Very High Cycle Fatigue Regime Research Articles

Ultrasonic fatigue and microstructural characterization of carbon fiber fabric reinforced polyphenylene sulfide in the very high cycle fatigue regime

Carbon fiber reinforced polymers (CFRP) are increasingly used for high performance applications, especially for aircraft structures. These structural components are subjected higher than 108 loading cycles during their operation time of more than 20 years. To utilize the full mechanical performance of CFRP for lightweight applications, the very high cycle fatigue (VHCF) behavior has to be well understood. Therefore the VHCF behavior of a carbon fiber twill 2/2 fabric reinforced polyphenylene sulfide (CF-PPS) was analyzed systematically up to 109 loading cycles. To realize these investigations in an economic reasonable time period a novel ultrasonic fatigue testing facility for CFRP was used. This facility works with cyclic three-point bending at a frequency of 20 kHz. Lifetime-oriented investigations at stress ratios between R = 0.21 and 0.51 showed an exponential decrease of the bearable stress amplitudes in the range between 106 and 109 cycles. Interrupted constant amplitude tests were performed for the characterization of the fatigue damage development. Based on light optical as well as SEM investigations the fatigue damage mechanisms in the VHCF regime were characterized in detail. The surface crack density was determined for several load amplitudes and in different fatigue states by analyzing merged light optical micrographs. Furthermore the stiffness degradation was measured ex-situ during interruptions of the constant amplitude tests, which shows a good correlation with the surface crack density of ultrasonically fatigued CF-PPS.

Ultrasonic fatigue testing of thin MP35N alloy wire

MP35N (35 % Co, 35 % Ni, 20 % Cr, 10 % Mo; weight fraction) in the form of thin wires (100 µm diameter) is commonly used as conductors in cardiac leads, which require excellent corrosion resistance and high fatigue strength, in particular in the very-high cycle fatigue (VHCF) regime. This becomes apparent when one assumes a typical adult human heart rate of 72 beats per minute, which over 10 years of implant deployment will roughly yield 3.8 × 108 cycles. Tensile properties of MP35N wire are considerably enhanced through extensive cold-working. Static strength of the material is in the range of 2 GPa (Prasad et al., 2014). Fatigue testing of very thin wires is time consuming with conventional fatigue testing methods. In earlier investigations (Prasad et al., 2014) the wire was stressed at a cyclic frequency of 30 Hz in monotonic loading tests, which would require about 193 days for one single specimen to complete 5 × 108 cycles. A complete characterization of a material’s fatigue properties however requires many specimens to be tested well into the VHCF regime, calling for an accelerated testing method, especially with regard to development of new implant materials. For the first time, a method to test thin wires with the ultrasonic fatigue testing method is presented. Rather than vibrating in resonance as in conventional ultrasonic fatigue tests, the wire is stressed with cyclic tension loads. Results of fatigue tests at a cycling frequency of around 20 kHz up to lifetimes of 109 cycles at load ratio R = 0.3 are shown. The influence of secondary phase particles on crack initiation is discussed. Microstructural observations and lifetimes measured at 30 Hz from earlier studies and 20 kHz are compared and discussed.

Development of a probabilistic model for the prediction of fatigue life in the very high cycle fatigue (VHCF) range based on inclusion population

The aim of the present work is to develop a statistical approach for the correlation between the quality of metallic materials with respect to the size and arrangement of inclusions and fatigue life in the VHCF regime by using the example of an austenitic stainless steel of type AISI 304 containing martensite from predeformation. For this purpose, the size and location of about 60000 inclusions in cross sections of an AISI 304 sheet in both longitudinal and transversal direction were measured and subsequently modeled using conventional statistical functions. This way a statistical model of inclusion population in AISI 304 was created. The model forms a basis for the subsequent statistical prediction of inclusion distribution in fatigue specimens and the corresponding fatigue lives. The size and location distributions of the crack initiating inclusions were defined based on the modeled inclusion population and the stress distribution in the fatigue specimen, using the probabilistic Monte Carlo framework. The experimentally obtained information about the correlation between the size and location of the crack initiating inclusions and corresponding fatigue lives allows an assessment of fatigue life of the specimens corresponding to the modeled crack initiating inclusions. Reasonable agreement was obtained between modeling and experimental results.

Influence of intrinsic and artificial defects on the VHCF properties of 17-4PH stainless steel

In the present work, fatigue test results in the very high cycle fatigue (VHCF) regime are discussed. Experiments were performed with smooth specimens in which crack initiation mostly originated at non-metallic inclusions. Additionally, the influence of small artificial defect, i.e. circumferential notches, drilled holes and corrosion pits, was investigated. Fatigue tests in the VHCF regime were conducted using ultrasonic fatigue testing technique. Further experiments in the high cycle fatigue regime were performed using rotating bending and servo-hydraulic testing machines.For smooth specimens, fatigue fracture was observed even at more than 1010 number of cycles. In contrast, no fatigue failure occurred above approximately 107 cycles in the presence of artificial defects, i.e., a pronounced fatigue limit was determined. Furthermore, it was found that the fatigue limit is not only depending on the defect size but also on the notch root radius of a defect. Consequently, different approaches were used to predict the fatigue limit in the presence of different defects.

Very high cycle fatigue behavior under constant and variable amplitude loading

Components and structures are often exposed a very high number of cycles. The investigations in the field of very high cycle fatigue (VHCF) are mainly focused on experiments without mean stresses and with constant amplitude loading. Therefore, within the scope of this paper, investigations with constant and variable amplitudes with different mean stresses will be presented. For studying variable amplitude loadings in the VHCF regime systematic two-step block loading experiments have been performed, in which the maximum load amplitudes of the high block and the number of cycles of the low block with amplitudes below the fatigue strength have been varied. Moreover, the standardized load-time-histories Felix/28 and WISPER have been used. The influence of different reconstructions as well as the amount of the amplitudes beneath the fatigue strength of the investigated high strength steel on the initiation site, the S-N curve and the lifetime prediction has been investigated. Due to the variable amplitude loadings arrest marks are produced within the fish-eye surrounding the inclusion. The sizes and the area, where arrest marks are observable, as well as the spacings between the arrest marks are influenced by the different load sequences. By counting and measuring the arrest marks an average crack growth rate for the crack propagation within the fish eye can be calculated. Moreover, the short crack growth curve is used for calculating lifetimes using fracture mechanical approaches.

Investigation of the damage mechanisms during very high cycle fatigue (VHCF) of a tempered carbon steel

The study is an overview of recent investigations dealing with the fatigue damage in the HCF and VHCF regime of 0.5C-1Cr martensitic steel (German designation: 1.7228) in a tempered condition. The experimental part is focussed on the microstructural characterization regarding crystallographic orientation, reconstruction and grain size distribution of prior austenite grains as well as metallographic investigations. Electrolytically polished cylindrical bulk specimens have been loaded under fully reversed (R=-1) loading condition using an ultrasonic fatigue testing machine (f=20000Hz) and an electro-mechanical resonance fatigue testing machine (f=95Hz). By means of a light microscope attached to the testing systems, fatigue crack initiation and propagation were correlated to the material microstructure and the specimens fatigue life. The application of a fast and high resolution thermography camera system in combination with the resonance testing system allows to use the heat dissipation determined on the surface of the fatigue specimen as a measure of specific fatigue damage. The tested specimens have been carefully investigated by means of scanning electron microscopy (SEM) in combination with automated electron back-scatter diffraction (EBSD) and energy dispersive X-ray (EDX) analysis. It was found that under HCF and VHCF loading conditions crack initiation is caused by slip band formation between the martensitic laths structure, which leads to microcrack initiation and propagation. The propagation of microcracks is sensitive to changing crystallographic orientations when crossing a grain boundary. In case of run-out specimens (109 cycles) microcracks of the length of several microns have been found that were blocked by prior austenite grain boundaries which act in this case as effective barriers against microcrack propagation.

Crack initiation characteristics and fatigue property of a high-strength steel in VHCF regime under different stress ratios

Crack initiation characteristics and fatigue property of a high-strength steel in very-high-cycle fatigue (VHCF) regime under different stress ratios were investigated in this paper. Fatigue tests were performed at stress ratios of -1, -0.5, 0.1 and 0.3 by using an ultrasonic fatigue testing machine. The difference of S-N data and the characteristics of crack initiation under different stress ratios were examined. It is shown that the magnitude of stress ratio has a substantial effect on the fatigue strength that decreases with the increase of stress ratio. However, the variation tendency of the S-N data from low-cycle fatigue to VHCF regime is similar for the four cases. SEM observations of the fracture surface indicate that fatigue crack initiates from the surface of specimen in low-cycle regime and initiates mostly from the inclusion in the interior of specimen in high-cycle and VHCF regimes, which is irrespective of the state of stress ratio. By means of Focused Ion Beam technique, the samples of crack initiation region were prepared then observed via TEM. The observations show the microscopic morphology of the crack initiation region, showing the different crack initiation characteristics and revealing the mechanism of crack initiation for different stress ratios. Moreover, the effect of inclusion size on fatigue life is discussed with the results showing that the effect of stress ratio and inclusion size on fatigue strength is well described by our proposed formula.

Very High Cycle Fatigue Failure Analysis and Life Prediction of Cr-Ni-W Gear Steel Based on Crack Initiation and Growth Behaviors.

The unexpected failures of structural materials in very high cycle fatigue (VHCF) regime have been a critical issue in modern engineering design. In this study, the VHCF property of a Cr-Ni-W gear steel was experimentally investigated under axial loading with the stress ratio of R = −1, and a life prediction model associated with crack initiation and growth behaviors was proposed. Results show that the Cr-Ni-W gear steel exhibits the constantly decreasing S-N property without traditional fatigue limit, and the fatigue strength corresponding to 109 cycles is around 485 MPa. The inclusion-fine granular area (FGA)-fisheye induced failure becomes the main failure mechanism in the VHCF regime, and the local stress around the inclusion play a key role. By using the finite element analysis of representative volume element, the local stress tends to increase with the increase of elastic modulus difference between inclusion and matrix. The predicted crack initiation life occupies the majority of total fatigue life, while the predicted crack growth life is only accounts for a tiny fraction. In view of the good agreement between the predicted and experimental results, the proposed VHCF life prediction model involving crack initiation and growth can be acceptable for inclusion-FGA-fisheye induced failure.

Fisheye failure analysis and life design approach for case-carburized gear steel based on statistical evaluation of defect size

The high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of a case-carburized gear steel were investigated under tension and compression, and the distribution characteristics of defect size was analyzed using four statistical functions including Statistics of Extreme Values (SEV), Generalized Pareto distribution (GP), Generalized Extreme Values (GEV) and Exponential Generalized Pareto distribution (EXPGP). Results show that the interior inclusion-induced failure with the existence of fisheye becomes the predominant failure mode in HCF and VHCF regimes. With the increasing of applied stress amplitude, the effect of crack size within the fisheye on fatigue life is gradually reduced. Based on this, a fatigue strength prediction model associated with the variation of crack size was developed. At a given probability, the predicted fatigue strength is the highest by the estimated maximum inclusion size using the GEV distribution, then followed by using Gumbel distribution and by using GP distribution, and the lowest by using EXPGP distribution, which is inversely proportional to the evaluated maximum inclusion sizes. Based on the good agreement between the predicted and experimental results, the proposed approach based on statistical evaluation of defect size can be well used to predict the HCF and VHCF fatigue lives with interior fisheye failure.

Fatigue behavior and mechanism of FV520B in very high cycle regime

BACKGROUND: High cycle fatigue (HCF) and very high cycle fatigue (VHCF) induced by wake flow and other causes are the main failure modes of impellers which are the key component of centrifugal compressors. Research on the fatigue behavior and mechanism of impeller material in the VHCF regime is beneficial to fatigue design and the remanufacturing of impellers, and it also has important significance to ensure a long operation life of centrifugal compressors. OBJECTIVE: The VHCF behavior and mechanism of impeller material FV520B has been studied. METHODS: Fatigue tests were conducted on a Shimadzu USF-2000 at a resonance frequency of 20 kHz and room temperature in air ambient. The fractures were observed by SEM. RESULTS: For FV520B-I, the S–N curve continuously declined, and there was no fatigue limit. Most of cracks initiated from subsurface inclusions in the VHCF regime. For FV520B-S, the S–N curve was far below that of FV520B-I, and there was a conventional fatigue limit. CONCLUSIONS: The fatigue behavior and mechanism of FV520B-I and FV520B-S are very different and determined by heat treatments. Prediction of the fatigue life in the VHCF regime according to the Paris law is not ideal. A new method for prediction of the fatigue life in the VHCF regime should be developed.

Very High Cycle Fatigue and Damage Behavior of Ti6Al4V

High frequency fatigue tests were carried out with a 20 kHz ultrasonic testing facility to investigate the cyclic deformation behavior of Ti6Al4V in the Very High Cycle Fatigue (VHCF) regime in detail. The S,Nf -curve at the stress ratio R = -1 shows a significant decrease of the stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 107 cycles. Microscopic investigations of the distribution of the α-and β-phase of Ti6Al4V indicate that inhomogeneities in the phase distribution are reasons for the internal crack initiation. Scanning electron microscopy as well as light microscopy were used to investigate the internal crack initiation phenomenon in the VHCF-regime. Beside the primary fatigue crack additional defects like micro cracks and crack clusters were observed in the fatigued specimens. SEM-investigations of specimens which were loaded up to 1010 cycles without failure show irreversible microstructural changes inside the specimens. Two step tests were performed to evaluate the influence of internal fatigue induced defects observed in specimens which did not fail within 1010 cycles.

Essential Characteristics and Frequency Effect for Very High Cycle Fatigue Behavior of Steels

With the increase of design fatigue life of many critical mechanical components and engineering structures, research on very high cycle fatigue (VHCF) has become a new topic for engineering components failure. This paper summarizes works of VHCF of high strength steel, such as the observations on fish-eye, which is one of the typical characteristics in VHCF regime; Characteristics of crack initiation and crack propagation are analyzed based on fracture surface; The present work also analyzes the fatigue mechanism and related models. Loading frequency effect on the VHCF behavior is also discussed. Some prospective aspects of future researches are proposed.

Evaluation of Very High Cycle Fatigue Properties of Low Temperature Nitrided Ti-6Al-4V Alloy Using Ultrasonic Testing Technology

Fatigue tests were carried out at the stress ratio R = -1 using a 20 kHz ultrasonic testing facility to investigate the effects of low temperature nitriding on the fatigue properties of Ti-6Al-4V alloy in the very high cycle fatigue (VHCF) regime in detail. The oscillation and fatigue behavior of the nitrided Ti-alloy were characterized by measuring parameters like the ultrasonic generator power, the displacement of the specimens and dissipated energy under ultrasonic cyclic load. Moreover, the surface microstructure of the nitrided Ti-alloy was characterized using a micro-Vickers hardness tester, an optical microscope, scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron backscatter diffraction technique (EBSD) to clarify the fatigue fracture mechanism. The Ti-alloy nitrided at the temperature of 873 K showed duplex S-N properties consisting of the respective fracture modes of the surface fracture and the subsurface fracture. The low temperature nitriding reduced the surface fatigue life of Ti-alloy in comparison to the un-nitrided one due to the formation of a brittle titanium nitride (Ti2N), whereas the subsurface fatigue life in the VHCF regime was increased by the low temperature nitriding. In addition, the fatigue fracture mechanisms of the low temperature nitrided Ti-alloy were discussed from viewpoints of fractography and fracture mechanics.

Through thickness property variations in friction stir welded AA6061 joint fatigued in very high cycle fatigue regime

Ultrasonic fatigue tests were performed on friction stir welded AA6061 joint to investigate very high cycle fatigue (VHCF) behaviors. As a result, almost all the fatigue cracks are initiated from local plastic slip markings around the boundary between thermo-mechanically affected zone and heat affected zone. The fatigue strength decreases from the top to root of the welded joint, owing to the variation of plastic deformation history and temperature distribution through the thickness. In fractography, the fatigue crack initiation site is surrounded by a semicircular flat zone, of which the formation in VHCF regime accounts for more than 98% of the total fatigue life.

Interior crack initiation and growth behaviors and life prediction of a carburized gear steel under high cycle fatigue and very high cycle fatigue

Abstract

Dominant factors for very‐high‐cycle fatigue of high‐strength steels and a new design method for components

AbstractDominant factors affecting fatigue failure from non‐metallic inclusions in the very‐high‐cycle fatigue (VHCF) regime are reviewed, and the mechanism for the disappearance of the conventional fatigue limit is discussed. Specifically, this paper focuses on the following: (i) the crucial role of internal hydrogen trapped by non‐metallic inclusions for the growth of the optically dark area (around the non‐metallic inclusion at fracture origin), (ii) the behaviour of the crack growth from a non‐metallic inclusion as a small crack and (iii) the statistical aspects of the VHCF strength, in consideration of the maximum inclusion size, using statistics of extremes. In addition, on the basis of the aforementioned findings, a new fatigue design method is proposed for the VHCF regime. The design method gives the allowable stress, σallowable, for a determined design life, NfD, as the lower bound of scatter of fatigue strength, which depends on the amount of components produced.

A two-parameter model to predict fatigue life of high-strength steels in a very high cycle fatigue regime

In this paper, ultrasonic (20 kHz) fatigue tests were performed on specimens of a high-strength steel in very high cycle fatigue (VHCF) regime. Experimental results showed that for most tested specimens failed in a VHCF regime, a fatigue crack originated from the interior of specimen with a fish-eye pattern, which contained a fine granular area (FGA) centered by an inclusion as the crack origin. Then, a two-parameter model is proposed to predict the fatigue life of high-strength steels with fish-eye mode failure in a VHCF regime, which takes into account the inclusion size and the FGA size. The model was verified by the data of present experiments and those in the literature. Furthermore, an analytic formula was obtained for estimating the equivalent crack growth rate within the FGA. The results also indicated that the stress intensity factor range at the front of the FGA varies within a small range, which is irrespective of stress amplitude and fatigue life.

Mean stress sensitivity of spring steel in the very high cycle fatigue regime

The influence of load ratio on the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of shot-peened VDSiCr spring steel is investigated. S–N curves are measured with ultrasonic fatigue testing equipment at load ratio R = −1, R = 0.1 and R = 0.5 up to 1010 cycles. Failures either occur below 5 × 106 or above 5 × 108 cycles. In the HCF regime cracks start exclusively at the surface. In the VHCF regime crack initiation occurs solely in the interior, at grain boundaries, inclusions or (rarely) in the matrix. The S–N curves continue to decrease beyond 109 cycles, most pronounced for R = −1 where the mean cyclic strength at 1010 cycles is 5 % lower than at 109 cycles. VHCF strength presented in a Haigh diagram can be very well approximated with a straight line. The mean stress sensitivity factor is M = 0.47 for load ratios between R = −1 and R = 0.5.

Fatigue testing of CFRP in the Very High Cycle Fatigue (VHCF) regime at ultrasonic frequencies

Carbon fiber reinforced polymers (CFRP) are more and more used for high performance applications such as aircraft structures. These structural components are subjected with more than 108 cycles during their lifetime. At present the fatigue behavior of CFRP and the corresponding failure mechanisms are not sufficiently investigated in the VHCF regime. To obtain comprehensive knowledge about the fatigue behavior as well as the failure mechanisms of CFRP in the Very High Cycle Fatigue (VHCF) regime, a new Ultrasonic Testing Facility (UTF) for cyclic three point bending of CFRP at 20 kHz has been developed at the Institute of Materials Science and Engineering (WKK), University of Kaiserslautern. This UTF with combined online nondestructive testing via laser vibrometry and IR-thermography enables VHCF experiments up to 109 cycles in twelve days. All experiments are carried out at a stress ratio between 0.29 and 0.49. To keep the specimen temperature below the glass transition temperature of the CFRP the cyclic bending load is divided in pulse pause sequences. The specimen material used in this investigation is a commercially available carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). For the accurate calibration of the loads as well as for the visualization of the strain distribution and the oscillation mode, measurements with 3D-scanning-laser-vibrometry have been carried out. Furthermore light optical micrographs plus SEM investigations have been performed upon completion and in defined interceptions of the experiments. Based on the results of the microscopic investigations the failure mechanisms of CF-PPS for the VHCF are described.

Cyclic torsion very high cycle fatigue of VDSiCr spring steel at different load ratios

Cyclic torsion fatigue tests with superimposed static torsion loads are performed with VDSiCr spring steel with shot-peened surface in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Fatigue properties are investigated at load ratios R=0.1, R=0.35 and R=0.5 up to limiting lifetimes of 5×109cycles with a newly developed ultrasonic torsion testing method. Increasing the load ratio reduces the shear stress amplitude that the material can withstand without failure. Fatigue cracks are initiated at the surface in the HCF regime. In the VHCF regime, cracks are preferentially initiated internally in the matrix, below the surface layer with compression residual stresses, and less frequently at the surface. Cyclic and mean shear stresses with 50% survival probability in the VHCF regime are presented in a Haigh diagram. Linear line approximation delivers a mean stress sensitivity of M=0.33 for load ratios between R=−1 and R=0.5.