Very High Cycle Fatigue Behavior Research Articles

Ultrasonic Fatigue of CFRP - Experimental Principle, Damage Analysis and Very High Cycle Fatigue Properties

Structural aircraft components are often subjected to more than 108 loading cycles during their service life. Therefore the increasing use of carbon fiber reinforced polymers (CFRP) as primary lightweight structural materials leads to the demand of a precise knowledge of the fatigue behavior and the corresponding failure mechanisms in the very high cycle fatigue (VHCF) range. To realise fatigue investigations for more than 108 loading cycles in an economic reasonable time a novel ultrasonic fatigue testing facility (UTF) for cyclic three-point bending was developed and patented. To avoid critical internal heating due to viscoelastic damping and internal friction, the fatigue testing at 20 kHz is performed in resonance as well as in pulse-pause control resulting in an effective testing frequency of ~1 kHz and the capability of performing 109 loading cycles in less than twelve days. The fatigue behavior of carbon fiber twill 2/2 fabric reinforced polyphenylene sulfide (CF-PPS) and carbon fiber 4-H satin fabric reinforced epoxy resin (CF-EP) was investigated. To study the induced fatigue damage of CF-PPS and CF-EP in the VHCF regime in detail, the fatigue mechanisms and damage development were characterized by light optical and SEM investigations during interruptions of constant amplitude tests (CAT). Lifetime-oriented investigations showed a significant decrease of the bearable stress amplitudes of CF-PPS and CFEP in the range between 106 to 109 loading cycles. The ultrasonically fatigued thermoset matrix composite showed a significantly different VHCF behavior in comparison to the investigated thermoplastic matrix composite: No fiber-matrix debonding or transversal cracks were present on the specimen edges, but a sudden specimen failure along with carbon fiber breakage have been observed. The fatigue shear strength at 109 cycles for CF-PPS could be determined to τa, 13 = 4.2 MPa and to τa, 13 = 15.8 MPa for the thermoset material CF-EP.

The potential of spinodal ferrite decomposition for increasing the very high cycle fatigue strength of duplex stainless steel

Duplex stainless steels (DSS) have become candidate materials for structural applications, where conventional austenitic stainless steels fail due to very high cycle fatigue (VHCF) in combination with corrosive attack. It seems that DSS exhibit a fatigue limit, which can be attributed to the two-phase austenitic–ferritic microstructure. Ultrasonic VHCF testing revealed that the phase boundaries are efficient obstacles for the transmission of slip bands and microstructural fatigue cracks up to 109 cycles and even beyond. The barrier strength is determined by the misorientation relationship between neighbouring grains but also by the strength of the individual phases. By thermal treatment at 475°C, spinodal decomposition of the ferrite phase results in the formation of Cr-rich α′ precipitates. While during static loading these precipitates give rise to a loss in ductility (475°C embrittlement), it was shown that the HCF strength can be increased and that there is also a tendency towards a beneficial effect on the VHCF behaviour. A more detailed analysis of the local plasticity sites by means of atom probe tomography (APT) revealed a dissolution of the α′ precipitates within operated slip bands. The dissolution might be an indication for a local softening mechanism that limits the VHCF strengthening effect of spinodal decomposition.

Effect of stress ratio on VHCF behavior for a compressor blade titanium alloy

Effect of stress ratio on fatigue properties of a titanium alloy (TC-17) in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) were investigated by electromagnetic and ultrasonic fatigue testing. The S–N curves at R=−1, 0.1, 0.5 and 0.7 at 110Hz and 20kHz were obtained and discussed. The effects of frequency on fatigue strength was also investigated. It was concluded that the fatigue strength with 50% fatigue failure probability at R=0.1, 0.5 and 0.7 is lower to the Goodman line and shows a bilinear decreasing trend. Cleavage fracture of primary grains in the surface and interior initiation zone were observed. The formation of the facets induced by the basal or prismatic slips of the H.C.P grains decreased the fatigue strength with variation in mean stress.

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.

Fatigue Assessment of Laser Additive Manufactured AlSi12 Eutectic Alloy in the Very High Cycle Fatigue (VHCF) Range up to 1E9 cycles

Selective laser melting (SLM) is a novel technique in laser additive manufacturing which uses laser energy to melt powder material according to the geometry of the computer aided design (CAD) model provided to the SLM system. The process uses a layer-wise manufacturing process which ensures that the manufactured parts possess good mechanical properties. The process is specifically suitable for complex geometries and customized parts which otherwise would be costly and, even, impossible to be manufactured using conventional manufacturing processes. However, for the application of the SLM process for aerospace applications, their performance needs to be investigated in very high cycle fatigue (VHCF) region. This study aims at determining the VHCF behavior of the AlSi12 alloy manufactured by the SLM process. Fatigue characterization has been carried out at frequencies of 20 Hz for high cycle fatigue (HCF) range and 20 kHz for the VHCF range until 1E9 cycles. Optical and scanning electron microscopes were used for microstructural and fracture analysis. The results show that the SLM parts outperform that of cast materials. However microstructural features as well as process-induced defects need to be controlled for a reliable fatigue performance.

VHCF Response of H13 Steels Produced with Different Manufacturing Processes

Experimental results have recently shown that failures at stress amplitude below the conventional fatigue limit and at Very High Cycle Fatigue (VHCF) are possible. In case of high-strength steels, VHCF failures generally originate from defects/inclusions present within the material and, consequently, the defect population properties (size and quantity) significantly affect the VHCF response of high-strength steels.Different refinement processes are commonly adopted in order to improve steel cleanliness: among the other, the ElectroSlag Remelting (ESR) process allows to obtain very clean high-strength steels, thus possibly inducing a significant enhancement of the VHCF response.The present paper aims at investigating the actual effect of the ESR process on the VHCF behavior of an AISI H13 steel. Fully reversed ultrasonic tension-compression tests are carried out on hourglass specimens manufactured with and without the ESR process. The estimated P-S-N curves highlight the positive effects of the ESR process on the VHCF response of the investigated H13 steel.

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.

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.

The Top 10 Influential Articles in Very High Cycle Fatigue

The top 10 most influential articles in Very high cycle fatigue (VHCF) have been indentified from web of science data. The attributes of the top 10 papers have been discussed. It was found that specialty area of fatigue called as “VHCF” is an emerging field. The most cited papers discussed the two the fatigue crack mechanism in fatigue. It was found that crack initiation shifts from surface to subsurface if the material beyond 107 cycles. There are some models which can predict the fatigue life of the material however the exact estimation is still challenging. Hence, it was found that still further efforts are required in the field to accurately understand the VHCF behavior.

Effect of microstructure on the very high cycle fatigue behavior of a bainite/martensite multiphase steel

We describe here the effect of microstructure on the very high cycle fatigue (VHCF) behavior of a high-strength bainite/martensite (B/M) multiphase steel (0.2C–1.4Si–2.2Mn–0.8Cr) studied through ultrasonic fatigue test, where the different microstructures were obtained via control of heat treatment. The study clearly points to that the microstructure had a significant effect on the VHCF behavior of B/M steel. The endurance limit of the steel after oil-cooling was enhanced by 75MPa because of change in the microstructure. Three types of failure modes were observed depending on the inclusion and microstructure, i.e., surface defect-induced failure, inclusion-induced failure, and non-inclusion induced failure, where the non-inclusion induced failure was influenced by microstructure. Large plastic deformation occurred within the bainite lath, leading to debonding from the adjacent martensite, and the crack initiated at grain boundaries between large bainite laths and martensite. There was a competition between inclusion-induced failure and non-inclusion induced failure modes, and the crack initiated at relatively weak sites that existed between the matrix and inclusions. The present study underscores a potential approach for improving VHCF properties of next generation of advanced high strength steels, where microstructure can be tuned to improve the VHCF performance.

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

The VHCF behaviour of metallic materials containing microstructural defects such as non-metallic inclusions is determined by the size and distribution of the damage dominating defects. In the present paper, the size and location of about 60.000 inclusions measured on the longitudinal and transversal cross sections of AISI 304 sheet form a database for the probabilistic determination of failure-relevant inclusion distribution in fatigue specimens and their corresponding fatigue lifes. By applying the method of Murakami et al. the biggest measured inclusions were used in order to predict the size of failure-relevant inclusions in the fatigue specimens. The location of the crack initiating inclusions was defined based on the modeled inclusion population and the stress distribution in the fatigue specimen, using the probabilistic Monte Carlo framework. Reasonable agreement was obtained between modeling and experimental results.

High Temperature Fatigue of Nickel-based Superalloys during High Frequency Testing

Abstract Microstructural features acting as stress raisers lead to localised and inhomogeneously distributed irreversible deformation in the very high cycle fatigue (VHCF) regime although load amplitudes are well below the classical fatigue limit. Hence, changes in the microstructure due to thermally activated processes can result in an overall change of damage mechanism compared to the fatigue behaviour at room temperature. The influence of isothermal testing at elevated temperatures up to 800 °C during high frequency testing was studied with regard to the VHCF behaviour of the nickel-based superalloys Nimonic 75 and 80A. This material combination allowed a distinction between the influence of the stability of the initial precipitation condition (precipitation-free, peak-aged, overaged) on the one hand and the formation of oxide layers on the other hand. High frequency tests were accompanied by extensive metallographic analysis by means of high resolution scanning and transmission electron microscopy. Early failure was primarily ascribed to the formation of microcracks in the emerging oxide layers. In this respect, the relevance of high frequency testing regarding the true fatigue behaviour, which in the case of thermally activated microstructural changes implies a likely time dependence of damage mechanisms, will be critically reviewed.

High-frequency cyclic testing of welded aluminium alloy joints in the region of very high cycle fatigue (VHCF)

In the present investigation high-frequency fatigue tests on welded specimens with trimmed surfaces were carried out by means of an ultrasonic fatigue testing system (UFTS). The wave-like distribution of the load amplitude along the longitudinal axis of the UFTS specimens with a maximum in the minimum cross-section (at about the half length) of the sample permits a selective testing of the different weld zones (weld seam, heat-affected zone and base metal). In addition, the influence of microscopic and macroscopic notch effects on the VHCF behaviour was analysed systematically for a precipitation-hardened Al-alloy in different heat-treatment conditions, representing the different zones of a welded joint. “Artificially” notched and heat-treated samples with the same notch factor compared to the weld reinforcement represent the macroscopic notch effect of the welded joint. A comparison of VHCF results of the welded samples with those based on notched specimens is presented and discussed. The weld metal with its various welding defects proved to be the weakest link of the joint structures, provided the reinforcement is trimmed. Welded fatigue samples show the lowest fatigue results in the VHCF regime compared to the base material and heat-affected zone. The VHCF behaviour of artificially notched samples is prone to microstructural effects as could be proven for various heat-treatment conditions.

Diversity of damage evolution during cyclic loading at very high numbers of cycles

Over the last decade, it has been shown for a number of metals that failure occurs even beyond the classical fatigue limit. High frequency testing techniques make it possible to conduct fatigue tests up to 109 or even 1011 cycles for various alloys, ranging from aluminium to high strength steels. As a consequence, the characterisation of fatigue life and damage mechanisms at N>107 cycles [very high cycle fatigue (VHCF)] has become a major research issue. Fatigue life in this regime is dominated by crack initiation. With the overall strain being in the purely elastic range, microstructural features acting as stress raisers lead to localised and inhomogeneously distributed irreversible deformation. Hence, microstructural discontinuities become the leading features controlling fatigue life at very high numbers of cycles. The present survey will focus on dislocation arrangement, grain orientation, grain size and surface roughening and their implications on the VHCF behaviour for selected virtually defect free metals, thus providing a sound basis for a detailed understanding of the relevant deformation and damage evolution mechanisms. It will also focus on the VHCF behaviour of materials representing a ‘transition’ between non-defect related damage evolution and defect based crack initiation, thus pointing out the complexity of damage evolution in the VHCF range. In this context, the term defect is limited to hard non-metallic inclusions, which can be found, among others, in high strength steels as well as pores in casting materials, both dominating the VHCF behaviour of these material types. In contrast, second phases, precipitates or intermetallic particles are considered as irregularities of the microstructure and will not be classified as defects. The current review will show that a true understanding of the VHCF behaviour requires a careful differential analysis of the possible microstructural features leading to localised plastic deformation and that not only crack initiation, but also crack growth behaviour analysis is essential to gain a sound basis for a reliable fatigue life prediction.

Very high cycle fatigue for GCr15 steel with smooth and hole-defect specimens

Very high cycle fatigue (VHCF) properties of a low temperature tempering bearing steel GCr15 with smooth and hole-defect specimens are studied by employing a rotary bending test machine with frequency of 52.5 Hz. Both smooth and hole-defect specimens break in VHCF regime with some difference in fatigue crack initiation. For smooth specimens, a fine granular area (FGA) is observed near the grain boundary in the fracture surface of the specimens broken after 107 cycles. But no FGA is observed in the hole-defect specimens broken in VHCF regime, and the VHCF crack does not initiate from the small hole at the surface as it does at low or high cycle fatigue regime. Internal stress is employed to explain the VHCF behavior of these two types of specimens. At last, an advanced dislocation model based on Tanaka and Mura model is proposed to illustrate the internal stress process and to predict fatigue crack initiation life with FGA observed in the fracture region.

Effect of precipitation condition, prestrain and temperature on the fatigue behaviour of wrought nickel-based superalloys in the VHCF range

The influence of a monotonic predeformation at room temperature on the very high cycle fatigue (VHCF) behaviour was studied in polycrystalline nickel-based superalloys (Nimonic 75 and Nimonic 80A) in three characteristic precipitation conditions (precipitation-free, peak-aged and overaged) after various degrees of predeformation. Isothermal fatigue tests were executed at temperatures up to 800 °C. In the intermediate temperature range (400–600 °C) the effect of predeformation becomes weaker with increasing temperature. This can be partially explained by the beginning of recovery. However, additional effects not related to the strain-hardened microstructure play a role, such as the formation of oxide layers and the decrease in notch sensitivity. The latter effect is important because of the surface roughening resulting from prestraining. Even for the overaged condition, 4% predeformation led to a higher VHCF fatigue resistance at 600 °C, due to the relatively stable recovered dislocation arrangement. Isothermal cyclic deformation at 800 °C revealed a pronounced decrease in cyclic lifetime for all precipitation conditions with or without a mechanical prehistory. Transmission electron microscopy was carried out in order to characterize the influence of the microstructure, the resulting dislocation slip behaviour and the relevant dislocation/particle interaction mechanisms on the VHCF behaviour. Furthermore, scanning electron microscopy observations revealed that all precipitation conditions show, independent of their predeformation condition, a transition from transgranular (room temperature to 600 °C) to intergranular (600–800 °C) microfatigue crack formation in the emerging oxide layers at the surface after VHCF tests.

Design of Dog-Bone-Shaped Ultrasonic Vibrational Fatigue Specimen and its Application in Study on VHCF Behavior of 6063 Aluminium Alloy

Based on the design of specimen with standard hourglass shape, a modified dog-bone-shaped ultrasonic vibrational fatigue specimen was proposed and studied in this paper. The resonance length and the distribution functions of axial displacement and stress of such specimen were obtained analytically. A comparison of the very high cycle fatigue (VHCF) life of 6061 aluminium alloy with dog-bone-shaped specimen and hourglass-shaped specimen was done and it indicate that the forer is obviously lower than the latter and the data obtained by specimen with proposed shape are conservative. Then, the ultrasonic vibrational fatigue test of 6063 aluminium alloy specimen with dog-bone-shape was done, showing that the fatigue failure occured at the middle of the specimen with invariable section where has predicted maximum stress; the S-N curve descends continuously and there is no fatigue limit as the traditional fatigue conception describes; the fatigue crack grows in the form of shearing.

Competition mechanism between microstructure type and inclusion level in determining VHCF behavior of bainite/martensite dual phase steels

The very high cycle fatigue (VHCF) behaviors of bainite/martensite (B/M) dual phase high-strength low-alloy (HSLA) steels were investigated using ultrasonic fatigue technology. Both non-inclusion and inclusion induced crack initiations occurred in the designed steels under VHCF test. It is shown that the VHCF behaviors are determined by the competition between the inclusion level of steels and the microstructure type consisted of different kinds of phases, microstructure homogeneity and refinement degree. The physical and chemical metallurgy methods could be coordinated to achieve excellent VHCF properties of high strength steels through microstructure optimization and inclusion size reduction.

Prehistory effects on the VHCF behaviour of engineering metallic materials with different strengthening mechanisms

Engineering materials often undergo a plastic deformation during manufacturing, hence the effect of a predeformation on the subsequent fatigue behaviour has to be considered. The effect of a prestrain on the microstructure is strongly influenced by the strengthening mechanism. Different mechanisms are relevant in the materials applied in this study: a solid-solution hardened and a precipitation-hardened nickel-base alloy and a martensite-forming metastable austenitic steel. Prehistory effects become very important, when fatigue failure at very high number of cycles (N > 107) is considered, since damage mechanisms occur different to those observed in the range of conventional fatigue limit. With the global strain amplitude being well below the static elastic limit, only inhomogeneously distributed local plastic deformation takes place in the very high cycle fatigue (VHCF) region. The dislocation motion during cyclic loading thus depends on the effective flow stress, which is defined by the global cyclic stress-strain relation and the local stress distribution as a consequence of the interaction between dislocations and precipitates, grain boundaries, martensite phases and micro-notches. As a consequence, no significant prehistory effect was observed for the VHCF behaviour of the solid-solution hardening alloy, while the precipitation-hardening alloy shows a perceptible prehistory dependence. In the case of the austenitic steel, strain-hardening and the volume fraction of the deformation-induced martensite dominate the fatigue behaviour.

Influence of prestraining on the high-temperature fatigue behaviour of polycrystalline nickel-based superalloys in the VHCF range

The influence of a monotonic predeformation at room temperature on the very high cycle fatigue (VHCF) behaviour was studied on polycrystalline nickel-based superalloys (Nimonic 75 and Nimonic 80A) in three characteristic precipitation (precipitationfree, peak-aged and overaged) and different predeformation conditions. Isothermal fatigue tests were executed at temperatures up to 800 °C. With increasing intermediate temperature (400 °C–600 °C) the effect of predeformation becomes weaker which might partially be explained by the beginning of a recovery of the dislocation arrangement as well as attributed to effects not related to the strain hardened microstructure, such as the formation of oxide layers and the materials notch sensitivity for the testing conditions used given pronounced surface roughening due to the prestraining. Isothermal cyclic deformation at 800 °C revealed a pronounced decrease of cyclic lifetime for all precipitation conditions with or without a mechanical prehistory. Transmission electron microscopy was carried out in order to characterize the influence of the microstructure, the resulting dislocation slip behaviour and the relevant dislocation/particle interaction mechanism on the VHCF behaviour.