Fatigue Specimens Research Articles

Prediction of fatigue crack propagation and fractography of rail steel

The aim of the present research is to determine the initiation and propagation of cracks and its geometric specifications. Metallurgical factors, especially the non-metallic inclusions have a significant impact on the fatigue life of railways. Since all non-metallic inclusions in a measure of steel alloy is not dangerous, determining how and which one of them affects the fatigue process of the rails is of the present studies' concern. Afterwards, the results of the research on fracture surface of fatigue specimens will be presented. Numerical and experimental methods are used to investigate the behavior of fatigue crack growth in the specimen. For this purpose, the fatigue crack growth and hardness tests were carried out and have been checked by fractography studies on fractured specimens. The hardness of the specimens was measured in different points by using Rockwell micro-hardness experiment. Afterwards, a three-dimensional boundary element method is used for fatigue crack growth under stress field. The modified Paris model is used to estimate fatigue crack growth rates. Finally, three-dimensional boundary element analysis results obtained show good agreement with those achieved in experimental tests.

Tensile and Fatigue Analysis of 3D-Printed Polyethylene Terephthalate Glycol

This paper investigates the tensile and fatigue behavior of polyethylene terephthalate glycol (PETG) parts manufactured by fused filament fabrication (FFF). PETG is a thermoplastic polyester that is available as a filament for commercial desktop 3D printers. As the functionality of 3D printing components for end-part use grows, it is essential to characterize mechanical properties of available materials in a quasistatic and dynamic loading condition. One of the main issues of FFF components is the anisotropy of mechanical properties that is induced by part-build orientation and raster orientation. The objective of this study was to quantify anisotropy of PETG coupons by testing against four raster orientations—longitudinal, transversal, diagonal and crosshatched. Quasistatic tensile tests were performed on two specimen types that yielded the highest ultimate tensile strength and modulus of elasticity for longitudinal specimens. Measured strength varied from 41.58 to 48.04 MPa for the weakest and strongest orientations which correspond to 83–96% of PETG strength processed by injection molding. Tensile–tensile fatigue tests with a stress ratio of R = 0.1 were performed at 90, 80, 70 and 60% of nominal UTS of each specimen type. S–N curves displayed the highest fatigue life of the longitudinal specimen followed by the crosshatched specimen at 80–90% UTS stress level that were both eventually outperformed by the diagonal specimen at the 60% UTS stress level. Finally, a fractography analysis conducted on fatigued specimens displayed signs of plastic failure for all orientations. This paper provides an experimental analysis of PETG tensile and fatigue properties that have not been found in the literature. Results show that PETG is a good candidate for FFF technology because of the competitive mechanical properties and lesser anisotropy.

Monotonic and cyclic behavior of polyamide 66/hectorite nanocomposites in marine environment

Product design for safe performance and operation in harsh environments triggers the importance of understanding property degradation due to prolonged exposure of advanced composite materials to such environments. The effect of seawater immersion on monotonic tensile, stress relaxation, and cyclic bending fatigue response of polyamide 66 nanocomposites containing 0%, 1%, and 5% hectorite organoclay were investigated. Seawater equilibrated nanocomposites demonstrated less moisture absorption compared to their pure counterparts. The imbibed seawater significantly increased the ductility, and reduced the tensile modulus and strength of nanocomposites. A nine-parameter Prony series viscoelastic constitutive model was used to estimate the influence of seawater on the viscoelastic behavior based on stress relaxation data. The phenomenological model evaluated a drop of 82% in the instantaneous and the residual elastic moduli indicating a significant plasticizing effect of seawater. However, the amount of increase in the rubbery nature of polyamide is observed from the reduction in stress relaxation time of first Maxwell element by about 40%. Nanocomposites exhibited improved fatigue life with an increase in organoclay content even after equilibration in seawater. The fracture surface observation of seawater equilibrated bending fatigue specimen revealed rubbery behavior indicating the positive difference between cyclic steady-state temperature and material glass transition temperature.

Investigating Discrepancies in Vibration Bending Fatigue Behavior of Additively Manufactured Titanium 6Al-4V

The vibration bending fatigue life uncertainty of additively manufactured titanium (Ti) 6Al-4V specimens is studied. In this investigation, an analysis of microscopic discrepancies between ten fatigued specimens paired by stress amplitude is correlated with the bending fatigue life scatter. Through scanning electron microscope (SEM) analysis of fracture surfaces and grain structures, anomalies and distinctions such as voids and grain geometries are identified in each specimen. These data along with previously published results are used to support assessments regarding bending fatigue uncertainty. The understanding gained from this study is important for the future development of a predictive vibration bending fatigue life model.

Influence of Residual Stress on Fatigue Weak Areas and Simulation Analysis on Fatigue Properties Based on Continuous Performance of FSW Joints

The fatigue weak area of aluminum alloy for a friction stir-welded joint is investigated based on the hardness profile, the residual stress measurement and the simulation analysis of fatigue property. The maximum residual stresses appeared at the heat-affected zone of the joint in the fatigue damage process, which was consistent with the fracture location of the fatigue specimen. The fatigue joint model of continuous performance is established ignoring the original negative residual stress; considering that it will be relaxed soon when the joint is under tension-tension cyclic loading. The fatigue parameters of joint model is based on the static mechanical properties of the joint that obtained from the micro-tensile tests and four-point correlation method. The predicted results for the fatigue weak locations and fatigue lives based on the continuous performance joint model are closer to the fatigue experimental results by comparison with the simulation results of the partitioned performance joint model.

Low Cycle Fatigue Behavior of CP-Ti at Different Temperatures

Low cycle fatigue (LCF) tests are performed on CP-Tiat different temperatures (293K,423K and 523K). It is found that the fatigue life of CP-Tidecreases with temperature. A short cycle hardening phenomenon occurs at the beginning of cyclic deformationat 293K and 423K, followed by cyclic softening untilfailure. At 523K, cycle hardening isexhibited throughout the entire cycle until thefracture. The fatigue-life curves obtained from the tests are constructed using Coffin-Manson-Basquin model. According to the relationship between the four parameters of Coffin-Manson-Basquin model and temperature, the temperature-based life prediction model is further proposed. Scanning electron microscopy observation of fatigue fractures showsthat the fatigue cracks of CP-Tiat 423K and 523K under different strain amplitudes initiate on the surface of fatigue specimens and extend to the fracture zone by the transgranular mode.

Effects of Building Orientation on Fatigue Behavior of Ti-6Al-4V Alloy Produced by Selective Laser Melting

Addictive manufacturing (AM) allows for the layer-by-layer fabrication of components via sequential material deposition and it is of immediate interest in many applications, in particularly aviation field. This work is tackling the issue that the influence of the inner-defects and building orientations on the fatigue behavior of Ti-6Al-4V Alloys produced by Selective Laser Melting (SLM). Specimens were built in two orientations (horizontal and vertical to the substrate) in order to evaluate the impact of the induced anisotropy of fatigue properties. A series of fatigue tests at five stress ratios ranged from-1 to 0.8 are conducted at 400°C. Scanning Electron Microscopy (SEM) is used to examine the fracture surfaces of fatigue specimens to qualify the failure mechanism and crack initiation sites, which are most likely attached to the surface defects. The fracture surface analysis of HCF specimen tested at two temperatures reveals that near 85% of the crack initiates from the defect under subsurface. The results of this study imply that the fatigue properties at 400°C are highly dependent on the specimen orientations relative to build directions, as the defects close to surface are the mainly cause of the crack initiations.

Effect of cyclic hardening on fatigue performance of slide burnished components made of low‐alloy medium carbon steel

AbstractThe slide burnishing process causes cyclic loading of the surface being treated, which provokes cyclic hardening. Using a forced‐controlled indentation test, the sixth “loading‐unloading” cycle was stabilised. The effect of the number of passes and the cyclic loading coefficient (CLC) on the fatigue performance of slide burnished specimens was investigated. Rotating bending fatigue tests were conducted using nine groups of hourglass shaped specimens, which were slide burnished through a different number of passes and CLC values. A stabilised cycle of the surface layer achieved with six passes, lead to largest fatigue limit, whereas the CLC exerted negligible influence on the fatigue performance. The observed phenomenon was explained through different residual stress relaxation rates, due to the rotating bending load, as well as with the obtained surface layer microstructure. The residual stress relaxation was investigated through rotating bending fatigue tests, using cylindrical fatigue specimens, followed by X‐ray stress analysis.

Very high-cycle fatigue properties and microstructural damage mechanisms of selective laser melted AlSi10Mg alloy

The influence of cooling rates in selective laser melted AlSi10Mg on fatigue properties is studied. The failure mechanisms during quasistatic and fatigue loading were analyzed using electron microscopy, X-ray computed tomography and ultrasonic fatigue testing systems. It was found that even when densification mechanism was not significantly enhanced, the morphology of defects was adopting more regularly spherical shape which is less critical under structural loading. The controlled cooling introduced microstructural homogeneity and further property enhancement of the microstructure which contributed to higher quasistatic and fatigue strength in this study and compared to the literature on the same alloy. Elimination of semi-coherent Si agglomerates on the melt pool boundaries impeded crack propagation and forced it to adjust to a perpendicular orientation to the columnar dendrites formed instead. The strengthening mechanism was shown effective in quasistatic, servohydraulic and ultrasonic fatigue tests in very high-cycle fatigue. The influence of testing at ultrasonic frequencies was analyzed through the fatigue data and influence was not significant. Analysis of the interrupted fatigue specimens in the X-ray computed tomography revealed a pore-crack interaction mechanism which was hypothesized earlier in the literature. Although impedance of crack propagation and theoretical drop of stress intensity factors at crack tips was expected, it was implied that recommence of crack propagation and the end life of the specimen will still be dependent on the local strength of the microstructure in the vicinity of the pore.

Analysis of the effect of structural defects on the fatigue strength of RFSSW joints using C‐scan scanning acoustic microscopy and SEM

AbstractSolid‐state refill friction stir spot welding (RFSSW) technology offers significant benefits in the fabrication of aluminium structures in the transport and aerospace industries. In this paper, the joining of 1.6‐mm‐thick Alclad 7075‐T6 aluminium alloy sheets is investigated. High‐cycle fatigue strength tests of single‐lap welded joints were carried out on an Instron E10000 testing machine with a limited number of cycles equal to 2 × 106. The welding of overlap fatigue specimens was conducted using an RPS100 spot welder by Harms & Wende GmbH & Co KG. C‐mode scanning acoustic microscopy (C‐SAM) and scanning electron microscopy (SEM) were utilised to evaluate the joint quality and characterise the microstructure. The paper discusses the effect of the maximum load force and defects (voids, hook, kissing bond, bonding ligament, etc) associated with the material flow in the weld on the failure mechanism. Insufficient plasticisation of sheet material and mixing of the material in the weld area are crucial defects that influence the number of destructive cycles. The weld defects in the joint structure are a source of a decrease in the fatigue life compared with the fatigue life of defect‐free welds. It was also found that RFSSW joint defects can be effectively detected by the nondestructive C‐SAM method.

Effects of Strain Rate on Low‐Cycle Fatigue Behaviors of Fe–22Mn–0.6C TWIP Steel

The influence of strain rate on low‐cycle fatigue (LCF) behavior of Fe–22Mn–0.6C twinning‐induced plasticity (TWIP) steel is investigated by conducting LCF tests. The LCF tests are performed at a strain amplitude of 0–1.4% with strain rates of 8 × 10−4 s−1, 2 × 10−3 s−1, 8 × 10−3 s−1, and 2 × 10−2 s−1. The corresponding fatigue fracture morphologies and microstructure are also investigated. The results indicate that the TWIP steel shows initial cyclic hardening and cyclic saturation and then cyclic softening until final fracture at all strain rates. In addition, the fatigue life decreases with increasing strain rate due to enhanced dynamic strain aging (DSA). Moreover, the generation of persistent slip bands (PSBs) at high strain rates is also favorable for crack nucleation and fatigue crack growth, which is one of the primary reasons that lead to the premature failure of the fatigue specimens. At all given testing conditions, the fracture morphologies occur in a transgranular fracture mode.

Assessment of the fatigue strength of thick-walled nodular cast iron with Dross

Dross in large castings made of nodular cast iron reduces the quasi-static and the fatigue strength of the materials significantly, which causes afford to remove the Dross or to reject the complete component, if a secure assessment of the component’s lifetime cannot be performed.The aim is thus to find appropriate methods to determine the quasi-static and cyclic material behavior of Dross and to set up an assessment concept for components affected by Dross with the help of non-destructive testing.For this purpose, cast blocks made of EN-GJS-400-15 were casted with Dross. These blocks were investigated with different NDT methods to achieve information about the position and the characteristics of Dross present. Afterwards fatigue and tensile specimens were removed from the blocks taken from the Dross and the baseline material, which were tested under strain control to determine the cyclic strain based material behavior. Additionally, all fatigue specimens were investigated concerning their Dross distribution with X-ray computer tomography and partly also with magnetic particle inspection to correlate the NDT and the destructive test results with the forms of Dross detected. Further, a fatigue assessment concept including these research results was developed to assess large cast components with Dross.

The effect of platinum-aluminide coating features on high-temperature fatigue life of nickel-based superalloy Rene®80

Low cycle fatigue is the most important failure mode in Aviation/Industrial engine rotary turbine parts. In this paper, the influence of Pt-aluminide coating parameters on high temperature low cycle fatigue behavior of superalloy Rene?80 which is used to manufacture turbine blades, has been investigated. For this purpose, initial platinum layers of different thicknesses (2?m and 8?m) were coated on fatigue specimens. Then the aluminizing process was performed with two conditions of low temperature-high activity and high temperature-low activity. The results of microstructure investigations performed by scanning electron microscope and X-ray diffraction phase analysis indicated a three-layer structure for the coating (bi-phase (Ni,Pt)Al+PtAl2, single-phase (Ni,Pt)Al and interdiffusion zone) with different chemical compositions at both thicknesses of the platinum layer and using both aluminizing methods. Also, the results of low cycle fatigue tests at 871?C, R=0 and strain rate of 2?10 -3 s-1 showed a decline in fatigue properties in coated specimens as compared to uncoated sample, at total strains of 0.4, 0.8, and 1.2%. This reduction was lower in the low temperature-high activity with platinum layer thickness of 2?m, while it was more significant in the high temperature-low activity with the platinum layer thickness of 8?m. The fractography studies on coated and uncoated specimens indicated a mixed mode of ductile and brittle fracture.

Defect sensitivity in additively manufactured aluminium alloys: contribution of CAD artificial defects

The prediction of the fatigue resistance of additively manufactured parts is a current issue for the materials and process qualification in aerospace industry. Despite a continuous improvement of AM process, the presence of defects cannot yet be completely avoided, and the latter are still one of the main causes of fatigue damage in AM materials. In this framework, the present work focused on the influence of defects on the uniaxial fatigue behavior of AlSi7Mg0.6 alloy produced by Selective Laser Melting (SLM). Uniaxial fatigue tests have been performed. Fatigue specimens were subjected to a T6 treatment, and then machined in order to avoid the influence of surface roughness. Besides, for some specimens, artificial defects were directly introduced through CAD. The introduction of artificial defects, whose sizes and positions are precisely controlled, aims to provide a proper assessment of defect sensitivity. X-ray tomography was used to characterize both natural and artificial defects. Finite-element calculations of the local stress fields in the vicinity of defects were conducted, accounting for the real defect geometries obtained with CT scans. The application of a non-local multiaxial fatigue criterion then allowed to analyze defect criticity.

Multiaxial fatigue behavior of additive manufactured Ti-6Al-4V under in-phase stresses

The development and application of additive manufacturing (AM) technologies is constantly increasing. However, in many applications, AM parts are subjected to multiaxial loads, arising from operating conditions and/or complex geometries. These make AM components serious candidates for crack initiation and propagation mechanisms. Therefore, a deep understanding of the multiaxial fatigue behavior of AM parts is essential in many applications where durability and reliability are core issues. In this study, multiaxial fatigue of Ti6Al4V thin-walled tubular specimens, made by Selective Laser Melting (SLM) process, was investigated by combined axial-torsional loads. Infrared thermography (IR) was also used to investigate the temperature evolution during fatigue tests. Results highlighted different damage mechanisms and failure modes in the low- and high-cycle fatigue regimes.

Fatigue Performances of Chemically Etched Thin Struts Built by Selective Electron Beam Melting: Experiments and Predictions

Thin Ti-6-Al-4V tensile fatigue specimens were specifically designed to mimic struts of a lattice structure. Those samples were submitted to HIP and chemical etching that are thought suitable for complex geometries (machining was excluded). The effect of each post-treatment, as well as their combination on the fatigue performance is evaluated and quantified. Fatigue cracks are found to systematically initiate from surface defects. An original approach based on a full-3D non-destructive characterization by X-ray microtomography is developed to identify the population of those defects and rank them based on their size. Kitagawa-Takahashi diagrams are then used as a predictive tool regarding fatigue failure of samples containing defects. We show that ranking the defect size alone does not fully reflect the harmfulness of the identified defects.

Fatigue behaviour of selected materials under multiaxial asynchronous loadings

Four types of materials: PA38 aluminium alloy, E235 steel, E355 steel and 1.4301 austenitic steel were subjected to low-cycle multiaxial loadings. All tests were strain-controlled and typical, thin-walled, hollow specimens were used. Various synchronous and asynchronous loadings were applied. The analysis of experimental results involved: cyclic stress-strain response, fatigue life and observation of microcracks behaviour on the surfaces of fatigued specimens. Obtained results indicate that the difference in the strain components frequency of the asynchronous loadings has a significant influence on the fatigue behaviour of the materials.

Dynamic response and fatigue life of Vacuum cast Polyurethane polymer material

The objective of this paper is to measure the dynamic response transmissibility and fatigue life of vacuum cast Polyurethane (PU) polymer material used in the construction of automotive lamps. Polymers used in automotive lamps, if mass produced, are injection moulded. For ultra-low volume production, however, the cost of tooling for these traditional manufacturing methods is often very high. Hence, alternative approaches such as Silicone Tool – Vacuum (STV) casting are used. The parts produced using alternative approaches may have inferior dynamic performance. The specimens for dynamic response and fatigue testing were cut-out from a vacuum cast polyurethane plate. An instrumented beam was mounted on a shaker table and using sine sweep base excitation the dynamic response was measured. Using measured acceleration, response transmissibility was calculated. The bending fatigue properties were measured on a vibration shaker using a 4-point bending based resonance test apparatus. A series of tests were carried out to obtain average performance parameters. The dynamic response shows nonlinearity, as expected of polymers, and a large variation in dynamic properties is observed. The obtained fatigue life curve shows significant scatter at 40% fatigue load level. The assessed fracture surface micrograph of the fatigued specimens at three load levels shows surface tear at 80% and 60% load levels. Overall, the Vacuum cast polyurethane shows less fatigue performance than the conventional polymers of automotive lamps.

Influence of internal imperfections on the fatigue resistance of cast steel – testing methodology

Tensile fatigue specimen of G20Mn5 and G22NiMoCr5-6 were tested to quantify the influence of internal defects on the fatigue resistance of cast steel components. Defects with varying sizes, geometric shapes and distribution were enforced by influencing the solidification and recorded by computer tomography (CT). Besides the characteristics of the detected cavities, the surrounding fungoid microstructure is classified and evaluated. Later the specimens were tested under cyclic tension and S/N-curves are derived. These data form the basis for extensive numerical simulations of the damage process and the crack growth of every individual specimen. Both processes are affected by the local multiaxial stress states and have their origin in the inside of the specimen. For validation, knowledge of the crack initiation time and propagation properties are essential. Therefore, all specimens respectively the properties of the internal defects are monitored during testing with three different state-of-the-art non-destructive testing (NDT) methods. Background and application of these NDT techniques are described within this paper. Finally, fracture surface analyses show different failure modes and provide further information for model validation.

Fatigue crack onset by Finite Fracture Mechanics

The paper investigates the fatigue crack/notch sensitivity by the coupled criterion of Finite Fracture Mechanics (FFM). The approach involves two parameters: the plain specimen fatigue limit, and the threshold value of the stress intensity factor range for fatigue crack growth. Useful analytical relationships are presented. The accuracy of FFM is verified by considering experimental data available in the Literature, showing the potentiality of the coupled approach to predict size effects.