Very High Cycle Fatigue Tests Research Articles

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.

Very High Cycle Fatigue Properties and Damage Evolution in Al-Si Cast Alloys

Abstract The steady progress in electro mobility increases the demand for lightweight space frame and engine solutions. Advanced aluminum-silicon cast alloys with additions of magnesium, copper, or both for precipitation hardening are promising candidates for lightweight structures because of their high strength-to-weight ratio. For a reliable design, the fatigue performance in the very high cycle fatigue (VHCF) regime must be understood. Therefore, this study deals with the influence of frequency on VHCF properties as well as fatigue damage evolution in AlSi7Mg and AlSi10Mg sand-cast alloys. The VHCF tests were performed using a resonant (70 Hz), a high-frequency resonant (1 kHz), and an ultrasonic (20 kHz) fatigue testing system. Thereby, a significant frequency effect could be determined that resulted in an increased fatigue lifetime by 1 decade at 1 kHz and by 2 decades at 20 kHz, whereas no significant change in fatigue limit could be determined. Moreover, the fatigue crack initiation mechanisms change from surface crack initiation at 70 Hz and 1 kHz to volume crack initiation at 20 kHz. Using the defect-based Murakami approach, including a lightweight extension of Noguchi, a defect-induced (size, location) frequency effect could be excluded. Based on damage monitoring, the effect could be related to the crack propagation phase, which is accelerated at low frequencies (<100 Hz) because of a humidity reaction with the fatigue crack surface and the formation of atomic hydrogen resulting in a saturated hydrogen embrittlement per cycle. At high frequencies of 1 kHz and 20 kHz, no saturated hydrogen embrittlement per cycle can be reached.

Comparative study of the effects of conventional shot peening and ultrasonic shot peening on very high cycle fatigue properties of GH4169 superalloy

This study investigates the effects of two different shot peening techniques, conventional shot peening (CSP) and ultrasonic shot peening (USP), on the very high cycle fatigue (VHCF) properties of the GH4169 superalloy. The study evaluates and compares surface roughness, X-ray diffraction patterns, microhardness, and residual stress in specimens treated with CSP and USP. The VHCF tests reveal that both CSP and USP treatments improve the VHCF properties of the GH4169 superalloy. However, while the USP-treated specimens have deeper work hardening and compress residual stress layers, their VHCF lives are significantly shorter than those of the CSP-treated specimens. Additionally, the study finds that the stress triaxiality, which inhibits dislocations at the crack tip, is higher closer to the center of the specimen. The transmission electron microscopy (TEM) results further confirm that the reduced dislocation density and increased stress concentration in the plastic zone of the high stress triaxiality region promote fatigue short crack propagation and reduce the fatigue properties of the material.

High Cycle Fatigue and Very High Cycle Fatigue of Orthorhombic (O + α<sub>2</sub>)-Type Ti–27.5Nb–13Al Alloy with and without B and Fatigue Striation Analysis in Comparison with (α + β)-Type Titanium Alloys

The lightweight and high strength Ti–27.5Al–13Nb intermetallic alloy, based on the orthorhombic Ti2AlNb phase (O phase) and the α2 phase incorporated, was previously developed by the authors. This alloy would seem to have good potential for applications where fatigue behavior is a main concern, such as automobile and aircraft engine parts. The minor addition of boron (B) is known to refine the ingot grain size and thus to improve the subsequent mechanical properties. For these reasons, the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of B-free and 0.1 pct B-modified Ti–27.5Al–13Nb alloy were examined in the present study. HCF tests were performed at room temperature (RT) in tension-tension mode at an R of 0.1 and a frequency of 10 Hz, while VHCF tests were performed using an ultrasonic resonance fatigue test machine at an R of −1 and a frequency of 20 kHz. In both fatigue tests, hourglass-shaped specimens were used. With the addition of 0.1 pct B, the prior B2 grain size of an ingot was reduced drastically, from 600∼1000 µm for the B-free alloy to 100∼250 µm. The 0.1 pct B-modified Ti–27.5Al–13Nb alloy with a duplex microstructure consisting of a globular α2 phase and a lamellar microstructure exhibited superior elongation of 6.1 pct at RT. The HCF curve for this alloy with a duplex microstructure was almost the same as that for a Ti–6Al–4V alloy with a fully lamellar microstructure. Although prolonged fatigue life was previously reported in the HCF region in the 0.1 pct B-modified Ti–6Al–4V alloy, the addition of 0.1 pct B to the Ti–27.5Al–13Nb, Ti–6Al–4V and Ti–4Al–2.5V–1.5Fe alloys had no such effect in the VHCF region. VHCF strength for a lamellar microstructure was ranked in the order of Ti–4Al–2.5V–1.5Fe, Ti–6Al–4V and Ti–27.5Al–13Nb from the highest. Well-defined striations were observed at the propagation stage area of the fatigue fracture surface of B-free Ti–27.5Al–13Nb, and the measured striation spacing kept a constant of 0.29 µm through the propagation distance of 300 µm. The calculation based on this observation showed that the fatigue life spent in the propagation stage was very short and thus almost 100 pct of HCF life was thought to be spent in the fatigue initiation stage. For the B-free Ti–6Al–4V alloy with an equiaxed microstructure, the striation spacing increased from 0.06 µm to 4 µm as the fatigue crack propagated for a distance of 1,000 µm. Calculation based on the striation spacing revealed that, similar to the case with the Ti–27.5Al–13Nb alloy, the fatigue initiation stage consumed almost 100 pct of fatigue life regardless of the B addition.

Effect of shot peening on high cycle and very high cycle fatigue properties of Ni-based superalloys

In this work, the GH4169 superalloy specimens were shot-peened at two different intensities (0.15 mmA and 0.3 mmA) to improve the fatigue properties, and the fatigue behavior of the shot-peen-treated specimens was investigated in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. The results of the HCF and VHCF tests indicated that shot peening improved the fatigue properties of the material. In addition, it has shown that the tensile residual stresses introduced by excessive shot peening intensity reduce the benefits of shot peening in terms of improving fatigue properties.

Artificial intelligence modeling of ultrasonic fatigue test to predict the temperature increase

The temperature behavior in very high cycle fatigue (VHCF) testing as well as the influence of the intermittent loading is not completely understood. In many cases the high frequency causes the specimens to heat up and may interfere in the material's fatigue performance. In order to address this issue, this study proposed an experimental test with different stress levels and intermittent driving (pulse-pause) with the aid of non-destructive testing (NDT) using a thermography camera. Specimens were coated with black spray to improve the emissivity to 0.93 and conducted to fully reversed condition (R = −1) up to 107 cycles. A large amount of raw data of pulse, pause, stress amplitude, number of cycles and temperature were recorded. These raw data were used to develop tree-based machine learning models called extreme gradient boosting (XGBoost), capable of predicting the temperature throughout the VHCF tests. The result presented a high performance model with determination coefficients (R2) above 0.98, proving the model to be an important ally for ultrasonic fatigue tests. Additionally, Shapley additive explanation (SHAP) method was adopted to assist in the interpretation of the model results.

Estimation of stress in specimens loaded with ultrasonic fatigue machines

Piezo-electric ultrasonic fatigue machines are used to carry out fatigue tests more rapidly than what is possible using other technologies, at a frequency of 20kHz. The very high cycle fatigue (VHCF) domain can be studied with these machines as 109 cycles are reached within 14 h when specimens are loaded at stress amplitudes below the yield stress or conventional fatigue strength. The estimation of stress in specimens fatigued at high frequency is a current challenge when adopting this technology. This paper discusses the accuracy and reliability of three methods used to estimate stress amplitudes in specimens subjected to VHCF tests at a high loading frequency. Two historically used methods using strain gauges and a laser vibrometer are discussed and compared with a third, recently developed method based on time-resolved in situ X-ray diffraction (XRD). The three methods are applied to estimate the stress amplitude in a pearlitic steel specimen. The experimental artifacts and uncertainties are evaluated quantitatively to compare the benefits and limits of the methods. The experimental results show that the three methods correctly estimate the stress amplitudes applied to fatigued specimens.

Ultrasonic VHCF Tests on Very Large Specimens with Risk-Volume Up to 5000 mm3

The research on the size-effects in Very-High-Cycle Fatigue (VHCF) has recently drawn the attention of several scholars. The fatigue cracks in VHCF originate from the largest defect present within the loaded material volume (risk-volume) and the larger the risk-volume, the larger the probability of critical defects affecting the VHCF response (size-effect). Many models have been proposed in the literature to deal with size-effects in VHCF. However, the proposed models cannot be validated on full-scale components, since VHCF tests are typically carried out with ultrasonic fatigue testing machines. The authors have proposed a specimen geometry, the so-called Gaussian specimens, to enlarge as much as possible the risk-volume in ultrasonic VHCF tests. In this study, fully reversed tension–compression ultrasonic VHCF tests up to 109 cycles were carried out on AISI H13 steel Gaussian specimens with a risk-volume of 5000 mm3, two times larger than the largest tested in the literature. The stress distribution and the absence of bending loads were verified with strain gages, proving that VHCF tests on risk-volumes of 5000 mm3 can be reliably carried out. Ultrasonic VHCF tests were also carried out on small hourglass specimens, confirming that larger risk-volumes allow for a more reliable design against VHCF failures.

Fatigue Life Assessment of KC-1 Membrane Considering the Effects of Cryogenic Temperature and Plastic Deformation

In this study, fatigue life assessment was conducted on a KC-1 membrane, considering cryogenic operation temperature, effect of stamping, and very long service period. KC-1 membranes are produced through a process of stamping, and they make direct contact with LNG filled within a containment system at − 162 °C. The high waves that LNG carriers encounter during sailing, and even when at anchor, cause rolling motions of the hull, which result in resonant fluid motions that generate a sloshing phenomenon; such sloshing motions expose containment systems to a high level of repeated stress. To assess the very high cycle fatigue (VHCF) life of the membrane, the effects of plastic deformation and the mechanical properties of STS304L, a membrane material, were examined at cryogenic temperatures. To identify the effects of plastic deformation and cryogenic temperatures on their mechanical properties, tensile and VHCF tests were conducted at cryogenic temperatures on a sheet to which plastic strain was applied through cold-rolling. Through forming analysis, changes in the thickness and plastic strain of a membrane caused during the process of stamping were examined. The results of earlier studies that performed flow analysis and fluid–structure interaction analysis to measure the stress applied to each component of the KC-1 containment system were cited, and the VHCF life of the membrane was assessed based on the surveyed mechanical properties, and the results of forming analysis.

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.

Correlation between fractographic aspects and stress intensity factor in very high cycle fatigue

Very high cycle fatigue (VHCF) tests carried out at high frequency made it possible for one to evaluate the behavior of materials, particularly steels, for fatigue lives exceeding 106 - 107 cycles in relatively short time intervals compared to conventional fatigue tests. This reduction in the test duration is due to the ultrasonic frequencies, usually situated in the range 15 - 30 kHz, provided by the test machines. In the VHCF regime, the fatigue crack tends to start internally or in subsurface from internal defects, intrinsic to the material. The evolution of fatigue cracks occurs in different stages: crack initiation, crack growth within the fish-eye, crack growth outside the fish-eye, and then final fracture. In addition, another phenomenon that may occur inside the fish-eye and nearby the initial defect, is known as fine granular area (FGA). VHCF tests of a crankshaft steel were performed at a frequency of 20 kHz and loading ratio of -1. The S-N curve indicates an increase in fatigue life as the applied stress decreases. The fracture surfaces analyzed have also indicated the presence of fish-eye which includes an FGA region. Aspects referring to fish-eye and FGA are presented and related to stress intensity factor (SIF).

VHCF response of heat-treated SLM Ti6Al4V Gaussian specimens with large loaded volume

Among the materials used for the production of components through Additive Manufacturing (AM) processes, the Selective-Laser-Melting (SLM) Ti6Al4V alloy is widely employed in aerospace applications for its high specific strength and in biomedical applications for its good biocompatibility. Actual structural applications are generally limited to static loading conditions where the large defects originating during the SLM process do not play a significant role for the static failure. On the contrary, the same defects strongly affect the fatigue response of the parts since they act as crack initiation sites that rapidly lead to fatigue failure. In the literature, a lot of research has been carried out to investigate the quasi-static and the High-Cycle Fatigue properties of the SLM Ti6Al4V alloy but there are still few studies on its Very-High-Cycle Fatigue (VHCF) response.In the paper, the VHCF response of Ti6Al4V specimens, which are vertically orientated during the SLM building and then subjected to a conventional heat treatment (2 hours heating in vacuum at 850°C), is experimentally assessed. Ultrasonic VHCF tests are carried out on Gaussian specimens with a large risk-volume (2300 mm3). Fracture surfaces are investigated for revealing the defect originating the fatigue failure. The Stress Intensity Factor Threshold associated to the experimental failures is finally estimated.

An innovative testing technique for assessing the VHCF response of adhesively bonded joints

AbstractIn the last few years, the use of adhesive joints for structural applications has rapidly increased and adhesives are more often subject to fatigue loads during their in‐service life. In presence of a rapidly varying load, such as a high‐frequency vibration, adhesively bonded joints may undergo fatigue lives in the Very High Cycle Fatigue (VHCF) region that are significantly larger than those investigated in usual high‐cycle fatigue tests. The present paper proposes an innovative testing technique for performing accelerated fully reversed tension‐compression VHCF tests on adhesive butt‐joints. The procedure for the design of the adherends is described and then experimentally validated. Ultrasonic VHCF tests are finally carried out on a cyanoacrylate butt‐joint up to 109 cycles: experimental results show that the proposed testing equipment permits an effective assessment of the VHCF response of the adhesive in a limited testing time.

Effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue regime

This work aims to investigate the effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue (VHCF) regime. The size and type of inclusions in the steel were quantitatively analyzed, and VHCF tests were performed. Some fatigue cracks were found to be initiated in the gaps between inclusions (Al2O3, MgO-Al2O3) and the matrix, while other cracks originated from the interior of inclusions (TiN, MnS). To explain the related mechanism, the tessellated stresses between inclusions and the matrix were calculated and compared with the yield stress of the matrix. Results revealed that the inclusions could be classified into two types under VHCF; of these two, only one type could be regarded as holes. Findings in this research provide a better understanding of how inclusions affect the high cycle fatigue properties of bearing steel.

VHCF response of as‐built SLM AlSi10Mg specimens with large loaded volume

AbstractIt is well known in the literature that fatigue is particularly critical for Additive Manufacturing (AM) parts, because internal defects originating during the AM process represent a critical site for crack initiation. In the literature, the High‐Cycle‐Fatigue (HCF) response of AM parts has been extensively investigated; however, there are few results on the Very‐High‐Cycle‐Fatigue (VHCF) behavior of AM parts, even if the number of machinery components that may sustain VHCF is rapidly increasing in the last years.The present paper investigates the VHCF response of an AlSi10Mg alloy produced through Selective Laser Melting. Ultrasonic tests are carried out on Gaussian specimens with a large loaded volume and show that fatigue failures in VHCF originate from surface and sub‐surface defects with the same mechanism of HCF failures. P‐S‐N curves and fatigue strength at 109 cycles are finally estimated to show the effect of defect size on the VHCF strength.Research highlights Ultrasonic VHCF tests on AlSi10Mg Gaussian specimens produced through SLM. Identification and analysis of critical AM defects inducing VHCF failure. Statistical estimation of models for the prediction of the VHCF response of AM parts. Statistical prediction of effect of defect size on the VHCF response of AM parts.

Micro-crack initiation and propagation in a high strength aluminum alloy during very high cycle fatigue

The behavior of the new AA2198-T8 high strength aluminum alloy during tensile and very high cycle fatigue (VHCF) tests is studied. The results of the tensile tests show that the alloy in this heat treatment state has excellent mechanical properties compared with the same alloy with other heat treatment conditions and the 2024 and 7075 conventional aluminum alloys. The VHCF tests show a fatigue life plateau with the fatigue life reaching 109 cycles under a stress amplitude of 200MPa, suggesting the existence of an endurance limit. Two different crack initiation modes are observed under scanning electron microscopy (SEM). Cracks tend to initiate from the interior of the specimen when fatigue life is in the VHCF regime. The interior crack initiation mode has a characteristic of a fish-eye, at the center of which is a white rough area (WRA) where the original micro-crack initiates. The thermo-mechanical process during ultrasonic fatigue is also studied. Quantitative analyses on micro-crack initiation and propagation show that the threshold stress intensity factor for stable micro-crack propagation is 1.66MPam1/2, and the threshold stress intensity factor for unstable macro-crack propagation is 3.17MPam1/2. Finally, analyses based on the Paris-Hertzberg law suggest that the micro-crack initiation stage likely consumes most of the fatigue life. The analyses do not apply to fatigue lives less than 106 cycles. The prediction of fatigue life is confirmed by the analyses of the thermo-mechanical process.

Effect of Surface Mechanical Attrition Treatment on the very high cycle fatigue behavior of TC11

As an important engine component material, TC11 (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si) titanium alloy is subjected to high frequency cyclic loading and its failure occurs beyond 109cycles. It is thus essential to investigate the very high cycle fatigue (VHCF) behavior of this alloy. Surface Mechanical Attrition Treatment (SMAT) is a promising surface treatment technique to improve fatigue strength by modifying the surface microstructure. Therefore, it is important to understand the fatigue damage and failure process of SMATed titanium alloy in the VHCF regime. In this work, VHCF tests of TC11 before and after SMAT under fully reversed loading were conducted at room temperature by using an ultrasonic fatigue testing machine at a frequency of 20 kHz. The preliminary results seem to indicate that SMAT can reduce fatigue strength and fatigue life of TC11. Fracture surface analysis of the specimens before and after SMAT was performed using scanning electron microscope (SEM) to investigate the mechanisms of crack initiation and propagation. Particular attention was paid to fatigue crack initiation sites. The effect of SMAT on damage mechanism of SMATed TC11 in the VHCF regime was discussed.

Numerical Computation of Stress Intensity Factors in Ultrasonic Very-High-Cycle Fatigue Tests

The correct computation of the Stress Intensity Factor (SIF) in ultrasonic Very-High-Cycle Fatigue (VHCF) loading conditions is a key issue when investigating the crack growth rate curve with pre-cracked specimens or when evaluating critical SIF values from fracture surfaces of failed specimens. Dynamic conditions related to the resonance of the vibrating specimen, contact nonlinearity between crack faces and stress singularity at the crack tip make the SIF computation difficult and cumbersome. Generally, numerical computation through Finite Element Models (FEMs) under non-linear dynamic conditions makes use of direct integration methods (implicit or explicit). However, in the high frequency regime of ultrasonic VHCF tests, the procedure may lead to an unacceptable computational time. In order to reduce the computational time, a hybrid procedure based on the Harmonic Balance Method (HBM) and on the Virtual Crack Closure Technique (VCCT) is originally presented and applied in this paper. The dynamic field parameters calculated with the HBM are used as input data for the computation of the SIF through the VCCT.

VHCF response of Gaussian specimens made of high‐strength steels: comparison between unrefined and refined AISI H13

AbstractIt is well‐known in the literature that the characteristic defect size of high‐strength steels strongly influences their Very‐High‐Cycle Fatigue (VHCF) response. Currently, several refining processes permit to reduce the characteristic defect size and permit to remove large defects in high‐strength steels, thus inducing a possible significant enhancement of the VHCF response.The present paper investigates the effects of a specific refinement process, the Electro‐Slag Remelting (ESR), on the VHCF response of an AISI H13 steel. Ultrasonic VHCF tests are carried out on large Gaussian specimens made of unrefined and refined AISI H13 steels. Fracture surfaces are investigated to compare inclusion populations before and after the ESR process. Finally, the fatigue limit and the design S‐N curves are statistically estimated to quantify the beneficial effects induced by the ESR process.

Experimental-Numerical Assessment of Critical SIF from VHCF Tests

The continuous enhancement of reliability and durability requirements for many machinery components is significantly pushing the experimental research on the Very-High-Cycle Fatigue (VHCF) response of metallic materials. In order to significantly reduce testing time, ultrasonic testing machines are widely adopted when carrying out VHCF tests. Several fundamental material properties can be estimated from the fracture surfaces of specimens failed during ultrasonic VHCF tests. In the VHCF literature the critical Stress Intensity Factor (SIF) is generally estimated by applying analytical SIF formulations to the typical semi-circular surface crack geometry revealed by fracture surfaces at final failure. However, when subjected to ultrasonic VHCF tests, analytical SIF formulations valid for static loading conditions could eventually lead to significant estimation errors. The present paper aims at comparing the critical SIF at failure estimated from conventional analytical formulations and from Finite Element Models (FEMs). The fracture surfaces of two specimens with different shapes (Hourglass and Gaussian) are taken into account for modeling crack geometry at final failure. Through an implicit solving procedure, the critical SIF in resonance condition at 20 kHz is computed from the 3D geometry of the cracked specimens and compared with the corresponding analytical prediction.