Showing papers on "Fatigue limit published in 2015"

Fatigue strength of severely notched specimens made of Ti–6Al–4V under multiaxial loading

Abstract: The present paper deals with multiaxial fatigue behaviour of severely notched components made of titanium grade 5 alloy (Ti–6Al–4V). The experimental tests have been carried out under combined Mode I and Mode III loadings, both in phase and out of phase. Cylindrical specimens weakened by circumferential notches have been employed. Different nominal load ratios have been applied in the tests (R = −1, 0 and 0.5). The specimens had a notch-tip radius smaller than 0.1 mm, a notch depth equal to 6 mm and an opening angle of 90°. The results obtained by multiaxial fatigue testing are depicted in comparison with data from pure modes of loading on smooth and notched samples, characterized by a load ratio in the range −3 ≤ R ≤ 0.5. A large bulk of new fatigue data (more than 160) is summarized in the manuscript. The data are first plotted in terms of the nominal stress amplitudes, and then they are reanalysed by means of the local energy measured in the control volumes surrounding the notch tip. The dependence of the size of the control radius as a function of the loading mode is analysed. A very different behaviour is found for tension and torsion, corresponding to a different notch sensitivity.

Improved mechanical properties of an epoxy glass–fiber composite reinforced with surface organomodified nanoclays

Abstract: An organomodified surface nanoclay reinforced epoxy glass-fiber composite is evaluated for properties of mechanical strength, stiffness, ductility and fatigue life, and compared with the pristine or epoxy glass-fiber composite material not reinforced with nanoclays. The results from monotonic tensile tests of the nanoclay reinforced composite material at 60 °C in air showed an average 11.7% improvement in the ultimate tensile strength, 10.6% improvement in tensile modulus, and 10.5% improvement in tensile ductility vs. these mechanical properties obtained for the pristine material. From tension–tension fatigue tests at a stress-ratio = +0.9 and at 60 °C in air, the nanoclay reinforced composite had a 7.9% greater fatigue strength and a fatigue life over a decade longer or 1000% greater than the pristine composite when extrapolated to 109 cycles or a simulated 10-year cyclic life. Electron microscopy and Raman spectroscopy of the fracture and failure modes of the test specimens were used to support the results and conclusions. This nanocomposite could be used as a new and improved material for repair or rehabilitation of external surface wall corrosion or physical damage on piping and vessels found in petrochemical process plants and facilities to extend their operational life.

Unified failure criterion for asphalt binder under cyclic fatigue loading

Abstract: Defining failure and developing a unified failure criterion for the fatigue testing of asphalt materials remain a challenge. This study seeks to develop a failure criterion for the fatigue testing of asphalt binders under cyclic loading in the dynamic shear rheometer. Newly developed pseudo-strain energy (PSE)-based failure analysis is introduced for both the time sweep fatigue test and the accelerated linear amplitude sweep (LAS) test (AASHTO TP101). The presented methodology builds upon recent advances in the simplified viscoelastic continuum damage (S-VECD) modelling of asphalt mixtures. Trends in stored PSE have been proven to be effective in defining failure for the LAS tests of asphalt binders. This new proposed failure definition is material-dependent and, thus, is effective in capturing the benefits of asphalt modification for binder fatigue resistance. In addition, it is found that a unique relationship that is independent of loading history exists between the PSE release rate and fatigue life. T...

Fatigue Performance of Laser Additive Manufactured Ti–6Al–4V in Very High Cycle Fatigue Regime up to 109 Cycles

Abstract: Additive manufacturing technologies are in the process of establishing themselves as an alternative production technology to conventional manufacturing such as casting or milling. Especially laser additive manufacturing (LAM) enables the production of metallic parts with mechanical properties comparable to conventionally manufactured components. Due to the high geometrical freedom in LAM the technology enables the production of ultra-light weight designs and therefore gains increasing importance in aircraft and space industry. The high quality standards of these industries demand predictability of material properties for static and dynamic load cases. However, fatigue properties especially in the very high cycle fatigue regime until 109 cycles have not been sufficiently determined yet. Therefore this paper presents an analysis of fatigue properties of laser additive manufactured Ti-6Al-4V under cyclic tension-tension until 107 cycles and tension-compression load until 109 cycles. For the analysis of laser additive manufactured titanium alloy Ti-6Al-4V Woehler fatigue tests under tension-tension and tension-compression were carried out in the high cycle and very high cycle fatigue regime. Specimens in stress-relieved as well as hot-isostatic-pressed conditions were analyzed regarding crack initiation site, mean stress sensitivity and overall fatigue performance. The determined fatigue properties show values in the range of conventionally manufactured Ti-6Al-4V with particularly good performance for hot-isostatic-pressed additive-manufactured material. For all conditions the results show no conventional fatigue limit but a constant increase in fatigue life with decreasing loads. No effects of test frequency on life span could be determined. However, independently of testing principle, a shift of crack initiation from surface to internal initiation could be observed with increasing cycles to failure.

Effects of stress ratio on high-cycle and very-high-cycle fatigue behavior of a Ti-6Al-4V alloy

Abstract: The effects of stress ratio on high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) behavior of a Ti-6Al-4V alloy were systematically investigated in this paper. Fatigue tests with ultrasonic frequency (20 kHz) were performed on specimens of a bimodal Ti-6Al-4V alloy with stress ratios of -1, -0.5, -0.1, 0.1 and 0.5. Three types of crack initiation mode were observed on the fracture surfaces of the specimens that failed in the HCF and the VHCF regimes, which were explicitly classified as surface-without-facets, surface-with-facets and interior-with-facets. With the increase of stress ratio from 1 to 0.5, the number of specimens for surface-without-facets decreased, that for surface-with-facets increased, and that for interior-with-facets increased first and then decreased. For the failure types of surface-with-facets and interior-with-facets, the fatigue strength decreased sharply in the VHCF regime, and the S-N curve switched from an asymptote shape to a duplex shape. Then, a new model based on Poisson defect distribution was proposed to describe the effects of stress ratio on the occurrence of different failure types, i.e., the competition of alternative failure types. The observations also showed that there is a rough area at the crack initiation region for interior initiation cases, and the values of the stress intensity factor range for the rough area are within a small range, with the mean value being close to the threshold for the crack starting to grow in vacuum environment of the alloy. The estimated value of plastic zone size at the periphery of rough area is close to the average diameter of the primary cc grains of the alloy. (C) 2014 Elsevier B.V. All rights reserved.

Fatigue Strength Prediction for Titanium Alloy TiAl6V4 Manufactured by Selective Laser Melting

Abstract: Selective laser melting (SLM), as a metalworking additive manufacturing technique, received considerable attention from industry and academia due to unprecedented design freedom and overall balanced material properties. However, the fatigue behavior of SLM-processed materials often suffers from local imperfections such as micron-sized pores. In order to enable robust designs of SLM components used in an industrial environment, further research regarding process-induced porosity and its impact on the fatigue behavior is required. Hence, this study aims at a transfer of fatigue prediction models, established for conventional process-routes, to the field of SLM materials. By using high-resolution computed tomography, load increase tests, and electron microscopy, it is shown that pore-based fatigue strength predictions for a titanium alloy TiAl6V4 have become feasible. However, the obtained accuracies are subjected to scatter, which is probably caused by the high defect density even present in SLM materials manufactured following optimized processing routes. Based on thorough examination of crack surfaces and crack initiation sites, respectively, implications for optimization of prediction accuracy of the models in focus are deduced.

Enhanced fatigue behavior of powder metallurgy Ti–6Al–4V alloy by applying ultrasonic impact treatment

Abstract: The effect of severe plastic deformation induced by multiple sliding impacts by the specimen surface produced at ultrasonic impact treatment (UIT) on the stress-controlled fatigue response of the powder metallurgy Ti–6Al–4V alloy is studied in this paper. Specimens of Ti–6Al–4V alloy were produced from Ti hydride precursor powders via the cost-effective blended elemental powder metallurgy technique. Structure investigations were performed by XRD, TEM, LM and SEM techniques. After UIT, fatigue strength was increased by about 60% on the base of 107 cycles, and lifetime was prolonged by two orders of magnitude at applied stress amplitudes of 300–400 MPa. The UIT process leads to approx. four times decrease in the surface roughness parameters. Increased by 65 and 20% microhardeness magnitudes are respectively registered on the top surface and on the depth of 100 μm. The UIT induced compressive stresses achieve about two thirds of the alloy yield stress. The hardness increase is shown to be coupled with the increased dislocation density, essentially refined of α+β microstructure and with randomization in α-grains orientations. Observations of cross-sections of the UIT processed specimens revealed the pores free near-surface layer of approx. 200 μm thick, which is formed thanks to micro-pore closure process promoted by high shear strains produced at the UIT induced sliding impacts. Analysis of fracture surfaces revealed subsurface cracks initiations and numerous fracture steps indicating on the cracks branching and deflection in the surface layers of the UIT processed specimens instead of superficial crack initiation and the grain boundary cleavages mainly observed in the pristine samples. Experimentally registered magnitudes of fatigue limit were successfully predicted by accounting for the effective stress intensity factor range ΔKth and observed by TEM microstructural units responsible for fatigue fracture (α-phase colonies, α-grains or micro-pores) and compared with literature data on PM Ti–6Al–4V alloy. Enhanced fatigue strength and prolonged lifetime of PM Ti–6Al–4V alloy after the UIT process are concluded to be associated with (i) minimized surface roughness; (ii) compressive residual stresses; (iii) UFG and nano-scale α+β microstructure; and (iv) micro-pore healing.

State-of-the-art review on the local strain energy density concept and its relation to the J-integral and peak stress method

Abstract: The local average strain energy density (SED) approach has been proposed and elaborated by Lazzarin for strength assessments in respect of brittle fracture and high-cycle fatigue. Pointed and rounded (blunt) V-notches subjected to tensile loading (mode 1) are primarily considered. The method is systematically extended to multiaxial conditions (mode 3, mixed modes 1 and 2). The application to brittle fracture is documented for PMMA flat bar specimens with pointed or rounded V-notches inclusive of U-notches. Results for other brittle materials (ceramics, PVC, duraluminum and graphite) are also recorded. The application to high-cycle fatigue comprises fillet-welded joints, weld-like shaped and V-notched base material specimens as well as round bar specimens with a V-notch. The relation of the local SED concept to comparable other concepts is investigated, among them the Kitagawa, Taylor and Atzori–Lazzarin diagrams, the Neuber concept of fictitious notch rounding applied to welded joints and also the J-integral approach. Alternative details of the local SED concept such as a semicircular control volume, microrounded notches and slit-parallel loading are also mentioned. Coarse FE meshes at pointed or rounded notch tips are proven to be acceptable for accurate local SED evaluations. The peak stress method proposed by Meneghetti, which is based on a notch stress intensity factor consideration combined with a globally even coarse FE mesh and is used for the assessment of the fatigue strength of welded joints, is also presented.

The Effects of Sulfide Inclusions on Mechanical Properties and Failures of Steel Components

Abstract: A literature review was performed to assess the effects of inclusions in carbon and alloy steels on their mechanical properties. Inclusions, including brittle oxides and more ductile manganese sulfides (MnS), affect fatigue endurance limit, fatigue crack propagation rate, fracture toughness, notch toughness, transverse tensile properties, and anisotropy of these properties with respect to the rolling direction. Significant property anisotropy has been documented, which needs to be taken into account in the design phases. Typical fracture morphologies and metallographic appearances of MnS-containing materials that the failure analyst will encounter are illustrated.

Effect of shot peening on metastable austenitic stainless steels

Abstract: In this work, shot peening was performed in a metastable austenitic stainless steel EN 1.4318 (AISI 301LN) in order to evaluate its effect on austenite to martensite phase transformation and also the influence on the fatigue limit. Two different steel conditions were considered: annealed, i.e., with a fully austenitic microstructure, and cold rolled, consisting of a mixture of austenite and martensite. X-ray diffraction, electron back-scattered diffraction and focus ion beam, as well as nanoindentation techniques, were used to elucidate deformation mechanisms activated during shot peening and correlate with fatigue response. Results pointed out that extensive plastic deformation and phase transformation developed in annealed specimens as a consequence of shot peening. However, the increase of roughness and the generation of microcracks led to a limited fatigue limit improvement. In contrast, shot peened cold rolled specimens exhibited enhanced fatigue limit. In the latter case, the main factor that determined the influence on the fatigue response was the distance from the injector, followed successively by the exit speed of the shots and the coverage factor.

Fatigue behavior of cold spray coatings: The effect of conventional and severe shot peening as pre-/post-treatment

Abstract: Cold spray coating is gaining increasing attention as a solid state additive technique for repairing structural components. The fatigue behavior of Al 6082 cold spray coating with the same material as substrate is studied. Furthermore, the possibility of improving the load bearing capacity of the cold spray coated specimens by shot peening process is explored. Conventional and severe shot peening were applied before and after cold spray depositions. The coated, coated-peened and peened-coated specimens were characterized by optical microscopy observation, surface roughness and X-ray diffraction residual stress measurements. The specimens were subjected to rotating bending fatigue tests in order to determine the fatigue strength. Unlike the fixed-cantilever, constant-deflection thin specimens, often used to measure fatigue strength of cold spray coatings, the present approach does not considerably reduce the applied stress at the interface compared to the nominal stress and thus represents a more critical and realistic response of the coatings to fatigue loading conditions. The fractured surfaces were examined by scanning electron microscopy. Cold spray coating increased the fatigue strength by 15% compared to the as-received specimens. Further improvement was achieved by the hybrid peening and coating treatment. It was found that shot peening is more efficient if it is performed prior to cold spray deposition. A 26% improvement in the fatigue strength was obtained by performing severe shot peening prior to cold spray deposition. This improvement was attributed to the integrity of coating/substrate interface without any signs of delamination, crack initiation from the surface instead of the interface, crack propagation mainly by transcrystalline mechanism and not through particle boundaries and finally high depth of compressive residual stresses.

Fatigue improvement in low temperature plasma nitrided Ti–6Al–4V alloy

Abstract: In this study a low temperature (600 °C) treatment was utilized to improve the fatigue performance of plasma nitrided Ti–6Al–4V alloy by optimization of microstructure. In order to study the fatigue properties, rotation bending tests were conducted, the S–N curves were constructed, and the results were compared with those obtained by an elevated temperature treatment (900 °C) as well as conventional gas/plasma nitriding treatments reported in literature. The plasma nitrided alloy at 600 °C showed an endurance limit of 552 MPa which was higher than those achieved by conventional nitriding treatments performed at 750–1100 °C. In contrast, plasma nitriding at 900 °C resulted in the reduction of fatigue life by at least two orders of magnitude compared to the 600 °C treatment, accompanied by a 13% reduction of tensile strength and a 78% reduction of ductility. The deterioration of mechanical properties after the elevated temperature treatment was attributed to the formation of a thick compound layer (~6 µm) on the surface followed by an α-Case (~20 µm) and phase transformation in the bulk microstructure from fully equiaxed to bimodal with coarse grains (~5 times higher average grain size value). The microstructure developed at 600 °C consisted of a thin compound layer ( 105 cycles) and the nitrided alloy endured cyclic loading until the tests were stopped at 107 cycles. The thin morphology of the compound layer in this study restricted the extent of premature crack initiation from the surface. Moreover, a deep diffusion zone with a well-bonded interface decreased the likelihood of fatigue initiation at (or below) the compound layer interface. Another notable feature was that the fatigue strength of the nitrided alloy was correlated with the surface roughness and in fact when the nitrided surfaces were polished, a higher number of cycles were dedicated to the formation of fatigue cracks compared to the as-treated condition resulting in an improved fatigue life.