VHCF Regime Research Articles

Comparison of crack paths in a forged and extruded aeronautical titanium alloy loaded in torsion in the gigacycle fatigue regime

This paper discusses features of fatigue crack initiation and growth in an extruded and forged VT3-1 titanium alloy loaded in pure torsion in the gigacycle regime. Torsion fatigue tests were performed at 20kHz up to a fatigue life of 109 cycles. It has been shown that regardless of the manufacturing process both surface and subsurface crack initiation may appear under ultrasonic torsion loads despite the fact that the maximum shear stress amplitude is located at the specimen surface. In the two cases cracks initiate on planes of maximum shear stress amplitude (mode II). After reaching a certain length the cracks turn and propagate in mode I. Surface crack initiation and growth mechanisms in VHCF regime are similar to HCF ones. Subsurface crack forms a typical “fish-eye” pattern on fracture surface with three different zones: (1) small crack growth in the plane of maximum shear stress, (2) crack growth on the plane of maximum normal stress with the formation of a quasi-flat failure surface covered by fretting debris, (3) crack branching under mixed mode loading conditions (I+II+III) with the formation of several secondary cracks. Subsurface crack initiation sites are located below the specimen surface (at a depth of approximately 200μm) for both the forged and the extruded materials. Subsurface crack growth is observed on the failure surface as being less rough. The mechanisms of subsurface crack initiation seems to be similar to these observed under fully reversed tensile loads.

The prediction for fatigue strength in very high cycle regime of high strength steel

BACKGROUND: Fatigue strength is an important parameter of evaluating the material fatigue property. There are many prediction models for fatigue strength in the VHCF regime. Murakami’s model is extensively applied in numerous prediction models. The initiation mode of VHCF cracks in high strength steel is very different from the assumption in Murakami’s model that the internal inclusion is considered as a disc crack. OBJECTIVE: We tried to analyze the effect of inclusion shape on the prediction by Murakami’s model and further examined the prediction effects of several generalized models. METHODS: We reviewed the prediction models for fatigue strength in the VHCF regime. On the assumption that the fatigue crack which initiated from the internal inclusion is a circumferential crack around a spherical cavity and crack propagation is the critical condition, we modified Murakami’s model. The predicted values of Murakami’s model, the modified model and Liu’s model were compared with the test results of 28 groups of high strength steel in the literature. RESULTS: The deviations between predictions by several generalized models and test results are mostly within 20%. The predicted values of four groups of high strength steel by the modified and Murakami’s models and six groups of high strength steel by Liu’s model have large deviations from the test results. The predicted fatigue strength values of Liu’s model are higher than that of the modified model, and the predicted values of the modified model are slightly higher than that of Murakami’s model. The mean value of fitting coefficient C of all types of high strength steel is closer to the coefficient of the modified model, and the sums of squares of coefficient deviations of the modified model is smaller than that of Murakami’s model. CONCLUSIONS: The effects of inclusion shape on the prediction of fatigue strength in the VHCF regime seem small. Murakami’s model, the modified model and Liu’s model coincide well with the test results in the literature. The prediction effect of the modified model is higher than that of Murakami’s model, and the prediction effect of Murakami’s model is higher than that of Liu’s model.

Cyclic deformation behavior of austenitic Cr–Ni-steels in the VHCF regime: Part I – Experimental study

A stable (AISI 316L) and a metastable (AISI 304L) austenitic stainless steel were investigated with respect to their VHCF behavior. The focus of the paper lies on the investigation of the cyclic deformation behavior of the two materials at very low stress amplitudes. The 304L steel is characterized by a pronounced cyclic softening during its initial stage of cyclic deformation. In the course of the following loading cycles, a phase transformation (γ-austenite→α′-martensite), accompanied by volume expansion is associated with the reduction of the global plastic strain amplitude and induces compressive stresses in the near surface layer. As a consequence, the material shows no failure up to 109 cycles at 240MPa. In contrast, the type 316L steel has a higher stacking fault energy and the microstructure remains fully austenitic during cyclic deformation when analyzed by means of magneto-inductive methods. In this case, very localized plastic shear occurs and the slip band topography reveals the formation of pronounced intrusions. Microcracks initiate from these intrusions in the VHCF regime and samples failed also beyond 107 cycles. This study presents a comparative investigation of the damage evolution – including dislocation morphology and phase transformations – during cyclic loading for both materials. The combined effect of the individual deformation mechanisms is investigated for both materials in the context of a microstructure-sensitive simulation discussed in Part II of this study.

Ultrasonic fatigue testing in the scanning electron microscope

Abstract The continuing trend to increase the performance and durability of machines requires the use of materials, whose structural integrity must be ensured far beyond the classical fatigue limit. In this so-called „very high cycle fatigue“ regime (VHCF), the materials show a strong scatter in fatigue life, which makes life service assessment difficult. Present studies on austenitic-ferritic duplex steel 1.4462 (X2CrNiMoN22-5-3) have shown that the reason of this scatter in fatigue life can be found in the microstructural length scale mainly due to the barrier effect of grain and phase boundaries. The integration of an ultrasonic fatigue testing system in a high-resolution scanning electron microscope allows observing and evaluating the active microstructural mechanisms of fatigue crack initiation and early crack propagation, such as the formation and propagation of slip bands, during the majority of VHCF life. Additional micro texture measurements by electron backscatter diffraction (EBSD) provide data for the development of a mechanism-oriented numerical short crack simulation. In the context of crack initiation and propagation in the VHCF regime in duplex stainless steel, the focus of the present paper is put on the technical realization of the in-situ ultrasonic fatigue testing in the scanning electron microscope.

Time Saving Method for Measuring VHC Fatigue and Fatigue Crack Growth Data with the Ultrasonic Fatigue Technique

The principles of the ultrasonic 20 kHz technique are shortly described and its peculiarities explained. Considering the technical requirements, such as an on-line amplitude and frequency feedback control and use of appropriate cooling allow obtaining data of high accuracy and reliability. Life-time data at constant and variable amplitudes, as well as fracture-mechanical values, such as (Δa/ΔN) vs. ΔK curves can be obtained, and especially endurance limit and ΔK thresholds can be obtained in a highly time-saving way. Some examples of performing measurements under superimposed loading conditions are shown. One is on-line observation of specimens during loading and simultaneous mechanical measurements plus fractography after final fracture so that, the correlation of these results allows quantitative information on (Δa/ΔN) and ΔK values in the VHCF regime. Another new technique led to a two-step model of small-crack initiation and propagation from corrosion pits. Investigations on polycrystalline copper likewise rendered interior fatigue-crack formation from persistent slip bands and corroborated that, arrest of small cracks is mainly responsible for several unexpected results in the VHCF regime.

Modeling and simulation of temperature-dependent cyclic plastic deformation of austenitic stainless steels at the VHCF limit

The exploration of fatigue mechanisms in the VHCF regime is gaining importance since many components have to withstand a very high number of loading cycles due to high frequency or long product life. In this regime, the period of fatigue crack initiation and thus the localization of plastic deformation play an important role. The material that was investigated in this study is the metastable austenitic stainless steel AISI 304 in the initially purely austenitic condition. The experimental investigations during quasi-isothermal fatigue tests revealed that a moderate increase of temperature from room temperature up to 150°C led to a reduced VHCF strength. At both temperatures the 304 grade still undergoes a pronounced localization of plastic deformation in shear bands accompanied by a deformation-induced martensitic phase transformation from the γ-austenite to the α’-martensite during VHCF loading. In the present study, the experimental study is extended by modeling and simulation of the relevant temperature-dependent VHCF deformation mechanisms in order to provide a more profound understanding of the observed cyclic deformation. For this purpose, two-dimensional (2-D) morphologies of microstructures are modeled in the mesoscopic scale by the use of the boundary element method (BEM), and cyclic plastic deformation is considered by certain mechanisms defined in a simulation model. It describes the localization of plastic deformation in shear bands taking into account the formation, plastic sliding deformation and cyclic slip irreversibility of each shear band. The deformation-induced martensitic phase transformation is represented by introducing martensitic nuclei into the modeled microstructure depending on the plastic deformation in shear bands. The influence of temperature is incorporated into the simulation model by the use of empirical data and a kinetic model, each related to the tensile test. The simulated cyclic deformation and phase transformation is compared to experimental observations and allows for assessing the individual influence of deformation mechanisms on the temperature-dependent fatigue behavior. Finally, a temperature-independent ‘limit curve’ for the accumulated irreversible plastic sliding deformation regarding failure in the VHCF regime is proposed.

Influence of Corrosive NaCl solution on Life times of 7075 Aluminum alloy under Combined Fatigue loading in the VHCF Regime

The loading histories of aircrafts and rockets are complex and the components are exposed to combined loading conditions with high numbers of high-frequency cycles superimposed to lower-frequency carrier vibrations. The amplitudes of both frequency ranges usually are variable. The small-amplitude cycles arise from several discrete sources such as structural vibrations which are especially critical at special components and places of an aircraft. The environment plays another important role and ambient air and sea-water have to be considered. Almost no knowledge exists of the material response in the VHCF regime, especially concerning the environmental influence. In this study, experiments were performed on aluminum alloy 7075-T651 in a 3.5%- sodium-chloride solution and in laboratory air of 22 °C of 50% relative humidity at high (105 – 108) and very high numbers of cycles (5 × 108 – 1010). The loading sequences consisted of a low-frequency square wave (0.4 to 1 Hz), being superimposed with a high frequency 20 kHz random vibration. The random vibrations were simulated by a Gauss distribution allowing stress ranges of 10 to 70 MPa or alternatively 50 to 90 MPa. Stress/strain vs. life time curves were measured. The results were correlated with in-situ microscopy observations of the specimen surfaces and with post-experimental fracture-surface images. This technology allowed identifying fatigue crack initiation and propagation stages being needed for an interpretation of the relevant fatigue-life mechanisms under environmental influences.

Gigacycle fatigue behavior of a cast aluminum alloy under biaxial bending: experiments with a new piezoelectric fatigue testing device

A new ultrasonic fatigue testing device has been developed to test specimens under biaxial loading at 20 kHz. A flat smooth specimen with a disc geometry is placed on a circumferential frame and cyclically loaded at the center of its upper face. Disc bending generates a biaxial proportional stress state at the center of the lower face. Any positive loading ratio can be applied. A cast aluminum alloy (used to produce cylinder heads) has been tested under biaxial loading using this device in order to determine its fatigue strength at 109 cycles under high hydrostatic pressure. Self-heating is moderate but macroscopic fatigue cracks after testing are very long. First results in VHCF regime are consistent with literature results obtained under similar stress state for the same material, but in HCF regime and at 20 Hz.

Fatigue crack initiation and growth on an extruded titanium alloy in gigacycle regime: comparison between tension and torsion loadings

This paper is focused on the analysis of fatigue crack initiation and growth mechanisms in defect free VT3-1 titanium alloy (similar to Ti6Al4V) in VHCF regime under tensile and torsion loadings. Fully reversed fatigue tests were carried out between 107 and 109 cycles at 20 kHz under constant amplitude loadings (no pulse-pause). SEM observations of the specimens fracture surfaces were carried out in order to compare the crack initiation mechanisms and the different crack growth stages under different loadings. It has been shown that subsurface crack initiation may appear under tension (as usual in gigacycle regime) but in our experiments no inclusion was observed in the “fish-eye”. Furthermore, subsurface crack initiation was observed under torsion loading too, despite the maximum shear stress location at the specimen surface. Under this loading, a “fish-eye” was observed too, but again without any inclusion in its center. The suspected microstructural reasons responsible for subsurface crack initiations under torsion loading could not be observed due to significant destruction of the fracture patterns (crack lips friction under mixed mode). The results of the fatigue tests show a principal difference in crack initiation and early crack growth stage between tensile and torsion loadings. Torsion crack initiates on a plane of maximum shear stress (like in HCF regime) while tensile crack is on a plane of maximum normal stress. Sequences of changes in fracture surface roughness is the same for tensile and torsion loadings.

Effect of martensite content and geometry of inclusions on the VHCF properties of predeformed metastable austenitic stainless steels

The effect of inclusions on the VHCF properties of a metastable austenitic stainless steel in the predeformed condition was studied. The material contains an inhomogeneous distribution of oxide inclusions with an elongated geometry in the rolling direction. Samples were monotonically predeformed at -80°C start temperature at a strain rate of 0.1 %/s resulting in a martensite content of about 60 vol-% and subsequently fatigued by means of high frequency testing machines. On the one hand the high martensite content results in an increase of the HCF strength, on the other hand fatigue failure occurs even beyond 107 loading cycles. The higher notch sensitivity of the martensite phase leads to internal crack initiation from inclusions accompanied by the formation of a fine granular area (FGA). A direct correlation between the size of the FGA-and the number of cycles to failure can be shown. In order to find the optimal martensite content for both HCF and VHCF regime, the notch sensitivity of the material in different predeformed conditions was investigated. The results show a significantly higher notch sensitivity for 30 vol% α’ martensite, whereas higher martensite contents do not show any further significant increase. Since mechanical components are in practice subjected to complex cyclic loading situations, samples were extracted and tested both parallel and transversal to the rolling direction, in order to reproduce the relation between rolling and loading direction. The change in testing direction perpendicular to the rolling direction results in a larger projection area of the inclusions and reduces the number of cycles to failure due to the increased stress intensity factor. In this case, the area of the inclusions (but not of the FGA) correlates with the number of cycles to failure. These findings are discussed on the basis of a detailed microstructural characterization of the material focusing on the effect of martensite content, the inclusion morphology with respect to the rolling direction and the load axis applied.

Study of Dissipation Properties and Structure Evolution in Metals with Different Grain Size under HCF and VHCF Loadings

The dissipation properties, evolution of the dilatation and elastic properties in sub-microcrystalline titanium Grade 4 samples loaded in HCF and VHCF regime were investigated. The dissipation properties were studied based on self-heating (Risitano) test. The mechanical properties (elastic modulus, amplitude-independent damping) and dilatation for a set of samples with different degrees of the life time exhaustion were studied based on the acoustic resonance method. The dilatation was studied by the method of hydrostatic weighing. The experimental results show the increasing of energy dissipation in submicrocrystalline titanium and absence the characteristic knee point on the graph of dependence of temperature rise versus stress amplitude for this material in self-heating test. The structural investigation shows the decreasing of elastic properties during fatigue loading and increasing of dilatation.

Very high cycle fatigue crack initiation mechanisms in different engineering alloys

The fatigue crack initiation mechanisms prevalent in high strength martensitic steel grades, hot rolled plate duplex stainless steels, cold rolled strip duplex stainless steel and a super alloy grade were compared. The fatigue testing of all the grades was conducted in the VHCF regime using an ultrasonic fatigue testing equipment operating at 20 kHz. Scanning electron microscope (SEM) observations of the fracture surfaces revealed the presence of a microstructure controlled initial growth of short fatigue cracks in all the tested grades. Fracture surfaces of the failed specimens of a high strength martensitic steel grade revealed the typical fine granular area (FGA) within the fish-eye area around the internal inclusions. Fatigue crack initiation in the cold rolled strip duplex stainless steel grade occurred at surface defects left over by the cold rolling process of this grade. However, the presence of FGA around the surface crack initiating defect was observed similar to the internal crack initiations in the high strength martensitic steels. By mapping the FGA size development during VHCF loading, as obtained from fracture surfaces, FGA growth results were obtained. A similar study on hot rolled plate duplex stainless steel grades, 2304 SRG and LDX 2101, revealed the presence of an initial crystallographic growth region (CGR) in which crack growth direction is changed by microstructural barriers such as phase and grain boundaries. The early plastic fatigue damage accumulation occurred predominantly in one phase or at the austenite-ferrite phase boundaries. On the other hand, an initial transcrystalline fatigue crack growth was observed in the Ni-based super alloy grade Inconel 718.

On the formation mechanisms of fine granular area (FGA) on the fracture surface for high strength steels in the VHCF regime

The fine granular area (FGA) plays a crucial role in the VHCF regime. However, so far, the uniform understanding or conclusion about the formation mechanism of FGA has not been drawn. In view of the significance of FGA, the current studies on FGA, i.e., the FGA size, the stress intensity factor range at the periphery of FGA, the crack initiation and growth process in FGA, and the formation mechanisms of FGA, are reviewed in the present paper. The role of internal hydrogen in the VHCF regime is further discussed. In the end, two important definite conclusions are drawn based on the present investigations.

Study on Very-High-Cycle-Fatigue Property of Aero-Engine Blades Based on Subcomponent Specimen

The cyclic load number of aero-engine blade during its service life is very likely beyond 107, which is regarded as the conventional fatigue limit. Moreover, surface strengthening is very often used in the manufacturing process of blade. The conventional testing method in the VHCF regime cannot exactly reflect the stress state of the blade, including the mechanism of crack initiation. To study the fatigue behavior and effects of laser shock peening, a kind of bending fatigue subcomponent specimen was designed and the laser shock peening model was established. Experiment about TC17 was accomplished by the Ulra-High Cycle bending fatigue system. It is found that the fatigue damage occurs beneath the surface and the S-N curve is continuously rather than multi-step declining in the VHCF regime. Process of surface strengthening has a significant effect on fatigue performance of TC17 titanium alloy.

Simulation of the Interaction of Plastic Deformation in Shear Bands with Deformation-Induced Martensitic Phase Transformation in the VHCF Regime

The experimental observation of the microstructural deformation behavior of a metastable austenitic stainless steel tested at the real VHCF limit indicates that plastic deformation is localized and accumulated in shear bands and martensite formation occurs at grain boundaries and intersecting shear bands. Based on these observations a microstructure-sensitive model is proposed that accounts for the accumulation of plastic deformation in shear bands (allowing irreversible plastic sliding deformation) and considers nucleation and growth of deformation-induced martensite at intersecting shear bands. The model is numerically solved using the two-dimensional (2-D) boundary element method. By using this method, real simulated 2-D microstructures can be reproduced and the microstructural deformation behavior can be investigated within the microstructural morphology. Results show that simulation of shear band evolution is in good agreement with experimental observations and that prediction of sites of deformation-induced martensite formation is possible in many cases. The analysis of simulated shear stresses in most critical slip systems under the influence of plastic deformation due to microstructural changes contributes to a better understanding of the interaction of plastic deformation in shear bands with deformation-induced martensitic phase transformation in the VHCF regime.

Three Point Bending Fatigue of Carbon Fiber Fabric Reinforced Polyphenylensulfide in the Very High Cycle Fatigue Regime

Carbon fiber reinforced polymers (CFRP) are getting more and more important for structural components for automotive and aerospace applications. These components are subjected to 1011and more loading cycles during their time in service. Therefore the VHCF behavior and the corresponding failure mechanisms have to be well understood. To obtain a comprehensive knowledge about the fatigue behavior and failure mechanisms of CFRP in the VHCF regime, a new Ultrasonic Testing Facility (UTF) for cyclic 3-point bending at 20 kHz has been developed at WKK. This UTF with combined online nondestructive testing via laser vibrometry and IRthermography enables VHCF experiments up to 109cycles in twelve days without overcritical heating up to the glass transition temperature of the polymer. The chosen material in this research project is the commercially available and aircraft qualified carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). To determine the fatigue characteristics of CF-PPS load increase tests and based on these results constant amplitude tests up to 109cycles have been carried out. Light optical and SEM microscopy have been performed in defined fatigue states or finally after reaching 109cycles with shear stress amplitudes of at least 44% of the monotonic ultimate shear strength. The induced fatigue damage of CF-PPS in the VHCF regime was studied in detail.

VHCF Behavior and Word Hardening of a Ferritic-Martensitic Steel at High Mean Stresses

Low-pressure steam turbine blades undergo VHCF-loadings induced by inhomogenous flow behind the vanes resulting in excitation frequencies of ≈ 2 kHz for rotational speeds of 50 Hz and a typical number of stator vanes of ≈ 60. The VHCF loading is superimposed by considerable mean stresses caused by centrifugal forces. In the present study, the VHCF-behavior of the ferritic-martensitic turbine blade steel X10CrNiMoV12-2-2 is investigated using an ultrasonic fatigue testing system up to cycle numbers of 5∙109 at stress ratios from R = -1 up to 0.7, i.e. up to very high mean stresses. Generally, crack initiation changes from the surface to internal inclusions at fatigue lives around 4∙107. The transition between fatigue failure and run-outs is shifted to higher lifetime with increasing R, and fine grained areas (FGAs) at the crack initiation sites only occur at R < -0.1. However, the fracture mechanics approach proposed by Murakami consistently describes the lifetime behavior for all load ratios over 4 decades of lifetime. At R up from 0.5 considerable cyclic creep occurs, even for lifetimes above 108 cycles, resulting in cyclic hardening which was proved by microhardness measurements at longitudinal sections. This effect at least partially explains the high maximum stresses close to the tensile strength of the material occurring in the VHCF regime at load ratios ≥ 0.5.

Influential Factors for Very High Cycle Fatigue Behavior of Metallic Materials

In recent years, very high cycle fatigue has become a major concern in design and durability of engineering components and structures such as railway wheels, rails, offshore structures, bridges, load bearings, etc. There are some factors which have been assumed to influence the fatigue behavior for metallic materials in VHCF regime. But most factors influencing the VHCF behavior have not been studied thoroughly. In order to deeply understand the essence of material fatigue, the experimental and theoretical aspects of the factors influencing on very high cycle fatigue for metallic materials should be studied further more. This paper deals with an overview on the effect of factors on the property in very high cycle fatigue regime. Research trends and some conclusions in this field are briefly discussed and obtained.

Very high cycle fatigue of a high strength steel under sea water corrosion: A strong corrosion and mechanical damage coupling

This paper is focused on the effect of sea water corrosion on the gigacycle fatigue strength of a martensitic–bainitic hot rolled steel R5 used for manufacturing off-shore mooring chains for petroleum platforms in the North Sea. Crack initiation fatigue tests in the regime of 106 to 1010cycles were carried out on smooth specimens under three different environment conditions: (i) without any corrosion (virgin state) in air, (ii) in air after pre-corrosion, and (iii) in-situ corrosion-fatigue under artificial sea water flow. A drastic effect of sea water corrosion was found: the median fatigue strength beyond 108cycles is divided by 5 compared to virgin state specimens. The crack initiation sites were corrosion pits caused by pre-corrosion or created during corrosion-fatigue under sea water flow. Furthermore some sub-surface and internal crack initiations were observed on specimens without any corrosion (virgin state). Crack propagation curves were obtained in mode I in air and under sea water flow. Calculation of the stress intensity factor at the tip of cracks emanating from hemispherical surface pits combined with the Paris–Hertzberg–Mc Clintock crack growth rate model showed that fatigue crack initiation period represents most of the fatigue life in the VHCF regime. Additional original experiments have shown physical evidences that the fatigue strength in the gigacycle regime under sea water flow is mainly governed by the corrosion process with a strong coupling between cyclic loading and corrosion.

Constant and Variable-amplitude Loading of Aluminum Alloy 7075 in the VHCF Regime

The aim of this study is to quantify the influence of VHCF cycles on fatigue life of 7075-T6 Al-alloys using both constant and variable amplitude loading. Constant amplitude loading tests with and without superimposed mean levels were performed to characterize the material behavior with minimal effect of load interactions. The effects of combined cycle fatigue loadings were studied by using a spectrum composed by a low frequency carrier wave at 0.01, 0.4, 2 and 5Hz and a superimposed high-frequency wave at 20kHz. The low frequency wave is sinusoidal or rectangular, the high-frequency wave has constant and variable amplitudes, and the effects of different loading sequences are investigated. Correlating the measurement results with microscopic surface observations allowed identification of fatigue crack initiation and propagation.