Fatigue Notch Research Articles

Notch fatigue behavior of a titanium alloy in the VHCF regime based on a vibration fatigue test

The notch fatigue behavior of a titanium alloy TC17 in the very high cycle fatigue (VHCF) regime is investigated by a vibration-based bending fatigue test. A complete S-N relation of notched specimens with high resonance frequencies is obtained, covering the lifetime up to 109 cycles, and the fracture surfaces are observed. The results show the TC17 exhibits relatively high fatigue notch sensitivity. Several fatigue crack initiation mechanisms relevant to the notch are then revealed. Furthermore, a notch fatigue prediction model based on a modified theory of critical distance is proposed by introducing a special weight function, which is demonstrated to have good prediction accuracy.

Notch Fatigue Life Prediction Model Considering Stress Gradient Influence Depth and Weight Function

Notch characteristics significantly affect the fatigue performance of engineered components, for which the stress gradient effect is worth careful consideration. The traditional stress gradient analysis method based on the Coffin–Manson equation does not take into account the stress gradient influence range regarding the definition of the stress gradient correction factor, nor the high-stress gradient region, which has a greater influence on fatigue life. To address the aforementioned problems, a new notch fatigue life model is proposed in this paper. First, the stress–strain field at the root of the notch is analyzed to define the depth of stress gradient influence, following which the influence of the low-stress gradient region is reduced by a weighting function in the calculation of the stress gradient correction factor. Finally, to validate the method, three sets of experimental data, including TC4, GH4169, and EN8B, were used and compared with three other models. The results demonstrate that the predicted lifetimes of the new model are all within a 2-fold dispersion band, and the prediction ability is better than that of the other three models.

Effect of carburizing process on bending fatigue performance of notched parts of 18CrNiMo7-6 alloy steel

Since stress concentration is easily formed at the notch, improving the fatigue performance of notched parts is critical. The carburizing process is widely used to manufacture notched parts. In this research, the effects of carburizing process and notch stress concentration on fatigue performance were analyzed by bending fatigue tests. In addition, the microstructure and hardness evolution processes of the carburized notched 18CrNiMo7-6 specimens were observed quasi-in situ. The results showed that macro-plasticity has little effect on the bending fatigue performance of notched samples and that the improvement in fatigue performance is primarily attributable to the high hardness and residual compressive stress introduced by carburizing. The fatigue notch sensitivity is therefore reduced after carburizing. Compared to the uncarburized specimens, the fatigue limit of the carburized specimens is increased by up to 128%. With the increase of the effective hardening layer depth, the fatigue limit increases first and then decreases. Moreover, the brittle phase transformation product (twinning martensite) of metastable retained austenite is easy to produce stress concentration. Conversely, the 'film-like' retained austenite with an extremely high stability can effectively avoid the phase transformation products.

Study of Local Fatigue Methods (TCD, N-SIF, and ESED) on Notches and Defects Related to Numerical Efficiency

The fatigue strength of structural components is strongly affected by notches and imperfections. Both can be treated similarly, as local notch fatigue strength methods can also be applied to interior defects. Even though Murakami’s √area approach is commonly used in the threshold-based fatigue design of single imperfections, advanced concepts such as the Theory of Critical Distances (TCD), Notch Stress Intensity Factors (N-SIF), or Elastic Strain Energy Density (ESED) methods provide additional insight into the local fatigue strength distribution of irregularly shaped defects under varying uniaxial load vectors. The latter methods are based on the evaluation of the elastic stress field in the vicinity of the notch for each single load vector. Thus, this work provides numerically efficient methods to assess the local fatigue strength by means of TCD, N-SIF, and ESED, targeting the minimization of the required load case count, optimization of stress field evaluation data points, and utilization of multi-processing. Furthermore, the Peak Stress Method (PSM) is adapted for large opening angles, as in the case of globular defects. In detail, two numerical strategies are devised and comprehensively evaluated, either using a sub-case-based stress evaluation of the defect vicinity with an unchanged mesh pattern and varying load vector on the exterior model region with optimized load angle stepping or by the invocation of stress and strain tensor transformation equations to derive load angle-dependent result superposition while leaving the initial mesh unaltered. Both methods provide numerically efficient fatigue post-processing, as the mesh in the evaluated defect region is retained for varying load vectors. The key functions of the fatigue strength assessment, such as the evaluation of appropriate planar notch radius and determination of notch opening angle for the discretized imperfections, are presented. Although the presented numerical methods apply to planar simulation studies, the basic methodology can be easily expanded toward spatial fatigue assessment.

An analysis of the dependence of the material length parameter a0 on the stress ratio

Safe life approaches in notch fatigue of metallic components rely on the knowledge of the material response to fatigue stresses at notches, which is typically quantified by a material-dependent length parameter. As a general material behaviour, such material parameter depends on several factors and one of the most important of them is the mean stress. The present study is focused on the El Haddad-Smith-Topper material length parameter a0 for analysing under which circumstances and to which extent it may vary with the applied mean stress or stress ratio. The analysis starts by considering the broader problem of the mean stress influence on the fatigue limit of plain material, blunt and sharp notches and cracks. Eventually, an analysis of previous sets of experimental data on wrought steels led to simple relations for estimating the a0 parameter at different stress ratios on the basis of the sole Vickers Hardness

A physics-informed machine learning approach for notch fatigue evaluation of alloys used in aerospace

Within this work, a unified physics-informed machine learning (PIML) framework is proposed for notch fatigue life prediction and key feature parameters identification of aerospace polycrystalline alloys. The unified PIML approach reaches a capable accuracy in notch fatigue life prediction of the polycrystalline alloys under a wide range of notch geometries and loading conditions compared with physics-based life models. In addition, the global sensitivity analysis method is used to accurately identify the key feature parameters that affect the notch fatigue life. The nominal maximum stress and unnotched specimen reference life are recognized as highly relevant features for notch fatigue life whereas the notch root radius is the key parameter among the notch parameters. Finally, life uncertainty induced by the notch geometry parameters is performed by using the proposed PIML model based on Latin hypercube sampling, which accomplishes the probabilistic estimation of notch fatigue life well. In practical applications, the PIML framework can be applied efficiently to notch fatigue life prediction of a new material dataset due to its powerful generalization ability without additional parameter fitting or finite element analysis. The well-trained PIML model and the uncertainty assessment method provide potential tools for notch fatigue evaluation under complex loading conditions.

Notch fatigue fracture and crack growth behaviors on a steel sheet under out-of-plane bending

For industrial products made of sheets, it is crucial to prevent the formation of fatigue cracks and the rapid destruction by stress risers. To carry out tests under out-of-plane bending load, this study used not only smooth sheet specimens made of a common structural steel, but also specimens with a through-wall circular hole in the center and specimens with a V-shape notch on one edge of the plate. The fracture mechanics parameters were investigated while fatigue crack initiation, growth, and the process leading up to a break were observed. In the high cycle region, the notch factor of the Hole specimen was 1.07, and that of the V-notch specimen was 2.22. With the same initial set stress amplitude, the V-notch specimen had fewer cycles at the time of crack initiation and faster crack growth rate than the Hole specimen. The state and the extent of crack initiation and growth were affected by the strain concentration and cyclic loading levels with different notch geometries. The threshold and unstable crack growth stress intensity amplitudes were identified, while the fracture mechanism and affecting factors were examined. By extending the Paris-Erdogan equation, a crack growth rate prediction model was proposed and validated with good approximation accuracy. It becomes possible to quantitatively predict the occurrence of fatigue, crack propagation, fracture, and fatigue life in industrial machinery and structures made of similar materials in out-of-plane bending.

Intrinsic-crack modelling of the notch fatigue limit of a solid with an elliptical-cylindrical or a spheroidal cavity

Based on an approximate stress-intensity factor for a crack emanating from an elliptical-cylindrical or spheroidal cavity, the El Haddad intrinsic-crack method for modelling the notch fatigue limit of a solid containing a cavity has been extended from a straight through-crack or a circular crack to a wider range of cavity shapes. Thus, predictions of the notch fatigue limit have been presented for elliptical cylinders, ranging from a through-crack to a circular cylinder, and for limiting cases of spheroids, viz. the circular crack and the sphere. These predictions have been shown to be in good to excellent agreement with those based on the theory of critical distances.

Fatigue strength and notch sensitivity of high-manganese flake graphite cast iron

To increase the recovery rate of high-tensile-strength steel (HTSS) scrap, the effect of manganese (0.5% and 2.0% Mn) on the fatigue properties of several flake graphite cast iron (FGI) samples is investigated, along with the thickness and notch sensitivity of high-manganese (Hi-Mn) FGIs. The results reveal an increase in the pearlite fraction, an increase in the crystallization of Type-A graphite, and a decrease in the interlamellar spacing owing to the addition of Mn, which results in a significant increase in fatigue strength. Moreover, Mn does not worsen the notch sensitivity of the Hi-Mn FGIs. However, Hi-Mn FGIs have a higher thickness sensitivity because a longer cooling time results in wider interlamellar spacing and increased graphite length. Overall, HTSS can be recycled as a raw material for Hi-Mn FGIs without removing Mn, and Hi-Mn FGIs can be used for thinner parts with high-fatigue-strength requirements.

Notch fatigue life prediction of micro-shot peened 25CrMo4 alloy steel: A comparison between fracture mechanics and machine learning methods

In this work, fracture mechanics and machine learning methods are separately employed to predict the high cycle fatigue (HCF) life of micro-shot peened 25CrMo4 alloy steel containing artificial notches. The influences of notch size, stress level, and surface condition on fatigue life are incorporated into the prediction model. The results shows that the fatigue life of shot-peened specimens is enhanced by the compressive residual stress (CRS) compared with that of the un-peened specimens. The prediction result using fracture mechanics method is within the error band of ±2, while that using machine learning method almost lies in the error band of ±1.5. The stress level and notch size are the most critical factors affecting the HCF fatigue life. The applicability and limitation of the two methods are finally discussed.

A simple and convenient fatigue analysis method considering the effect of plasticity on fatigue

Continued plastic accumulation at the notch location changes the stress distribution of the notch, causing the position of the maximum stress gradient to change. In this study,a method for analyzing notch fatigue combining maximum notch stress and the relative stress gradient at the location of yield stress was developed. Experimental data on various materials are applied for model validation and comparison. The results show that the prediction accuracy of the proposed model is significantly better than that of the classical fatigue analysis method, and has the same prediction accuracy as the line method of the critical distance theory.

A hierarchical mechanism-informed neural network approach for assessing fretting fatigue of dovetail joints

In the present paper, a hierarchical mechanism-informed neural network (HMNN) life prediction method was proposed. The fretting fatigue was decomposed into different fatigue problems and considered in four neural network layers, which were hierarchically and progressively established for proportional multi-axial fatigue, non-proportional multi-axial fatigue, notch fatigue and fretting fatigue, respectively. Each layer can be used to assess the fatigue life of the previous layer based on the progressive construction of fatigue complexity. The HMNN approach can predict all kinds of fatigue implemented in the method with reasonable accuracy and provides a new approach for complex fatigue assessment.

Fatigue performance of laser powder bed fusion hydride-dehydride Ti-6Al-4V powder

Hydride-dehydride (HDH) Ti-6Al-4V alloy with particle size distribution of 50–120 µm is laser powder bed fusion ( L -PBF) processed using optimum processing parameters and a near-fully dense structure with a density of 99.9 % is achieved. Microstructural observations and phase analyses indicate formation of columnar β grains with acicular α/α′ phases in as-built condition. The roughness of the as-fabricated samples is significant with an average roughness of R a = 15.71 ± 3.96 µm and a root mean square roughness of R rms = 108.4 ± 24.9 µm, however, both values are reduced to R a = 0.19 ± 0.04 µm and R rms = 4.9 ± 0.6 µm after mechanical grinding. Mechanical tests are carried out on as-fabricated specimens followed by stress relief treatment. All samples are tested to failure in fatigue, under fully-reversed tension-compression conditions of R = −1. The as-built samples failed from the surface with crack initiation mainly at micro-notches, whereas after mechanically grinding, crack initiation changed to subsurface defects such as pores. Minimizing surface roughness by mechanically grinding eliminates surface micro-notches which improves fatigue strength in the high cycle fatigue region. Fatigue notch factor calculations showed that the effect of surface roughness was significantly lower when HDH powder is used compared to standard spherical powder. X-ray diffraction analysis revealed an in-plane compressive stress, micro-strain and grain refinement on the surface of the mechanically ground samples. Fractography observations (macroscale) revealed a fully brittle fracture in the first stage of crack growth with a transition to a dominantly ductile fracture in the third stage of crack growth. On the other hand, at the micro scale, even the brittle fracture regions showed evidence of ductile fracture within the α′ martensite laths.

Evaluating the fatigue property of S355J2W steel butt-welded joint: Multiple notch effects

The fatigue fracture mechanisms and the fatigue property evaluations of S355J2W steel butt-welded joints relieved welding residual stresses with and without welding reinforcement were systematically investigated by microstructure characterization and fatigue tests. Experimental results show that the complex evolution of microstructure at welded joint results in the notable degradation of mechanical properties. The fatigue performance of the as-welded specimen with welding reinforcement is significantly poorer than that of the smooth-welded specimen without welding reinforcement. Stress concentration under the fish-scale pattern at the weld toe caused by geometric dimension change is the major reason for the fatigue fracture of the as-welded specimen. By comparison, the cementite segregation at grain boundary in the partial recrystallization zone of the heat affected zone caused by the sharp change of microstructure should be responsible for the fatigue fracture of the smooth-welded specimen. The fatigue notch coefficient (Kf=1.89) calculated by the effective notch stress method is consistent with that (Kf=1.9) measured by the fatigue tests, which shows that the effective notch stress method can well evaluate the multiple notch effect under the coupling of the weld toe and the fish-scale pattern. Furthermore, the fatigue risk factor (Rf) model was employed to evaluate the fatigue performance of welded joint, and the intuitive Rf curves show that the fatigue fracture occurs at the fatigue life exceeding 2 × 106 cycles, i.e., Rfmax exceeds 0.7.

Fatigue life prediction of notched components under size effect using critical distance theory

Notch and size effects have shown significant influences on fatigue performance of engineering components, which deserve special attentions when performing structural health assessment. Thus, a possible size effect on critical distance is identified, when evaluating fatigue lives of notched specimens using the theory of critical distance.. In this work, by introducing the relative stress gradient concept, a novel reformulation of critical distance theory is proposed to consider the size effect in notch fatigue analysis. Experimental data of GH4169 Ni-based alloy, Al 2024-T351 alloy, and En3B low carbon steel notched specimens are applied for model validation and comparison. Results indicate that all predicted lives based on the proposed method agree well with the experimental values.

Stress gradient effect in metal fatigue: Review and solutions

Under external loadings, notched components always show varying degrees of stress concentration. Accordingly, different kinds of approaches have been presented to characterize notch fatigue behaviours under complex loading histories. Among them, the considering of the stress gradient effect is a popular perspective, together with the fatigue damage zone and critical distance concepts. This work systematically reviews recent advances on notch fatigue analysis considering the stress gradient effect. Specifically, four approaches are summarized and explored, namely the local stress/strain-based approaches, critical distance-based approaches, critical plane-based approaches, and weighting control parameters-based approaches. This review aims to provide a systematic review and comparison on these methods, therefore to offer a reference for subsequent research on notch fatigue.

Predicting of Process Parameters for Theoretical Concentrated Stress of Fatigue Notch Coefficient of Auto Parts Using Virtual Recognizable Performance Evaluation Research.

This paper analyzes the structure of the key parts of the car belt guide, and the average stress of the vulnerable parts is simulated by analysis software. The theoretical stress of the section is calculated. The theoretical stress concentration factor () is given. The relation between the gap radius and the notch coefficient () was studied according to a previous calculation formula. The tensile tests of real products are used as reference data. The results showed that and are linear in most cases, but there are also cases of non-compliance. The relationship between the fatigue notch coefficient and the theoretical stress concentration coefficient was closely related to the service life and fatigue strength of the product. In addition, we found that the size and direction of warpage improved significantly with the increase of fillet size, which was not consistent with the effect of adding glass fiber material. The rounded corners of ordinary PP materials usually displayed forward warping, but the addition of glass fiber into PP materials made the degree of warping smaller, or even led to reverse warping. The size of rounded corners is an important optimization parameter. The relationship between and was studied from the perspectives of virtual measurement (VM) and recognizable performance evaluation (RPM). According to abnormal filling pressure, these relationships were compared with filling data to generate a fracture initiation control model. Based on a large amount of normal process data and quality inspection data, the historical data (causes) and quality inspection data (results) were combined.

Fatigue life prediction method for central hole of needled ceramic matrix composite

Based on the consideration of matrix porosity and representative volume element of needled structure, the macroscopic elastic constants of needled C/SiC ceramic matrix composite and the stress concentration factor of the hole-edge were calculated by the finite element method first. Then, based on the fatigue test data of the smooth and central hole specimens, the fatigue notch factors corresponding to different stress levels were obtained. In view of the different characteristics of the fatigue notch factor obtained under different limit lives, the relationship between the fatigue notch and stress concentration factors was established, and the influence of nominal stress on the fatigue notch factor was also considered. Based on this, a fatigue life prediction method for central hole of needled C/SiC composite was proposed. The proposed method can predict the fatigue life of central hole by using the S–N curve of smooth specimen and the stress concentration factor of central hole specimen. The comparison between predicted and experimental lives showed that the proposed method can better predict the fatigue life of central hole specimen for needled C/SiC composite.

Study of the notch fatigue behaviour under biaxial conditions of maraging steel produced by selective laser melting

The current work aims to characterise the fatigue behaviour of an additively manufactured maraging steel, AISI 18Ni300. This is a class of high-strength steel widely used in biomedical, aircraft, aerospace, offshore, and military industries thanks to its good performance in terms of strength, toughness, ductility, dimensional stability, and weldability. Laser-beam powder bed fusion (additive manufacturing) is used to fabricate this type of steel and endows it with properties that make it an excellent candidate for producing prosthetic parts, thereby facilitating a reduction in manufacturing material consumption, labour, and machining time. Given the wide range of loads biomedical components are often subjected to, the current study focused on the multiaxial behaviour of this type of steel. To this end, Fatemi–Socie (FS) and Smith–Watson–Topper (SWT) critical plane methods combined with the theory of critical distances were applied to predict the fatigue life and cracking orientation of this material, with and without notches, under three biaxial loading scenarios. Cylindrical specimens were used and these were fabricated on the base plate in the vertical orientation using a linear printing system equipped with a Nd:YAG fibre laser. The building strategy involved the deposition of 40 μm thick layers at a scan speed of 800 mm/s. The two critical plane models returned good results at low life cycle fatigue but the FS model obtained better results at high life cycle fatigue. Regarding the crack angles, SWT model produced the best predictions.

Effect of Different Heat Treatments on Tensile Properties and Unnotched and Notched Fatigue Strength of Cold Work Tool Steel Produced by Powder Metallurgy

The present study investigates the effect of two heat treatments on the microstructure, the tensile and the fatigue properties of a powder metallurgy tool steel that has undergone two heat treatments: quenching and multiple tempering (conventional for powder metallurgy tool steel), and quenching and multiple tempering with an intermediate cryogenic step at −80 °C (new solution). The findings of the research indicated that the new heat treatment promotes the development of a homogeneous distribution of carbides in the martensitic matrix, with an increase of about 10% in tensile strength and about 7% in elongation to failure. This combination of exceptional strength with a high degree of toughness leads to an improvement in the fatigue behaviour of the steel, which exhibits a higher unnotched and notched fatigue strength (about 15% and 25% respectively) and a lower fatigue notch factor (about 15%) compared to conventionally heat-treated steel. These results highlight that the powder metallurgy tool steel, with the new heat treatment, could be a viable option for the production not only of tools and dies, but also for high-performance automotive components, including even those with complex geometries, such as camshafts or crankshafts.