Fatigue Strength Of Specimens Research Articles

Fatigue behavior assessment of heavy-duty freight railway axles under different heat treatments

The safety of heavy-duty freight railway axles is threatened by defects, including inclusions, corrosion pits, fretting cracks and foreign object impact damages. Accordingly, surface strengthening techniques are applied to reduce the detrimental effect of defects on the structural integrity of railway axles. This work studied the effectiveness of heat treatment, in which the specimens machined from a heavy-duty freight railway axle were induction hardened under three types of processing parameters and tempered. The fatigue strengths of specimens with electrical discharge machined (EDM) notches and impact craters on their surfaces were tested. The fatigue behavior of induction-hardened (IH) specimens was compared with that of as-received (AR) specimens and fully-quenched (FH) specimens. The fatigue limit of smooth IH specimens is close to that of FH specimens while almost twice as much as that of AR specimens. Inclusions were found on the crack initiation sites on the surface or subsurface of AR and FH specimens. For IH specimens, the crack initiation sites moved to the zone between the case and the core. Fatigue strengths of IH and FH specimens decline with increasing EDM notch size but remain higher than AR specimens’. The influence of impact damage on fatigue properties depends on damage size and heat treatment process.

Influence of metal structure on the fatigue strength of products obtained by additive growth

The results of experimental studies of the fatigue strength of specimens obtained by 3D printing from various metal wires during additive growth using the WAAM technology are presented. It is established that the fatigue strength of specimens made of 08Х18Н9 corrosion-resistant steel is higher than that of specimens made of low-carbon 08Г2С steel and АМг5 aluminum alloy. It is shown that the fatigue strength of 08Х18Н9 steel is related with its high plasticity. Ways of increasing the plasticity of metals are proposed. Keywords: 3D printing, specimens, wire, metals, fatigue strength, plasticity, fracture. uru.40@mail.ru

Study on the Effect of Pre-strain on Fatigue Properties of Dual-phase Steel

The uniaxial tensile test investigated the fatigue performance of DP780 steel without pre-strain and with 5% and 10% uniaxial pre-strain. Topography of fractured surfaces was observed to study the relationship between pre-strain and fatigue performance. The results showed that pre-strain the specimens would affect the fatigue performance of DP780 steel. It would cause an increase in fatigue life when the stress was comparatively high and the fatigue life was less than 100,000 cycles. However, the fatigue life was reduced when the stress level was comparatively low, and the fatigue life was more than 100,000 cycles. Pre-strain reduced the fatigue strength of samples. Fatigue strength of specimens without pre-strain and 5% and 10% pre-strain were 318 MPa, 310 MPa, and 303 MPa, respectively. The sample fracture was a brittle fracture with a pre-strain of 5%, while it was a ductile fracture with a pre-strain of 10%.

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.

Fatigue behavior of zirconia with microgrooved surfaces produced using femtosecond laser.

Femtosecond laser is a promising surface treatment tool for zirconia implant. In this study, the fatigue behavior of zirconia specimens with microgrooved surfaces formed by femtosecond laser is reported. One hundred sixty CAD/CAM zirconia bars (20mm × 4mm × 1.4mm) were evenly divided into four groups with different surface: as sintered; sandblasted with 110μm Al2O3; femtosecond laser produced microgrooves having 50μm width, 30μm depth, and 100μm pitch; microgrooves having 30μm width, 20μm depth, and 60μm pitch. The femtosecond laser formed micro/nanostructured microgrooves with precise size on zirconia surfaces. XRD analysis indicated that microgrooved surface showed no obvious tetragonal-to-monoclinic phase transformation. The fatigue strength of sandblasted specimens (728MPa) was significantly higher than that of as sintered specimens (570MPa). However, the fatigue strength of specimens with microgrooved surface decreased to about 360-380MPa. The results suggest femtosecond laser is an effective technique to regulate the surface microtopography of zirconia, while further investigations are needed to improve its fatigue behavior.

切削摩擦加工材の疲労特性に及ぼす負荷形式の影響

Aiming for a result to improve fatigue strength, “cutting and rubbing process” which is a kind of surface treatment was used for forming a round-bar smooth-specimen, showing drastically enhanced rotating-bending fatigue strength. On the other hand, actual machine components and structures invariably contain geometrical irregularities, such as fillets, keyways, screw threads, and holes. Fatigue cracks initiated from such stress raisers, indicating that the validity of the cutting and rubbing process on fatigue strength of specimen with a stress raiser should be examined. In this study, fatigue tests of smooth and notched specimens finished by the cutting and rubbing process were conducted under some loading types including rotating-bending, tension-compression and torsion loading. Results showed that fatigue strength of notched specimens was significantly enhanced compared to that of smooth specimens under rotating-bending and tension-compression loads. In other words, the cutting and rubbing processing was more effective in stress raisers than the smooth surface. The physical background on the effect of loading type on the fatigue strength of the cutting and rubbing processed specimen was discussed based on the fracture origin, microstructure in the surface layer, stress gradient caused by both loading type and notch.

High-temperature failure mechanism and defect sensitivity of TC17 titanium alloy in high cycle fatigue

• Internal crack initiation with nanograins is found in TC17 alloy at 400 °C. • Oxygen-rich layer formed at high temperature affects surface crack initiation. • Fatigue strength of notched specimens is insensitive to high temperature. • A fatigue strength model is proposed including effect of temperature and defect. Crack initiation is an essential stage of fatigue process due to its direct effect on fatigue failure. However, for titanium alloys in high-temperature high cycle fatigue (HCF), the crack initiation mechanisms remain unclear and the understanding for the defect sensitivity is also lacking. In this study, a series of fatigue tests and multi-scale microstructure characterizations were conducted to explore the high-temperature failure mechanism, and the coupled effect of temperature and defect on TC17 titanium alloy in HCF. It was found that an oxygen-rich layer (ORL) was produced at specimen surface at elevated temperatures, and brittle fracture of ORL at surface played a critical role for surface crack initiation in HCF. Besides, internal crack initiation with nanograins at high temperatures was a novel finding for the titanium alloy. Based on energy dispersive spectroscopy, electron backscatter diffraction and transmission electron microscope characterizations, the competition between surface and internal crack initiations at high temperatures was related to ORL at surface and dislocation resistance in inner microstructure. The fatigue strengths of smooth specimens decreased at elevated temperatures due to the lower dislocation resistance. While the fatigue strengths of the specimens with defect were not very sensitive to the temperatures. Finally, a fatigue strength model considering the coupled effect of temperature and defect was proposed for TC17 titanium alloy.

The anisotropic fatigue properties of extruded Al–Zn–Mg–Cu alloy

The effect of specimen orientation on the fatigue properties of extruded Al–Zn–Mg–Cu alloy was investigated, and the relationship between tensile properties and fatigue strength was established. The results show that the fatigue properties of specimens vary largely with the orientation. The specimens with 0° orientation (relative to extrusion direction) have the highest fatigue strength, i.e. 301.3 MPa, whereas the fatigue strength of specimens with 45°and 90° orientation was reduced by 13.3% and 33.5%, respectively. The fatigue strength is linearly proportional to the elongation. It was found that the crack propagation modes vary with specimen orientation, a result of the grain orientation and intergranular coarse second phase particles, leading to the difference in fatigue lives. In addition, Basquin equations for specimens in the three directions were proposed for fatigue life prediction.

Torsional-loaded notched specimen fatigue strength prediction based on mode I and mode III critical distances and fracture surface investigations with a 3D optical profilometer

Torsional and axial fatigue tests were performed on steel 42CrMo4+QT and aluminium alloy 7075-T6, and a 3D optical profilometer was used for the investigations of the fracture surfaces. The mode I and mode III critical distances of the two materials were regarded as reference scales for evaluating the initial orientation of the cracks and then identifying the tests valid for the calibration of the normal and those for the shear-fatigue models. The torsional strength of blunt notched specimens was then predicted with the Smith-Watson-Topper and the Fatemi-Socie criteria, combined with the critical distances and the Chaboche model for the steel.

Fatigue Life of Miniature SAC Solder Specimen Affected by Distribution Morphology of Primary Tin Crystal

Miniature specimens of Sn-3.0Ag-0.5Cu (SAC) solder show large variation in tensile strength even though they are under the same loading conditions. We have found that this phenomenon can be correlated to the difference in the distribution morphology of primary Tin crystals in the specimens. Such a different could affect the fatigue strength of the miniature specimen. Then, in this study, we carried out fatigue tests with cyclic tension-compression loading using the miniature SAC solder specimen to investigate the relationship between the fatigue life and the distribution morphology of primary Tin crystals. The results show that the stress amplitudes of the hysteresis loops under the same loading condition show large variations and the fatigue life becomes shorter with the increase in the stress amplitude. The specimens having a high-stress amplitude in their hysteresis loops contain a high proportion of the primary Tin crystals which grow in directions vertical to or parallel to the loading direction. Namely, the fatigue strength of the specimen containing a large number of such primary Tin crystals tends to become lower.

Evaluating size effects on fatigue life of 42CrMo4+QT steel using a statistical S-N model with highly-stressed volume and surface

A statistical S-N model based on the highly-stressed volume/surface concept is proposed for evaluating the size effects on fatigue strength of specimens from 42CrMo4+QT steel. The proposed model extends an existing physically valid model (Castillo et al. 2009) and allows the S-N curve to be extrapolated from small-scale to full-scale components. The model is validated against experimental fatigue data obtained by testing solid and hollow smooth cylindrical specimens under push-pull constant amplitude loadings. After checking that the proposed model agrees with experimental fatigue data, it is possible to apply the model to estimate the S-N curve of specimens of the highly-stressed volume up to 1802 mm3. For this case, an S-N expression is provided that can directly be used in fatigue analysis without the necessity of running a non-linear optimization.

Variation of fatigue strength of parts manufactured by laser powder bed fusion

ABSTRACT This study reports the variability of the fatigue strength of specimens manufactured by the laser powder bed fusion process with respect to their location on the build plate. Specimens from the right-hand and left-hand halves of the build plate were tested under high cycle fatigue. Comparison of the fatigue data suggests that the specimens manufactured on the right-hand half of the build plate have a higher fatigue strength than those manufactured on the left-hand half. One reason for the observed discrepancy in fatigue strength was the higher accumulation of spattered powder particles on the left-hand side as compared to the right-hand side of the build plate. These spattered particles are oxidised, and form defects such as inclusions within the specimen.

Fatigue and damage tolerance assessment of induction hardened S38C axles under different foreign objects

Surface damages caused by the impact of flying objects are important factors responsible for fatigue performance degradation of high-speed railway axles, both qualitative and quantitative assessment of these damages are necessary for the maintenance of these axles. In this paper, cubical projectiles were used to reproduce actual damages on S38C axle specimens by a compressed-gas gun at varying velocities. Morphologies of simulated impact damages were characterized, and fatigue strengths of specimens under different impact damages were determined by the step-loading method. Results show that fatigue strength of impacted specimens reduces with the increase of damage depth; besides, other factors, including damage orientation, damage shape, and microstructural damage also exert their contributions. Finally, evaluation on fatigue strength reduction has been performed based on the damage tolerance philosophy.

Fatigue assessment of EA4T railway axles under artificial surface damage

Foreign object damage (FOD) introduced in service, indentations, and nicks encountered during maintenance are typical defects leading to pre-scrap of high-speed railway axles. A comprehensive assessment of the influence of these defects on the structural integrity of axles is critical for ensuring their operation safety with low maintenance costs. Three types of defects were produced artificially on specimens extracted from EA4T axles, electronic discharge machine (EDM) crater, indentation via compressing balls, and FOD by tungsten steel balls and cubes. Rotating bending fatigue tests were performed for fatigue strength evaluation of smooth and defected specimens. Morphologies of defects and fracture surfaces were studied. EDM defect surface is rough and the secondary notches are the fatigue crack initiation sites, while fatigue crack of indented and impacted specimens initiates from the sharp point of defect rims. Fatigue strength of specimens with indentation or tungsten ball impact damage is superior to that of EDM specimens and cubical projectile impacted specimens under the same depth. A large dispersion was found in the shape of impact damages by cubic projectiles and scattered fatigue strength as a consequence. Except for the defect depth, other parameters including produce modes and shape of the defect, should be considered to give a reliable evaluation on the fatigue strength of defected specimens.

Influence of artificial defects on fatigue strength of induction hardened S38C axles

Impact damage are critical for damage tolerance assessment of high-speed train axles. Flying ballast impact damage on railway axles produce geometry discontinuities, which become preferential sites for fatigue crack initiation and usually lead to premature failure. According to this, the formation of surface impact damage and its influence on structural integrity of induction hardened S38C axles are herein explored. In particular, the morphology of impact damage on induction hardened S38C axles were analyzed as well as fatigue properties of specimens with artificial defects, electronic discharge machine (EDM) defects and impact defects fabricated by shooting metallic balls to the surface with compressed-gas gun. Results show that surface impact damage of the axles include two types, namely scratches and notches. Fatigue strength of the specimens with EDM defects are larger than the Murakami’s model predictions due to existence of compressive residual stresses. The influence of shallower impact damage (less than 200 μm) on the fatigue strength is negligible. With a larger depth, the rim is extracted out of the original surface and fatigue crack starts from this point with a decline of fatigue strength. Microcracks and loss of materials appear at the rim of impact damage, and microcracks along adiabatic shear bands lies beneath the surface when the impact depth is about 300 μm resulting in 50% reduction of fatigue strength. Fatigue strength of impacted specimens are larger than that of EDM specimens by 200 MPa with the equivalent area due to the local residual stresses.

Fatigue Strength Evaluation of Notched Ductile Steel Specimens Using Critical Distances

In the case of AISI 304L austenitic stainless steel it was found that the fatigue strength of specimens characterized by the presence of blunt notches cannot be rationalized either in terms of linear elastic local stress or nominal stress amplitude. This fact was related to the high level of ductility shown by this material also in the case of high cycle fatigue regime. Therefore, in this paper some classical fatigue design methodologies are extended to account for this aspect also in the case of severe notches. On this basis a FEM oriented extension of the concept of geometric elastic stress concentration factor is proposed.

Investigation of fatigue strength of aluminium 2024-T3 subjected to shot peening process by Ni shots

Aluminium alloys are most suitable for light weight applications. This research paper investigates the fatigue strength of Aluminium alloy 2024-T3 after shot peeing process using nickel shots. Nickel shots are used in this research to investigate the influence of nickel on the fatigue strength of aluminum 2024-T3 alloy. Compressed air is used for shot peening process. Shot peening process parameter values are chosen based on response surface method (RSM) to conduct the experiments. Desirability approach is used to get the optimized values of process parameters. High cycle fatigue testing is performed to find the fatigue strength of specimens before and after shot peening process. An increase in the fatigue strength up to 34% is observed after peening using the nickel shots. FESEM and EDS tests revealed the presence of nickel on the surface of aluminium 2024-T3 specimen after shot penning with nickel shots.

Fatigue strength prediction of laser powder bed fusion processed Inconel 625 specimens with intentionally-seeded porosity: Feasibility study

Inconel 625 fatigue testing specimens with different levels of intentionally-seeded porosity in their gauge section (≤0.1, 0.3, 0.9 and 2.7%) were manufactured by laser powder bed fusion and subjected to stress relief annealing. The morphology, number and distribution of seeded defects were characterized using the micro-computed tomography technique. The effect of the different levels of porosity on the high cycle fatigue behavior of these specimens was studied using room temperature force-controlled fatigue testing according to the ASTM E466 standard (frequency of 30 Hz; R = 0.1) with a runout at 107 cycles. The fatigue strength of specimens with the lowest level of porosity (≤0.1%) was found to be 590 MPa. Next, using El-Haddad’s formulation of the Kitagawa-Takahashi model defining the limiting conditions for fatigue failure in the presence of defects and Murakami’s parameter for defect size rating, the fatigue strengths of specimens with 0.3, 0.9 and 2.7% porosity were predicted at 280, 190 and 160 MPa, respectively. The predicted values were found to deviate by less than 8% from the experimental results, which were, respectively, 288, 207 and 170 MPa.

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

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

Probabilistic Control Volume Method for Evaluating the Effects of Notch Size and Loading Type on Fatigue Life

The probabilistic control volume method has great prospects in correlating the effects of specimen size, notch and loading type on fatigue life or fatigue strength. In this work, the effects of notch size and loading type on fatigue life are investigated by using the probabilistic control volume method. Rotating bending and axial loading fatigue tests are at first performed on the hourglass specimen, circumferential V-notch specimen and V-notch plate specimen of 30CrMnSiA steel. Experimental results indicate that the notch reduces the fatigue strength of specimens in terms of nominal stress amplitude while in terms of local stress amplitude, the notch specimen could endure higher fatigue strength. Then, the probabilistic control volume method is used to evaluate the effects of notch size and loading type on fatigue life. It is shown that the probabilistic control volume method correlates well the effects of notch size and loading type on fatigue life, even for the local stress of the notch root exceeding the yield stress of the material.