High Cycle Fatigue Strength Research Articles

High cycle fatigue strength of hot isostatically pressed and chemically etched laser powder bed fusion produced Ti-6Al-4V

Metal additive manufacturing (AM) parts are likely to fail in fatigue when subjected to cyclic loading due to inherent manufacturing defects, including a rough surface finish, internal pores, high residual stress, and martensitic microstructure. In particular, a rough surface finish is understood to limit fatigue strength most severely since surface aberrations act as severe stress raisers where stresses are typically the highest. As such, post-surface finishing processes are recommended to reduce surface roughness. Conversely, one main advantage of AM is the ability to produce complex geometries without the need for machining. This makes it impractical to use mechanical surface finishing techniques. One solution is to use chemical etching techniques that are not bound by geometry constraints. This study further investigates the effect chemical etching, with an HF-HNO3 solution, has on the fatigue strength of laser powder bed fusion-produced Ti-6Al-4V when immersed for different time intervals (5, 15 and 90 min) in the chemical solution. The reduction in surface roughness and the increase in fatigue strength determined the effectiveness of the different chemical etching times. The results show that a 90-minute etching interval provides the highest surface roughness reduction of 75.9 % and fatigue limit increase of 46 % compared to the as-built surface roughness. However, the internal pores limited the fatigue strength increase because the subsurface pores were brought to the surface by the etching process. Combining hot isostatic pressure (HIP) with chemical etching resulted in a significant 92 % increase in the fatigue limit that is comparable to a wrought defect-free specimen.

Influence of notch root radius on high cycle fatigue properties and fatigue crack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure

Notch high cycle fatigue (HCF) properties and microcrack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure under various notch radii were systematically investigated. Results indicate that the reduction of notch root radius significantly promotes fatigue microcrack initiation, and then dramatically reduces the HCF life and strength of the alloy. Cyclic deformation of the alloy is mainly controlled by the slipping and deformation twinning in α plates. The primary fatigue crack initiation micro-mechanism is α/β interface cracking induced by slipping and twinning at all notch HCF specimens. Moreover, the volume fraction of twinning would increase with decreasing of the notch root radius. Interestingly, when the notch root radius is the smallest (R = 0.14 mm) and the stress concentration factor is the largest (Kt = 4), in addition to slipping and twinning, basal stacking faults promoting the cracking of α/β interface could be another crucial HCF microcrack initiation mechanism of the alloy. Furthermore, with decreasing of notch root radius and increasing of stress concentration factor, the size of cycle plastic deformation zone at notch root gradually reduces to the size of α colony and even the α plate. Therefore, the order of influential degree of three different levels microstructure on the crack initiation mechanism of notched HCF can be arranged as α plate > α colony > β GB.

"High-cycle fatigue and tensile strength of AZ91 Mg alloy with MAO-epoxy duplex coating "

"High-cycle fatigue and tensile strength of AZ91 Mg alloy with MAO-epoxy duplex coating "

Influences of Manufacturing‐Related Microstructural Variations on Fatigue in Carbide‐Rich Tool Steels

In article number 2200578, Lennart Mirko Scholl and co-workers present a correlation between manufacturing-related defect sizes and fatigue strength. Powder metallurgy (PM) steels show significantly higher high-cycle fatigue (HCF) strength than ingot cast steels. The main factor influencing HCF strength is the size of fracture mechanical defects, whereby improvements can be obtained by reducing their size.

High cycle fatigue properties and fatigue crack propagation behavior of a bainite railway axle steel

Carbide-free bainite (CFB) steel with metastable austenite and refined microstructures has been demonstrated to be a promising alternative to conventional pearlitic rail and wheel steels, due to its outstanding static mechanical properties and performance in service. However, the exploration on its potential application in railway axles is scarce, and the effect of microstructures on the fatigue crack growth (FCG) behavior of CFB steels is not well understood. In this work, a CFB axle steel containing retained austenite (RA) and refined bainite is designed by austempering and low-temperature tempering. The static mechanical properties, high cycle fatigue (HCF) properties and FCG properties of the conventional high-speed railway axle steel (grade DZ2) and the CFB steel are evaluated. The tensile strength, uniform elongation and HCF strength of CFB steel are about 87%, 24% and 60% higher than that of the DZ2 steel, respectively. The FCG behavior of the two axle steels is almost equivalent in the Paris law region. In the DZ2 steel, crack deflection and secondary cracks inhibit the main crack growth, the grain bridging caused by the intergranular crack growth delays the crack opening at the crack tip. The crack propagates transgranularly in the CFB steel. When the crack propagation direction is parallel to the long axis of the bainite lath, the crack propagates preferentially along this direction. However, when the directions are nearly perpendicular, a zig-zag crack path formed near the fine bainitic lath can effectively prevent crack growth. Furthermore, the deformation induced martensitic transformation ahead of the crack tip enhances the crack-tip shielding.

Prediction of high cycle fatigue strength for additive manufactured metals by defects incorporated crystal plasticity modeling

The durability assessment of additive manufacturing (AM) components is an open issue due to the large scatter in their high cycle fatigue performance. The intrinsic defects inherited from AM processes, especially the irregular defects, pose great challenge to numerical modeling of AM metals and prediction of their fatigue behavior. In this paper, a computational method is proposed to predict the high cycle fatigue (HCF) properties of AlSi10Mg alloy fabricated by laser powder bed fusion (LPBF). A virtual laboratory framework including model generation, crystal plasticity calculation and fatigue performance evaluation is established. In the framework, a representative volume element (RVE) model integrating the polycrystal matrix and defects is developed; the defects are reconstructed based on X-ray Computed Tomography (XCT) measurement and then incorporated into the synthetic polycrystal matrix as the second phase; a crystal plasticity (CP) model using a FFT solver is employed to calculate the mechanical response under cyclic loading; finally, the fatigue strength is determined by using the staircase method. The proposed method well predicts the varied fatigue strengths of specimens containing different defects.

Tensile and fatigue properties of the binder jet printed and hot isostatically pressed 316L austenitic stainless steel

The binder jet printed (BJP) metallic components are not yet considered for widespread structural applications that involve cyclic loading conditions due to the high levels of porosity typical to them. In the present study, the porosity in BJP 316L stainless steel (SS) was reduced from 3.5-5.6% to 1–1.2% by improving process parameters first and then employing the post-processing hot iso-static pressing (HIP) treatment. The reduced porosity in the as-sintered conditions resulted in ⁓15 and 5% improvement in tensile yield and ultimate tensile strengths, respectively. However, the high cycle fatigue strength (⁓250 MPa) did not improve even after a ⁓fivefold reduction in the porosity. HIP resulted in a further reduction in the porosity, which improved the elongation to failure by 30% while reduced the yield strength by ∼10% (due to grain coarsening that occurred during HIP). It, however, did not improve the unnotched fatigue strength. The microstructural coarsening during HIP also affected the short fatigue crack growth resistance of the BJP 316L SS specimens. Moreover, after reduction in overall porosity and the size of the pores, the void and fatigue crack nucleation mechanisms during tensile and fatigue loadings changed from pore assisted to inclusion assisted.

Effects of building direction and loading mode on the high cycle fatigue strength of the laser powder bed fusion 316L

The present study aims to investigate the high cycle fatigue (HCF) performance of steel 316L fabricated by the laser powder bed fusion (LPBF) process. Bending and torsional fatigue test specimens built horizontally (0°), inclined (45°), and vertically (90°) have been prepared and tested. Stress-relieving heat treatment was carried out to reduce the residual stresses. The 90°, 45°, and 0° built near-net-shape and polished specimens have fatigue strengths between 140 and 250 MPa under bending loading and 150 and 170 MPa under torsion loading. This difference illustrates a more pronounced defect sensitivity in bending compared to torsion. Fractographical analyses revealed the fatigue failure mechanisms. As the building direction inclines from vertical to horizontal, the effective areas of inherent defects diminish contributing to improving fatigue strength under bending loading whilst the differences from building directions in torsional fatigue strengths are minor. Microstructural features are seen to compete with inherent defects to affect fatigue performance in the condition that the effective defect sizes are close to the critical fatigue crack size.

Influence of abrasive water jet and conventional drilling on the fatigue performance of hybrid carbon/flax laminates

Most of the research on hybrid natural fiber composites has been focused on non-structural applications. The aim of this research is to improve the understanding of how to use hybrid natural fiber composites for structural applications, such as in the aerospace and automotive industries. The joining of subassemblies with rivets, bolts, or screws is a prominent feature in these industries, which implies that the presence of holes in the structural members is unavoidable. The study of the effect of holes on the mechanical behavior of these members is therefore imperative, especially given that holes are stress risers. An important factor in the design of open-hole hybrid flax composites is their fatigue behavior under cyclic loading because it accounts for most failures in composite structures. The investigation of this behavior is reported in this study where three types of 16-ply symmetric hybrid composites that were manufactured using woven carbon (C) and unidirectional flax (F), namely, unidirectional (CF[0]), cross-ply (CF[0/90]) and angle-ply (CF[[Formula: see text] 45]) are employed. A circular hole was made in the center of each specimen using either an abrasive water jet or conventional drilling methods. The observed machining-induced critical defect was delamination, which manifested in three forms: peel-up, push-out, and secondary. Stepwise fatigue tests were conducted to determine the damage evolution and high-cycle fatigue strength of each layup. The damage evolution of the material was assessed via stiffness damage, thermographic methods, and dissipated energy per cycle. It was observed that CF[0] layups are unaffected by the machining process and they have an identical high-cycle fatigue strength. For the CF[0/90] and CF[[Formula: see text] 45] layups, conventional drilling specimens have a higher high-cycle fatigue strength than their abrasive water jet counterparts.

Investigations of Fatigue Damage in a Nitriding Low-Carbon Bainitic Steel for High-Performance Crankshaft

In the automotive environment, the need to increase the performance of materials requires extra engineering efforts. The possibility of developing new materials is strategically important. Indeed, alternative solutions in terms of material choice allow designers to optimise their projects and keep competitive production costs. Traditional quenched and tempered steels are usually used for highly stressed components, and possible alternatives could be important competitive opportunities. One possible substitute is using bainitic steels to exploit their economic advantages while maintaining acceptable mechanical performances. This paper explores the fatigue life behaviour of a new low-carbon bainitic steel for applications requiring case hardening treatment obtained by the nitriding process. A high-cycle fatigue (HCF) strength assessment is conducted through a test campaign to compare treated and untreated material. The improvement in fatigue strength is evaluated as well as the study of fracture surfaces, residual stress, and microhardness profiles to assess in detail the effectiveness of the nitriding process. It is found that the nitriding leads to an improvement in fatigue life but not as much as expected because of the low ductile behaviour of this steel, the high speed of stress application added, and the embrittlement of the nitriding treatment, as confirmed through fracture surface analysis.

Porosity-related high-cycle fatigue strength of nickel-base single crystals: Fatigue experiments and electron back-scattered diffraction analysis

The fatigue limit of nickel-base single crystal (NBSX) is experimentally studied at an elevated temperature of 980 °C. The crack initiation position is revealed as the casting pores in the single crystal under scanning electron microscope (SEM), and distinct “fish-eye” region is observed on the long cycle fatigue fracture surface. The failure mechanism is explored around the “fish-eye” by adopting the focused ion beam (FIB) and electron backscatter diffraction (EBSD) technique. The crystalline motion and the fatigue crack initiation mechanism around the initiation pore are revealed by conducting microscopic analyses.

Influences of Manufacturing‐Related Microstructural Variations on Fatigue in Carbide‐Rich Tool Steels

In cold-work applications, tool steels with high carbide contents are used as cutting and stamping tools. The tool service life is limited by wear resistance and fatigue strength. The relationship between manufacturing-related microstructural influences and fatigue strengths of tool steels has not yet been adequately investigated. To investigate these influences on high-cycle fatigue (HCF) strength (NG = 107), rotating bending tests are performed on AISI D2 and AISI M2/M3. Raw materials are produced by conventional ingot casting and subsequent hot working (HW) as well as in a powder metallurgy (PM) process with hot isostatic pressing (HIP) and forging. Herein, a statistically validated correlation of process-related defect size and the resulting fatigue strength is presented. Both PM steels show significantly higher HCF strength than the HW steels. Critical defects in PM appear to be exclusively small oxide inclusions. In contrast, fatigue cracks in HW are typically initiated by the fracture of large, blocky eutectic carbides. The main factor influencing HCF strength is defect size. Other critical features of the microstructure include matrix hardness, circularity, and defect type. Improvements in fatigue strength can be obtained by reducing the size of fracture mechanical defects, inclusions for PM, and eutectic carbides for HW microstructures.

High-Cycle Fatigue Behavior and Corresponding Microscale Deformation Mechanisms of Metastable Ti55511 Alloy with A Basket-Weave Microstructure.

High-cycle fatigue (HCF) is a critical property of metastable β Ti alloys in aerospace applications. In this work, the HCF behavior and corresponding microscale deformation mechanisms of a metastable Ti-5Al-5Mo-5V-1Cr-1Fe (Ti55511) alloy with a basket-weave structure were investigated. HCF and its deformation mechanisms of a Ti55511 alloy were systematically studied in the deformed condition by using a scanning electron microscope (SEM), a transmission electron microscope (TEM), and electron backscatter diffraction (EBSD). It was found that the Ti55511 alloy exhibited an excellent HCF strength (107 cycles, Kt = 1, R = 0.06) of 738 MPa. The fractographic investigation demonstrated that fatigue striations and secondary cracks were the main features in the crack initiation zone. Dislocation analyses indicated that the HCF deformation of the basket-weave microstructure was mainly affected by the dislocation slipping of the primary α (αp) phase. In addition, the dislocation pile-up at the αp/βtrans interface led to crack initiation. EBSD analyses indicated that the prismatic <a> type slip system of the αp phase was preferentially activated during the HCF deformation process of the Ti55511 alloy, followed by the basal <a> type and pyramid <a> type systems.

High cycle fatigue strength of additively manufactured AISI 316L Stainless Steel parts joined by laser welding

The present paper deals with the high cycle fatigue behavior of 316L stainless steel parts, obtained either by rolling or by LPBF additive manufacturing process, and their assemblies by autogenous laser welding. The aim is to study the effects of this assembly process on the fatigue strength, assessed at a 106 cycles, of additively produced parts, compared to that of raw rolled products. An original campaign of high cycle fatigue tests based on seven batches of specimens, manufactured additively or cold rolled, one-pieced or assembled, raw or polished, is carried out with a positive load ratio (R= 0.1). A stress relieving heat treatment is applied to all specimens to reduce residual stress levels. Particular attention is paid to identifying the damage mechanisms associated with each batch. Preliminary results allowed an original conclusion that laser welding does not degrade the fatigue strength of additively produced parts. In addition, the fatigue behavior of the studied system depends essentially on the stress concentration generated on the surface or a few micrometers in the underlayer, mainly by the roughness of the LPBF produced parts combined with the lack of fusion (LoF) defects, and in a second order by the geometry of the laser welding channel. Furthermore, additively produced parts showed higher sensitivity to the presence of defects than rolled parts.

High cycle and very high cycle fatigue of TC17 titanium alloy: Stress ratio effect and fatigue strength modeling

The fracture surface of TC17 alloy tends to exhibit facet characteristic at R > 0, but it presents rough area morphology at R = −1 for the internal crack initiation and early growth region. The former is attributed to basal slip or cleavage of α grains. The later is due to the cracks caused by the nanograin formation and the cracks formed irrespective of nanograins. The present TC17 alloy and various alloys in literature indicate that Walker’s equation [(1-R)/2]a is more accurate than Goodman’s equation and Smith-Watson-Topper’s equation [(1-R)/2]1/2 for the effect of stress ratio on high cycle and very high cycle fatigue strength.

Failure analysis and design improvements of steel piston for a high-power marine diesel engine

In this study, the failure analysis and design improvements of a steel piston for a marine engine were performed. In order to assess the high-cycle fatigue performance of the steel piston, the high-cycle fatigue durability test was performed and the failure happened in steel piston. Therefore, the present analysis focuses on simulating durability test and assessing high-cycle fatigue strength of the steel piston. For this purpose, the high-cycle fatigue load spectrum of the steel piston was determined by numerical simulation method, the mean stress and stress amplitude of high-cycle fatigue cyclical loads were obtained by critical plane method. Then the fatigue assessment model was built to explore the failure cause of the piston. And a new scheme was designed by improving the local structure of the object. The research results showed that the excessive stress amplitude caused by the alternating gas pressure was the root cause of steel piston failure. The new scheme satisfied the fatigue strength assessment and passed the engine durability test successfully, so the design improvements were effective. The simulation method and fatigue strength assessment method in this work may provide an effective way for the design and service life of steel piston.

Effect of end milling, grinding and tartaric-sulfuric anodizing on the fatigue behavior of AA7050 alloy

The present study investigates the effect of the high-speed milling (HSM), grinding and anodizing processes on the high cycle fatigue strength of the AA7050 aluminum alloy. These processes are systematically applied to certain aircraft components as specified by aeronautical standards, to attain a good surface finish. In order to understand the effect of each process on the fatigue strength, a vast experimental campaign consisting of fully reversed plane bending fatigue tests has been conducted on specimens with different surface states. A polished batch, three milled batches with different surface roughness, a grinded batch and two milled then anodized batches with different surface roughness have been tested. The results show that the roughness must be highly degraded (Sa > 3 \(\mu m\)) in order to impact the fatigue strength. It is also shown that anodizing has slight beneficial effect on the fatigue strength in the high cycle fatigue domain studied. The experimental results show a dependence of the fatigue strength on the surface roughness via the parameter \(\sqrt{Sa}\), previously proposed by the authors.

Effect of welding processes on high cycle fatigue behavior for naval grade HSLA joints: A fatigue strength prediction

Naval grade high strength low alloy steel is designed especially for shipbuilding applications due to its high strength-to-weight ratio. The fatigue failure of welded structures is susceptible to cyclic loading through heterogeneities of consumables, microstructure deformation, and complicated local stress-strain conditions. In this investigation, the influence of welding processes on high cycle fatigue behavior and fatigue strength prediction of DMR249 A grade steel. The fatigue test was conducted for five different stress levels from 50 to 90 % yield strength with a stress ratio R = −1. The evaluated fatigue properties are correlated with mechanical properties and microstructural characteristics. The maximum obtained fatigue strength of SMAW and GMAW joints is about 342 MPa and 355 MPa for the unnotched specimens. From the experimental results, it is found that the fatigue strength of the fusion welded joints relative to 55 % (SMAW) and 59 % (GMAW) of tensile strength of the parent metal. Therefore, the fatigue strength of GMAW joints is appreciably better than SMAW joints. Finally, the constructed SN curves of base metal and welded joints were lying above the IIW standard curve.

Mean-stress sensitivity of an ultrahigh-strength steel under uniaxial and torsional high and very high cycle fatigue loading.

The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = -1, 0.05, 0.3, and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2 × 105 cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean-stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode-I crack growth. Furthermore, the mean-stress sensitivity is independent of loading condition and can be expressed by and .

Effect of the notched foreign object damage depth on the HCF strength of titanium alloy TC11

In this study, the effects of different incident foreign object damage (FOD) depths on the high-cycle fatigue (HCF) performance of TC11 titanium alloy specimens with compressor leading edge blades were studied. FOD values were obtained through high-speed ballistic impact tests at different aiming positions under the same impact conditions. HCF tests of smooth and FODed specimens were conducted via the step-loading test method, and the HCF strength was determined via experiments and numerical analysis. The results indicated that with increasing damage depth, the stress concentration factor significantly increased. There occurred greater material losses on the exit side of the resultant notch damage than those on the incident side. The maximum damage depth ranged from 0.23 to 1.51 mm, and the fatigue strength of the FODed specimens reached approximately 21.77–84.99 % of that of the smooth specimens. With increasing depth, the material loss increased, and the fatigue strength decreased. According to the Kf = 3 empirical criterion for FOD tolerance design, the determined serviceable limit of notch-type damage was 0.84 mm, which is expected to be useful for quick assessment of the notch-type FOD degree of TC11 titanium alloy leading edge blades.