Bending Fatigue Tests Research Articles

Influence of specimen geometry and notch on fatigue lifetime and fracture behavior of aluminum-based nanocomposite under stress-controlled fully-reversed bending loads.

In the present article, the effect of the specimen geometry and the sample notch was studied on the high cycle fatigue lifetime and fracture behavior of the aluminum-based nanocomposite. For such an objective, rotary fully-reversed bending fatigue tests were performed on smooth and notched specimens, with the frequency of 100 Hz. Then, simulated results using the MSC Fatigue software were calculated and compared to the fatigue lifetime in the experiments for validation. Moreover, scanning electron microscopy was utilized to observe the fracture surface of failed samples after testing. Obtained results indicated that the fatigue lifetime increased by enhancing the sample diameter. However, the fatigue lifetime reduced when the stress concentration factor changed from 1.0 to 2.9. All samples with three geometries had a brittle fracture due to cleavage and quasi-cleavage marks on the fracture surface.

Evaluation of Hydrogen Embrittlement of Electroless Ni–P Plated 6061-T6 Aluminum Alloy by Three-Point Bending and Rotating Bending Fatigue Tests

Evaluation of Hydrogen Embrittlement of Electroless Ni–P Plated 6061-T6 Aluminum Alloy by Three-Point Bending and Rotating Bending Fatigue Tests

Методика циклических испытаний образцов-моделей коленчатого вала судового двигателя внутреннего сгорания на изгиб с кручением

It is proposed to consider the bending fatigue strength of the crankshaft as the main criterion of its operability during operation. It is noted that thermomechanical straightening with heating by stress relaxation method is most acceptable for straightening critical shafts that have received a curvature of the longitudinal axis during operation. Bending fatigue tests of round samples with a diameter of 20 mm in a dangerous section were carried out to substantiate the correction technology, test modes and evaluate the results of the correction. It was found that an increase in diameter leads to a decrease in the endurance limit of structural steels. The loading scheme of the experimental sample is adopted – transverse bending with torsion. A semicircular notch in the sample was used to focus the fracture site, and the radius of the notch was calculated. The size of the sample tuck is determined by the formula based on the similarity criterion adopted in the statistical theory of similarity of fatigue failure of parts, proposed by V. P. Kogaev. The requirements for the roughness of the surface of a circular groove of at least Ra 0.16 are established. The need for control of the manufacturing technology of the experimental sample is noted. An experimental installation for testing flexural fatigue strength is proposed. A cone cartridge was used to securely fasten and prevent the sample from turning during the experiment. Experimental samples were used-models with deflection arrows of 1, 2, 3, 4 and 5 mm. The method and equipment for obtaining samples with the necessary deflection arrows have been determined. Diagrams of a model sample for testing cantilever bending with torsion, an experimental installation for testing model samples, bending to a given deflection arrow, and a model sample with a curved longitudinal axis are presented.

Fatigue Resistance Assessment of WAAM Carbon Steel

This study presents a comprehensive exploration of the fatigue resistance of wire arc additive manufacturing (WAAM) carbon steel for lattice structures. Microstructural analysis unveils substantial grain dimensions characterized by a distinctive crystallographic configuration. These grains exhibit equiaxed characteristics, demonstrating uniform dimensions in all directions. The prevailing microstructure is dominated by ferrite grains. In tandem with the microstructural insights, hardness evaluations were conducted in correspondence with the part's deposition direction. The analysis of these measurements unveiled a consistent base material hardness of approximately 159 HV. The uniform distribution of hardness profiles supports the deduction that WAAM carbon steel uniformly embodies strength attributes. This congruence aligns harmoniously with the uniform microstructure evident in microscopic analyses. The yield strength of the WAAM carbon steel exhibits higher values in the build direction, peaking at 392 MPa. The bending fatigue tests revealed a fatigue limit approximating 180 MPa for WAAM carbon steel, evident in both the build and deposition directions. Fatigue strength of WAAM carbon steel mirrors that observed for reference material S355MC steel sheet.

Effect of surface coating on fatigue life and fatigue crack growth behavior of AISI D2 tool steel

To study the effect of the specimen surface coating on the fatigue life and fatigue crack growth behavior, rotating bending fatigue tests were performed with a specimen coated with a TiCN thin film on AISI D2 tool steel (JIS SKD11) and a bare specimen without coating. Fatigue cracks developed inside the specimen because compressive residual stress was present on the surface of the coated and bare materials. The fatigue crack growth behavior of the internal cracks was investigated with the two-stage double-stress experimental method proposed by the authors. The fatigue life estimated from the fatigue crack growth (FCG) behavior of the internal cracks and the experimental results agreed. Furthermore, a method was proposed that can easily evaluate the effect of residual stress on fatigue life and FCG behavior. The evaluation and experimental results are in good agreement. In addition, the effects of atmospheric and vacuum conditions on FCG behavior are discussed.

Road Performance and Self-Healing Property of Bituminous Mixture Containing Urea-Formaldehyde Microcapsules.

Urea-formaldehyde (UF) is a common shell material for self-healing microcapsules; however, the influence of urea-formaldehyde microcapsules (UFMs) on the road performance of bituminous mixtures and the sensitivity of their healing abilities remains unclear. In this paper, UFMs were prepared via in situ polymerization (ISP), followed by an investigation into the road performance of UFM self-healing bituminous mixtures through various tests, including wheel tracking, immersed Marshall, freeze-thaw splitting, low-temperature bending, and three-point bending fatigue tests. Subsequently, the impact of the damage degree, healing duration, and temperature on the self-healing property was discussed. The results indicated that incorporating 3 wt% UFMs into bitumen significantly improved the high-temperature stability and fatigue resistance of the bituminous mixture; for example, its dynamic stability and fatigue life could be increased by about 16.5% and 10%, respectively. However, it diminished the thermal crack resistance, as evidenced by decreases in bending tensile strength and strain by 3.7% and 10.1%, respectively. And it did not markedly improve the moisture susceptibility. Additionally, the maximum improvement observed in the healing rate was about 9%. Furthermore, the healing duration and temperature positively influenced the bituminous mixture's self-healing, whereas the degree of damage exerted a negative impact, with a relatively significant effect.

Topology optimization of wheel spoke under fatigue tests considering the rotation characteristics of automobile wheels

The topology structure of wheel spoke has a great impact on the safety and lightweight of automobile wheels. However, the rotation process of the wheels is difficult to be reflected in the design and optimization. In this paper, the topology optimization of wheel spoke under fatigue tests considering the rotation characteristics is studied. The bending fatigue test and radial fatigue test are first introduced and the corresponding computation models are presented to calculate the mechanical performances of the wheel hub. The relationship between the internal stress and the number of the wheel spokes is established, and the mechanical performances with different number of wheel spokes are analyzed to guide the selection to the number. Then, the rotation characteristics of the wheel hub are simplified as static loads at different directions or positions according to the configuration of the wheel spokes. A comprehensive evaluation function which includes the test and load levels is defined by the compromise programming method to simulate the stress during the rotation process. Two levels of topology optimization methods are determined. Next, a mathematical model of multi-objective topology optimization for the wheel spoke is established based on the Solid Isotropic Material with Penalization, and a new wheel hub is obtained. Finally, the mechanical performances are compared with the original wheel hub. The obtained results show that the stiffness and strength of the wheel hub are improved under two test conditions, while the weight is reduced by 2.75%.

Effect of surface roughness and polishing orientation on fatigue life and fracture fatigue entropy

AbstractThe effect of surface roughness and polishing orientation on fatigue life is investigated. Fully reversed bending fatigue tests are conducted on ASTM A1008 specimens having surface roughness asperities ranging from mirror finish (Ra = 0.05 μm) to coarse (Ra = 1.5 μm). Two polishing directions (i.e., orientations) are examined. For the range of roughness values tested, the results suggest that polishing orientation has a strong influence on fatigue life. Specifically, within the range of surface roughness values tested, fatigue life was found to be relatively insensitive to surface roughness asperities when the roughness orientation was parallel to the direction of surface stress but sensitive to surface roughness asperities when the orientation crossed the direction of surface stress. Results are generalized using the concept of fracture fatigue entropy (FFE). It is shown that, unlike fatigue life, FFE is independent of polishing orientation, a noteworthy result. To explain the behavior of FFE in relation to surface roughness asperities, a fatigue thermodynamics theory is presented that hypothesizes that all specimens of the same material fail at equivalent entropy levels.

Impact of Corrosion in Simulated Body Fluid on Fatigue Characteristics of 3D-Printed Polylactic Acid-Coated AM60 Magnesium Alloys

In this research, the pure fatigue behaviors of AM60 magnesium alloy with polylactic acid (PLA) coating (PF-AM60-PLA) and the corrosion fatigue behaviors of magnesium alloy with PLA coating (CF-AM60-PLA) were evaluated. Polymer coating was made by fused deposition modeling (FDM) with a 3D printer and attached to standard fatigue test specimens with glue. Then, after 27 days of immersion in the simulated body fluid (SBF), the high-cycle bending fatigue test was performed on samples. Due to corrosion, the weight of the specimens was reduced by an average of 35%. The corrosion rate decreased in the first 7 days and then increased. PF samples with a coating had an average 49% increase in fatigue lifetime. Regarding the CF samples, despite the use of a 10-times stronger solution, the fatigue lifetime of these samples decreased by only 35%. The field-emission scanning electron microscopy (FESEM) results also showed cleavage plates and striations. In addition, the separation of the glue from the coating and Mg was observed. Corrosion products, in addition to microcracks and holes, were seen on the fracture surface of CF specimens, which caused the stress concentration and the crack initiation. Holes caused by the release of gases were also observed in polymer coatings, which were fabricated by 3D printing.

Optimization of 3D printing parameters in polylactic acid bio-metamaterial under cyclic bending loading considering fracture features

3D printing has become a crucial additive manufacturing technique with the applications in various industries. Fused deposition modeling (FDM) is a common additive manufacturing process that offers considerable flexibility in the component fabrication through multiple parameters, which strongly influence the properties of the produced parts. This study focused on the impact of different printing parameters on the fatigue behavior of polylactic acid (PLA). The standard samples were 3D-printed with varying speed (5, 10, and 15 mm/s), print temperature (180, 210, and 240 °C), and nozzle diameter (0.2, 0.4, and 0.6 mm). The fatigue properties were evaluated through rotating bending fatigue tests, and a model was developed based on the results with a statistical analysis. The model accuracy was validated and the interactions between the parameters were analyzed. The optimization study found that a print speed of 5 mm/s, print temperature of 210 °C, and nozzle diameter of 0.2 mm were optimal. The fracture surfaces were also examined using a scanning electron microscopy, revealing the presence of crazing despite the brittle behavior of PLA.

Bending fatigue performance of high-strength seven-wire monostrand

Large-amplitude cable vibrations are remarkably common in cable-stayed bridges subjected to wind and mechanical loading due to low inherent damping of these structures. Although the high slenderness ratio of cables means that bending moments are not normally significant over their free length, local effects near the cable anchorage induce non-negligible bending stress variation under dynamic loading, which could lead to fatigue failure of stay cables. In modern parallel strand stay cables, the fatigue performance depends on the fatigue properties of the individual seven-wire monostrands which together make up the cable. We introduce a new bending fatigue testing device for seven-wire monostrand which can control the fatigue loading in the axial and transverse directions of the cable independently. The fatigue endurance of monostrand with guide deviators is significantly longer than without guide deviators present, especially for smaller bending fatigue amplitudes. We found that bending fatigue life of monostrand is also reduced by increased constant or dynamic axial stress, but over-tensioning of the monostrand (e.g. during construction) had no consistently adverse effect on fatigue life. For monostrand loaded in severe bending, fatigue occurs due to the bending stress variation caused by the combination of bending moments and axial forces at the extreme edges of the monostrand cross-section, rather than due to relative movement and fretting between adjacent wires within the monostrand.

Flexible UV detectors based on in-situ hydrogen doped amorphous Ga2O3 with high photo-to-dark current ratio

Amorphous Ga2O3 (a-Ga2O3) has been attracting more and more attention due to its unique merits such as wide bandgap (∼4.9 eV), low growth temperature, large-scale uniformity, low cost and energy efficient, making it a powerful competitor in flexible deep ultraviolet (UV) photodetection. Although the responsivity of the ever-reported a-Ga2O3 UV photodetectors (PDs) is usually in the level of hundreds of A/W, it is often accompanied by a large dark current due to the presence of abundant oxygen vacancy (V O) defects, which severely limits the possibility to detect weak signals and achieve versatile applications. In this work, the V O defects in a-Ga2O3 thin films are successfully passivated by in-situ hydrogen doping during the magnetron sputtering process. As a result, the dark current of a-Ga2O3 UV PD is remarkably suppressed to 5.17 × 10−11 A at a bias of 5 V. Importantly, the photocurrent of the corresponding device is still as high as 1.37 × 10−3 A, leading to a high photo-to-dark current ratio of 2.65 × 107 and the capability to detect the UV light with the intensity below 10 nW cm−2. Moreover, the H-doped a-Ga2O3 thin films have also been deposited on polyethylene naphtholate substrates to construct flexible UV PDs, which exhibit no great degradation in bending states and fatigue tests. These results demonstrate that hydrogen doping can effectively improve the performance of a-Ga2O3 UV PDs, further promoting its practical application in various areas.

Fatigue life analysis based on the groove angle reduction in the butt welding of excavator booms

This study analyzes the effect of changes in groove angle on the fatigue strength of the V-groove butt weld joint made of SM490A steel used in excavators. Butt-type specimens with different groove angles of 40° and 30° were manufactured by CO2 welding at a speed of 300 mm/min on a 13.5 mm thick steel plate. Four-point bending fatigue tests were conducted at room temperature with a frequency of 5 Hz and stress ratio of R = 0.1, and fatigue limits were set to 2 × 106 cycles. The high-cycle fatigue life of VG40 was approximately 7% higher than that of VG30. The stress concentration coefficient calculated using the effective notch stress method was 1.62 for the VG40, and 1.78 for the VG30, with an approximate difference of 9%. This decrease in fatigue life was caused by stress concentration due to the notch effect at the weld root, which was attributed to the adjacent backplate. Although the fatigue life decreased slightly owing to the improvement angle of the welded part, no difference was observed in microstructure and hardness. Therefore, it was confirmed that a sound weld was formed. The results of this study can be used as important basic data for determining the V-groove angle.

Ti-6Al-4V L-PBF chemically etched components: from the surface micro-geometric characteristics to the fatigue strength

Additive manufacturing is nowadays already used on aeronautical serial parts but very few of them are highly loaded critical components. Titanium alloy Ti-6Al-4V and laser powder bed fusion (L-PBF) manufacturing process are both part of the most studied materials and processes. Many studies have been dedicated to L-PBF produced Ti-6Al-4V on a large range of subjects from defect generation and microstructure evolution to fatigue life. Finishing processes are also a significant part of the studied processes as the micro-geometry of additively manufactured components remains the principal drawback for the production of fatigue subjected critical parts. In this work, fatigue bending tests are undertaken on Ti-6Al-4V L-PBF made coupons. For each of them, the loaded surface has been scanned using an optical proflometer to identify the population of micro-geometric surface features. The effect of a chemical etching process on this population is described and the killer feature is highlighted in the population. Different indicators are discussed to understand if and how the killer feature can be predicted, knowing the loading conditions. It is concluded that Murakami's √area parameter may not be sufficient to determine the critical surface feature. A large scatter is observed in the fatigue strength, however, an approach based on the stress intensity range seems the most appropriate to identify the potentially critical surface micro-geometric features from the characterized population.

Polymer Fatigue Evaluation Methodology for Unbonded Flexible Pipes

Polymer layers have been used in the offshore Oil & Gas industry for many years in unbonded flexible pipes for static and dynamic applications. The evaluation of fatigue performance for polymer grades in such applications is complex and no qualification method is well defined nor standardized at the time this article is written.Though historically no reported polymer fatigue field failures the complexity of unbonded flexible pipes, the conservatism of this industry makes the introduction of new polymer grades in risers for dynamic applications challenging. Development of a polymer fatigue methodology will allow the industry to quantitatively evaluate risks of fatigue failure of liners and thereby be important in introducing new material grades.In this paper, a design fatigue methodology is built based on inputs from a specific project and results from small- and full-scale tests. The pipe curvature ranges, extracted from the project location, are used to obtain an accumulated tensile strain which is then converted into an accumulated tensile stress. The fatigue damage is then determined, using a design curve established from bending or tension-tension fatigue testing performed on dumbbells from pipe samples. Parameters are included to reflect geometrical effects from manufacturing process but also service life conditions with several temperature profiles. This method can be applied for pressure sheaths as well as sealed outer sheaths with relevant inputs related to their operating conditions. The obtained fatigue damage can then be used to validate the use of polymer grades for specific unbonded pipe designs under their service conditions.

Infrared imaging surface roughness criticality assessment of Wire Arc Additive Manufactured specimens

Infrared imaging surface roughness criticality assessment of Wire Arc Additive Manufactured specimens

Quantification of structural components of Alloy 718 and their effect on dynamic mechanical properties

Ni-based superalloy 718 is especially suitable for high-temperature, high-pressure environments in aviation, where corrosion, creep, and thermal shock resistance are necessary for safe operation. The listed properties of alloy 718 are obtained by heat treatment - precipitation hardening. Heat treatment significantly affects the grain size of the basic gamma solid solution and the structural components of the alloy such as L12 gamma prime, DO22 gamma double prime and BCT delta phase. Their morphology, distribution and degradation affect the mechanical and thermo-mechanical properties of the alloy. The article aims to characterize and quantify the influence of the mentioned structural components on the dynamic properties of the alloy. For this purpose, methods of quantitative metallography using optical (differential interference contrast – DIC and dark field – DF) and scanning electron microscopes (EDS analysis, EDS elemental mapping, etc.) were used. To consider the influence of δ-phase on the dynamic properties (fatigue life) of alloy, blocky shape specimens at starting stage and after applying additional annealing (to increase the volume fraction of the δ-phase) at 800°C ± 5°C/72h + cooling in the furnace and the three-point bending fatigue test with the cycle asymmetry parameter R < 1, loading frequency f ≈ 145 Hz and Nf = 2.107 as a run-out were used.

Surface Integrity induced by the Belt Finishing Process and Effects on the Fatigue Limit of a 27MnCr5 Carburized Steel

Among superfinishing processes, belt finishing is well known for its ability to induce compressive residual stresses and low surface roughness. Moreover, it is promoted by industry, as this process requires limited investments and is easy to handle. Combined with hard turning, precise dimensions and fine surface roughness of the parts can be ensured. Therefore, the combination of these processes could replace the grinding operation performed on automotive gear shafts for example. The article proposes to study the effects of the belt finishing operation on surface integrity of 27MnCr5 carburized steel shafts. Residual stresses and surface roughness are investigated for hard machined and belt finished samples. Afterwards, rotating bending fatigue tests are performed, and the fatigue limit of the belt-finished samples is compared with hard machined ones.

Fatigue life prediction of the nitrided steel by multiaxial high cycle fatigue criteria

The present study aims to predict the fatigue strength of ion-nitrided 42CrMo4 steel, using multiaxial high cycle fatigue (HCF) criteria and considering the effects of stabilized residual stresses and surface hardening. The predicted fatigue strength was compared to experimental data obtained after three-points bending fatigue tests at two stress ratios (0.1 and 0.5) and for notched (Kt = 1.6) nitrided and untreated specimens. A 3D finite element (FE) model of a three-point bending fatigue test was developed under ABAQUS software in order to determine the actual applied cyclic stress state at the notch. The results show that ion-nitriding treatment led to 32% improvement in the fatigue strength at 106 cycles compared to the untreated material. This improvement is explained by the advantageous effect of ion-nitriding treatment in terms of compressive residual stresses and surface layer hardening. Both stabilized residual stress state and hardening effects were successfully implemented into various multi-axial HCF criteria, including Sines, Crossland, Dang Van and Matake criteria. A sensitivity analysis has shown that Crossland criterion has permitted to predict the fatigue limits in accordance with the experimental results.

Kaji Eksperimental Karakteristik Fatik Komposit Berpenguat 2 Lapis Serat Nenas dan Orientasi Serat 45 Derajat atau 90 Derajat

The aim of this study is to experimentally investigate the effects of fiber orientations on the fatigue strength, fatigue life and surface fracture of 2 ply Pineapple Leaf Fiber Composite. Fatigue test specimens with 45o and 90o fiber orientation are loaded using rotary bending fatigue test machine at several load levels. Fatigue strength and fatigue life are measured during fatigue test and fracture surface of fatigue specimens are observed using microscope. The results show that 2 ply Pineapple Leaf Fiber Composite with 45o fiber orientation have highest fatigue strength than 2 ply Pineapple Leaf Fiber Composite with 90o fiber orientation. The shape of the surface fracture in the test specimen varies widely which is generally caused by Multiple Origins or several fault starting points at once which can erode or tear the surface of the specimen before the specimen finally fails or breaks.