Fatigue Tests In Air Research Articles

Fatigue and corrosion‐fatigue behavior of the β‐metastable Ti‐5Al‐5Mo‐5V‐3Cr alloy processed by laser powder bed fusion

AbstractWe performed rotating bending tests and axial (tension‐compression) load‐increase and constant amplitude high‐cycle fatigue tests in air and Hanks' balanced salt solution (HBSS) on the β‐metastable titanium alloy Ti‐5Al‐5Mo‐5V‐3Cr, processed by laser powder bed fusion (LPBF‐M), solution‐treated and aged, and shot‐peened. Rotating bending loading in air revealed a strong influence of process‐induced flaws on fatigue endurance. Especially in the high‐cycle fatigue range and the transition region, the stochastic distribution of the flaws and flaw sizes led to a high scatter of the number of cycles to failure. The axial load‐increase tests yielded a good fatigue life estimation, with a negligible difference between air and HBSS. The cyclic deformation behavior in HBSS was also strongly influenced by the local microstructure and defect distribution, and, thus, by crack formation and propagation. Plastic deformation and microcrack growth interact, and their relative amount resulted in different progressions of the plastic strain amplitude over the number of cycles for different specimens. Changes in the free corrosion potential and the corrosion current were highly sensitive indicators for fatigue‐induced damage on the rough surfaces, which was correlated to the microscopic examination, fracture surface features, and the fatigue crack development.

Improvement of stress corrosion cracking resistance by low cycle fatigue of a CrNiMoV steel

Low cycle fatigue often interacts with stress corrosion cracking behavior. In this work, the effect of low cycle fatigue on stress corrosion cracking susceptibility of a CrNiMoV steel was investigated by interrupted fatigue tests in air and slow strain rate tensile tests in 3.5 wt% NaCl solution at 180 °C. Microstructure evolution characterization verified cyclic softening and was found to improve stress corrosion cracking resistance with crack deflection along lath martensites due to a decrease of residual stress, phase transformation and increase of low-energy structures. As a semi-quantitative damage indicator, the microstructure evolution due to low cycle fatigue damage can be well described by the Kernel Average Misorientation (KAM) from the Electron Backscatter Diffraction technique. The effect of fatigue damage on stress corrosion sensitivity was found predictable based on the linear relationship between KAM and stress corrosion sensitivity factor of reduction of area.

Corrosion fatigue behaviour of electrospun PCL/HA coated magnesium biocomposites

ABSTRACT Magnesium implants are susceptible to premature failure due to corrosion and cyclic loading within the body. Composite fibre coatings are effective at reducing corrosion in magnesium materials. In this study, an Mg/HA composite was fabricated, and microscopic and mechanical characterisation was performed. The samples were then electrospun with PCL/2.5%HA fibres, and corrosion behaviour was explored by immersion tests. Finally, rotary bending fatigue tests in air and SBF environments were conducted. According to the findings, the Mg/2.5%HA and coated Mg/2.5%HA samples can withstand more than 1 million cycles under 60 MPa. Compared to pure Mg, Mg/2.5%HA has better corrosion and corrosion fatigue resistance. Furthermore, a PCL/2.5%HA fibre coating can increase the corrosion fatigue resistance of Mg/2.5%HA as a result of the polymer scaffold, passive protective layer, and apatite adsorption capability. Results suggest that Mg/2.5%HA composites coated with PCL/2.5%HA fibres are potentially suitable for use in orthopaedic biomaterials.

Experimental insight on the fatigue resistance of FV520B-I stainless steel under corrosive environments

Corrosion fatigue is the main concern for FV520B-I steels in broad industries. However, the corrosion fatigue behavior has not been elucidated clearly. In this paper, the designed corrosion fatigue tests in air, water, 3.5% and 5% NaCl solution are carried out. It is found that with the transformation of test environments from the air to NaCl solution, the fatigue life decreases from VHCF to HCF; and in the water, it spans two ranges. With the comprehensive combination of test data and the Basquin formula, the fatigue S-N formula in the different environments are achieved, the undetermined formula parameters for different test environments have been estimated, which can reflect the influence of different corrosive mediums on the fatigue resistance. The observations of post-mortem specimens clearly show that, in the water, both the internal failure and surface failure are detected when the fatigue life reaches VHCF regime, and with the fatigue life decreases to HCF regime, the fatigue failure is mainly from the surface. While, in the NaCl solution, and corrosion fatigue failure only tends to originate from the surface crack, obvious corrosion traces are observed around the region of crack initiation sites.

Influence of Glucose on Corrosion Fatigue and Cytocompatibility of Mg–Zn–Zr–Y Alloy

This article mainly focuses on the effects of glucose on the mechanical safety and cytotoxicity of Mg alloys used as implant materials. Under the corrosive human body fluid and dynamic loading, especially in a high‐glucose environment, Mg alloys are inevitably subjected to the decrease of mechanical properties in the process of long‐time service and eventually faced with corrosion fatigue (CF) failure. Herein, a comparative study on the dynamic mechanical properties of the Mg–Zn–Zr–Y alloy is evaluated by carrying out fatigue tests in air and in Hank's solution. CF and cytocompatibility of Mg–Zn–Zr–Y alloys response to different glucose contents (1 and 3 g L−1) are also detected. The Mg–Zn–Zr–Y alloy exhibits an excellent fatigue limit of 119.43 MPa in air compared with 47.77 MPa in Hank's solution at 37 °C. The fatigue cracks initiate at the microstructural defects in air and nucleate at corrosion pits in Hank's solution. Better corrosion resistance and enhanced CF are obtained with the increasing glucose content in Hank's solution. The in vitro tests with MC3T3‐E1 cells demonstrate that the Mg–Zn–Zr–Y alloy shows excellent cytocompatibility under high‐glucose condition.

Gauge-Strain-Controlled Air and PWR Fatigue Life Data for 304 Stainless Steel—Some Effects of Surface Finish and Hold Time

We performed environmental fatigue testing in simulated primary water reactor (PWR) primary water and reference fatigue testing in air in the framework of an international, collaborative project (INCEFA-PLUS), where the effects of mean strain and stress, hold time, strain amplitude and surface finish on fatigue life of austenitic stainless steels in light water reactor environments are being studied. Our fatigue lives obtained on machined specimens in air at 300 °C lie close to the NUREG/CR6909 mean air fatigue curve and are in line with INCEFA-PLUS air fatigue lives. Our environmental fatigue lives obtained in simulated PWR primary water at 300 °C lie relatively close to the NUREG/CR6909 mean fatigue curve; derived from the NUREG/CR6909 mean air fatigue curve and the applicable environmental correction factor (Fen). The PWR results show that (1) a polished surface finish has a slightly higher and a ground surface finish a slightly lower fatigue life than the NUREG/CR6909 prediction; (2) the ratio of polished to ground specimen life is ~1.37 at 300 °C and ~1.47 at 230 °C; (3) holds—at zero strain after a positive strain-rate—have a slightly detrimental effect on fatigue life. These results are in line with the INCEFA-PLUS PWR fatigue lives. A novel gauge-strain extensometer was deployed in order to perform a true gauge-strain-controlled fatigue test in simulated PWR primary water.

Influence of equal-channel angular pressing on the corrosion fatigue behaviour of an Al-Mg-Si aluminium alloy for automotive conductors

The influence of equal channel angular pressing (ECAP) on the corrosion fatigue behaviour of an Al-Mg-Si alloy was studied. Preliminary fatigue tests in air showed an increase in fatigue lifetime for ECAP samples, as compared to as-received samples, related to the ECAP-induced grain refinement. After pre-corrosion, the fatigue lifetime was lower for ECAP samples than for as-received samples, because the fragmentation of coarse intermetallics during ECAP led to an increase in the density of corrosion defects. Corrosion fatigue tests demonstrated a synergy between cyclic mechanical loading and corrosion processes for all samples; a deleterious effect of ECAP was also noted.

Formation of a hydroxyapatite layer on Ti–29Nb–13Ta–4.6Zr and enhancement of four-point bending fatigue characteristics by fine particle peening

A hydroxyapatite (HAp) surface layer was generated at room temperature on a beta-titanium alloy (Ti-29Nb-13Ta-4.6Zr, TNTZ) using fine particle peening (FPP), as a means of enhancing fatigue resistance. This material is commonly employed in bio-implants, but has a suboptimal Young's modulus. Treated specimens were assessed using scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and electron backscatter diffraction, as well as by measuring the Vickers hardness. Performing FPP using shot particles made of HAp was found to transfer the particles to the TNTZ surface to rapidly produce a HAp layer that became thicker as the FPP process progressed. Room-temperature four-point bending fatigue tests in air showed that the HAp layer resulting from the FPP process increased the endurance limit of test specimens. This improved performance is ascribed to the appearance of a hardened surface layer having a significant degree of compressive residual stress.

Four-point bending fatigue behavior of an equimolar BCC HfNbTaTiZr high-entropy alloy: Macroscopic and microscopic viewpoints

High-entropy alloys (HEAs) have attracted a great deal of interest and high expectations not only from the scientific community, but also from the industrial point of view, mainly because of a remarkable combination of promising properties. However, the characterization of HEAs under fatigue loading is still very scarce, especially in the case of body centred cubic (BCC) HEAs. As an attempt to fill this gap, we report here the first study on the fatigue behavior of a refractory BCC HEA in four-point bending loading conditions. Equimolar HfNbTaTiZr HEA was subjected to load-control fatigue tests in air and at room temperature to assess its fatigue properties in the high-cycle regime. Experimental results show that the maximum stress level reached within one fatigue cycle exceeds the uniaxial yield stress of the material. This particularity is observed within the whole range of fatigue life investigated, i.e., from approximately 104 to 107 cycles, showing no macroscopic plastic deflection of the beam in the process. A fatigue endurance of approximately 430 MPa (at 107 cycles) is found, for a stress ratio of approximately 0.1. A fatigue crack initiation mechanism occurs at the specimen surface through an intergranular phenomenon and propagates in a transgranular crystallographic manner. The fatigued samples underlined the low local lattice rotation, implying a restrained cyclic strain hardening. Finally, the fatigue study of the HfNbTaTiZr HEA highlights a fatigue endurance over an ultimate tensile strength ratio of 0.43, which is significantly larger than the actual conventional structural metallic materials under similar conditions.

Corrosion fatigue testing: the combined effect of stress and high temperature corrosion

A corrosive environment can have a detrimental effect on the fatigue life of a material due to a change in failure mechanism. Attempts have been made to replicate this change on nickel-base superalloy CMSX-4 cast in the <001> orientation. Fatigue testing in air, of this material typically produces a fracture on an angle of approximately 55° which is consistent with the fracture having propagated on a {111} slip plane. The aim of the research was to fatigue test in a corrosive environment with the purpose of producing a crack/fracture which deviated from the typical angle and thus confirm that the corrosive environment had affected the fatigue mechanism. It was concluded that the change in mechanism to high temperature corrosion fatigue was associated with a reduced load application rate together with precorroding the test specimens to trigger the initiation of the corrosion fatigue mechanism.

Influence of Texture and Environmental Effects on Fatigue Behavior of Ti-6Al-4V Alloy

Ti-6Al-4V alloy in solutionized and aged condition was subjected to axial fatigue testing in air and corrosive environments respectively. Severity of the methanol damage as evidenced through fractographic studies, corroborates loss in fatigue strength of samples tested in methanol environment in contrast to samples tested in air. Samples subjected to fatigue loading in NaCl environment revealed extensive secondary cracks along alpha grain boundaries.

In-vitro biodegradation and corrosion-assisted cracking of a coated magnesium alloy in modified-simulated body fluid

A calcium phosphate coating was directly synthesized on AZ91D magnesium (Mg) alloy. Resistance of this coating to corrosion in a modified-simulated body fluid (m-SBF) was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Mechanical properties of the bare and coated alloy were investigated using slow strain rate tensile (SSRT) and fatigue testing in air and m-SBF. Very little is reported in the literature on human-body-fluid-assisted cracking of Mg alloys, viz., resistance to corrosion fatigue (CF) and stress corrosion cracking (SCC). This study has a particular emphasis on the effect of bio-compatible coatings on mechanical and electrochemical degradations of Mg alloys for their applications as implants. The results suggest the coating to improve the general as well as pitting corrosion resistance of the alloy. The coating also provides visible improvement in resistance to SCC, but little improvement in CF resistance. This is explained on the basis of pitting behaviour in the presence and absence of the coating.

Fatigue crack tolerance design for stainless steel by crack growth analysis

Fatigue damage of stainless steel was assessed by crack growth analysis. In order to characterize small crack initiation and growth, fatigue cracking behavior was observed by periodical replica investigations during a strain-controlled fatigue test in air at room temperature. It was shown that fatigue cracks with depths of several tens of micrometers were initiated in the early stage of the fatigue test and fatigue life could be estimated as a sum of the cycles for crack initiation of 100μm in depth and those for growth to 3mm. Use of the equivalent stress intensity factor allowed prediction of the crack growth for a given strain range. The crack growth analysis was also made to estimate the fatigue life prescribed by the design fatigue curve. Then, the growth prediction was made for thermal transient conditions to take advantage of the stress gradient in the depth direction and the relationship between the cumulative usage factor (CUF) and crack size was shown. It was concluded that fatigue damage accumulated in a stainless steel component can be estimated from an identified crack size, or the maximum CUF can be determined even if no crack was detected by the inspection.

Fatigue behaviors of HP-Mg, Mg–Ca and Mg–Zn–Ca biodegradable metals in air and simulated body fluid

The dynamic loading in human body, along with the corrosive body fluid, presents a great challenge for the practical use of biodegradable magnesium implants. In this study, a high purity magnesium (99.99wt.%) and two typical promising biodegradable magnesium alloys (binary Mg–1Ca and ternary Mg–2Zn–0.2Ca) were chosen as the experimental materials. Their dynamic mechanical performances were comparatively evaluated by carrying out fatigue tests in air and in simulated body fluid (SBF). The fatigue strengths of HP-Mg, Mg–1Ca and Mg–2Zn–0.2Ca were all around 90MPa in air, however, they decreased to 52MPa, 70MPa and 68MPa in SBF at 4×106cycles, respectively. The fatigue cracks initiated from the microstructural defects when tested in air, but nucleated from surface corrosion pits when tested in SBF. Cyclic loading significantly increased the corrosion rates of all the experimental materials compared to that in static SBF. Moreover, based on our findings, the fatigue failure processes and interactions between material, corrosion and cyclic loading were systematically discussed. Statement of SignificanceFatigue strength and life are vital parameters to the design of metallic implant devices. For the corrosion fatigue of biomedical magnesium alloys, we reported the corrosion fatigue behavior of AZ91D and WE43 in SBF (Acta Biomaterialia, 6 (2010) 4605–4613), and till now there is no other reports to our knowledge. We spent 3years to finish the fatigue testing and get S-N curves for three more magnesium biomaterials, and our significant finding is that the fatigue strengths of HP-Mg, Mg–1Ca and Mg–2Zn–0.2Ca are all around 90MPa in air but 52MPa, 70MPa and 68MPa in SBF at 4×106cycles, which will provide the first-hand data for the future magnesium implants design.

Bending Fatigue Testing of Commercial Purity Titanium for Dental Implants

High fatigue strength is one of the major requirements for dental implant materials. It was previously shown that the fatigue strength under conventional stress‐control tension–compression testing can be doubled for commercially pure (CP) titanium processed by equal channel angular pressing. However, the fatigue endurance of an implant exposed to cyclic loading in corrosive media (bodily fluids) may potentially be compromised. In this work, non‐conventional bending fatigue testing in air and in simulated body fluid (SBF) has been carried out for coarse‐grained and ultrafine‐grained CP titanium.

G0301505 冷間工具鋼の回転曲げ疲労強度に及ぼす一次炭化物の影響

SKD11 contains many coarse carbides to improve wear resistance and hardness. However fatigue strength of SKD11 decreased due to coarse carbides. Therefore new SKD11 was developed for improving adhesion to surface coating and less primary carbide than SKD11. Rotary bending fatigue test in air at room temperature using hourglass shaped specimen is conducted to clarify S-N property and effect of the carbide on fatigue strength of cold work tool steels. From the experimental results of fatigue test, it was found that the fatigue strength of improved showed high values in the range from 10^4 to 10^7 cycles than that of JIS SKD11. Fatigue strength was increased 220〜240 MPa in low cycle and 250〜260 MPa in high cycle, significant difference in fatigue strength was observed in high cycle. From the results of fracture surface observation by SEM and EDX, it was found that fracture origin of JIS SKD11 was internal coarse carbide. Fracture origin of new SKD11 was surface.

A model to predict S–N curves for surface and subsurface crack initiations in different environmental media

The influence of environmental media on crack propagation of a structural steel at high-cycle and very-high-cycle fatigue (VHCF) regimes was investigated with the fatigue tests in air, water and 3.5% NaCl aqueous solution. The fatigue strength in water and 3.5% NaCl solution is significantly decreased and the cracking morphology due to different driving forces is presented. A model is proposed to explain the influence of environmental media on fatigue life, which reflects the variation of fatigue life with applied stress, grain size, inclusion size and material yield stress. The model prediction is in good agreement with experimental observations.

OS1513 水素環境下における繰返し応力がTINI形状記憶合金の寿命に及ぼす影響

TiNi shape memory alloys (SMA) receive the great attention because of their unique shape memory properties and commercial potential. However, TiNi alloys are apt to absorb hydrogen and deteriorate in various hydrogen environments, and the behavior of TiNi alloy fatigue life under the hydrogen environment has not been elucidated yet. By comparing fatigue test in air and fatigue test in hydrogen environment, the fatigue life of later clearly decreased than the other. At this time, a brittle layer was not formed on the fracture surface of the specimen for fatigue test in hydrogen environment However, macroscopically fracture surface was showing signs of brittle fracture. Furthermore, by comparing fatigue test in hydrogen environment and constant stress test and SSRT, the relationship between maximum stress and charging time showed good agreement with each other in low stress area about 300 MPa. On the other hand, in high stress area about 1000 MPa, large difference can be seen in the life of each. Therefore, the destruction under high stress amplitude is controlled by the damage due to stress fluctuation. On the other hand, destruction under low stress amplitude is controlled by the hydrogen embrittlement and the maximum stress.

Occurrence of Single and Double Slip Crossover Bands in Fatigued Alloys C460 and C22

Aluminium lithium alloy C460 and nickel chromium alloy C22 are used extensively in the aerospace industry for various aircraft components. In this study, fine and coarse -grained specimens of these alloys were subjected to fatigue testing in air and at room temperature in a servo-hydraulic testing machine to study fatigue behavior and the microstructures developed by cyclically deforming the specimens in a series of stress-strain hysteresis loops. Microstructural characterization revealed the development of single and double slip crossover bands within grains. Further, it was found that double slips crossover band density increased with the number of cycles to failure, with double slip crossover bands being more pronounced in C22 than in C460. Double slip crossover bands in C22 were readily observed by scanning electron microscopy (SEM), whereas, for C460 the clear observation of double slip crossover bands required transmission electron microscopy (TEM).

Corrosion–Fatigue Life Prediction for 2024-T62 Aluminum Alloy Using Damage Mechanics-Based Approach

An approach based on the continuum damage mechanics was applied to predict corrosion–fatigue crack initiation life of 2024-T62 aluminum alloy. A fatigue test in air, a pre-corrosion–fatigue test, and a corrosion–fatigue test on smooth standard specimens were performed. The fatigue lives are strongly reduced by the corrosive environment of 5 wt.% NaCl continuous salt spray compared with non-corroded specimens. Damage evolution models for fatigue in air and pre-corrosion–fatigue of smooth specimens were established, which forms the basis for solving the corrosion–fatigue problem. Finally, the method of corrosion–fatigue life prediction was presented. The predictions comply with the experimental data.