Fatigue Life Limit Research Articles

Mean Stress Correction of S45C Carbon Steel Based on Crack Growth Concept (Proposal of Non-Closure Model)

Abstract Influence of the tensile mean stress on fatigue life and fatigue limit was investigated for carbon steel. Uni-axial fatigue tests were conducted under stress and strain-controlled conditions at room temperature. The fatigue life and fatigue limit were reduced by applying the mean stress for the same stress amplitude. The fatigue life exhibited a better correlation with the strain range than with the stress amplitude. Increase in strain range caused by applying the mean stress correlated well with the decrease in the fatigue life. It was proposed that the mean stress effect on the fatigue life was brought about by the change in crack growth rate caused by applying the mean stress. The mean stress enhanced crack opening and accelerated the crack growth. The reduction in the fatigue limit was also brought about by the same effect. It was shown that the effective strain range considering crack closure effect gave good prediction of fatigue life and fatigue limit with and without the mean stress.

Fatigue strength improvement of additively manufactured 316L stainless steel with high porosity through preloading

This work investigates the influence of a single tensile preload, applied prior to fatigue testing, on the fatigue strength of 316L stainless steel parts manufactured using laser-based powder bed fusion (PBF-LB) with a porosity of up to 4 %. The specimens were produced in both the horizontal and vertical build directions and were optionally preloaded to 85 % and 110 % of the yield strength before conducting the fatigue tests. The results indicate a clear tendency of improved fatigue life and fatigue limit with increasing overload in both cases. The fatigue limits increased by 25.8 % and 24.6 % for the horizontally and vertically built specimens, respectively. Extensive modelling and experiments confirmed that there was no significant alteration in the shape and size of the porosity before and after preloading. Therefore, the observed enhancement in fatigue performance was primarily attributed to the imposed local compressive residual stresses around the defects.

Influence of post-processing treatments on fatigue limit of notched additive manufactured Ti-6AL-4V determined by rapid thermographic methodology

This article explores how post-processing treatments affect the fatigue limit of additive manufactured Ti-6Al-4V in the presence of a stress gradient. Additive manufacturing (AM) technology enables manufacturers to create customized designs that can withstand specific loads, but it often results in complex shapes with stress gradients. Investigation into the influence of microstructure, internal porosity, and surface integrity on the fatigue limit of AM Ti-6Al-4V was conducted. Infrared imaging was used to detect and measure self-heating from which an estimate of the materials fatigue limit was derived. The study indicated that surface integrity is the most important factor to the fatigue life limit, and showed that the infrared thermography analysis provides a quick and accurate way to determine the fatigue limit, compared to the conventional staircase method.

Effect of Thickness of Ti Coating Deposited by Vacuum Arc Melting on Fatigue Behavior of Aluminum Alloy Al–5%Si

Fatigue strength tests of Ti-coated aluminum alloys with a thickness of 1 µm, 3 µm, and 5 µm were conducted to investigate the effect of the coating thickness on fatigue strength. Under the same applied stress amplitude, the optimum thickness with the most-extended fatigue life was around the coating thickness of 5 µm. This may be attributed to the good resistance to surface cracks under repeated loads. The results suggested that a lower fatigue life of a coating thickness of 1 µm results from the fracture of the coating layer under the strong influence of the deformation of the substrate. This could be due to the higher tensile residual stress induced in the substrate near the coating layer and substrate interface. The titanium coating restricted the initiation of offsets and cracks beneath the surface of the specimen, which may be attributed to the high strength of the Al–5%Si substrate, good flexibility, and strong adhesion, which provided sufficient compressive stress to suppress slip band protrusions. The fatigue life and fatigue limit increased proportionally to the thickness of the titanium coating due to changes in the surface roughness and adhesion capability.

Grain refinement effect on fatigue life of two grades of commercially pure titanium

The fatigue life and fatigue limit of commercially pure Ti grade 2 and 4 in coarse-grained and ultrafine-grained states were experimentally determined. Ultrafine structures were prepared by Conform severe plastic deformation method and by conventional Equal channel angular pressing, respectively. The differences in the improvement of fatigue life and the influence of both procedures on the fatigue limit based on 107 cycles to fracture are discussed in terms of grain refinement, microstructure, differences in fatigue damage mechanism and influence of surface roughness on the initiation of fatigue cracks. Improvement in fatigue life was observed for both materials, however, the improvement of the fatigue limit was found only for Ti grade 2.

Comparison between Fractal and Statistical Approaches to Model Size Effects in VHCF

Size effects concern the anomalous scaling of relevant mechanical properties of materials and structures over a sufficiently wide dimensional range. In the last few years, thanks to technological advances, such effects have been experimentally detected also in the very high cycle fatigue (VHCF) tests. Research groups at Politecnico di Torino are very active in this field, observing size effects on fatigue strength, fatigue life and fatigue limit up to the VHCF regime for different metal alloys. In addition, different theoretical models have been put forward to explain these effects. In the present paper, two of them are introduced, respectively based on fractal geometry and statistical concepts. Furthermore, a comparison between the models and experimental results is provided. Both models are able to predict the decrement in the fatigue life and in the conventional fatigue limit.

Flexural and Fatigue Analysis of Blend Matrix Composite Reinforced by Natural Fibers

In this study, the mechanical properties, fatigue life, and endurance limit values were investigated for composite materials containing natural fiber reinforcement (flax, wool), which were arranged by 0°/90° Angle orientation and a combination of 0°/90° and 45°/-45° angle orientation, then the wool woven mixed by flax woven and unidirectional flax interfered with jute woven. Binary blends containing epoxy and polyurethane at v were mixed at various weight percent (0%, 5%, 10%, and 15 %) vacuum infusion technique was used to prepare the specimens. ANSYS Workbench was used as numerical software to investigate the fatigue behavior and endurance limit values of the unreinforced and reinforced blend. It was found that the weight percentage of 10% polyurethane in the blends was the optimal percentage providing the best fatigue life and the endurance limit value, endurance limit values of wool composite improved when hybridized by flax woven and unidirectional flax interfered with jute woven by 132.895 % and 66.549 % respectively

Study on the relation between surface integrity and contact fatigue of carburized gears

Surface integrity is critical for gear contact fatigue performance. The relation between gear surface integrity and contact fatigue remains unclear, which is a challenge for gear anti-fatigue design. This study investigates the relation between surface integrity and contact fatigue of 18CrNiMo7-6 carburized gears through fatigue experiments and data-driven modeling. A series of gear contact fatigue tests, with approximately 110 × 106 running cycles in total, has been conducted. P-N curves and fatigue limits of the tested gears are investigated for four typical manufacturing processes: carburizing and grinding, shot peening, barrel finishing, and barrel finishing after shot peening. The influence of different surface integrity components on contact fatigue is explored with a Pearson correlation coefficient analysis and a random forest algorithm. Formulae of gear contact fatigue life and fatigue limit considering surface integrity are proposed. Results show that a high surface integrity state with surface hardness of 686.5 HV, maximum compressive residual stress of −1162 MPa, and surface roughness Sa of 0.36 μm, exhibits the highest gear contact fatigue limit, which is 15.1% higher than the carburizing and grinding state, indicating the benefits of improving surface integrity. For both the gear contact fatigue life and fatigue limit, the most significant surface integrity components are surface hardness, maximum compressive residual stress, and surface roughness. The proposed formulae considering surface integrity illustrate reasonable prediction accuracy, with 1.5 times dispersion band for the predicted fatigue life and a maximum relative error of 2.1% for the predicted fatigue limit.

Accelerated fatigue life diagram characterization of CFRP

There are several ways to obtain the fatigue life behavior of composite materials, such as the application of classical theories or complex progressive damage models. Recently, constant life diagrams have demonstrated good effectiveness in predicting the fatigue life of CFRPs. Nevertheless, the large number of tests required, and the time necessary for the characterization, makes this method very expensive in terms of time and resources. The aim of this work is the evaluation of accelerated characterization techniques in addiction with statistical methods, to obtain constant fatigue life diagrams of CFRPs, with a reduced number of tests and a shorter characterization time. The results show that, combining universal and accelerated testing techniques with statistical methods, it is possible to obtain constant life diagram for the whole range of tension-tension, tension-compression, and compression-compression loading modes, with a reduced number of tests. Moreover, it is observed that, the accelerated characterization techniques show a fatigue life limit similar to the conventional ones.

Modifying fatigue performance of reactive powder concrete through adding pozzolanic nanofillers

The concrete fatigue performance that determines service life of concrete structure are closely related to its microstructure. Incorporating pozzolanic nanofillers into concrete can yield dense C-S-H gel to increase the logarithmic creep modulus, and modifies the microstructure of concrete to enhance the cracking resistance, which can help to modify the fatigue performance of concrete. This paper investigates the impact of pozzolanic nanofillers on the compressive fatigue performance of reactive powder concrete (RPC), with investigation into the modifying mechanisms. Compressive fatigue test results show that adding nanofillers significantly improve fatigue life and fatigue limit of RPC, whose maximum enhancement is 120.1% and 19.4%, respectively. The addition of nanofillers also leads to the increase in ultimate compressive strain of RPC and the decrease in strain rate at the creep-fatigue coupling stage. The image processing results on morphology at cracking surface of composites demonstrates that incorporating nanofillers approximately reduces the porosity of RPC by half. The scanning electron microscope image indicates the nanofillers are beneficial for producing more and dense calcium silicate hydrate gel. According to the line scanning results of energy dispersive spectroscopy, the average ratio of calcium oxide to silicon dioxide in interfacial transition zone of RPC with pozzolanic nanofillers is obviously lower than that of plain RPC. These are the main underlying mechanisms contributing to the improvement of RPC fatigue performance. In addition, a fatigue equation that reflects the relation between fatigue life and strain rate at creep-fatigue coupling stage is established, making it possible to predict the fatigue life by measuring the strain increase within limited fatigue loading circles.

The effect of artificial defects on the fatigue characteristics of AISI 4340 steel

This study examined the fatigue life (Nf) and fatigue limit (FL) of AISI 4340 steel in relation to surface roughness, artificial defects and shot peening. Artificial surface defects on fatigue specimens were obtained by electrical discharge machining (EDM) or by pre-corrosion in 3.5% NaCl and perchloric acid solutions, followed by a fatigue test. The presence of artificial defects of various sizes, taken into account by the area parameter, led to a significant decrease in fatigue characteristics, while shot peening contributed to their noticeable improvement. Defects resulting from electrochemical corrosion had a much greater negative effect on fatigue life than defects introduced by EDM.

Machine learning-based predictions of fatigue life and fatigue limit for steels

To predict the fatigue life for oblique hyperbola- and bilinear-mode S–N curves of metallic materials with various strengths, a machine-learning approach for direct analysis was employed. Additionally, to determine the fatigue limit of the utilized materials (AISI 316, AISI 4140 and CA6NM series) with different S–N curve modes using finite-fatigue life data, a Bayesian optimization-based inverse analysis was performed. The results indicated that predictions of the fatigue life for the utilized datasets via the random forest (RF) algorithm for AISI 4140 and CA6NM, and artificial neural network (ANN) for AISI 316, distribute within 2 factor error lines for most data. In the Bayesian optimization-based inverse analysis, the specific explanatory variables corresponding to the optimized maximum fatigue life were treated as the fatigue limits. The predicted fatigue limits either approximated to or slightly underestimated the experimental results, except for several cases with large errors. Using the inverse analysis to predict the fatigue limit for both S–N curve modes is applicable for current employed data-set. However, the explored maximum fatigue lives via BO corresponding to the predicted fatigue limit were underestimated for AISI 4140 and CA6NM, and was overestimated for AISI 316 because of effect of shape of S–N curves. By combining the ANN or RF direct and BO inverse algorithms, whole S–N curves (including the fatigue limit) were evaluated for the S–N curve shapes of the oblique hyperbola and bilinear modes.

Fatigue strength analysis of desilting platform structure

In order to ensure the fatigue reliability of the dredging platform and key equipment structure, the fatigue of the platform is analyzed by using the analysis method based on S-N curve and fatigue cumulative damage criterion. Firstly, according to the wave characteristics of the Pearl River Basin, the periodic load of the dredger is calculated by using hydrodynamic analysis tools. Then, the fatigue life of dangerous parts is analyzed by using cumulative damage theory and S-N characteristic curve. The results show that the joint of desilting equipment and conveying device is easy to suffer fatigue damage, and the fatigue life limit is 2328h, and the other equipment and structure meet the fatigue requirements.

EXPERIMENTAL AND NUMERICAL FATIGUE ANALYSIS OF BRASS SHAFT SPECIMEN UNDER CYCLIC BENDING MOMENTS

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses, where failure can occur at a stress level significantly lower than the tensile or yield strength of a static load under these circumstances. The term “fatigue” is used because, after a long period of repetitive stress or stress cycling, this form of failure typically occurs. Fatigue is important because it is the single largest cause of metal failure, estimated to account for about 90% of all metal failures; polymers and ceramics (except glasses) are also prone to this form of failure. This research is studying the failure analysis, fatigue life and endurance limit of brass metal experimental and numerical under cyclic bending moments

Effect of Nano clay on the fatigue of epoxy and glass fiber composites

This study describes the experimental results of the fatigue behavior manifested by fatigue strength, fatigue life, and fatigue limit composites for epoxy resin and its E-glass fiber (2,4,6) laminates, nano clay prepared in laboratory reinforced in with (2%, 4%, 6%) weight fraction. The evaluation of (S-N) curves of the tested specimens was depending according to the type of E-glass fibers, laminates number, and addition nano clays small granular volume which prepared in laboratory to a polymer. The fatigue tests showed high resistance to all samples after being immersed in water. Adding the nano clay reduced the brittle nature of the Epoxy resin as it is improved drastically.

Essential structure of S-N curve: Prediction of fatigue life and fatigue limit of defective materials and nature of scatter

Historically, S-N curve has been expressed in the form of an exponential relationship between applied stress σ and number of cycles to failure Nf. This paper elucidates the essential structure of S-N curve based on the prediction method for fatigue life and limit of materials containing defects from the viewpoint of mechanics of small crack. The extended application of the proposed method to step loading and variable amplitude loading is discussed. The question about the applicability of Miner’s rule is also discussed from the viewpoint of continuous decrease in fatigue limit with number of stress cycles and crack growth.

Influence of surface enhanced treatment on microstructure and fatigue performance of 6005A aluminum alloy welded joint

Abstract The application of 6005A aluminum alloy in transportation is increasing, which puts forward higher requirements for the fatigue performance of welding process. In this study, friction stir welding (FSW) enhanced treatment was applied on the traditional metal inert gas (MIG) weld. The results show that the fatigue life of FSW plate is three times higher than that of original MIG welded plate at 140 MPa stress level. Under 107 cycles, the fatigue life limit was increased by 16 MPa compared with as-received MIG plate. The main reason of FSW leading to the increasing fatigue life are the reduction of porosity to reduce crack initiation and grain refinement to delay crack propagation.

Probabilistic fatigue model for cast alloys of aero engine applications

PurposeThe purpose of this study is to address the complexity involved in computing the fatigue life of casted structure with porosity effects in aero engine applications. The uncertainty of porosity defects is addressed by introducing probabilistic models.Design/methodology/approachOne major issue of casted aluminium alloys in the application of aerospace industries is their internal defects such as porosities, which are directly affecting the fatigue life. Since there is huge cost and time effort involved in understanding the effect of fatigue life in terms of the presence of the internal defects, a probabilistic fatigue model approach is applied in order to define the realistic fatigue limit of the casted structures for the known porosity fractions. This paper describes the probabilistic technique to casted structures with measured porosity fractions and its relation to their fatigue life. The predicted fatigue life for various porosity fractions and dendrite arm spacing values is very well matching with the experimentally predicted fatigue data of the casted AS7G06 aluminium alloys with measured internal defects. The probabilistic analysis approach not only predicts the fatigue life limit of the structure but also provides the limit of fatigue life for the known porosity values of any casted aluminium bearing support structure used in aero engines.FindingsThe probabilistic fatigue model for addressing porosity in casting structure is verified with experimental results.Research limitations/implicationsThis is grey area in aerospace and automotive industry.Originality/valueThis work is original and not published anywhere else.

Characteristics and life expression of fatigue fracture of G105 and S135 drill pipe steels for API grade

Characteristics and life expression of fatigue fracture of drill pipe steels for API G105 and S135 grade were investigated experimentally. S135 steel with higher tensile strength has higher fatigue strength and fatigue limit than that of G105 steel with lower tensile strength. A modified model for the S-N data, revealing the correlation between the fatigue life and fatigue limit, equivalent stress amplitude reflecting the effect of stress ratio, is suggested for predicting the fatigue damage. The fatigue limit and the fatigue strength coefficient are correlated to the tensile properties. The mechanism of the fatigue fracture was examined by scanning electron microscopy.

Study on Fatigue Test and Life Prediction of Polyurethane Cement Composite (PUC) under High or Low Temperature Conditions

There are many diseases in the deck pavement of long‐span steel bridges under the action of vehicles, rainwater, and freezing. It is necessary to study a new type of pavement material with high waterproof property, light weight, and high bonding performance for steel deck pavement. Polyurethane cement composite (PUC) can be used for steel deck pavement. In order to find out the temperature effect on fatigue properties of PUC, the four‐point bending fatigue test was carried out at different temperatures. In this paper, the optimum mix ratio of PUC was selected by compressive and flexural tests, and then the bending fatigue test was conducted under strain control mode. Under temperature and external force coupling condition, a method for predicting fatigue life of PUC is proposed by the combination of theoretical deduction and experimental research. The results show that the proposed formula can effectively describe the fatigue life and fatigue limit of PUC. Finally, compared with three different asphalt mixtures for steel deck pavement, it is found that the fatigue performance of polyurethane cement is better than that of asphalt mixture.