyclically deformed specimens at various cycles were examined by transmission electron microscopy (TEM). The variation of β with fatigue cycles was correlated with the microstructural changes and the results were explained through the variation in dislocation morphology and carbon content of the steel. 相似文献
14.
Literature datasets showed that gigacycle fatigue properties of materials may be affected by the specimen risk‐volume, i.e., the part of the specimen subjected to applied stress amplitudes above a prescribed percentage of the maximum applied stress amplitude. The paper proposes a Gaussian specimen shape able to attain large risk‐volumes for gigacycle fatigue tests, together with a general procedure for its design: wave propagation equations are analytically solved in order to obtain a specimen shape characterised by a uniform stress distribution on an extended length and, as a consequence, by a larger risk‐volume. The uniformity of the stress distribution in the Gaussian specimen is numerically verified through a finite element analysis and experimentally validated by means of strain gauge measurements. 相似文献
15.
The present paper aims to investigate the effect of ultrasonic peening treatment on the very high cycle fatigue resistance of an AA7075 friction stir welded joint. Microscopy observation, microhardness and X‐ray diffraction measurements were carried out to characterize the treated surface of peened specimens. Fatigue crack initiation sites were investigated through scanning electron microscope, and the role of enhanced surface on fatigue resistance was analyzed. The results indicate that a sensible fatigue strength improvement can be obtained through application of ultrasonic peening treatment and that fatigue cracks can initiate from the interior of the specimen. To clarify the fatigue failure mechanism, we analyzed the microstructure characteristics, compressive residual stress profile and intermetallic inclusion distribution in the surface layers, and we discussed the capability of ultrasonic peening treatment to hinder the surface crack initiation. 相似文献
16.
The pressurized disc fatigue (PDF) test technique was employed to obtain fatigue lives of Type 316 stainless steel under equi-biaxial stress conditions. In the PDF test, a disc-type specimen was subjected to the cyclic bulge test. The biaxial fatigue lives were successfully obtained by the PDF tests, and they were longer than those obtained by the uni-axial and plate bending fatigue tests under the same equivalent strain range. Observations of crack initiation and growth behavior during the PDF test revealed that the relatively large size of the disc-type specimens had only a minor influence on the fatigue lives. Finite element analysis results showed the PDF test was valid for evaluating the fatigue lives under equi-biaxial conditions. It was concluded that the influence of equi-biaxial condition was not necessary to be considered in the design fatigue curve. 相似文献
17.
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. 相似文献
18.
针对一种以Al作为主要强化元素的新型马氏体时效不锈钢,通过力学性能测试、光学显微镜观察和透射电子显微分析方法,研究不同的热处理温度对实验钢力学性能和微观组织的影响。结果表明:该实验钢的抗拉强度最高可达1876MPa,屈服强度可达1762MPa,具有良好的强韧性配合。固溶处理后形成了具有高密度位错的细小板条马氏体组织,在时效过程中,马氏体基体上弥散析出的NiAl相使其强度得到大幅度的提升。随着时效温度的提高,NiAl析出相颗粒逐渐长大粗化,从而使强度在到达峰值后迅速下降,出现了过时效现象。实验钢经过820℃固溶+(-70℃)冷处理+540℃时效处理后可获得良好的综合力学性能。 相似文献
19.
Constant-amplitude in-phase and 90° out-of-phase axial-torsional fatigue tests were conducted on tubular specimens made from a medium-carbon steel with three hardness levels obtained from normalizing, quenching and tempering and induction hardening to find the effect of hardness on multiaxial fatigue behaviour. In addition, the same loadings were applied on the normalized solid specimens to investigate the effect of specimen geometry on multiaxial fatigue life. Similar fatigue life variation as a function of hardness was found for in-phase and out-of-phase loadings, with higher ductility beneficial in low-cycle fatigue (LCF) and higher strength beneficial in high-cycle fatigue (HCF). Multiaxial fatigue data were satisfactorily correlated for all hardness levels with the Fatemi–Socie parameter. Furthermore, in order to predict multiaxial fatigue life of steels in the absence of any fatigue data, the Roessle–Fatemi hardness method was used. Multiaxial fatigue lives were predicted fairly accurately using the Fatemi–Socie multiaxial model based on only the hardness level of the material. The applicability of the prediction method based on hardness was also examined for Inconel 718 and a stainless steel under a wide range of loading conditions. The great majority of the observed fatigue lives were found to be in good agreement with predicted lives. 相似文献
20.
There is an increasing necessity to record the deformation characteristics of microelements containing freestanding bond wires. The data required are either mechanical or thermal such as Young's moduli, stress–strain values, fatigue‐ and thermal‐strain data, but the nominal strength of a structure changes by scaling its size. Due to this size effect, material data cannot be taken from macrospecimens, thus special testing procedures were introduced. Laser optical sensors based on the speckle correlation technique were applied to determine non‐contacting strain values with high strain resolution. For the mechanical properties tensile tests were used. A special ultrasonic resonance fatigue system is described for testing freestanding microwires. In this study the stress–strain and fatigue response of microwires of Cu with a purity of 99.99+% with diameters between 10 and 125 μm with a typical bamboo structure have been investigated. A size dependence of the yield strength which increased with decreasing diameter was observed, while the fracture elongation showed contrary behaviour. Fatigue life also decreased with reduced diameters. An explanation is presented based on formed oxide layers, strengthening effects by dislocation pile‐ups and a pronounced localization of formed slip steps acting as notches being most dominant in the very thin microwires. 相似文献
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