Very High Cycle Fatigue Behavior Research Articles

Very high cycle fatigue of fiber‐reinforced polymer composites: Uniaxial ultrasonic fatigue

AbstractThis review explores uniaxial ultrasonic fatigue (USF) testing as a common and dependable method for quantifying the extended fatigue life of fiber‐reinforced polymer (FRP) composites. The objective is to explain the complexities governing the fatigue life behavior of FRPs, particularly in the realm of very high cycle fatigue (VHCF) where the number of loading cycles exceeds 107. To this end, this review encompasses the analysis of VHCF behavior, including the derivation and interpretation of stress–life (S–N) data, the evaluation of various fatigue damage mechanisms (i.e., controlling mechanisms of crack initiation and propagation) exhibited in FRP composites, and a thorough investigation of the frequency‐dependent effects on fatigue responses. Furthermore, this review tries to analyze the microscopic intricacies intrinsic to the VHCF failure of FRP composites, encompassing aspects such as fiber‐matrix de‐bonding, matrix cracking, and delamination, unveiling their modes and effects in a detailed manner. This review also underscores the pivotal integration of simulations, machine learning, and modeling techniques, emphasizing their crucial role in explaining both macroscopic and microscopic interactions governing the VHCF of FRPs.

Very high cycle fatigue of laser powder bed fused Al-Cu-Mg-Ag-TiB2 (A20X) Alloy: Stress relief and aging treatments

This study presents a comprehensive exploration of the fatigue response in the very high cycle fatigue (VHCF) regime for an additively manufactured (i.e., laser powder bed fused) A20X aluminum alloy. Although the need for high-performance materials with exceptional fatigue qualities has increased dramatically, the VHCF behavior of Al-Cu-Mg-Ag-TiB2 (A20X) structures remains largely unknown. A series of ultrasonic fatigue tests were performed to assess the prolonged fatigue life of the A20X alloy (in the VHCF domain where the number of cycles to failure is beyond 10 million cycles). The VHCF response, assessed through ultrasonic fatigue testing, was investigated by examining the stress-life (S-N) curves in a statistical framework, the fatigue crack initiation and propagation behavior, and the fracture surfaces. An asymptotic trend was experimentally found at 109 cycles, with a stress amplitude of 110 MPa for stress-relieved (SR) and 125 MPa for artificially aged (T7) materials, indicating the presence of an endurance limit. Furthermore, fracture surfaces showed the typical fisheye morphology, with a fine granular area (FGA) containing an internal crack-initiating site. The findings of this paper can assist in optimizing fatigue and durability design allowable for applications for extended fatigue life in the VHCF domains.

Crack initiation induced nanograins and facets of a titanium alloy with lamellar and equiaxed microstructure in very-high-cycle fatigue

Fractographic and microstructure features in crack initiation region in very-high-cycle fatigue (VHCF) were characterized for an extruded titanium alloy VT3-1 under stress ratios R = −1 and 0.1. The material consists of lamellar and equiaxed microstructure. All specimens failed by internal crack initiation with rough area feature in VHCF. Within the rough area region, facets initiated from a big cluster of lamellar α at R = 0.1, and nanograin layers formed at some local areas underneath fracture surfaces due to insufficient contact of NCP (numerous cyclic pressing) process in the inhomogeneous microstructure at R = −1. Here, NCP mechanism still dominates the VHCF behavior of this titanium alloy under negative stress ratios, and facets dominate at R > 0.

Damage assessment during ultrasonic fatigue testing of a CF-PEKK composite using self-heating phenomenon

The knowledge and experience with the very high cycle fatigue (VHCF) behavior are limited compared to the low cycle and high cycle fatigue behavior of fiber-reinforced composite materials. Accelerated cyclic loading using an ultrasonic fatigue testing system is one potential solution to evaluate the longevity of composite materials within a reasonable time. Insufficient sampling rates and the inherent inaccuracies of non-contact measurement devices pose critical challenges in the adaptation of accelerated fatigue experiments. This study aims to overcome these challenges by combining the temperature response of the composite specimen and the input parameters for the dynamic response of the in-house developed ultrasonic fatigue testing system to detect damage before failure. Increasing- and constant-amplitude fatigue experiments were conducted on a carbon fiber satin fabric-reinforced poly-ether-ketone-ketone (CF-PEKK) composite material. These experiments were carried out under three-point bending loading conditions at a cyclic frequency of about 20 kHz. Various parameters, such as the surface temperature, the cyclic displacement, and the input resonance parameters of the ultrasonic generator, were recorded during these experiments. The VHCF behavior of the CF-PEKK material system was characterized based on these results and through digital light optical microscopy.

Very High Cycle Fatigue of Welds: A Review

The design life of welded structures and components extends into the very high cycle fatigue (VHCF) regime across various applications. However, the availability of data on the fatigue behaviour of welded joints in the VHCF regime is limited, particularly when compared to the low and high cycle fatigue regimes. The development of ultrasonic fatigue testing equipment has accelerated fatigue testing and allowed for the VHCF properties of welds to be investigated in a feasible timeframe. In the present review, the emerging research concerning the VHCF behaviour of welds of various steels and non-ferrous alloys are individually explored. Overall, it is observed that welded joints have significantly lower fatigue strength than the base metal in the VHCF regime and that welding defects have a considerable influence on fatigue strength. Through the discussion of the relevant literature, important findings concerning the effects of specimen geometry and fatigue improvement methods are underlined. Furthermore, the guidance provided within design standards is compared, and some examples of VHCF failures of in-service components are highlighted. Finally, perspectives on future directions of investigation are put forward with the aim of encouraging further research in the field of VHCF of welds.

Wire arc additive manufactured AWS ER100S-G steel: Very high cycle fatigue characterization

This paper presents a comprehensive study on the very high cycle fatigue (VHCF) characterization of a wire arc additive manufactured (WAAM) AWS ER100S-G steel. The demand for high-performance materials with superior fatigue properties has grown exponentially. However, the VHCF behavior of large-format WAAM’d structures remains relatively unexplored. In this study, a series of VHCF tests were conducted under fully reversed cyclic loading conditions to investigate the extended fatigue life (performance) of the WAAM ER100S-G steel. The VHCF properties, relative to conventional fatigue employing a servohydraulic testing system, of the WAAM ER100S-G steel were evaluated by analyzing the stress-life (S-N) curves, fatigue crack initiation, and propagation behavior, in a statistical framework, and the fracture surfaces. The results revealed the controlling mechanisms of VHCF failure of the WAAM ER100S-G steel and the fatigue response of the material beyond the conventional fatigue limit of 107 cycles. The findings provide valuable insights into the influence of WAAM-induced defects/microstructure on the extended fatigue performance of WAAM ER100S-G steel, which can aid in optimizing fatigue and durability design guidelines for additive manufacturing applications in high-cycle and very high-cycle fatigue domains.

Probabilistic Prediction Synthesizing Data and Science-Based Modeling for Very High Cycle Fatigue

Abstract Epistemic and aleatoric uncertainty are inherent in engineering modeling and experimentation. Uncertainty, which must be considered for accurate life estimation and prediction, is an undeniable aspect of very high cycle fatigue (VHCF). Frequently, limited data are available for VHCF. The magnitude and extent of uncertainty are increased for high reliability applications or gigacycle predictions. The purpose herein is to present a methodology for managing uncertainty satisfactorily. The procedure integrates data with science-based modeling (SBM) for VHCF. An extensive set of data for SUJ2 steel, which exhibits bimodal damage growth for some loading conditions, will be used. One mode is associated with damage nucleating from internal particles, and the other is surface-induced damage, both of which can have fatigue lives in excess of 108cycles. One important conclusion is that synthesis of SBM with data greatly improves reliability estimation and prediction. As the accuracy of the SBM increases, the effect of uncertainty is reduced; however, even basic approximations are more beneficial than empirical analysis alone. Consequently, an SBM that reasonably predicts VHCF behavior resulting from multiple modes of damage growth, which is tuned subsequently by infusing VHCF data, is warranted. The approach focuses on the estimation and prediction of the cumulative distribution functions for life given the loading condition and their statistical goodness of fit.

Very high cycle fatigue assessment at elevated temperature of 100 μm thin structures made of high-strength steel X5CrNiCuNb16-4

Many components and structures are exposed to very high number of cycles and challenging environmental conditions during operation. This study contributes to a better understanding of the very high cycle fatigue (VHCF) properties of high-strength steel X5CrNiCuNb16-4 at room temperature (RT) and 350 °C. For this purpose, conventional specimens and thin-walled structures are extensively examined with novel high-frequency fatigue testing techniques at elevated temperature. Tests with unnotched specimens at 350 °C show a 21.7% reduction in fatigue strength for 107 cycles and a different failure mechanism compared to RT. In contrast, no temperature influence is observed for mildly notched specimens and even a higher local fatigue strength is found for sharply notched specimens at 350 °C. The decrease in fatigue strength for 109 cycles is more pronounced at 350 °C (−10%) than at RT (−5%), and it is proven that notched specimens adequately represent the VHCF behaviour of structures. The transferability of specimen results to components and structures is given great attention. A new proposal for the VHCF strength assessment of structures with high stress gradients is presented, which is based on specimen results, an extended material-mechanical support factor and a VHCF reduction factor. The prediction model gives conservative fatigue strength estimates for 109 cycles with a maximum deviation of 5.8%. This demonstrates that even complex shaped structures can be successfully evaluated with suitable specimens and methods.

Interior crack initiation during the very‐high‐cycle fatigue of railway wheel steel under axial loading and rolling contact loading

AbstractIn this paper, a comparative study of the very‐high‐cycle fatigue (VHCF) behavior of railway wheel steel under axial loading and rolling contact loading was conducted. Fatigue tests were performed with an ultrasonic fatigue test machine under axial loading, and the fracture surfaces from the fatigue tests and shattered rims taken from the failed railway wheels were observed. The wheel steel under axial loading presents a VHCF behavior with Mode I crack, and that under rolling contact loading is a VHCF behavior with mix Mode II–III crack. For the VHCF behavior with Mode I crack, surface and interior crack initiation occurred with equal probability at both low and high stress levels and produced a dual linear S–N curve since the value of fatigue limits for the surface and interior crack initiation are close. For the VHCF behavior with mix Mode II–III crack, cracks were initiated from the interior Al2O3 inclusion, and the fatigue life was beyond 107 cycles. Fatigue bands were observed on the fracture surface under rolling contact loading. The ferrite nanograins formed due to the stress state of shear plastic strain with a large compressive stress. The formed nanograins were softer than the matrix caused by the redistribution of the carbon.

Effect of thermal induced porosity on high-cycle fatigue and very high-cycle fatigue behaviors of hot-isostatic-pressed Ti-6Al-4V powder components

• The regrowth of the residual pores in the as-HIPed Ti-6Al-4V powder component occurs after solution heat treatment. • Effect of thermal induced porosity on the very-high cycle fatigue behavior was firstly studied. • The competition of different failure types in the very-high cycle fatigue regime is related to the area density of pores (pore size >40 μm). The present work reports the effect of thermal induced porosity (TIP) on the high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) behaviors of hot-isostatic-pressed (HIPed) Ti-6Al-4V alloy from gas-atomized powder. The results show that the residual pores in the as-HIPed powder compacts present no obvious effect on the HCF life. The regrowth of the residual pores can be observed after solution heat treatment. The pore location ranks the most harmful for the fatigue life compared with the other initiating defects. The maximum stress intensity factors were calculated. The plastic zone size of fine granular area (FGA) is much less than the characteristic size of the microstructure, and the crucial size of the internal pores in this study is about 40 μm. The failure types of fatigue specimens in the VHCF regime were classified, and the competition of different failure types was described based on the modified Poisson distribution.

VHCF Behavior of Welded Joints with HFMI Treatment under Moisture Conditions

Fatigue tests of cruciform welded joints made of Q355B steel at very-high-cycle fatigue (VHCF) regimes were carried out on as-welded specimens using high-frequency mechanical impact (HFMI) treatment in dry air and water-spray environments, respectively. The influence of the environment on fatigue life was more obvious in the VHCF regime. It was found that S-N curves became flat over the range of 106–108 cycles for as-welded specimens, while a continuously decreasing S-N curve existed for HFMI-treated specimens. Fatigue cracks initiated from the weld toe of the as-welded specimens in dry air and water-spray environments. Due to residual stress, the crack initiation site transition of HFMI-treated specimens from the weld toe to the weld root and base metal was observed at lower stress levels. Moreover, hydrogen-assisted quasi-cleavage and intergranular fracture were captured using a scanning electron microscope and a hydrogen permeation test.

Ultrasonic Fatigue Device and Behavior of High-Temperature Superalloy Inconel 718 with Self-Heating Phenomenon

Ultrasonic resonance fatigue test method at 20 kHz related to the very high cycle fatigue (VHCF) aims to accelerate a time-consuming experiment. In this paper, an ultrasonic fatigue device with a data acquisition system was improved for monitoring and recording the data from fatigue tests in which self-heating phenomenon exists. Symmetric tension-compression sinusoidal vibrating mode (R = −1) was observed in this study. VHCF behavior and mechanism of Inconel 718 were carried out using this device. It was concluded that more than 99% of fatigue life is consumed in initiation duration. Specimen temperature increase was not a decisive factor in VHCF strength for Inconel 718, as long as it was far less than the design temperature limitation. A single initiation site existed at the subsurface facet or grain cluster, observed from scanning electron microscope (SEM) micrographs. Quasi-cleavage fracture in transgranular ductile mode emerged and then tended to trace grain boundaries in an intergranular manner by cleavage-dominated mixed mode.

Recent Advances in Very High Cycle Fatigue Behavior of Metals and Alloys—A Review

We reviewed the research and developments in the field of fatigue failure, focusing on very-high cycle fatigue (VHCF) of metals, alloys, and steels. We also discussed ultrasonic fatigue testing, historical relevance, major testing principles, and equipment. The VHCF behavior of Al, Mg, Ni, Ti, and various types of steels were analyzed. Furthermore, we highlighted the major defects, crack initiation sites, fatigue models, and simulation studies to understand the crack development in VHCF regimes. Finally, we reviewed the details regarding various issues and challenges in the field of VHCF for engineering metals and identified future directions in this area.

Internal crack characteristics in very-high-cycle fatigue of a gradient structured titanium alloy

Gradient structure (GS) is commonly designed and processed in engineering materials to improve mechanical properties especially fatigue performance by taking advantage of the strengthened surface. However, whether the very-high-cycle fatigue (VHCF) property can be improved by GS is questioning due to the different crack initiation mechanisms between low-, high-cycle and VHCF. In this paper, GS of a Ti-6Al-4V alloy is generated by pre-torsion and characterized by electron backscatter diffraction. Then the VHCF behavior of the GS specimen is studied. The fractography and synchrotron radiation X-ray microtomography presented detailed characteristics of the internal crack initiation region in VHCF of the titanium alloy with GS. The results indicated that, in contrast to the low- and high-cycle regimes, the VHCF strength is reduced for the specimens with GS. Thus, the GS induced by pre-torsion cannot enhance the VHCF strength of the titanium alloy. This implies that VHCF test (property) is an important consideration for the microstructural designed materials. The graphical abstract is available in Supplementary information.

Effect of Ultrasonic Peening Treatment on VHCF Behavior of Friction Stir Welded Joints in Aluminum Alloys

Studying the mechanical properties of aluminum alloy 6061 friction stir welded (FSW) joints after ultrasonic peening treatment (UPT) under very high cycle fatigue (VHCF) loading. The effect of joints ultrasonic peening treatment was explored. It can be seen from the results that the ultrasonic peening treatment can form a reinforced layer on the surface of joints, and the fatigue crack source is transferred from the surface of the specimen to the inside. However, the change of fatigue crack initiation site does not improve the fatigue strength of specimen through ultrasonic peening treatment. The fatigue strength of the aluminum alloy 6061 joint after ultrasonic peening treatment is even lower than as a received condition. The characteristics of fatigue crack initiation and the mechanism of surface strengthened layer on fatigue strength were analyzed and discussed.

Influence of the annealing and defects on the VHCF behavior of an SLM AlSi10Mg alloy

AbstractIn the paper, the influence of the annealing temperature on the Very High Cycle Fatigue (VHCF) of AlSi10Mg specimens produced through selective laser melting (SLM) is experimentally assessed. VHCF tests at 20 kHz are carried out on Gaussian specimens subjected to a heat treatment suggested by the system supplier (heating for 2 hours to 320°C and air cooling) and to a heat treatment proposed by the authors (heating for 2 hours to 244°C and air cooling). The defects originating failure and the conditional P‐S‐N curves are compared. Experimental results show that an annealing temperature of 320 ° C induces the spheroidization of the Si network, which enhances the ductility but has a negative effect on the VHCF response. On the contrary, by reducing the heating temperature to 244 ° C, the original as‐built microstructure is not altered and the minimization of the residual stresses permits to enhance the VHCF response.

VHCF Response up to 109 Cycles of SLM AlSi10Mg Specimens Built in a Vertical Direction

It is well-known that many manufacturing parameters affect the quasi-static and the fatigue response of additive manufacturing (AM) parts. In particular, due to the layer-by-layer production, the load orientation, with respect to the building direction, plays a fundamental role for the fatigue response. This paper investigates the fatigue response up to 109 cycles (very high cycle fatigue (VHCF)) of selective laser melting (SLM) AlSi10Mg specimens built in a vertical direction. Ultrasonic tension-compression tests (stress ratio of –1) are carried out on as-built Gaussian specimens with a large loaded volume (2300 mm3). Fracture surfaces are investigated with the scanning electron microscope to analyze the defects originating the VHCF failure. Probabilistic S-N curves are estimated and analyzed. Experimental results confirm that the defect size controls the VHCF response, thus highlighting the importance of testing large risk volumes for a reliable assessment of VHCF behavior. The average value of the VHCF strength is close to that of the hourglass specimen tested in the literature. The variability of the VHCF strength is instead significantly larger, due to the scattered size distribution of the defects located near the specimen surface, which is the most critical region for crack initiation.

An overview of very high cycle fatigue behavior of additively manufactured Ti-6Al-4V

This paper presents a brief review on the current state of knowledge of the very high cycle fatigue (VHCF) behavior of metallic parts fabricated using Additive Manufacturing (AM) processes. It has been shown that AM has significant potential to replace traditional manufacturing methods that impose geometric limitations to designs. Powder-based metallic AM methods allow for precise layer-wise processing of complex net-shape parts without the use of special tooling or molds. Among various metals commonly used in AM processes, titanium (Ti) 6Al-4V alloy is currently of great interest especially in aerospace applications that contain parts with complex geometries (i.e., turbine blades in jet engines). To safely adapt AM Ti-6Al-4V parts in these applications, their mechanical properties and fatigue behavior must be understood. Various studies have identified AM process-induced defects (i.e., entrapped gas pores, lack of fusion defects between build layers, etc.) to be the main cause of fatigue failure of AM Ti-6Al-4V parts. However, there are limited studies relating to the effects of these defects on the behavior of AM metals in the VHCF regime (beyond 107 cycles). Knowledge of the VHCF behavior of AM Ti-6Al-4V is needed for the aforementioned applications due to the high loading frequencies and long service lives required for these parts.

Effect of Artificial Defects on the Very High Cycle Fatigue Behavior of 316L Stainless Steel

Widely used for structural materials in nuclear engineering, 316L austenitic stainless steel undergoes very high cycle fatigue (VHCF) throughout its service life. Since defects caused by service conditions are unavoidable in many engineering components during service life, the effects should be properly understood. In the present study, the effect of surface defects on the VHCF behavior were investigated on solution annealed (SA) and cold-worked (CW) 316L. Surface defects were artificially created using indentation. The VHCF test was conducted using an ultrasonic fatigue testing system. The results showed that the fatigue crack initiation was independent of the indent with the applied range of depth in this research. Furthermore, the critical depth of the indent was evaluated based on an empirical formula (Murakami’s model). In the case of SA 316L, the VHCF strength was not affected when the indent depth was less than 40 μm, which is consistent with the value obtained from the empirical formula. In the case of 20% CW 316L, the VHCF strength was not affected when the indent depth was less than 80 μm. The experimental results, i.e., the critical depth of the indent, were much larger than the results obtained from the empirical formula, and might have been caused by the plastic deformation, residual stress, and probable deformation-induced martensite transition around the indent.

Effect of electroslag remelting on the VHCF response of an AISI H13 steel

AbstractExperimental results have shown that high‐strength steels can fail in the Very‐High‐Cycle Fatigue (VHCF) regime with cracks originating from internal defects. Material cleanliness thus plays a major role in the VHCF response of high‐strength steels and refinement processes (eg, electroslag remelting [ESR] and vacuum arc remelting) could significantly enhance their performance.The present paper aims at investigating the effect of the ESR process on the VHCF behavior of an AISI H13 steel by carrying out fully reversed ultrasonic tension‐compression tests on hourglass specimens manufactured with and without the ESR process. Size effect is also taken into account in the paper: the effectiveness in the prediction of the VHCF response of specimens with large risk‐volumes from experimental data on specimens with small risk‐volumes is discussed and experimentally validated.