Fatigue Notch Sensitivity Research Articles

Non-Propagating Cracks at the Fatigue Limit of Notches: An Analysis of Notch Sensitivity and Size Effect.

The issues of the high-cycle fatigue resistance of notches and the role of non-propagating short cracks in defining the fatigue notch sensitivity and fatigue limit of the configuration are addressed. A fracture mechanics approach is employed to determine the threshold configuration that defines the associated fatigue limit. The influence of notch sharpness, notch size, intrinsic fatigue limit, microstructural dimensions, and the threshold for crack propagation is examined. A simple expression is proposed to estimate the maximum fatigue notch factor, kfMax, which incorporates the influence of these non-propagating cracks. The fatigue limits for both blunt and sharp elliptical notches are analyzed and predicted based on experimental results reported in the literature. Additionally, shallow notches or small defects are analyzed, where it is found that the same hypothesis may not be applicable.

Understanding the Fatigue Notch Sensitivity of High-Strength Steels through Fracture Toughness

This study presents an innovative approach for selecting high-strength materials for fatigue dimensioning parts, considering both fracture toughness and fatigue performance. Warm and hot forming processes enable the construction of high-strength parts above 1000 MPa with complex geometries, making them suitable for lightweight chassis in automotive and freight applications. This research reveals that high-strength steels can experience up to a 40% reduction in fatigue performance due to manufacturing defects introduced during punching and trimming. Fracture toughness has been proposed as a good indicator of notch sensitivity, with a strong correlation of 0.83 between fracture toughness and fatigue notch sensitivity. Therefore, by combining fracture toughness measurements and fatigue resistance obtained through the rapid fatigue test, it becomes possible to quickly identify the most fatigue-resistant materials to deal with defects. Among the nine materials analysed, warm-formed steels show promising characteristics for lightweight chassis construction, with high fatigue resistance and fracture toughness exceeding the proposed fracture threshold of 250 kJ/m2.

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.

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.

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.

Improvement of traction-traction fatigue properties of A100 steel plate-hole-structure by double shot peening

To reduce the stress concentration and improve fatigue resistance of A100 steel plate-hole-structure, double shot peening (DSP) of the primary shot peening with strong intensity iron shots and the secondary shot peening with weak intensity ceramic shots is carried on material surface and hole and the effect of surface integrity on fatigue properties is investigated. The results show that the surface of hole presents severe surface plastic deformation (SSPD) with increasing DSP intensity. A high intensity compressive residual stress (CRS) distributed in deep layer is generated. Meanwhile, the compressive residual stress is much more stable during the fatigue process, which is beneficial to improve fatigue properties by inhibiting fatigue crack initiation and early propagation. In addition, due to the reduced stress concentration at the edge of hole and higher compressive residual stress, the fatigue resistance of A100 steel plate-hole-structure is remarkably improved. The average fatigue strength of DSP-treated specimens with medium intensity primary shot peening was at least improved by 59.6%, and the fatigue notch sensitivity was reduced by 61%. However, it is also discovered the secondary shot peening cannot repair the rough surface and surface damage caused by excessive primary shot peening, which compromise the improvement of fatigue resistance.

Static and fatigue behavior in presence of notches for polyamide 12 (PA12) additively manufactured via Multi Jet Fusion™ process

Static, fatigue and notch sensitivity tests were performed on Multi Jet Fusion™ (MJF) Polyamide 12 (PA12) specimens in unnotched conditions or with blunt/sharp notches. Stress concentration factors, and potential applicability of theory of critical distances (TCD) were investigated. Fracture surfaces were comparatively analyzed for quasi-static, fatigue and cryogenic failures. Stress-Life curves were determined and MJF-PA12 showed static and fatigue properties comparable to conventional PA12. The blunt notch had limited impact on fatigue strength, with failure onsetting from internal defects. The sharp notch showed low fatigue notch sensitivity but reduced static strength and fatigue life, with failure onsetting from notch tip.

Effect of heat treatment conditions on the fatigue resistance of press hardened 22MnB5 steel evaluated through rapid testing technique

Fatigue strength is considered as a crucial parameter for automotive applications subjected to cyclic loads during their long service life, as chassis parts. The high yield stress of press hardened steels poses them as good candidates for lightweight solutions with improved fatigue resistance. However, their high strength leads to an increase in notch sensitivity which can ruin the whole part’s integrity. This behaviour was observed in previous works on press hardened steels, where their high fatigue strength was significantly affected by the surface conditions and by heat treatment conditions. Nevertheless, press hardening steels are still good candidates to manufacture complex geometry parts reaching high performance.Aiming at increasing the existing knowledge on the fatigue behaviour of press hardened steels, this paper analyses the fatigue performance of boron steel (22MnB5) under different time austenitizing times. Fatigue resistance is evaluated using a novel rapid fatigue testing technique based on the stiffness evolution. The method permits a fast and reliable determination of the fatigue limit. Based on results obtained with this rapid testing method, the most suitable heat treatment to mitigate fatigue notch sensitivity and then achieving the best fatigue performance for chassis applications is discussed.

A novel thread connection structure of slimehole drilling tool in ultra-deep natural gas well.

The thread connection's root fillet radius of 0.038″ size is the greatest weakness of the API NC type joints and thread. During the slimehole drilling, especially in the deep and ultra-deep gas well, its stress concentration factor and notch sensitivity factor are very high A novel thread connection design (TM) of a drilling tool is proposed to decrease the fatigue failure of the slimehole drilling tool in the deep and the ultra-deep gas well in the Tarim oilfield China. The novelty in the TM thread structure is, reducing the threads per inch, extending the distance from the last engaged thread to the external shoulder of the pin and adding three threads to the conventional connection. The novel thread connection will improve the slimehole drilling tool's anti-fatigue life due to its improved elasticity and rigidity. Furthermore, the TM can transfer the maximum stress at the connection root to the loaded surface, which can effectively lower the fatigue notch's sensitivity coefficient. In this paper, the finite element method (FEM) is applied to carry out the detailed comparative analysis of the TM with existing thread connection NC38, TX60 and TH90. The TM has the lowest stress concentration factor and fatigue notch sensitivity coefficient, so its anti-fatigue life is the highest. In addition, TM is manufactured and is tested at Tarim oilfield in China.

High-Cycle Fatigue Properties of Ferritic-Pearlitic Medium-Carbon Forging Steels with Smooth and Notched Specimens

High-cycle fatigue properties of two ferritic-pearlitic medium-carbon forging steels 30MnVS and 49MnVS with smooth and notched (stress concentration factor Kt=2.1 and 3.9) specimens were studied by using rotating bending fatigue and fatigue crack growth (FCG) rate testing. Structural steels 40Cr and 30MnVS with quenched & tempered microstructures (40Cr-QT, 30MnVS-QT) were used for comparison. It was found that both fatigue strength and fatigue strength ratio at each Kt of the as-forged 30MnVS steel (30MnVS-AF), which are comparable to those of 30MnVS-QT and 40Cr-QT, are notably higher than those of the as-forged 49MnVS steel (49MnVS-AF), and this difference tends to decrease with an increase in Kt. However, 30MnVS-AF exhibits slightly higher fatigue notch sensitivity and FCG rate than 49MnVS-AF, and the steels with ferritic-pearlitic microstructure also have higher fatigue notch sensitivity than the steels with QT microstructure. A simple and practical method which incorporates simple microstructural parameters, and hardness is proposed to estimate the smooth fatigue strength (σw) of ferritic-pearlitic forging steels, while notched fatigue strength can be expressed by a power function of Kt with its coefficient equals to σw, which can be further classified into two groups corresponding to forging steels with relatively lower and higher C contents, respectively.

Comparison between EDM and grinding machining on fatigue behaviour of AISI D2 tool steel

Non-conventional machining processes, such as the Electrical Discharging Machining (EDM) offers the possibility to machine very hard materials with great potential to generate complex geometries. However, the EDM process involves significant microstructural changes in the as machined surfaces, which are known to have a significant impact on fatigue behaviour of mechanical parts. This study aims at investigating the fatigue behaviour of the AISI D2 tool steel subjected to EDM machining. Both smooth and notched specimens are tested aiming at assessing the fatigue notch sensitivity of this material and results compared with conventional grinding techniques in terms of fatigue resistance and residuals stresses. The AISI D2 steel shows a microstructure composed of large dendritic primary carbides detrimental to the fatigue resistance. Also, the EDM process is responsible for the generation of detrimental tensile residual stresses that justifies the lower fatigue resistance when compared with the ground material that exhibits beneficial compressive residual stresses. The fatigue test data of the notched specimens machined by EDM process show a high fatigue notch sensitivity of the AISI D2 tool steel. The heterogeneous structure of the material, showing large undissolved carbides, leads to a notch fatigue reduction factor that is higher to the apparent theoretical elastic stress concentration factor.

Effect of grain size on the very high cycle fatigue behavior and notch sensitivity of titanium

The very high-cycle fatigue performances of coarse-grained and ultrafine-grained titanium samples with different geometries at ambient temperature and various stress amplitudes were investigated. Severe plastic deformation improves monotonic strength of titanium at the cost of a loss in ductility. Ultrafine-grained titanium demonstrates a superior fatigue performance compared to that of coarse-grained counterparts in the high-cycle fatigue regime, however, suffers notch sensitivity. Furthermore, in the very high-cycle fatigue regime stress-life curves merge unexpectedly. Microstructural inhomogeneity in the ultrafine-grained titanium is expected to be the reason. Analysis of fracture surfaces reveals that the formation of fatigue slip marks is evident on the fatigued samples of both microstructural states. Ultrafine-grained titanium is more prone to the intergranular fracture.

Warp direction fatigue behavior and damage mechanisms of centrally notched 2.5D woven composites at room and elevated temperatures

Fatigue behavior, notch sensitivity and environmental effect are three of the principal concerns in composite applications, especially those for aeroengines that require mechanical fastening and fatigue resistance at elevated temperatures. As a new generation of woven composites, 2.5D woven composites (2.5DWC), have drawn increasing amounts of interest due to the excellent fatigue resistance of these materials. However, the lack of temperature-dependent fatigue explorations into centrally notched 2.5DWC (N-2.5DWC) significantly limits the engineering applications of these materials. Therefore, the objective of this paper is to investigate the temperature-dependent fatigue behavior of N-2.5DWC. Warp direction tension-tension fatigue tests of N-2.5DWC were performed at 20 °C and 180 °C for the first time. The fatigue lives and the related failure mechanisms were obtained experimentally. Interestingly, the fatigue life at the same temperature exhibited a sudden increase with a decreasing stress level. A temperature-dependent fatigue life prediction model was established, and the fatigue lives and damage propagation processes at various temperatures and stress levels were predicted. The predicted damage propagation process was a “L” shaped, which was consistent with the corresponding experimental fracture morphologies. Finally, the effects of temperature and warp arranged density on the warp direction fatigue life were discussed.

Directed Energy Deposition versus Wrought Ti‐6Al‐4V: A Comparison of Microstructure, Fatigue Behavior, and Notch Sensitivity

Laser Engineered Net Shaping (LENS), a Direct Energy Deposition (DED) additive manufacturing process is a 3D manufacturing process generally used to produce fully dense parts or to repair/add additional material to an existing component. The main aim of this work is to evaluate the fatigue behavior of LENS specimens in the presence of geometrical discontinuities and to compare its performance to the one obtained from wrought specimens. For this aim, axial fatigue tests are carried out on three sets of specimens namely, smooth, semi‐circular and V‐notched specimens to determine the fatigue strength and notch sensitivity of the LENS and wrought Ti‐6Al‐4V materials. The LENS material shows higher fatigue strength and notch sensitivity compared to wrought material which is attributed to the unique microstructural features leading to different fatigue failure mechanisms. Further, the fatigue data is assessed by use of strain energy density as a failure criterion.

Influence of microstructure on fatigue property of ultra high-strength steels

Ultra-high-strength steels (with tensile strength higher than 980 MPa) are widely used in automobile manufacturing owing to their lightweight that contributes to fuel efficiency. The fatigue strength of ultra-high-strength steels with a notch tends to decrease, which is known as the effect of notch sensitivity. In this study, 4-point bending fatigue tests were performed to examine the fatigue strength and notch sensitivity of four steels; namely 590 MPa class steel, 980 MPa class martensitic steel, 980 MPa class bainitic steel, and 980 MPa class precipitation hardening steel plates with three different stress concentration factors. The results indicate that the fatigue strength and notch sensitivity of 980 MPa class steel specimens were higher than those of 590 MPa class steel specimens. The notch sensitivities of tested plate specimens were lower than those reported for cylindrical specimens of bainitic ultra-high-strength steels. Fatigue crack observation revealed that the cracks initiated in 590 MPa class steel, 980 MPa class bainitic, and martensitic steel propagated southward from the lowest bottom of notch. Although similar initial crack propagation pattern was detected in precipitation hardening steel, the crack changed direction when it reached the central part of the specimen.

Effect of HIP Treatment on Fatigue Notch Sensitivity of Ti–6Al–4V Alloy Fabricated by Electron Beam Melting

The effect of hot isostatic pressing (HIP) treatment on the fatigue notch sensitivity of Ti–6Al–4V alloy fabricated by electron beam melting (EBM) was investigated. The results indicate that the fatigue notch sensitivity of the as-fabricated samples is much lower than that of HIPed samples. The variation of $$\alpha$$ lamella thickness and the distribution of pore defects have an effect on the fatigue strength in smooth and notched EBM Ti–6Al–4V samples, resulting in the divergence of the as-fabricated and HIPed samples on fatigue notch sensitivity.

Effect of organic and inorganic reinforced particulates for fatigue behaviour of Al–Si7–Mg0.3 hybrid composite: V-notched and un-notched specimen experiments with microstructural constituents

The design of industrial cast parts Al–7Si–0.3Mg alloy is widely used as structural components of aircraft and automotive safety components. The investigation mainly focuses on fatigue life influenced by organic and inorganic particles with different weight fractions measured at different stress levels over a wide range spectrum of three different materials which are evaluated and compared. The notch and un-notch specimens of these materials are described fatigue strength and crack mechanism using high cycle fatigue. According to Basquin constants, fatigue notch factor and notch sensitivity factor were studied. The A356/7.5% rice husk ash-7.5% fly ash hybrid composite increases fatigue strength of 30% and lower the fatigue notch factor to 25% compared to the matrix alloy. Porosity is the main factor for estimation of fatigue life. The micro-/macro-cracks and fracture surface are revealed under the SEM and optical microscope analysis.

Influence of as-built surface on fatigue strength and notch sensitivity of Ti6Al4V alloy produced by DMLS

Direct Metal Laser Sintering (DMLS) is one of the additive manufacturing technologies involving a metal powder bed and a laser source to obtain components of complex geometry. The fatigue life of an as-built DMLS part is negatively influenced by the rough surface compared to a machined surface. Since most DMLS components are expected to have also stress concentrations, here the combined effect on fatigue of a as-built surface and a geometrical notch in as-built DMLS Ti6Al4V was investigated using a new test methodology. Miniature specimens having three different directions with respect to build direction were fabricated and tested determining the directional fatigue notch sensitivity of as-built DMLS Ti6Al4V Optical inspection of notch surface quality and roughness measurements demonstrated a ranking of the directional notches that inversely correlated to the measured fatigue performance. The link among the curved surface orientation, the peculiar layer-wise additive manufacturing fabrication and the fatigue notch sensitivity is clarified.

Weak fatigue notch sensitivity in a biomedical titanium alloy exhibiting nonlinear elasticity

It is well known that metallic materials exhibit worse fatigue damage tolerance as they behave stronger in strength and softer in modulus. This raises concern on the long term safety of the recently developed biomechanical compatible titanium alloys with high strength and low modulus. Here we demonstrate via a model alloy, Ti-24Nb-4Zr-8Sn in weight percent, that this group of multifunctional titanium alloys possessing nonlinear elastic deformation behavior is tolerant in fatigue notch damage. The results reveal that the alloy has a high strength-to-modulus (σ/E) ratio reaching 2% but its fatigue notch sensitivity (q) is low, which decreases linearly from 0.45 to 0.25 as stress concentration factor increases from 2 to 4. This exceeds significantly the typical relationship between σ/E and q of other metallic materials exhibiting linear elasticity. Furthermore, fatigue damage is characterized by an extremely deflected mountain-shape fracture surface, resulting in much longer and more tortuous crack growth path as compared to these linear elastic materials. The above phenomena can be explained by the nonlinear elasticity and its induced stress relief at the notch root in an adaptive manner of higher stress stronger relief. This finding provides a new strategy to balance high strength and good damage tolerance property of metallic materials.

Fatigue Properties of the Ultra-High Strength Steel TM210A.

This paper presents the results of an experiment to investigate the high cycle fatigue properties of the ultra-high strength steel TM210A. A constant amplitude rotating bending fatigue experiment was performed at room temperature at stress ratio R = −1. In order to evaluate the notch effect, the fatigue experiment was carried out upon two sets of specimens, smooth and notched, respectively. In the experiment, the rotating bending fatigue life was tested using the group method, and the rotating bending fatigue limit was tested using the staircase method at 1 × 107 cycles. A double weighted least square method was then used to fit the stress-life (S–N) curve. The S–N curves of the two sets of specimens were obtained and the morphologies of the fractures of the two sets of specimens were observed with scanning electron microscopy (SEM). The results showed that the fatigue limit of the smooth specimen for rotating bending fatigue was 615 MPa; the ratio of the fatigue limit to tensile strength was 0.29, and the cracks initiated at the surface of the smooth specimen; while the fatigue limit of the notched specimen for rotating bending fatigue was 363 MPa, and the cracks initiated at the edge of the notch. The fatigue notch sensitivity index of the ultra-high strength maraging steel TM210A was 0.69.