Evaluation of giga-cycle fatigue properties of some maraging steels by intermittent ultrasonic fatigue testing

ABSTRACT In evaluating the giga-cycle fatigue strength of some high strength steels, information on the size distribution of nonmetallic inclusions contained in the material is indispensable. To save time and effort of obtaining such data concerning the inclusions, a convenient dissolution method to evaluate the maximum inclusion size is proposed, in place of a conventional method of measuring the inclusion sizes on many cross-sectional areas. Meanwhile, to save time-consuming work of obtaining giga-cycle fatigue properties of some metallic materials, an intermittent ultrasonic fatigue testing method has also been developed. In the present paper, these two newly developed methods were successfully combined to assess the long life fatigue properties of maraging steels as a function of inclusion size.     

The effect of two-frequency low-cycle loading on the fatigue life of steels and welded joints

Some parts of nuclear power equipment (NPE) are subjected mostly to two-frequency loading during their service. Oscillations with a lower stress amplitude are superimposed on the basic slow loading with high stress (or strain) amplitude. The damage cumulation law, which is valid relatively well for random loading, seems to be less suitable for two-frequency loading. According to the design specifications [1] the service life of parts at two-frequency loading may be 10 to 20 times lower than that at single-frequency loading. The decrease in life depends on both the ratio of loading frequencies and amplitudes, and the material characteristics.Published inProblemy Prochnosti, Nos. 1–2, pp. 118–125, January–February, 1995.     

Simulation of fatigue life of constructional steels within the mixed modes I and III loading

The authors analysed influence of a component of the torsional moment M_as under the complex loading state, that is under bending with torsion, on fatigue life during initiation and propagation of fatigue cracks. Simulation of specimen life was performed according to the relationships describing the crack propagation rate and including the equivalent stress intensity factor range K_eq. Under complex loading, increase of amplitude of the torsional moment M_as for a given initial value of the resultant moment M_aw0 caused a higher fatigue life of specimens made of 10HNAP and 18G2A steels. This fatigue life increase was described by a nonlinear equation, the parameters of which had been determined from the experimental results. The fatigue lives estimated according to the assumed models were compared with those obtained from tests.     

Characteristics of rolling contact fatigue of steels produced by thermomechanical processing

0.27C-1.97Cr-1.65Mn-0.30Mo-0.21Ni steels produced by thermomechanical processing were rolling contact fatigue tested in the elastohydrodynamic lubricating condition at the rotating speed of 8,000 rpm under the applied load in the range of 25–100 kgf. A mixture of lower bainite and martensite was formed during thermomechanical processing, and it was found in transmission electron microscopy that fine lower bainite was formed by splitting in two after formation of martensite. The zone of maximum shear stress was found to be 195.0–339.3 m in depth from the contact surface by comparing the regions of hardness increase, microstructural change and the contact width during rolling contact fatigue. Resistance to crack initiation during rolling contact fatigue is the main reason for improved fatigue life. This is confirmed from the result that the shortest fatigue life was shown in the specimen with the largest crack length and crack depth.     

Modeling cyclic ratcheting based fatigue life of HSLA steels using crystal plasticity FEM simulations and experiments

This paper develops a plastic ratcheting based fatigue failure model for HSLA steels from a combination of results from experiments and finite element simulations using crystal plasticity constitutive relations. It predicts the nucleation of major cracks in the microstructure in ratcheting. Subsequently, the total life is limited by the growth of ductile fracture in the microstructure, which is factored in by comparing the simulated results with experiments. A crystal plasticity based FEM (CPFEM) model is used in this paper to predict the local plastic strain in the microstructure which plays a role in the ratcheting life. Orientation imaging based microstructural information (orientation and misorientation distributions) is incorporated in CPFEM. The model proposed has the ability to represent a range of behavior from low and high cycle behavior in the life models. The predictions from it are found to be in excellent agreement with experimental data.     

An approach to fatigue life modeling in titanium-matrix composites

A review of the procedures developed by the author and his colleagues over the last several years for predicting elevated-temperature fatigue life of metal-matrix composites is presented. Modeling approaches involve concepts of both linear and non-linear summation of damage from cycle-dependent as well as time-dependent mechanisms. The analyses, further, treat the micromechanical stresses in the constituents as parameters in the life prediction models. The material characterized is SCS-6/Timetal®21S, a metastable beta titanium alloy reinforced with continuous SiC fibers. Modeling is applied to isothermal fatigue at different frequencies and temperatures, and thermomechanical fatigue (TMF) under both in-phase and out-of-phase loading conditions at different temperature ranges and maximum temperatures. Experimental data are used as the basis for determining the parameters embedded in the models. The numerical results, in turn, provide insight into the dominant mechanisms controlling fatigue life under a given condition. The capability to correlate experimental data from a wide variety of test conditions for several versions of a damage summation model is demonstrated.     

The fatigue properties of some cast steels

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.     

An effect of strength of railway axle steels on fatigue resistance under press fit

High-cycle fatigue tests with an evaluation of fatigue limit were carried out on large model components of bars with press fitted hubs of diameter 63/59 mm. Bars were made of three railway axle steels EA1N, EA4T and 34CrNiMo6 with considerable different strength from 586 MPa to 1041 MPa, respectively. Detection and measurement of crack growth under hubs by ultrasonic method was performed during the tests. In spite of the differences in strength and alloying of tested bars, differences in mean value of fatigue limit were not significant. This result was connected with specific damage mechanism and microcracks initiation under hubs with fretting effects. Short fatigue crack growth under hubs occurred at stress intensity factor range ΔK considerably bellow threshold value ΔK_th of long cracks. Simultaneous growth of main cracks from more than one point of surface circumferential area under hub was quite frequently observed.     

An efficient fatigue and creep‐fatigue life prediction method by using the hysteresis energy density rate concept

Fatigue damage, time‐dependent creep damage and their interaction are considered as the main failure mechanisms for many high temperature structural components. A generalized methodology for predicting both the high temperature low cycle fatigue (HTLCF) and creep‐fatigue lives by using the hysteresis energy density rate (HEDR) and fatigue damage stress concepts was proposed. Experimental data for HTLCF and creep‐fatigue in Alloy 617, Haynes 230 and P92 steel were respectively collected to validate the method. A better prediction capacity and most of the data points that fall within a 1.5 scatter band were obtained compared with the traditional energy‐based method, time fraction rule and ductility exhaustion model. Moreover, a creep‐fatigue damage diagram was also constructed by using the proposed approach.     

Detrimental effect of nitride and aluminium oxide inclusions on fatigue life in rotating bending of bearing steels

Abstract

Rotating bending fatigue tests were performed on hardened AISI type 52100 bearing steel. Fracture surfaces after testing at a stress amplitude of 950 MPa showed that the Ti(C,N) inclusions which caused fatigue failure were significantly smaller than the corresponding alumina inclusions. The smallest crack initiating Ti(C,N) inclusion had a size of 3 μm and the smallest alumina inclusion was 17 μm. It was also shown that fatigue life was significantly shorter for a steel which showed cracked alumina inclusions on the fracture surfaces than for a steel which had non-cracked inclusions. Finite element calculations were performed to determine the driving forces of short cracks at Ti(C,N) and alumina inclusions. Two configurations were studied in each case, based on both non-cracked and cracked inclusions. The calculations incorporated heat treatment simulation and cyclic loading with successive growth of cracks. It was found that the Ti(C,N) configurations gave the highest driving forces for crack growth. The alumina configuration with a non-cracked inclusion gave the lowest driving force. It was concluded based both on experimental evidence and theoretical considerations that Ti(C,N) inclusions are more detrimental to fatigue life than alumina inclusions of the same size. It is their shape and thermal properties which make Ti(C,N) inclusions more detrimental than alumina inclusions. Internal cracking of alumina inclusions leads to reduced fatigue life.