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This paper reviews quantitively the multiaxial fatigue assessment of steel and aluminum welded joints, focusing on Eurocodes (ECs) and the recommendations of the International Institute of Welding (IIW). Extensive fatigue data under constant and variable amplitude loading are reanalyzed using stress analysis approaches such as nominal stress, hot spot stress, and effective notch stress. Evaluation of the ECs and IIW criteria reveals an effective assessment of multiaxial fatigue, with a satisfactory level of conservatism. Further research is needed especially for variable amplitude (VA) loading to enhance the precision and reliability of assessments, contributing to improved design practices and structural integrity in welded joint applications. Highlights: The multiaxial fatigue criteria proposed by EC3 and the IIW are reviewedDesign criteria are applied in terms of nominal, hot spot, and effective notch stressBoth constant and variable amplitude load histories are consideredStandard approaches are characterized by a large level of conservatism [ABSTRACT FROM AUTHOR]

In the present work, multiple regression models are established for assessing the fatigue life of nanoclay nanocomposites comprising natural rubber (NR) and styrene‐butadiene rubber (SBR). Quadratic (for two factors) and reduced cubic (for three factors) regression models have been applied to create the NR/SBR/nanoclay durability models, and the goodness of fit, homoscedasticity, and normality of the models were evaluated. Also, an analysis of the influence factors on the fatigue life of NR/SBR/nanoclay nanocomposites was done. Various factors were chosen for the investigation: strain amplitude, nanoclay loading, and test frequency. By creating the effects model, the main effects and the interactions of each factor on the fatigue life of the nanocomposites were considered. A statistical analysis of the quadratic model revealed that the strain amplitude has the most significant effect on the fatigue life of rubber samples. The variance analysis of the reduced cubic model showed that the nanoclay loading and the test frequency have the most negligible effect on the rubber samples' fatigue life, respectively. Despite the significant main impact of the studied factors, the 2‐way interaction, as well as the 3‐way interaction of the variables, was not effective in the reduced cubic fatigue life model. Highlights: Multiple regression models are used for the evaluation of the fatigue life.The effect of nanoclay loading, the strain, and the test frequency are studied.Statistically fatigue life prediction model of nanocomposite is created.Nanoclay loading has the most effect on fatigue life. [ABSTRACT FROM AUTHOR]

Foreign non‐metallic inclusions can significantly reduce the low‐cycle fatigue (LCF) life of powder metallurgy (PM) superalloy and greatly affect the safety and reliability of aeroengines. In this paper, LCF experiments on PM FGH96 superalloy with and without inclusions were conducted. Effects of inclusions on the fatigue life and damage mechanism of FGH96 alloy are discussed by fractography analysis. Parameters related to inclusion characteristics such as strength–inclusion coefficient are proposed. By introducing characteristic parameters of inclusions, the LCF life prediction models are established based on the Manson–Coffin relationship, which significantly improved the prediction accuracy and reduced the scattering band by a factor of 2. Highlights: By performing strain‐controlled low‐cycle fatigue tests at different strain amplitudes, statistical analyses were carried out to assess the influences of size, location, and type of inclusions on LCF life, with a quantitative relationship established between them.The change law of LCF life of powder metallurgy superalloy under different inclusion conditions is summarized. Effects of inclusions on the LCF life of FGH96 alloy were clarified and applied to the establishment of fatigue life prediction models considering characteristic parameters of non‐metal inclusions.A modified Manson–Coffin life prediction model considering inclusion characteristics is proposed to support the safety and reliability assessment of turbine disks made of PM superalloy FGH96.By introducing characteristic parameters of inclusions, the LCF life prediction models are established based on the Manson–Coffin relationship, which significantly improved the prediction accuracy and reduced the scattering band by a factor of 2. [ABSTRACT FROM AUTHOR]

The paper attempts to predict the probabilistic fatigue strength scatter bands of a welded bicycle frame joint. The experimental data of the base material and the weld material were approximated by a nonlinear probabilistic model of the Weibull distribution. The fatigue strength for the material at the fatigue crack location was estimated considering the mean stress, residual stress, fatigue notch factor, and hardness profile. The output finite element numerical data of the real welded joint were compared with a nonlocal multiaxial fatigue criterion. The implementation of the volumetric method takes into account the stress gradient effect in the three‐dimensional geometry of the weld toe. The comparative localization of the minimum stress and the real crack indicates the validity of the numerical model. The predicted fatigue strength determines the possible failure of a welded joint at 5·105 cycles. Highlights: The P–S–N curves are obtained for miniature specimens of the base and weld material.The Crossland equivalent stress distribution is averaged over a curvilinear weld toe.The failure probability is predicted for a welded bicycle frame joint. [ABSTRACT FROM AUTHOR]

Lattice materials are a subset of cellular solids with a periodic microstructure and are of interest in a number of applications because of their potential weight‐saving properties. In this study, circular lattices are machined out of plates made of Ti‐6Al‐4V and fatigue tested to generate an S–N curve. These results are compared to a FEM with fatigue data based on MIL‐HDBK‐5H. A good agreement with the FEM was found, suggesting that the fatigue life of lattices can be modeled without specifically detailed fatigue data. The model accurately predicted the point of failure in most of the lattice specimens and was within 4% of the scatter band. Fractographic analysis showed significant shear features in the fracture surfaces, indicating a rapid shear fracture is likely the dominant mode of failure. Finally, the relationship between equivalent stress and relative density was determined using a variety of optimization schemes. Highlights: This study demonstrated that commercial FEA code is capable of calculating the point of failure and life expectancy of circular lattice materials based on generic fatigue data.Fractographic analysis indicates that LEFM is applicable to the fatigue failure of lattice materials.This study demonstrated that optimization of circular lattices for fatigue life can be accomplished with computationally inexpensive methods and has a linear behavior. [ABSTRACT FROM AUTHOR]

The fatigue behavior of woven ply (0/90°) glass fiber reinforced plastic (GFRP) laminates filled with 2 wt.% of tungsten carbide nano‐powder at different stress ratios, 0.1 and 0.3, was investigated. Specimens were subjected to constant amplitude fatigue loading for three stress levels, 80%, 70%, and 60%, of ultimate tensile strength at each stress ratio for both neat GFRP and tungsten carbide modified GFRP (GFRP‐WC). The fatigue life was enhanced at all stress levels in neat GFRP composites with an increase in stress ratio from 0.1 to 0.3. In addition, incorporation of nanoparticles also increases fatigue life at all stress levels for both stress ratios. The addition of nanoparticles acts as a barrier to crack propagation, which leads to a significant effect on the evolution of fatigue stiffness, cyclic creep strain, and dissipated energy during cyclic loading at both stress ratios. Highlights: Effect of stress ratio (R) on fatigue behavior of woven GFRP laminate.Effect of tungsten carbide (WC) on the fatigue behavior of GFRP for R = 0.1 and 0.3.Effect of R and WC on damage, fatigue stiffness, cyclic creep, and dissipated energy.Analytical models are able to reliably predict the fatigue life of GFRP and GFRP‐WC. [ABSTRACT FROM AUTHOR]

Aluminium‐2magnesium alloy castings find its application in engineering sectors where fatigue property is a key requirement. Few researches have been carried out in the past to improve its fatigue property by micro, nano reinforcements. However, the requirement of intended application has not yet attained and hence this current research study has been taken up to provide a solution. This research work focuses through a novel approach for synthesizing in‐situ magnesium aluminate particles in aluminium‐2magnesium alloy by the addition of manganese dioxide and copper oxide precursor addition (1 wt.%, 1.5 wt.% and 2 wt.%) during the casting process. Microstructure of fabricated composites were examined through field emission scanning electron microscopy to characterize the morphology and distribution of magnesium aluminate particles. The fabricated aluminium‐2magnesium/magnesium aluminate in‐situ composite specimens were subjected to low cycle fatigue testing at two different strain amplitudes of 0.3 % and 0.4 % and a frequency of 0.3 Hz. Among all composite specimens, specimens reinforced with 2 wt.% manganese dioxide and 2 wt.% copper oxide outperformed when tested in strain amplitudes of 0.3 % and 0.4 % respectively. Fractography studies were performed on failed composite specimens to study the modes of failure that resulted after fatigue testing. [ABSTRACT FROM AUTHOR]

The modelling of fatigue using machine learning (ML) has been gaining traction in the engineering community. Among ML techniques, the use of probabilistic neural networks (PNNs) has recently emerged as a candidate for modelling fatigue applications. In this paper, we use PNNs with nonconstant variance to model fatigue. We present two case studies to demonstrate the developed approach. First, we model the fatigue life of cover‐plated beams under constant amplitude loading, and then we model the relationship between random vibration velocity and equivalent stress in process pipework. The two case studies demonstrate that PNNs with nonconstant variance can model the distribution of the data while also considering the variability of both distribution parameters (mean and standard deviation). This shows the potential of PNNs with nonconstant variance in modelling fatigue applications. All the data and code used in this paper are openly available. Highlights: Probabilistic neural networks with nonconstant variance are used to model fatigue.Two case studies are presented to demonstrate the developed approach.The proposed method accurately models fatigue and accounts for aleatoric uncertainty. [ABSTRACT FROM AUTHOR]

This study is focused on experimental and numerical estimation of the fatigue life of a flange yoke. The material's stress‐life (fatigue) curve (S−N curve) for axial loading is constructed using specimens extracted from actual flange yokes. The torsional fatigue lives of components are determined by torsional fatigue tests of the complete cardan shafts. Simulation of the cardan shaft model including the flange yoke is carried out in Catia V5 R6 2016 software, which is then transferred to Altair Hyperworks 2017 software for the stress analyses. To predict the number of cycles to failure, the results are transferred into nCode 2018 DesignLife analysis module. An aim is to employ the stress‐life curves as input in numerical calculations and finite element analyses to minimize the costly and time‐consuming experimental work. The results show that physical testing and analysis results are in accordance when S−N curve of actual component material, as well as surface roughness factors measured from real component surface affecting fatigue life are incorporated into the analysis. [ABSTRACT FROM AUTHOR]

By substituting Equation (4) into Equation (3), one can get 5 HT ht Generally, I i SB f sb ' equals the true fracture strength of specimens approximately,5 so, the 1/2-( I i SB b sb / I i SB f sb ') SP 1/ I b i sp is defined as a constant of I B i . [Extracted from the article]

The fatigue behavior of 28 amorphous and semi‐crystalline thermoplastic polymers and elastomers is tested under strain controlled sinusoidal tension‐tension (TT) and torsion (T) at room temperature and analyzed via strain‐life (total strain amplitude versus fatigue lifetime) and crack propagation rate versus total strain amplitude curves, analogous to Paris' law. Investigating fatigue is extremely time‐ and resources consuming, so universal relationships between the exponents of the strain‐life power‐law and material properties are of high importance. For a brittle failure mechanism (I), the strain‐life curves are found to have fixed exponents of BTT,I = −0.27 and BT,I = −0.22, respectively, while the crack propagation versus strain amplitude in TT has an exponent of mda/dN,I = 4. For ductile failure (II), fixed strain‐life curve exponents in TT of BTT,II = −0.35 and in torsion of BT,II = −0.48 with mda/dN,II = 2.8 are obtained. In torsion, most semi‐crystalline polymers show brittle and ductile failure depending on the applied strain amplitude, so the strain‐life curve exponent changes accordingly. The universal exponents for strain‐life and crack growth‐strain amplitude curves offer a significant simplification to rapidly estimate, predict, and simulate the fatigue behavior of polymers. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of stress gradient on the fatigue life of U‐notched specimens under multiaxial proportional loading by studying the shear stress gradient and normal stress gradient on the crack initiation plane separately. First, based on the energy method and critical plane method, an energy‐critical plane method for determining the direction of crack initiation is proposed. Second, with the help of the concept of stress field intensity, the shear stress gradient and normal stress gradient on the critical plane are studied. Finally, a multiaxial fatigue life prediction model is established by combining the equivalent stress field intensity, composed of the shear stress field intensity and normal stress field intensity, with the Manson–Coffin equation. Theoretical analysis is verified by the experiments of Q345 steel notched specimens, and the results show that the predicted accuracy of the proposed method is not conservative and is superior to the local stress–strain method. [ABSTRACT FROM AUTHOR]

A new calculation approach is suggested to the fatigue life evaluation of notched specimens under multiaxial variable amplitude loading. Within this suggested approach, if the computed uniaxial fatigue damage by the pure torsional loading path is larger than that by the axial tension–compression loading path, a shear strain‐based multiaxial fatigue damage parameter is assigned to calculate multiaxial fatigue damage; otherwise, an axial strain‐based multiaxial fatigue damage parameter is assigned to calculate multiaxial fatigue damage. Furthermore, the presented method employs shear strain‐based and axial strain‐based multiaxial fatigue damage parameters in substitution of equivalent strain amplitude to consider the influence of nonproportional additional hardening. The experimental data of GH4169 superalloy and 7050‐T7451 aluminium alloy notched components are used to illustrate the presented multiaxial fatigue lifetime estimation approach for notched components, and the results reveal that estimations are accurate. [ABSTRACT FROM AUTHOR]

Low‐cycle fatigue testing of a lead‐free solder (InnoLot) based on Sn‐3.8Ag‐0.7Cu (SAC387) with three simultaneous additions of bismuth, nickel and antimony was conducted using miniature‐sized fatigue specimens at different temperatures and strain amplitudes. The experiments show a decline of the load capacity of the solder alloy with the number of loading cycles. The fatigue life of the solder is also decreased by the level of imposed temperature. The temperature‐modified Coffin‐Manson and Morrow models were used to analyse the behaviour under fatigue and predict lifetime. The parameters in the two fatigue models were determined by considering experiments at different temperatures and total strain amplitudes. In comparison with other reference lead‐free solders, the InnoLot solder shows much better fatigue strength. The better fatigue strength is found to result from the effect of BiNiSb elements. Also, lifetime predictions were made with both models for the solder alloy under different conditions. [ABSTRACT FROM AUTHOR]

In the literature there are many experimental results of multiaxial fatigue testing, usually generated by the combination of two or more cyclical loads. The cases in which samples are randomly stressed are rarer. Moreover, to generate particular stress states, the use of specific machinery for fatigue tests is usually required. For these reasons, the authors have created a particular geometry of specimens, which, when solicited by a single input of a random type, guarantees the creation of specific multiaxial stress states without using complex and costly instruments. The experimental tests were finally used for the validation of the multiaxial reduction method developed and currently utilized in the authors' design phase, though potentially used to verify all the other methodologies present in the literature. [ABSTRACT FROM AUTHOR]

Infrared thermography allows an alternative energy‐based approach for studying the fatigue behaviour of materials to better understand damage phenomena. In particular, the methodology of infrared thermography can explain the complex dissipative mechanisms promoted by the input parameters, such as the loading ratio, can rapidly provide information about the fatigue strength, and has low cost. In this work, analysis of the thermographic sequences of ASTM A 182 grade F6NM steel obtained during fatigue testing provided four thermal indexes that were used to investigate the thermoelastic and plastic behaviour of material. Fatigue tests at two opportunely chosen loading ratios (R = −0.1, R = 0.5) were performed to investigate the relation between the material behaviour and each index at a specific loading ratio. Finally, estimation of the fatigue strength by means of suitable analysis procedures allowed for an investigation of the damage behaviour of materials under specific loading conditions. [ABSTRACT FROM AUTHOR]

An analytically formulated structural strain method is presented for performing fatigue evaluation of welded components by incorporating nonlinear material hardening effects by means of a modified Ramberg‐Osgood power law hardening model. The modified Ramberg‐Osgood model enables a consistent partitioning of elastic and plastic strain increments during both loading and unloading. For supporting 2 major forms of welded structures in practice, the new method is applied for computing structural strain defined with respect to a through‐thickness section in plate structures and cross section in piping systems. In both cases, the structural strain is formulated as the linearly deformation gradient on their respective cross sections, consistent with the "plane sections remain plane" assumption in structural mechanics. The structural strain‐based fatigue parameter is proposed and has been shown effective in correlating some well‐known low‐cycle and high‐cycle fatigue test data, ranging from gusset‐to‐plate welded plate connections to pipe girth welds. [ABSTRACT FROM AUTHOR]

Abstract: Mechanical structures in service are often subjected to irregular loads, which are typically idealized as stationary random processes. However, the service loads in real applications are often nonstationary ones, in which case conventional frequency‐domain methods are no longer applicable. Here, a novel frequency‐domain approach based on empirical mode decomposition is developed for the fatigue analysis of nonstationary processes. A simplified approach is introduced to calculate the rainflow cycle distributions of non‐Gaussian intrinsic mode functions. Two typical nonstationary processes in engineering practice are simulated numerically to validate the proposed method. The proposed method is applied to fatigue life estimation of a dynamical structure under nonstationary excitations. Reliable life estimations are obtained within much less computation time than the time‐domain method. The results verify the applicability and effectiveness of the proposed method for the fatigue analysis of nonstationary processes. [ABSTRACT FROM AUTHOR]

Abstract: A series of symmetric torsional fatigue with axial constant stress tests, a kind of multiaxial fatigue test, was conducted on oligo‐crystalline 316L stainless steel thin wire, which was less than 3.5 grains across diameter of 200 μm. The material presents significant cyclic hardening under symmetric torsion cycling, and hardening is more obvious with the increasing shear strain amplitude. However, symmetric torsional cycle with constant axial stresses tests characterize rapid initial hardening and then gradually softening until fatigue failure. The axial stress has a great effect on torsional fatigue life. Fractography observation shows a mixed failure mode combined torsional fatigue with tensile strain because of axial tensile stress. A newly proposed model with axial stress damage parameter is used to predict the torsional fatigue life with constant axial stress of small scale thin wire. [ABSTRACT FROM AUTHOR]

Abstract: The present study examines ratcheting response of steel samples with various notch diameters through conducting several cyclic tests. Ratcheting strain values were measured through strain gauges mounted at different distances from the notch root. Local ratcheting at the notch region was highly influenced by notch diameter, notch shape, distance from the notch root, and magnitude of the nominal mean/amplitude of loading cycles. Nominal force‐controlled cycles were kept below the yield point and the Neuber's rule accommodated for the maximum/minimum local stress components along those local strains measured through the strain gauges at the notch region. Plastic strains at the vicinity of notch root over loading cycles were further accumulated by means of the Chaboche hardening model. The local ratcheting strain while progressed at the notch root plastic zone over loading cycles resulted in mean stress relaxation controlled by the model. [ABSTRACT FROM AUTHOR]

Abstract: Cables are used in many engineering applications, whether considered as stationary or running cables. In the latter, ropes are subjected to repeated tension and bending as they run over a sheave wheel. Such loading scenarios are seen for large mining equipment such as draglines and shovel hoist ropes. Fretting fatigue failure often occurs after several cycles of loading because of wires rubbing against each other and external wires rubbing against sheave wheels. It is also pertinent to understand the behaviour of cables subjected to bending over sheaves, to be able to predict fretting fatigue life, so as to set preventive maintenance activities to avoid catastrophic failure in such systems. In this paper, the behaviour of 2 specific configuration strands, composing either 7 or 19 wires, bent over a sheave is investigated numerically. To aid preventive maintenance inspection, critical locations of stress concentrations are identified as a function of applied load or tensile stress. The investigation also considers the impact of groove size, diameter of sheave to diameter of cable ratio, and contact length, enabling the application to infer fatigue life. [ABSTRACT FROM AUTHOR]

Abstract: To study crack dynamic propagation behaviour and rock dynamic fracture toughness, a single cleavage triangle (SCT) specimen was proposed in this paper. By using these specimens and a drop‐weight test system, impact experiments were conducted, and the crack propagation velocity and the fracture time were measured by using crack propagation gauges. To examine the effectiveness of the SCT specimen and to predict the test results, finite difference numerical models were established by using AUTODYN code, and the simulation results showed that the crack propagation path agrees with the test results, and crack arrest phenomena could happen. Meanwhile, by using these numerical models, the crack dynamic propagation mechanism was investigated. Finite element code ABAQUS was applied in the calculation of crack dynamic stress intensity factors (SIFs) based on specimen dimension and the loading curves measured, and the curves of crack dynamic SIFs versus time were obtained. The fracture toughness (including initiation toughness and propagation toughness) was determined according to the fracture time and crack speeds measured by crack propagation gauges. The results show that the SCT specimen is applicable to the study of crack dynamic propagation behaviour and fracture toughness, and in the process of crack propagation, the propagation toughness decreases with crack propagation velocity, and the crack arrest phenomena could happen. The critical SIF of an arrest crack (or arrest toughness) was higher than the crack propagation toughness but was lower than the initiation toughness. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, the stress gradient modified critical distance method is proposed to predict the low‐cycle fatigue lifetime of notched TA19 specimens by introducing a weight function of relative stress gradient into conventional critical distance theories. The accuracy and reliability of the proposed method were validated based on the experimental data by performing low‐cycle fatigue tests on smooth bar TA19 specimens as well as geometrically similar plate specimens containing a bore hole (BHP). Predicted results demonstrate that the mean absolute errors for entire tested specimens using the stress gradient modified critical distance method on the basis of the point method and the line method are 7.49% and 7.41% respectively. Furthermore, the fatigue striation width at the fracture surface was determined by scanning electronic microscope observations to examine the stress gradient effect on the crack growth rate. [ABSTRACT FROM AUTHOR]

Abstract: The fatigue strength of ferritic, pearlitic, and solution strengthened ferritic ductile irons taken from heavy sections and characterized by long solidification times has been assessed. Starting from the idea of Murakami and co‐workers, a new model for the prediction of the fatigue strength is proposed. It allows a sound fatigue assessment of the fatigue strength of as‐cast ductile irons containing solidification defects, such as low nodule count, exploded, chunky and spiky graphite, or microshrinkage porosities. The newly developed equation validated by means of an extensive benchmarking with data taken from the literature has shown a very high potential for applications to thick‐walled components. [ABSTRACT FROM AUTHOR]

Abstract: A new computational methodology is proposed for fatigue life prediction of notched components subjected to variable amplitude multiaxial loading. In the proposed methodology, an estimation method of non‐proportionality factor ( F) proposed by authors in the case of constant amplitude multiaxial loading is extended and applied to variable amplitude multiaxial loading by using Wang‐Brown's reversal counting approach. The pseudo stress correction method integrated with linear elastic finite element analysis is utilized to calculate the local elastic‐plastic stress and strain responses at the notch root. For whole local strain history, the plane with weight‐averaged maximum shear strain range is defined as the critical plane in this study. Based on the defined critical plane, a multiaxial fatigue damage model combined with Miner's linear cumulative damage law is used to predict fatigue life. The experimentally obtained fatigue data for 7050‐T7451 aluminium alloy notched shaft specimens under constant and variable amplitude multiaxial loadings are used to verify the proposed methodology and equivalent strain‐based methodology. The results show that the proposed methodology is superior to equivalent strain‐based methodology. [ABSTRACT FROM AUTHOR]

Abstract: An innovative approach for predicting the reliability of a structure that is subject to a variable‐amplitude dynamic load is presented. In this approach, a Gassner durability curve with its scatter is modelled using a 2‐parametric Weibull's probability density function (PDF). The trend of the Gassner durability curve is modelled with a general hyperbola equation in a log‐log scale. The hyperbola equation is applied to represent the durability curve for the 63.2% probability of fatigue failure that describes the dependency of the Weibull's scale parameter on the loading spectrum's maximum stress. Equations are derived to link the parameters of the hyperbola curve to the material's S‐N curve and the loading spectrum. The Weibull's shape parameter is estimated from the scatter of the material's S‐N curve. The proposed Gassner‐curve model is applied to calculate the fatigue reliability from the PDF of the loading spectrum's maximum stress and the PDF of the durability‐curve's amplitude stress for the selected number of loading‐cycles‐to‐failure. [ABSTRACT FROM AUTHOR]

The classical Palmgren-Miner (PM) rule, despite clearly approximation, is commonly applied for the case of variable amplitude loading, and to date, there is no simple alternative. In the literature, previous authors have commented that the PMhypothesis is based on an exponential fatigue crack growth law, ie, when da-dN is proportional to the crack size a, the case that includes also Paris law for m = 2, in particular. This is because they applied it by updating the damage estimate during the crack growth. It is here shown that applying PM to the "initial" and nominal (Stress vs Number of cycles) curve of a cracked structure results exactly in the integration of the simple Paris power law equation and more in general to any crack law in the form da-dN = HΔ-han. This leads to an interesting new interpretation of PM rule. Indeed, the fact that PM rule is often considered to be quite inaccurate pertains more to the general case when propagation cannot be simplified to this form (like when there are distinct initiation and propagation phases), rather than in long crack propagation. Indeed, results from well-known round-robin experiments under spectrum loading confirm that even using modified Paris laws for crack propagation, the results of the "noninteraction" models, neglecting retardation and other crack closure or plasticity effects due to overloads, are quite satisfactory, and these correspond indeed very closely to applying PM, at least when geometrical factors can be neglected. The use of generalized exponential crack growth, even in the context of spectrum loading, seems to imply the PM rule applies. Therefore, this seems closely related to the so-called lead crack fatigue lifing framework. The connection means however that the same sort of accuracy is expected from PM rule and from assuming exponential crack growth for the entire lifetime. [ABSTRACT FROM AUTHOR]

Fatigue damage and life of SS304, S45C, and SNCM630 steel samples tested at various irregular axial-torsional loading spectra were evaluated by means of 7 Smith-Watson-Topper (SWT) type damage descriptions. The overall damage was calculated over peak-valley events of counted cycles by means of the product of stress and strain corresponding to the areas within stress-strain hysteresis loops. Fatigue lives of 304 steel samples predicted by the SWT, Lorenzo-Laird, Szolwinski-Farris, and Chen-Xu-Huang models were found noticeably larger than those of experimentally obtained at various loading spectra. Predicted lives by these descriptions were found moderately in agreement with experimental data of S45C and SNCM630 steel samples. The predicted lives of steel samples by Socie, Lv et al, and the one presented in this study were closely agreed with experimental data due to their different descriptions. The modified SWT model further included the product of maximum shear stress and shear strain amplitude and related the overall damage to fatigue life through Coffin-Manson equation. The choice of model descriptions for damage assessment was discussed based on their terms, areas underneath of hysteresis loops, and material damage mechanism. [ABSTRACT FROM AUTHOR]

Abstract: A critical aspect of standard test methods for fatigue characterization is that they do not provide any information on heat dissipation in the material and involve very expensive experimental campaigns in time and costs. In recent years, thermographic methods capable of reducing testing time have been developed, also providing more information on damage occurring in the material. A commonly used approach is based on the assessment of the temperature plateau during a stepwise loading procedure. At times, however, this approach can fail if temperature stabilization is not achieved. In this regard, in this paper, a new approach based on the assessment of 3 different thermal indexes was proposed to estimate the fatigue limit of 3 stainless steels: AISI 316, 17‐4PH, and ASTMA890 grade 4a, respectively, exhibiting fully austenitic, fully martensitic, and duplex biphasic microstructure. The fatigue tests were carried out by using a stepwise loading procedure, under loading ratio of 0.5. The analysis demonstrated the possibility to further reduce testing time and, consequently, the fatigue experimental campaign. Moreover, some ideas are discussed about how to justify the different thermal behaviour of a biphasic stainless steel, in total temperature variation. Moreover, a discussion of results regarding the various thermal behaviours of the investigated steels and a possible correlation with the microstructure has been proposed. [ABSTRACT FROM AUTHOR]

Abstract: The present study is focused on the fatigue strength of 15–5 PH stainless steel, built by Direct Metal Laser Sintering. Six‐specimen sets were manufactured, mechanically and thermally treated and tested under rotating bending fatigue. The study investigates the effects of the build orientation (parallel, perpendicular, or 45° inclined with respect to the vertical stacking direction) and of allowance for machining (1 mm or 3 mm at gage). The results, processed by an ANOVA methodology, indicate that allowance for machining has a beneficial effect on the fatigue response. Removing the surface irregularities, averagely leads to a 19% enhancement of the fatigue limit. The build orientation also becomes beneficial, when the slanted samples are included in the experiment. In this case, a fatigue strength increase up to 20% can be achieved. Further developments will include the investigation of the effects of heat and surface treatments, involving also further materials in the study. [ABSTRACT FROM AUTHOR]

Translation abstract Hot work tools are subjected to complex thermal and mechanical loads during hot forming processes. Locally, the stresses can exceed the material's yield strength in highly loaded areas. During mass production, this leads to cyclic plastic deformations and thermomechanical fatigue of the tools, which can be a major lifetime limiting factor. However, established concepts for thermomechanical fatigue life assessment of hot work tools do not exist, since this aspect first reached attention in the last years with the needs for higher resource and energy efficiency as well as optimized manufacturing processes (e. g. in the frame of Industrie 4.0). Hence, in this paper, the contemporary industrially used concepts for dimensioning hot forming tools regarding the tooling fatigue life are presented. Furthermore an overview of existing plasticity and lifetime models is given. The models are divided in phenomenological and mechanism based models. The review shows that further research is essential in this field. [ABSTRACT FROM AUTHOR]

A new critical plane-energy model is proposed in this paper for multiaxial fatigue life prediction of metals. A brief review of existing methods, especially on the critical plane-based and energy-based methods, is given first. Special focus is on the Liu-Mahadevan critical plane approach, which has been shown to work for both brittle and ductile metals. One potential drawback of the Liu-Mahadevan model is that it needs an empirical calibration parameter for non-proportional multiaxial loadings because only the strain terms are used and the out-of-phase hardening cannot be explicitly considered. An energy-based model using the Liu-Mahadevan concept is proposed with the help of the Mróz-Garud plasticity model. Thus, the empirical calibration for non-proportional loading is not needed because the out-of-phase hardening is naturally included in the stress calculation. The model predictions are compared with experimental data from open literature, and the proposed model is shown to work for both proportional and non-proportional multiaxial loadings without the empirical calibration. [ABSTRACT FROM AUTHOR]

The aim of the present work is to investigate the influence of different stress concentration factors (1.09, 1.31, and 2.42) and several R-values on the fatigue behaviour of martensitic steel X10CrNiMoV12-2-2 up to 2 · 109 load cycles at ambient temperature. The tests were performed using an ultrasonic fatigue testing system developed at the authors' institute. The S-N curves of specimens with the lowest investigated stress concentration factor of 1.09 show for both R = −1 and R = 0.5 a flat slope with transition from surface to volume crack initiation at about 1-2 · 107 load cycles. The maximum number of load cycles where fracture occurs increases with increasing R-value. The √area approach by Murakami describes lifetime behaviour within about 4 decades of lifetime and a wide range of load ratios. In case of internal crack initiation at R = −1, the fracture surface contains a fine granular area (FGA) around the inclusion within the typical 'fish-eye' fracture. With increasing notch factor, the fatigue strength up to 2 · 109 load cycles decreases and the slope of the S-N curve becomes steeper in the low cycle fatigue and high cycle fatigue regime, especially for α k = 2.42. In this case, damage at internal defects is suppressed and fatigue cracks are generally initiated at the notch root. [ABSTRACT FROM AUTHOR]

The scope of this study is to characterize the mechanical properties of a novel Transformation-Induced Plasticity bainitic steel grade TBC700Y980T. For this purpose, tensile tests are carried out with loading direction 0, 45 and 90° with respect to the L rolling direction. Yield stress is found to be higher than 700 MPa, ultimate tensile strength larger than 1050 MPa and total elongation higher than 15%. Low-cycle fatigue (LCF) tests are carried out under fully reverse axial strain exploring fatigue lives comprised between 102 and 105 fatigue cycles. The data are used to determine the parameters of the Coffin-Manson as well as the cyclic stress-strain curve. No significant stress-induced austenite transformation is detected. The high-cycle fatigue (HCF) behaviour is investigated through load controlled axial tests exploring fatigue tests up to 5 × 106 fatigue cycles at two loading ratios, namely R = −1 and R = 0. At fatigue lives longer than 2 × 105 cycles, the strain life curve determined from LCF tests tends to greatly underestimate the HCF resistance of the material. Apparently, the HCF behaviour of this material cannot be extrapolated from LCF tests, as different damage, cyclic hardening mechanisms and microstructural conditions are involved. In particular, in the HCF regime, the predominant damage mechanism is nucleation of fatigue cracks in the vicinity of oxide inclusions, whereby mean value and scatter in fatigue limit are directly correlated to the dimension of these inclusions. [ABSTRACT FROM AUTHOR]

In this paper, several multiaxial fatigue damage parameters taking into account nonproportional additional hardening are reviewed. According to the way nonproportional additional hardening is considered in the model, the damage parameters are classified into 2 categories: (1) equivalent damage parameters and (2) direct damage parameters. The equivalent damage parameters usually define a nonproportional coefficient to consider nonproportional additional cyclic hardening, and make a combination of this nonproportional coefficient with stress and/or strain quantities to calculate the equivalent damage parameters. In contrast, the direct damage parameters are directly estimated from the stress and strain quantities of interest. The accuracy of 4 multiaxial fatigue damage parameters in predicting fatigue lifetime is checked against about 150 groups of experimental data for 10 different metallic materials under multiaxial fatigue loading. The results revealed that both Itoh's model, one of equivalent damage parameters, and Susmel's model, which belong to direct damage parameters, could provide a better correlation with the experimental results than others assessed in this paper. So direct damage parameters are not better than the equivalent damage parameters in predicting fatigue lifetime. [ABSTRACT FROM AUTHOR]

This paper presents current research results for polygonal connections with hypocycloidal profiles (H-profiles). It proposes a design concept including permissible stress with respect to limits of plasticity. Moreover, notch effect figures are specified for use in the nominal stress concept. A comparison with conventional shaft-hub connections reveals the benefits of new polygonal connections. [ABSTRACT FROM AUTHOR]

A prediction method to evaluate the effect of scratch geometry on fatigue life of aluminium structures containing scribe marks was developed on the basis of the experimental results described in Part I of this paper. Finite element calculations were performed on scribed samples to investigate the local stress around scribes. Elastic and elastic-plastic stress and strain distributions at the scribe root were computed under monotonic and cyclic tensile and bending loads evaluating the driving force behind initiation and propagation from scribes. Scribe shape, size and cladding regulated stress and strain distributions in the neighbourhood of scribe roots. Fatigue life of tested scribed samples was divided into initiation life, defined as the cycles spent to develop a 50 µm deep crack at scribe roots, and the remaining propagation life up to failure. Striation counting measurements were used to calculate propagation lives by integrating linear elastic da/ dN versus ΔK curves. Only up to a maximum of 38% of total fatigue life was spent to propagate an initial 50 µm deep crack from scribe roots. The theory of critical distances was successfully applied to predict initiation lives of scribed samples from elastic stress distributions. A plastic correction was also suggested, in the frame of the theory of critical distances, to correlate initiation lives of clad and unclad specimens. [ABSTRACT FROM AUTHOR]

A new mean stress fatigue model based on the distortional strain energy is proposed to account for the mean stress effects on fatigue life. The proposed model is compared with the Morrow and the Smith-Watson-Topper (SWT) mean stress correction models using a number of experimental data sets for one cast iron, two steels and two aluminium alloys under tensile and compressive mean stress loadings. It is found that both the proposed mean stress correction model and the SWT model yield similar results and provide very good correlation for positive mean stress data and moderate negative mean stress data. For high compressive mean stresses, the proposed model shows reasonably good correlations, while the SWT model fails to correlate the fatigue data. The Morrow model was found to give poor correlations for all fatigue data analysed by yielding conservative results for compressive mean stresses and non-conservative results for tensile mean stresses. [ABSTRACT FROM AUTHOR]

In this study, the residual stress relaxation behaviour occurring during low-cycle fatigue in shot-peened specimens with either a flat or a notched geometry has been studied. A representative low-pressure steam turbine material, FV448, was used. The residual stress and strain hardening profiles caused by shot peening were measured experimentally and were then incorporated into a finite element model. By allowing for both effects of shot peening, the residual stress relaxation behaviour was successfully simulated using this model and correlated well with the experimental data. Although more modelling work may be required to simulate the interaction between shot peening effects and external loads in a range of notched geometries, the model predictions are consistent with the specimens tested in the current study. The novelty of this study lies in the development of such a modelling approach which can be used to effectively simulate the complex interaction between shot peening effects and external loads in notched regions. Compared with the un-notched geometry, the notched geometry was found to be more effective in retaining the improvement in fatigue life resulting from shot peening, by restricting the compressive residual stress relaxation during fatigue loading. [ABSTRACT FROM AUTHOR]

This study considers the effect of bending loading and the symmetry of joints on the fatigue strength of transverse non-load carrying attachments. Conventionally, the fatigue strength of a welded joint has been determined without taking these factors into account. Experimental and finite element analyses were carried out and both methods showed that both loading type and symmetry have an influence on the fatigue resistance of a welded joint. Under tensile loading, the fatigue strength of asymmetric T-joints was higher than that of symmetric X-joints. Respectively, the fatigue resistance of tested joints improved explicitly when the external loading was bending. The finite element analysis was in good agreement with the test results in the joints subjected to tension but gave very conservative results in the joints subjected to bending. [ABSTRACT FROM AUTHOR]

High precision vibroseis are widely used in oil and gas exploration. The key component of a vibroseis is its baseplate, suffered from complicated external load. Cracks could be generated in the baseplate, and fractures could occur in the welding parts of baseplates because of an extreme loading. This failure not only reduces the baseplate life significantly, but also affects the quality and precision of the signal excitation seriously. The objective of this work is to study the fatigue behavior of the baseplate by the finite element analysis (FEA) and fatigue reliability analysis. At first, the solid model of the baseplate is built, and the dynamic characteristics are analyzed by FEA based on the orthogonal design scheme. Then the stress distribution and peak area of baseplate are obtained under a series of loadings, thereby producing the stress-time data of the risky areas. With the S-N curve and the Palmgren-Miner cumulative damage theory, we could therefore predict the baseplate life, and then its fatigue reliability is calculated with the Kriging response surface method. The sensitivity of fatigue reliability with respect to the design parameters is also obtained, and this provides useful information about the importance of design parameters to the fatigue reliability, therefore resulting in the new design that would lead to longer baseplate life, higher quality of the seismic signal excitation, smaller signal distortion and higher accuracy of oil and gas exploration ultimately. [ABSTRACT FROM AUTHOR]

The weld toe as well as the weld root of joints acts as a geometrical notch, which decreases the fatigue strength of welded components. Local approaches used for fatigue assessment account for the local stress concentration when referring to the notch stress as a fatigue parameter. This applies also to the approaches based on the notch stress intensity factor like, for example, the averaged strain energy density, neglecting the actual notch radius and considering a sharp notch as a simplification. A uniform S-N curve valid for different types of welded joints and failure locations was derived from re-analyses of fatigue test results as documented in literature. The fatigue tests described in this paper aimed at validating that energy-based S-N curve by dedicated tests on artificially notched specimens. At first, four parameters were investigated in order to estimate their influence on the fatigue strength and to select appropriate notch geometries for the final step of the test campaign. The advantages of these tests are that both the exact notch geometry and the local stress range at the notch, including misalignment effects, were identified and considered in experimental data analysis. This paper presents the results of the rather comprehensive testing activities and comparisons with the design-S-N curve mentioned, yielding unexpected fatigue behaviour. This can be explained by the short crack propagation life. [ABSTRACT FROM AUTHOR]

An energy density zone (EDZ) model is developed for the prediction of fatigue life. The microscopic effects can be involved in the EDZ model. Three scale transitional functions in the model are utilized to describe the trans-scale behaviours of fatigue failure from micro-scale to macro-scale. Fatigue failure behaviours of a low-alloy and ultra-high-strength steel material (i.e. 40CrNi2Si2MoVA steel) is investigated. Two fatigue parameters in the model are determined from the experimental S-N curves for the smooth cylinder specimens (the stress concentration factor, SCF, Kt = 1). Then, fatigue lives of notched specimens with SCFs Kt = 2 and Kt = 3 are predicted respectively by the proposed model. The predicted S-N curves are satisfactory in comparison with the experimental results. Scatter of the fatigue test data can be depicted when the microscopic effects are considered. Influences of microscopic effects on the fatigue behaviours are explored by means of numerical simulations. [ABSTRACT FROM AUTHOR]

Das Fließstreifenmodell ist ein verbreitetes Hilfsmittel zur Berechnung der zyklischen Rissspitzenöffnungsverschiebung und des Ermüdungsrisswachstums von Strukturen unter isothermer zyklischer Belastung. Im Zuge einer Modellerweiterung kommt das rheologische Masing-Modell zum Einsatz, das aus parallel geschalteten Reihenschaltungen von Feder und Reibelement besteht. Indem den Reibelementen unterschiedliche Fließspannungen zugeordnet werden, kann mit dem Masing-Modell die zyklische Spannungs-Dehnungs-Kurve des Werkstoffs beschrieben werden. Anstelle einer konstanten Fließspannung werden diese verschiedenen Fließspannungen zufällig den einzelnen Elementen, in die die Riss-Ligament-Linie im Fließstreifenmodell unterteilt ist, zugewiesen. Neben einer besseren Modellierung des mechanischen Materialverhaltens ist auf diese Weise die Berücksichtigung mikrostruktureller Aspekte des Ermüdungsrisswachstums möglich, wenn auf die Werte der Fließspannungen im Bereich des frühen mikrostrukturellen Risswachstums Einfluss genommen wird. Zur Untersuchung thermomechanischer Ermüdung werden die Fließspannungen in Abhängigkeit von der Temperatur formuliert. The strip-yield model is a common tool to calculate the cyclic crack tip opening displacement and the fatigue crack growth life of structures in case of isothermal fatigue loading. As part of an extension of the model the rheological Masing model consisting of spring-slider elements connected in parallel is used to describe the material's stress-strain curve. For this purpose, different yield stresses are allocated to the numerous sliding frictional elements. Instead of the usually applied constant yield stress, these different yield stresses are assigned to the elements into which the crack ligament line is divided. Afterwards, these elements are randomly arranged along the crack ligament line. Besides an improved modelling of the mechanical material behavior the consideration of microstructural aspects of fatigue crack growth becomes possible by influencing the values of the yield stresses near the crack initiation site. [ABSTRACT FROM AUTHOR]

In this study, friction stir welding of Ti-6Al-4 V was demonstrated in 24 mm thickness material. The microstructure and mechanical properties, fatigue, fracture toughness and crack growth of these thick section friction stir welds were evaluated and compared with electron beam welds produced in the same thickness material. It was found that the friction stir welds possessed a relatively coarse lamellar alpha transformed beta microstructure because of slow cooling from above the transus temperature of the material. The electron beam welds had a fine acicular alpha structure as a result of rapid solidification. The friction stir welds possessed better ductility, fatigue life, fracture toughness and crack growth resistance than the base meal or electron beam welds. Thus, even though friction stir welding is a relatively new process, the performance benefits it offers for the fabrication of heavy gage primary structure make it a more attractive option than the more well-established electron beam welding method. [ABSTRACT FROM AUTHOR]

This work investigates the possibility of designing fretting and notch fatigue experiments that are nominally equivalent in terms of damage evaluated by a multiaxial fatigue model. The methodology adopted to carry out this search considered a cylinder-on-flat contact geometry and a V-notched plate. The loading conditions and geometries of the experimental configurations were adjusted to obtain the same decay of the multiaxial fatigue index from the hot spot up to a critical distance. Aluminium alloy 7050-T7451 was used in the experimental evaluation of the methodology. Considering the well-known scatter of fatigue data and the limited number of specimens available, the obtained results suggest that the use of the notch analogy in fretting fatigue is appropriate. [ABSTRACT FROM AUTHOR]

The paper investigates the fatigue strength of laser stake-welded T-joints subjected to reversed bending. The fatigue tests are carried out with the load ratio, R ≈ −0.8. The experimental data is firstly analysed using the nominal stress approach and then by the J-integral as the local fatigue strength parameter in the finite element (FE) assessment. The nominal stress approach demonstrated that the fatigue strength of the investigated T-joints is lower than encountered for any other steel joint under reversed tensile loading. The results also showed that the fatigue strength of this joint under the load ratio R ≈ −0.8 increases with respect to R = 0 bending by 22.6% in the case of the nominal stress approach and 13% in the case of the J-integral approach. However, the slopes of the fatigue resistance curves for different load ratios appear very similar, suggesting that the load ratio has an insignificant influence to the slope. In contrast to the similar slopes, the scatter indexes were different. The nominal stress approach shows that the scatter index is 3.4 times larger for R ≈ −0.8 than R = 0 bending. The J-integral approach showed that the scatter index for R ≈ −0.8 is only 67% larger than in the R = 0 case because the weld geometry is modelled in the FE analysis. [ABSTRACT FROM AUTHOR]

The present study aims at explaining the synergistic effect of environmental media and stress/strain on fatigue lives of aluminium alloys. Rotating bending fatigue tests were carried out using four different aluminium alloys LY12-CZ, 2024-T4, 7475-T7351 and 7075-T651, at air state, 3.5% and 5.0% NaCl aqueous solutions. These results indicated that synergistic actions of the environmental media and cyclic loading accelerated the fatigue crack propagation of aluminium alloys. Furthermore, various influence factors (such as solution concentration, cyclic numbers, high (low) strength aluminium alloys etc.) of the fatigue life at synergistic actions of the environmental media and stress were quantificationally discussed in this paper. [ABSTRACT FROM AUTHOR]

Fatigue life assessment for two-phase steel SAE 1045 has been carried out by experimental and simulation techniques. Analytical approach, termed as fatigue lifetime calculation, was employed making use of a load increase testing procedure and constant amplitude tests equipped with measurement techniques - plastic strain amplitude, change in temperature and change in electrical potential difference. The predicted fatigue life has been validated by constant amplitude tests and compared with fatigue life estimation by microstructure-based simulation. Simulation has been carried out over the complete cross section of the specimen. The simulation uses damage accumulation in the gage section of the specimen culminating in the macro-crack propagation, taking into account the inhomogeneous fatigue resistance of the material element. The results show that at the initial intervals of high cycle fatigue range at relatively higher stress amplitudes, the experimental and simulation results are in agreement; whereas in the (high cycle fatigue) region at relatively low stress amplitudes, the simulation results were found more optimistic and the corresponding fatigue scatter is also increased. Each scatter is attributed to the relatively small number of analysed models of the material structure. Scanning electron microscope was used to determine volume fraction of the microstructure for simulation. Fatigue fracture surface analysis shows that crack initiated from internal defect of material and crack propagation is driven by silicon oxide inclusion. [ABSTRACT FROM AUTHOR]

In order to improve the rolling contact fatigue (RCF) behavior of gear steels, a concept to increase their damage tolerance is developed alternatively to the conventional approach of improving the degree of steel cleanliness. For that purpose, Cu is used as a main alloying element in order to trigger the precipitation of nano-sized Cu precipitates which shall improve the strain-hardening rate of the martensitic matrix of Cu-alloyed 18CrNiMo7-6 steel surrounding a non-metallic inclusion during plastic deformation. In this way, early component failure may be avoided and the maintenance costs of, e.g., wind energy converters may be kept low. The experimental analysis shows that nano-sized Cu precipitates influence the material's strength, ductility, and strain-hardening behavior under tension, depending on their coherence. Among others, the latter is related to strain-induced martensitic transformation of coherent Cu. The structure of the Cu precipitates is studied by TEM and SANS analysis. The Cu-alloyed steel also shows an increased hardening-exponentCHT studied by cyclic hardness test (CHT) PHYBALCHT. Fatigue tests of specimens with coherent precipitates show cyclic hardening until a critical stress amplitude. Above that, stress amplitude cyclic softening is detected. An increased damage tolerance could be obtained for a 1 mass-% Cu-alloyed 18CrNiMo7-6 steel. [ABSTRACT FROM AUTHOR]