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We examined the fatigue properties in very high cycle regime of large FV520B-I specimens in an ultrasonic fatigue test. The fatigue mechanism in very high cycle regime did not change and the fatigue properties obviously degraded as the specimen size enlarged. The fatigue life decreased and the S–N curve moved downward due to the increase in inclusion size in large specimens. The maximum inclusion sizes in specimens were predicted by the method of statistics of extreme value. The characteristic area sizes were obtained by experiment, including the distance from the crack origin to the specimen surface, the fish-eye diameter, the GBF diameter, and the inclusion diameter. According to these data, the relationships of characteristic area sizes with VHCF life were analyzed. The prediction of fatigue strength using the modified Murakami model was closer to the test result, and the fitting of fatigue life using the corrosion fatigue crack initiation life model was less effective compared with the fitting of small specimen test results. [ABSTRACT FROM AUTHOR]

This paper focuses on the fatigue reliability analysis and the development of a new life model of reliability and crack growth mechanisms in FV520B-I (high strength martensitic-type stainless steels) in the very-high cycle fatigue (VHCF) regime, which haven’t been studied well. First, the fatigue test was carried out to clarify the fatigue failure mechanism in the very-high cycle regime. Based on the test results and fatigue reliability theory, the fatigue life distribution and P-S-N curves were modeled. A new fatigue life evaluation model for FV520B-I is proposed according to the fracture mechanics and classic life evaluation method. With the comprehensive application of P-S-N curves and a new proposed fatigue life evaluation model, a new assumption of a P-Sc-N curve is developed and verified, to quantitatively express the relationship between fatigue life, reliability and fatigue cracking. This is novel and valuable for further fatigue study of FV520B-I.

Fatigue property of FV520B-I is affected by the ultrahigh loading frequency significantly, and the ultrasonic fatigue experimental data can't be employed directly to analyze the fatigue failure in the actual remanufacturing engineering. However few theories about the effect of loading frequency on the fatigue property of FV520B-I has ever been proposed. In this paper, both ultrasonic experiment and conventional experiment are conducted out to obtain the fatigue data. The effect of loading frequency on the fatigue data distribution is discussed firstly, its fatigue limit declines with the increase of the loading frequency. Then the fracture surface observations are captured, the fatigue property and fatigue behavior mechanism of FV520B-I is examined by analyzing the fracture surface features, crack initiation and failure observations. A new material frequency correction factor is proposed and introduced to eliminate the influence of the loading frequency on the FV520B-I fatigue property. FV520B-I empirical fatigue life conversion model and fatigue strength conversion model are established with comprehensive use of a fitting algorithm based on the combination of experimental data and classic formula. A clear understand of the effect of loading frequency on the fatigue property of FV520B-I is novel and has an important significance in guaranteeing the accuracy of the actual fatigue analysis of FV520B-I in the remanufacturing engineering. [ABSTRACT FROM AUTHOR]

The high-cycle fatigue (HCF) behavior is significantly affected by surface roughness, especially for high strength metal FV520B-I. However, with surface roughness effect, neither the fatigue property, nor the high-cycle fatigue life model about FV520B-I with surface roughness has been reported. In this paper, designed fatigue experiment using the specimen with different surface roughness is presented to study the effectiveness of the roughness to the fatigue. The observations of the fatigue crack initiation sites and the crack propagation. Then the high cycle fatigue behavior of FV520B-I affected by surface roughness is analyzed. The existing very-high-cycle fatigue life model is not well-fit for high-cycle fatigue model of FV520B-I. A NEW high-cycle fatigue life prediction model of FV520B-I, taking surface roughness as a main effective variable is proposed. The model is built up by a comprehensive use of experimental data and the traditional fatigue modeling theory. The new finding between the fatigue strength coefficient and stress amplitude, with surface roughness, is adopted, leading to a NEW modified life prediction model. Study on fatigue model of FV520B-I with surface roughness is a very beneficial effort in fatigue theory and fatigue engineering development. [ABSTRACT FROM PUBLISHER]

The fatigue properties of FV520B-I up to 10 9 cycles when the surface roughness R a ≈ 0.6 were tested and compared with two groups of previously obtained test results. The test results showed that the S - N curve continuously moved downward and the transition stress at which the crack origin changed from the surface to the subsurface decreased with an increase of surface roughness, and the conventional fatigue limit finally appeared. The initiation mechanism of subsurface cracks in a very high cycle fatigue regime was independent of surface roughness. The surface fatigue limit and the high cycle fatigue life were predicted by relevant models. The competition mechanism between surface cracking and subsurface cracking was further discussed. [ABSTRACT FROM AUTHOR]

Research of material giga-cycle fatigue shows that fatigue failure is caused by a comprehensive effect of the competition between surface defect and internal inclusion, and the competition is more significant with high strength steal FV520B-I. A clear understanding of fatigue failure mechanism can improve the accuracy of the fatigue life prediction for FV520B-I. There is currently no available fatigue-life model that describes the competition mechanism of giga cycle behavior for FV520B-I. In this paper, ultrasonic fatigue experiment is presented, which revealed the existence of the competition during the giga-cycle process. The giga-fatigue life prediction models for FV520B-I are established with the consideration of surface defect and internal inclusion. Based on fracture mechanics, the surface roughness is correlated to surface defect. The competition analysis is carried out on the basics of fatigue strength to obtain the critical defect value. The critical defect value is the direct criterion in practical engineering and the primary factor of final failure is determined by comparing the actual defect with the critical value. Then the competitive giga-fatigue model for FV520B-I is established and the actual fatigue life is calculated by substituting the actual defect value into the corresponding fatigue life model. [ABSTRACT FROM AUTHOR]

Surface roughness has proven to be an important factor on the fatigue life of FV520B-I. However few laboratory experiments on fatigue properties with the consideration of surface roughness are reported in literature. In this paper, giga-cycle fatigue experiment was conducted to obtain fatigue data and the mechanism of fracture development for FV520B-I. With a comprehensive use of energy theory, fracture developing mechanics and the impact of stress ratio, a life prediction model for FV520B-I with the synthesis of surface roughness information is established. Using the fatigue data, including life time and inclusion size, an FV520B-I empirical fatigue life prediction model is identified by both model fitting and model parameters estimation. This model of giga-cycle fatigue of FV520-I is novel to the study of FV520B-I because of the surface roughness considered comprehensively in modeling. This result will also highly enhance the research development in remanufacturing engineering. [ABSTRACT FROM AUTHOR]

FV520B-I which is a kind of martensitic precipitated hardening stainless steels is usually applied to manufacture centrifugal compressor impellers. The centrifugal compressor impellers may fail from ultrahigh cycle fatigue induced by wake flow. Aim to assess ultrahigh fatigue life of impellers, fatigue tests up to 10 9 cycles were carried out by ultrasonic fatigue testing machine to research ultrahigh cycle fatigue (UHCF) behavior and mechanism of FV520B-I. The results show that the S - N curve of FV520B- I can be divided into two segments at 10 7 cycles apparently. The fatigue cracks almost all initiated from the surface of an impeller when the fatigue life less than 10 7 cycles and then from the subsurface inclusion of an impeller when the fatigue life more than 10 7 cycles. In other words, the fatigue behavior of FV520B-I is in accordance with UHCF except one specimen's crack occurred at surface surprisingly when fatigue cycles greater than 10 7 . Through the fracture morphology to be observed and measured, the reasons of “fish eye” area and GBF area formation were discussed, and the relationship of fatigue strength, fatigue life, and characteristic area size were researched by relative models. [ABSTRACT FROM AUTHOR]

A specific fatigue life prediction model for FV520B-I blade with fatigue crack is developed with the comprehensive consideration of ultrasonic fatigue test, FEA and influencing factor of fatigue failure, including ultrahigh loading frequency, stress amplitude, surface roughness and dimension. All these factors are involved in the analysis process, to guarantee the applicability and rationality of the fatigue life prediction model in actual operational condition of FV520B-I blade. • A new quantitative formula between stress σ m and fatigue crack Sc is proposed. • A specific stress intensity factor mode is proposed with parameter as Y = 0.68. • The fatigue limit of FV520B-I is estimated as 450 MPa according to the test. • A specific fatigue life model for FV520B-I blade with fatigue crack is proposed. The fatigue life prediction of FV520B-I blade with fatigue crack is a critical study in the fatigue analysis of shrouded impeller. However, the relationship between fatigue crack and fatigue life of FV520B-I blade has not been well studied. In this paper, finite element analysis (FEA) for FV520B-I blade with fatigue crack is conducted; both centrifugal load and aerodynamic load involved, and the stress distribution and the fatigue failure at decisive positions are analyzed. The new correlation between mean stress and fatigue crack is found based on the results of FEA, which is helpful to the development of fatigue life model of FV520B-I blade. Then fatigue test for blade material FV520B-I is performed to determine the fatigue limit. A fatigue life evaluation model for FV520B-I blade is then achieved, by taking fatigue crack as main variable, combining with the comprehensive application of FEA, fatigue test, actual operating parameters and existing fatigue modeling theory. The model is proved to be reliable by comparing the theoretical predicted life with simulated one. Study on fatigue life evaluation method of FV520B-I blade with fatigue crack is novel and essential to improve precision of fatigue life prediction of shrouded impeller in engineering practice. [ABSTRACT FROM AUTHOR]

This paper focuses on the fatigue reliability analysis and the development of a new life model of reliability and crack growth mechanisms in FV520B-I (high strength martensitic-type stainless steels) in the very-high cycle fatigue (VHCF) regime, which haven't been studied well. First, the fatigue test was carried out to clarify the fatigue failure mechanism in the very-high cycle regime. Based on the test results and fatigue reliability theory, the fatigue life distribution and P-S-N curves were modeled. A new fatigue life evaluation model for FV520B-I is proposed according to the fracture mechanics and classic life evaluation method. With the comprehensive application of P-S-N curves and a new proposed fatigue life evaluation model, a new assumption of a P-Sc-N curve is developed and verified, to quantitatively express the relationship between fatigue life, reliability and fatigue cracking. This is novel and valuable for further fatigue study of FV520B-I. [ABSTRACT FROM AUTHOR]