Fatigue Strength Reduction Research Articles

Considerations about the fatigue strength assessment of local cut-out in the double bottom floor

The aim of this paper is to obtain the effect on the fatigue strength assessment of the different geometrical variables involved in the design of the local cut-out, resulting from the intersection between the longitudinal stiffener and the double bottom floor of a bulkcarrier vessel; a common ship structural detail considered to be prone to fatigue failure. Based on finite element models, different combinations between cut-out geometries and longitudinal stiffeners, in terms of size and shape, are analysed to establish the structural behaviour, expressed in accordance with the location and value of hot spot and fatigue life. A novel and valuable scantling tool, known as fatigue sensitivity curves, is developed that determines fatigue life under loading conditions that differ from the design load and reduce fatigue strength. A series of experimental tests are carried out to investigate the fatigue crack growth and validate the numerical results, from the most effective combination of the local weight determination.

Evaluating the strength of a beam-type electric switch mechanism housing

Aim. Choice of method and its validation for the purpose of confirming the feasibility of safe operation of a switch mechanism housing submitted to rolling stock axle loads of 30 tf. Method. As of today, there are neither regulations regarding the strength parameters of load-carrying elements of electric switch mechanisms, nor verification procedures. Given the above, when evaluating the strength of electric switch mechanism housing, the authors used the method used for the purpose of evaluating the strength of load-carrying elements of locomotives per GOST R 55513-2013. That method has a long history of application as part of calculation and testing of motive power and has shown good results. According to the method, strength is evaluated by comparing the safety factor of fatigue strength n with the allowed value for steel structures [n] = 2.0. Result. The activities resulted in the validation of the proposed method of strength estimation by the criterion of fatigue strength. The value of fatigue endurance of electric switch mechanism housing was estimated based on the results of fatigue benchmark tests of three items. Each was stressed by stepping up the amplitude of the applied force after the base number of cycles had been reached. At the first stage, the loading was equivalent to the operational value. After the base number of functions had been reached for each item, the following step was initiated. The tests continued until cracks have been found. In the process of testing, the amplitude of stress was recorded at each level of loading. For the purpose of identifying the fatigue strength characteristics, the reduced fatigue strength of the switch mechanism housing was calculated under the hypothesis of linear addition of fatigue damage subject to the condition of deterministic loading and subsequent processing of the findings using statistical methods. The value of fatigue limit has been obtained for the housing of a tie-type electric switch mechanism equal to 48.4 MPa. The safety factor of fatigue strength was found to be equal to 2.86, which is above the minimal allowed value. Conclusion. It has been shown that the housing of a tie-type electric switch mechanism has a sufficient strength as regards the operational static and dynamic loads. Its safe operation when exposed to axle loads of 30 tf has also been confirmed.

Very high cycle fatigue assessment at elevated temperature of 100 μm thin structures made of high-strength steel X5CrNiCuNb16-4

Many components and structures are exposed to very high number of cycles and challenging environmental conditions during operation. This study contributes to a better understanding of the very high cycle fatigue (VHCF) properties of high-strength steel X5CrNiCuNb16-4 at room temperature (RT) and 350 °C. For this purpose, conventional specimens and thin-walled structures are extensively examined with novel high-frequency fatigue testing techniques at elevated temperature. Tests with unnotched specimens at 350 °C show a 21.7% reduction in fatigue strength for 107 cycles and a different failure mechanism compared to RT. In contrast, no temperature influence is observed for mildly notched specimens and even a higher local fatigue strength is found for sharply notched specimens at 350 °C. The decrease in fatigue strength for 109 cycles is more pronounced at 350 °C (−10%) than at RT (−5%), and it is proven that notched specimens adequately represent the VHCF behaviour of structures. The transferability of specimen results to components and structures is given great attention. A new proposal for the VHCF strength assessment of structures with high stress gradients is presented, which is based on specimen results, an extended material-mechanical support factor and a VHCF reduction factor. The prediction model gives conservative fatigue strength estimates for 109 cycles with a maximum deviation of 5.8%. This demonstrates that even complex shaped structures can be successfully evaluated with suitable specimens and methods.

Actual weld profile fatigue performance by digital prototyping of defected and un‐defected joints

AbstractThis paper considers the effects of the actual weld profile on the fatigue behavior of welded joints by means of the implicit gradient approach. Different types of geometrical approximation are considered in the analysis. In order to define the fatigue strength reduction factor of the actual weld profile, FE analyses have been considered based on a three‐dimensional scan of the welded joints. Furthermore, the fatigue behavior of butt‐welded joints with a discontinuous undercut is considered, and a comparison has been made with experimental data taken from the literature.

Multiaxial fatigue life prediction model considering stress gradient and size effect

Stress gradient effect (SGE) and notch size effect (NSE) have important influences on the fatigue performance of engineering components. In this paper, a new multiaxial fatigue life prediction model is developed by considering the effects of both. Firstly, with the help of the coordinate transformation principle (CTP), the location of the dangerous critical plane is found according to the energy-critical plane (ECP) method, and the equivalent stress gradient factor (SGF) within the fatigue damage zone is calculated by analyzing the equivalent stress distribution on the critical plane. Secondly, considering the size of the notch, the effect of stress concentration at the notch is analyzed. Meanwhile, a size influence factor is introduced to characterize the NSE and construct a new fatigue strength reduction factor. Further, combining with the Manson-Coffin equation (MC equation), a new fatigue life prediction model for multiaxial notched components is proposed by considering SGE and NSE on fatigue damage. Finally, the experimental data of fatigue specimens with different notch sizes processed by En8 and GH4169 materials are used for model validation and comparison with other three classical models, and the results show that the prediction life of the new model for proportional and non-proportional loads is within two-fold dispersion band and the prediction ability is better than the other three classical models.

Stress assessment and fracture surface analysis in a foreign object damaged 7075-T6 specimen under rotating bending

The damage caused by impact of hard particles on aircraft components is called Foreign object damage (FOD) and it may cause premature failure. 7075-T6 is one of the alloys frequently used for manufacturing aircraft parts and components. In this paper, the failure mechanism of a damaged 7075-T6 specimen subject to rotating bending was explained from a phenomenological point of view using the Finite Element (FE) method and the fracture surface observation. The results showed that the crack initiation occurred in the locations of the highest residual tensile axial stresses after the impact, where also the tensile axial stresses in the simulated fatigue cycle are the highest, and at the surface of the FOD dent. A discontinuity was observed under the dent, in the area subject to compression after the impact and in the fatigue cycle. The damage induces a 30 ± 10% reduction in fatigue strength compared to undamaged specimens.

Notches, nominal stresses, fatigue strength reduction factors and constant/variable amplitude multiaxial fatigue loading

This paper deals with the reformulation of the Modified Wöhler Curve Method to make it suitable for being used along with nominal stresses to design notched components against constant/variable amplitude multiaxial fatigue loading. The accuracy of this design methodology is checked against a large number of experimental results taken from the literature. The outcomes of the performed validation exercise strongly support the idea that the proposed approach can safely be used to perform the multiaxial fatigue assessment of notched structural components. This holds true also when the necessary fatigue strength reduction factors are estimated via standard, classic formulas.

Corrosion fatigue mechanism and life prediction of railway axle EA4T steel exposed to artificial rainwater

High-speed railway axle is the critical safety component of modern ground vehicles, which experiences extremely complex environments. In this paper, the effect of corrosion on the fatigue behavior of EA4T axle steel was investigated in an artificial rainwater environment. Combining scanning electron microscopy (SEM) and synchrotron radiation X-ray micro computed microtomography (SR-μCT) with theoretical analysis, damage measurement enabled a full evolution characterization of the corrosion process for exploring the corrosion mechanism and its influence on the fatigue life. It is found that the corrosive environment significantly reduced fatigue strength and facilitated multiple crack initiation, while showed relatively little effect on the long fatigue crack growth. The fitness of the 3-P Weibull distribution was slightly higher than that of the Gumbel distribution in terms of crack population. The life prediction model incorporating the pit growth, short crack, and long crack stages showed a good agreement within the 95% and 5% probabilistic S-N curve.

Effects of voids and raster orientations on fatigue life of notched additively manufactured PLA components

In this study, the fatigue life of notched polylactic acid (PLA) samples fabricated through the fused deposition modeling (FDM) technique was studied experimentally and numerically. The volumetric method based on the theory of critical distance was employed for fatigue life predictions. The effects of influential process parameters including raster orientation and FDM-induced defects such as voids or gaps inside the parts were examined on the fatigue strength reduction factors and fatigue lives. Circular and elliptical-shaped notch geometries were considered with various dimensions. Fatigue tests were conducted on notched and un-notched samples at the load ratio of 0.1. Predicted results were compared with experimental fatigue test data. Results revealed that the raster orientation parameter had a substantial impact on fatigue strength reduction factors and fatigue lives. The stress concentrations induced by the FDM process on the surfaces and inside the parts for the samples with 90° raster angles acted similar to the sharp notches, resulting in no substantial difference in fatigue life of notched and un-notched 3D-printed samples. In contrast, un-notched 3D-printed specimens with 0° raster orientations possessed higher fatigue lives as compared to the notched samples. While the volumetric approach efficiently predicted the fatigue lives of the samples with 90° raster orientations, it moderately underpredicted the fatigue lives of the samples with 0° raster angles.

Fatigue Resistance of 3-Unit CAD-CAM Ceramic Fixed Partial Dentures: An FEA Study.

To estimate the fatigue life of 3-unit molar fixed partial dentures (FPDs) made from two different monolithic ceramic systems, zirconia cercon (ZC) and lithium disilicate (LD). The effect of the connector size on the fatigue resistance of the monolithic FPD was also investigated. Two models for the FPDs were built, a 3-unit all-ceramic and a porcelain-fused-to-metal. The porcelain-fused-to-metal FPD model was used as the control. Actual 3-unit FPDs (replacing the second lower premolar) were constructed using a computer aided design and computer-aided manufacturing (CAD-CAM) system. Finite element analysis (FEA) was executed. A hemispherical indenter was used to simulate occlusal load. The occlusal load phase of the chewing cycle was applied at the premolar pontic. The failure location for the monolithic FPD was always located at the distal connector. Connector size played a key role in determining the long-term survival of the prosthesis. The fatigue resistance was predicted to be 670 N for the ZC with a connector size 4 × 3 mm, while it was only 226 N for LD. As for porcelain-fused-to-metal (PFM), FEA predicts that fatigue resistance can reach up to 770 N. Under the cyclic load of 670 N, the fatigue life for the zirconia FPD with connector size 4 × 3 mm was 2.23 × 106 cycles while it survived only 3.1 × 105 cycles when the connector was reduced to 3.5 × 2.5 mm. The angle of the oblique load has a significant effect on the stress distribution. 3-unit monolithic FPDs made of ZC have superior fatigue performance compared to those made of LD. The fatigue life of the zirconia FPD was about three times longer than that made of LD with a connector size of 4 mm × 3 mm. The survival rates of ZC FPDs are comparable to porcelain-fused-to-metal. A significant reduction in fatigue strength is predicted for reduced connector size. Therefore, it is necessary to establish general guidelines for the minimum connector size.

Foreign object damage characteristics and their effects on high cycle fatigue property of Ni‐based superalloy GH4169

AbstractIn this study, high‐speed ballistic impact tests were conducted on GH4169 alloy samples with the aeroengine compressor blade leading‐edge feature to simulate the notch‐type foreign object damages (FOD). Macroscopic and microscopic characterization of FOD and high cycle fatigue tests were performed to investigate the effect of FOD depth on GH4169 alloy fatigue strength along with numerical analysis using the Kitagawa–Takahashi diagram. Results show that the incident side of notch‐type FODs is relatively smooth, whereas the exit side is rugged. The FOD depth ranges from 0.18 to 1.33 mm, and the fatigue strength of damaged samples is 37.93% – 97.04% of the undamaged samples. As the FOD depth increases, damage length, material losses, and the stress concentration coefficient of the FOD increase significantly and accompanied by the increasing of adiabatic shear bands, micro voids, and cracks, resulting in fatigue strength reduction. Numerical analysis indicates that the Kitagawa–Takahashi diagram can provide a basic model for the design of FOD tolerance.

Effect of machine processing scratch on fatigue strength of notched specimen of aluminum alloy

The purpose of this study is to investigate effect of machine processing scratches on fatigue strength of a notched specimen of aluminum alloy. In this study, strain-controlled fatigue tests were carried out for two types of notched specimens with machine processing scratches. The fatigue strengths of notched specimens decreased by more than the elastic stress concentration factor due to the notch. To discuss the fatigue strength reduction, the specimen and fracture surfaces were observed carefully. The observations showed that the fatigue cracks initiated from the machine processing scratches and they are aligned around the notch root due to the stress concentration. As a result, the fatigue strength reduction factor became greater than elastic stress concentration factor.

Fatigue Characterization on a Cast Aluminum Beam of a High-Speed Train Through Numerical Simulation and Experiments

The cast aluminum beam is a key structure for carrying the body-hung traction motor of a high-speed train; its fatigue property is fundamental for predicting the residual life and service mileage of the structure. To characterize the structural fatigue property, a finite element-based method is developed to compute the stress concentration factor, which is used to obtain the structural fatigue strength reduction factors. A full-scale fatigue test on the cast aluminum beam is designed and implemented for up to ten million cycles, and the corresponding finite element model of the beam is validated using the measured data of the gauges. The results show that the maximum stress concentration occurs at the fillet of the supporting seat, where the structural fatigue strength reduction factor is 2.45 and the calculated fatigue limit is 35.4 MPa. Moreover, no surface cracks are detected using the liquid penetrant test. Both the experimental and simulation results indicate that the cast aluminum beam can satisfy the service life requirements under the designed loading conditions.

Fatigue Property of Additively Manufactured Ti-6Al-4V under Nonproportional Multiaxial Loading

The low cycle fatigue strength properties of the additively manufactured Ti-6Al-4V alloy are experimentally investigated under proportional and nonproportional multiaxial loading. The fatigue tests were conducted using hollow cylinder specimens with and without heat treatments, at room temperature in air. Two fatigue tests were conducted: one for proportional loading and one for nonproportional loading. The proportional loading was represented by a push-pull strain path (PP) and the nonproportional loading by a circle strain path (CI). The failure lives of the additively manufactured specimens were clearly reduced drastically by internal voids and defects. However, the sizes of the defects were measured, and the defects were found not to cause a reduction in fatigue strength above a critical size. The fracture surface was observed using scanning electron microscopy to investigate the fracture mechanisms of the additively manufactured specimens under the two types of strain paths. Different fracture patterns were recognized for each strain paths; however, both showed retention of the crack propagation, despite the presence of numerous defects, probably because of the interaction of the defects. The crack propagation properties of the materials with numerous defects under nonproportional multiaxial loading were clarified to increase the reliability of the additively manufactured components.

Comparison of the bending fatigue performances of selective laser sintered and injection moulded nylon spur gears

This study compares the bending fatigue performance of a selective laser sintered Nylon 12 spur gear with an injection moulded Nylon 66 gear. Test gears were subjected to load-controlled, single tooth bending fatigue tests in a custom-built test setup. Cyclic pulsating loads were applied on the test gear using a steel driver gear. The bending fatigue life of selective laser sintered gears was superior compared to the injection moulded gears. In the high cycle fatigue region, the difference between the fatigue life of selective laser sintered and injection moulded gears was higher, whereas it was lower in the low cycle fatigue region. The variation in the fatigue strength of the selective laser sintered gears was due to the different thermal behaviour at low cycle fatigue and high cycle fatigue regimes. The lower surface temperature caused the higher fatigue strength of the selective laser sintered gears in the high cycle fatigue regime. On the contrary, the selective laser sintered gears’ surface temperature was higher than injection moulded gear in the low cycle fatigue regime, which reduced fatigue strength. The crack path was tortuous in selective laser sintered gears and smoother in injection moulded gears. In selective laser sintered gears, the layered structure of the part aided in impeding the propagation of crack.

Damage causes and failure analysis of a steam turbine blade made of martensitic stainless steel after 72,000 h of working

The harsh operating environment, and the extreme working condition, of the steam turbine blades have led to the use of alloys with special characteristics. Of these properties are high corrosion resistance and excellent rupture strength. The most widely used alloys in this category are heat treatable martensitic stainless steels. They provide a wide range of mechanical properties. In that sense, this research is focused on one of the most commonly used alloys of these series; i.e. 410 alloy. In this study, a turbine blade made of 410 stainless steel was under operation for 72,000 h, and its possible damage mechanisms were explored. To define the damage causes, microstructural analyses were performed by Optical Microscopy (OM), and a Field Emission Scanning Electron Microscopy (FESEM) instrument equipped with Energy Dispersive Spectroscopy (EDS) detector. As well, mechanical properties evaluation was performed using hardness testing. Results showed that the existence of foreign particles in the environment of the turbine has led to the initiation of damage in the blade through the erosion mechanism. Alternatively, such a damage was intensified by corrosion attack due to the presence of the fuel impurities. Such damages eventually led to the formation of fatigue cracks on the trailing edge of the blade. The latest phenomenon substantially reduced the fatigue strength of the blade. Finally, it is predicted that the reduced fatigue strength would ensue shortened service life for the blade.

Generation mechanism of residual stress at press-blanked and laser-blanking edges of 1.5 GPa ultra-high strength steel sheet

The generation mechanism of the residual stress at press-blanked and laser-blanked edges of a 1.5 GPa ultra-high strength martensite steel sheet was investigated. The residual stress at the press-blanked edges of the 1.5 GPa sheet is highly tensile, whereas the residual stress at the laser-blanked edges is compressive. The high tensile residual stress causes the occurrence of hydrogen-induced delayed cracking and the reduction in fatigue strength. In press blanking of the 1.5 GPa sheet, considerably high compressive stress was generated under the punch corner just before the separation due to a small critical punch stroke, and high tensile residual stress on the fracture surface of the blanked edge was induced by releasing this high compressive stress for the separation. Not only the high strength of the 1.5 GPa sheet but also the low ductility influences the generation of the high tensile residual stress on the fracture surface because of the small critical stroke. On the other hand, the compressive residual stress at the laser-blanked edges of the 1.5 GPa sheet was generated by the volume increase of the martensite transformation at a comparatively low temperature. In addition, an approach for determining the allowable amount of hydrogen for the occurrence of delayed cracking at the edges was proposed, and the relationship between the critical residual stress and the amount of hydrogen for the 1.5 GPa sheet was obtained.

Relationship between Residual Stress and Net Strain in Low-Temperature Transformation Weldments Considering Microstructure

Weldments inevitably shrink during cooling from the melt pool. Residual stresses then occur owing to surrounding constraints. Tensile residual stresses in weldments cause various problems, such as deformation and reduction of fatigue strength. Low-temperature transformation (LTT) welding consumables can reduce the tensile residual stress through volume expansion, which accompanies a phase transformation from austenite to martensite. In this study, the relationship between residual stress and net strain was examined, mainly by controlling the martensite start (Ms) temperature, and the result was related to the weld’s microstructure. The Ms temperature and the expansion accompanying the phase transformation were analyzed by the dilatometric method. A hole drilling test was carried out to measure the residual stress in the weldments. The highest compressive stress was observed in the most expanded weldment at room temperature, and a linear relationship between the net strain and residual stress was derived. This linear relationship was analyzed with a microstructural approach.

Effect of Platform Temperature and Post-Processing Heat Treatment on the Fatigue Life of Additively Manufactured AlSi7Mg Alloy

The effect of platform temperature combined with a T5 heat treatment on the fatigue life of additively manufactured aluminum alloy AlSi7Mg was characterized and understood. High-cycle fatigue tests were carried out on samples built with four platform temperatures (35 °C, 60 °C, 80 °C and 200 °C) and post-processing heat treatment strategies (F and T5). Microstructural and fractographic observations combined with microhardness measurements were performed. A log-normal statistical distribution regressed with 90% B-basis probabilities of survival revealed that specimens produced on a platform maintained at 80 °C and post-processed with a T5 heat treatment presented the highest fatigue life among the conditions tested. Precipitation of silicon within the aluminum cells during the T5 heat treatment is the proposed explanation for the improved fatigue life of the T5 samples. In the as-built condition, specimens produced at 200 °C were found to be less resistant to fatigue than the specimens built at lower temperatures. The coarser microstructure and lowest microhardness resulting from high-temperature manufacturing explain this reduced fatigue strength. All fatigue cracks initiated from manufacturing discontinuities. This led to a fatigue life prediction model based upon linear elastic fracture mechanics. The model was fitted to the experimental results of the F and T5 samples separately. With the exception of the 35 °C—T5 specimens, the predicted fatigue lives agree with the experimental results and literature.

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.