Very High Cycle Fatigue Regime Research Articles

Tensile and very high cycle fatigue behaviors of a compressor blade titanium alloy at room and high temperatures

Tensile and fatigue behaviors of a compressor blade titanium alloy TC17 at room and high temperatures (RT and HT) have been investigated up to very high cycle fatigue (VHCF) regime. Compared to RT, the HT tensile strength of TC17 titanium alloy decreases, but the elongation remains basically unchanged at HT. The fatigue S–N curves are changed from single linear mode at RT to bilinear mode at HT owing to the temperature effect, and the failure mode is transformed from the surface and subsurface crack initiations at RT to the surface crack initiation at HT. The brittle oxygen-enriched subsurface (BOES) layer on the specimen surface is responsible for the crack initiation at HT. The failure models of oxide shedding and oxide intrusion are proposed. Dislocation distribution indicates that the fatigue failure at HT is insensitive to the intrinsic material microstructure and is only related to the BOES layer on the specimen surface in the VHCF regime. • Very high cycle fatigue behavior of titanium alloy at room temperature and high temperature was investigated. • The oxide layer on the specimen surface is responsible for the fatigue cracks initiation at high temperature. • The dislocation distribution revealed that the material is insensitive to microstructure at high temperature.

Very-High-Cycle Fatigue Behavior of Inconel 718 Alloy Fabricated by Selective Laser Melting at Elevated Temperature.

This study investigates the very-high-cycle fatigue (VHCF) behavior at elevated temperature (650 °C) of the Inconel 718 alloy fabricated by selective laser melting (SLM). The results are compared with those of the wrought alloy. Large columnar grain with a cellular structure in the grain interior and Laves/δ phases precipitated along the grain boundaries were exhibited in the SLM alloy, while fine equiaxed grains were present in the wrought alloy. The elevated temperature had a minor effect on the fatigue resistance in the regime below 108 cycles for the SLM alloy but significantly reduced the fatigue strength in the VHCF regime above 108 cycles. Both the SLM and wrought specimens exhibited similar fatigue resistance in the fatigue life regime of fewer than 107–108 cycles at elevated temperature, and the surface initiation mechanism was dominant in both alloys. In a VHCF regime above 107–108 cycles at elevated temperature, the wrought material exhibited slightly better fatigue resistance than the SLM alloy. All fatigue cracks are initiated from the internal defects or the microstructure discontinuities. The precipitation of Laves and δ phases is examined after fatigue tests at high temperatures, and the effect of microstructure on the formation and the propagation of the microstructural small cracks is also discussed.

Effect of transformed β phase on fish-eye ductile crack initiation of a Ti-6Al-4V alloy in very high cycle fatigue regime

Very high cycle fatigue (VHCF) properties and damage mechanisms were investigated for a Ti-6Al-4V alloy with needle-like β phase. It was revealed that the fatigue damage is dominated by interior crack initiation with a fish-eye pattern. The center of the fish-eye area is covered by peaks, dimples, microvoids and grooves, exhibiting micro-ductile characteristics instead of the commonly reported brittle ones. It was found that the formation of this particular appearance resulted from the nucleation and coalescence of microvoids and grooves essentially occurring in the transformed β phase (βt). The βt phase played a dominant role in the crack initiation process of the Ti-6Al-4V in the VHCF regime.

Effect of Turning on the Surface Integrity and Fatigue Life of a TC11 Alloy in Very High Cycle Fatigue Regime

In this work, the effect of a turning process on fatigue performance of a Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) titanium alloy is studied in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. For this purpose, the surface characteristics including surface morphology, surface roughness and residual stress were investigated. Moreover, axial fatigue tests were conducted with an ultrasonic fatigue testing system working at a frequency of 20 kHz. The results show that the turning process deteriorated the fatigue properties in both HCF and VHCF regimes. The fatigue strength at 1 × 108 cycles of turned samples is approximately 6% lower than that of electropolished ones. Fracture surface observations indicate that turning marks play a crucial role in the fatigue damage process, especially in the crack initiation stage. It was observed that the crack of all the turned samples originated from turning marks. In addition, the compressive residual stress induced by the turning process played a more effective role in resisting crack propagation in the VHCF regime than in the HCF regime.

Crack initiation behavior and fatigue performance up to very-high-cycle regime of AlSi10Mg fabricated by selective laser melting with two powder sizes

The high-cycle and very-high-cycle fatigue (VHCF) behavior of AlSi10Mg specimens manufactured by selective laser melting with different printing powder sizes was investigated. The specimens with 20 μm powder size contained more defects than those with 50 μm powder size, and the latter have a higher fatigue property than the former. Unmelted powders were detected as the main source of crack initiation. The specimens printed with 50 μm powder size are more likely to feature subsurface crack initiation in the VHCF regime. Five different types of crack initiation modes were identified, and the fatigue strength was assessed by statistical model.

Influence of processing parameters of selective laser melting on high‐cycle and very‐high‐cycle fatigue behaviour of Ti‐6Al‐4V

AbstractOrthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti‐6Al‐4V were fabricated. The results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in high‐cycle fatigue (HCF) and very‐high‐cycle fatigue (VHCF) regimes. The S‐N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Then, the tests on the validation group were performed to verify the optimal combination of SLM processing parameters. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack‐of‐fusion defects in the cases of surface and internal crack initiation.

Comparison of the damage evolution in glass fiber-reinforced polyurethane and epoxy in the HCF and VHCF regimes investigated by intermittent in situ X-ray computed tomography

The aim of this work is the comparative characterization of the fatigue and damage behaviors of GFR-polyurethane and GFR-epoxy with application-relevant quasi-isotropic layer setup in the high cycle and very-high cycle fatigue regimes. Therefore, a high-frequency test method based on a resonant testing system (1 kHz) has been further developed and assessed with special consideration of self-heating. In intermittent test procedures, the damage state has been explored by in situ X-ray computed tomography analysis after certain numbers of cycles. It was shown that the overall damage state in the VHCF regime is reduced by a factor of three compared to the HCF regime and accompanied by delayed initiation and propagation of delamination. The latter was proven to be the main reason for a decreased inclination of the S/N-curve in the VHCF regime by 50-60%.

Variable amplitude very high cycle fatigue of 17-4PH steel with a stepwise S-N curve

High and very high cycle fatigue (VHCF) tests were performed with 17-4PH stainless steel under constant amplitude (CA) and variable amplitude (VA) loading at load ratios of −1 and 0.05. Stepwise (duplex) S-N curves were determined with failure from surface inclusions above the transition stress amplitude, σtr, and from the interior below σtr. Similar σtr for CA and VA loading were found. VA failure occurred at Miner damage sums between 0.1 and 0.9 (surface failure) and 0.2 and 2.2 (internal failure). Omission of stress amplitudes less than 60% of the maximum amplitude is allowable for VA studies in the VHCF regime.

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.

Microcrack initiation mechanism of a duplex stainless steel under very high cycle fatigue loading condition: The significance of load partitioning and micro residual stresses

Investigation on the cyclic response of metallic materials in very high cycle fatigue (VHCF) is a challenging problem, which hinders the development of fatigue theories. To overcome this difficulty, we initially applied several sensible techniques, e.g., High-energy X-ray diffraction (HEXRD), confocal laser scanning microscope (CLSM), nanoindentation and transmission electron microscope (TEM) to investigate the cyclic response of a duplex stainless steel (DSS) in the VHCF regime. In-situ XRD and in-situ digital image correlation (DIC) experiments were subsequently performed at observed changing stages, intending to explore the underlying mechanism of microcrack initiation. The first-hand results obtained revealed that the austenite phase exhibits cyclic softening-hardening-softening behavior during the VHCF process. The in-situ investigations performed at the cyclic softening and hardening stages showed a load partitioning and a load transfer between the two phases, implying the cyclic response can significantly affect the distribution of the applied load. Residual strain obtained by DIC technique after unloading exhibited strong variations at phase boundaries, suggesting micro residual stresses have developed pronouncedly. Based on all the experimental findings, a unified crack initiation mechanism for the investigated DSS during VHCF loading was proposed.

Very High Cycle Fatigue Behavior of Additively Manufactured 316L Stainless Steel.

The present paper is focused on an experimental study of the damage-to-failure mechanism of additively manufactured 316L stainless steel specimens subjected to very high cycle fatigue (VHCF) loading. Ultrasonic axial tension-compression tests were carried out on specimens for up to 109 cycles, and fracture surface analysis was performed. A fine granular area (FGA) surrounding internal defects was observed and formed a “fish-eye” fracture type. Nonmetallic inclusions and the lack of fusion within the fracture surfaces that were observed with SEM were assumed to be sources of damage initiation and growth of the FGAs. The characteristic diameter of the FGAs was ≈500 μm on the fracture surface and were induced by nonmetallic inclusions; this characteristic diameter was the same as that for the fracture surface induced by a lack of fusion. Fracture surfaces corresponding to the high cycle fatigue (HCF) regime were discussed as well to emphasize damage features related to the VHCF regime.

Microcrack Initiation Mechanism of a Duplex Stainless Steel Under Very High Cycle Fatigue Loading Condition: The Significance of Load Partitioning and Micro Residual Stresses

Investigation on the cyclic response of metallic materials in very high cycle fatigue (VHCF) is a challenging problem, which hinders the development of fatigue theories. To overcome this difficulty, we initially applied several sensible techniques, e.g., High-energy X-ray diffraction (HEXRD), confocal laser scanning microscope (CLSM), nanoindentation and transmission electron microscope (TEM) to investigate the cyclic response of a duplex stainless steel (DSS) in the VHCF regime. In-situ XRD and in-situ digital image correlation (DIC) experiments were subsequently performed at observed changing stages, intending to explore the underlying mechanism of microcrack initiation. The first-hand results obtained revealed that the austenite phase exhibits cyclic softening-hardening-softening behavior during the VHCF process. The in-situ investigations performed at the cyclic softening and hardening stages showed a load partitioning and a load transfer between the two phases, implying the cyclic response can significantly affect the distribution of the applied load. Residual strain obtained by the DIC technique after unloading exhibited strong variations at phase boundaries, suggesting micro residual stresses have developed pronouncedly. Based on all the experimental findings, a unified crack initiation mechanism for the investigated DSS during VHCF loading was proposed.

Effect of surface mechanical attrition treatment on high cycle and very high cycle fatigue of a 7075-T6 aluminium alloy

Effects of surface mechanical attrition treatment (SMAT) on fatigue performance of a 7075-T6 aluminium alloy were studied in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. Fatigue test results showed that SMAT can improve fatigue strength in HCF regime, but decrease it in VHCF regime. Fracture surface observations indicated that the low SMAT intensity is quite optimal, which can protect the surface from crack initiation. However, increasing SMAT intensity could lead to over-peening by inducing deep impact dimples, micro-cracks at the surface and high tensile residual stresses in the subsurface, which deteriorated fatigue performance in VHCF regime.

Very-high-cycle fatigue behavior of AlSi10Mg manufactured by selective laser melting: Effect of build orientation and mean stress

This paper studies the influence of build orientation and mean stress on the very-high-cycle fatigue (VHCF) behavior of AlSi10Mg fabricated by selective laser melting (SLM). Horizontally built specimens demonstrate better mechanical performance than vertically built specimens. Four crack initiation modes are identified for the specimens in high-cycle fatigue (HCF) and VHCF regimes, namely initiation by lack-of-fusion, initiation by internal pore, initiation with no obvious defect and initiation by tearing. Moreover, it turns out that the mean stress accelerates crack initiation, and hence reduces fatigue life. A model is proposed to describe the effect of crack initiation location on fatigue strength.

Very-high-cycle fatigue behavior of Ti-6Al-4V manufactured by selective laser melting: Effect of build orientation

The effect of building orientation on the very-high-cycle fatigue (VHCF) response of Ti-6Al-4V specimens produced through selective laser melting (SLM) process with three different building orientations (0°, 45° and 90°) has been experimentally assessed. The fatigue performance decreases with different building orientations from 0° to 90°. The fatigue crack origin has been found to be always an internal defect both at high-cycle fatigue and VHCF regime independent of building orientations. Size of defects induced fatigue failures and the stress intensity factor range decrease with the number of cycles to failure. By considering the VHCF strength at 109 cycles, the median value decreases from 217 MPa (0°) to 201 MPa (45°) and finally to 155 MPa (90°), with a 40% reduction from 0° to 90°. The building orientation significantly influences both the defect size and the resulting VHCF response.

Fatigue Life Evaluation Considering Fatigue Reliability and Fatigue Crack for FV520B-I in VHCF Regime Based on Fracture Mechanics

This paper focuses on the fatigue reliability analysis and the development of a new life model of reliability and crack growth mechanisms in FV520B-I (high strength martensitic-type stainless steels) in the very-high cycle fatigue (VHCF) regime, which haven’t been studied well. First, the fatigue test was carried out to clarify the fatigue failure mechanism in the very-high cycle regime. Based on the test results and fatigue reliability theory, the fatigue life distribution and P-S-N curves were modeled. A new fatigue life evaluation model for FV520B-I is proposed according to the fracture mechanics and classic life evaluation method. With the comprehensive application of P-S-N curves and a new proposed fatigue life evaluation model, a new assumption of a P-Sc-N curve is developed and verified, to quantitatively express the relationship between fatigue life, reliability and fatigue cracking. This is novel and valuable for further fatigue study of FV520B-I.

Investigation of crack initiation mechanism of a precipitation hardened TC11 titanium alloy under very high cycle fatigue loading

TC11 alpha-beta titanium alloy (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si) is widely used in the construction of aero-engine components. In service, these components are subjected to cyclic loading due to vibrations. This work aims to assess the fatigue properties of a TC11 alloy and to reveal the effect of silicide precipitates and α2 phase (Ti3Al) on fatigue behavior including failure mechanism in very high cycle fatigue (VHCF) regime. For this purpose, tension-compression fatigue experiments were carried out with an ultrasonic fatigue testing system. Fatigue crack initiation and early propagation mechanisms were finely investigated using Field Emission Gun Scanning Electron Microscopy (FEG-SEM). The results show that this alloy exhibits a high fatigue strength at 109 cycles with a fatigue limit up to 618 MPa. Fracture surface observation indicates that the fatigue failure can be categorized into three modes: (a) surface failure in short life regime, (b) subsurface failure and (c) internal failure with the presence of a fish-eye pattern for long life regime. The internal and subsurface crack initiation regions are characterized by microvoids, dimples and peaks without the presence of crystallographic facets in the VHCF regime. This signifies that ductile damage mechanism instead of brittle one is the dominant failure mode for this alloy in the VHCF regime. Detailed observation of fracture surfaces seems to indicate that this ductile damage mechanism can be attributed to the combined effect of the silicide precipitates and α2 phase. Moreover, the results show that the cuboid micro-silicide precipitates significantly deteriorate the fatigue life of this studied alloy.

Initiation and Early-Stage Growth of Internal Fatigue Cracking Under Very-High-Cycle Fatigue Regime at High Temperature

The initiation and early-stage crack growth under very-high-cycle fatigue (VHCF) at room temperature, 750 °C, and 850 °C on directionally solidified Ni-base superalloy have been investigated. There was little frequency effect of 20 kHz on fatigue lives when compared with 100 Hz, nor did the deformation and fracture mechanisms. Dislocation tangles re-arranged themselves to form well-defined networks at interface of γ/γ′, accounting for the enhanced fatigue strength at 850 °C in VHCF regime when compared to that at 750 °C. In most cases, internal casting pore was the crack initiation site. Crack initiation and early-stage growth occurred on one of the {111} planes or their intersecting planes, a characteristic of Stage I cracking. With the use of optimized intermittent loading conditions, both the initiation and early-stage crack growth processes were successfully tracked on the basis of fine but visible beach marks within the Stage I cracking region. The first registered fatigue beach mark can be as close as only 86 μm to the crack initiation site and the crack length increased steadily over the whole early-stage crack growth stage. The enhanced fatigue strength at 850 °C can be rationalized with the higher threshold for propagating the early-stage crack. The fraction of fatigue life consumed for early-stage crack growth reduces with the decreasing stress, eventually leading to the initiation-controlling VHCF fatigue failure. The implications of these results are discussed with respect to the model prediction of fatigue life and fatigue strength.

Mechanism of crack initiation and early growth of high strength steels in very high cycle fatigue regime

In this paper, we capture the evolution characteristic of interior crack initiation and early growth of a bearing steel (GCr15) with tensile strength bigger than 2000 MPa in very high cycle fatigue (VHCF) regime by variable amplitude loadings. The traces left on the fracture surface suggest that the equivalent crack growth rate in crack initiation and early growth stage is of the magnitude 10−12-10−11 m/cyc. Transmission electron microscopy (TEM) observation further shows that there are discontinuous refined grain regions beneath the fracture surface in the crack initiation and early growth region. Moreover, the compressive fatigue test is performed on the specimen of a martensitic stainless steel (AISI630) with a pre-crack, and no grain refinement phenomenon is observed in the vicinity of the crack tip and beneath the crack surface by the electron backscatter diffraction (EBSD). The present results support the mechanism of the crack initiation and early growth of high strength steels in VHCF regime: the crack initiation and early growth is attributed to the grain refinement caused by the dislocation interaction over a number of cyclic loadings followed by micro cracks along with the formation of micro cracks irrespective of the grain refinement during the cyclic loading.

Fretting behaviors of a steel up to very high cycle fatigue

In the present work, the fretting fatigue behaviors of GCr15 steel (Chinese GB/T-standard) have been studied up to the very high cycle fatigue (VHCF) regime by using an ultrasonic fatigue system. The main aims of the investigations were the analyses of S–N curves, worn morphology, as well as fracture morphology. As expected, the results of the fretting fatigue tests show a reduction in the lifetime compared to plain fatigue tests without fretting. 2-D and 3-D morphologies of the fretting worn surfaces are combined to study the evolution of the stick zone. With the increasing fatigue life, the stick zone will fully occupy the whole worn zone. Besides, fretting causes severe oxidation and debris in the worn surface. The crack initiation depth increases with the increase in fatigue life, especially in the VHCF regime, the depth increases drastically because of fretting delamination.