Cyclic Material Properties Research Articles

Thermo-mechanical fatigue analysis of ferritic stainless steel STS444LM for exhaust manifold application

This work presents thermo-mechanical fatigue (TMF) life analysis of ferritic stainless steel STS444LM recently developed to enhance the durability of the automotive exhaust manifolds. To determine cyclic material properties, isothermal tensile and low cycle fatigue (LCF) tests at temperature ranging from 200 °C to 900 °C under two different strain rates of 0.1 %/s and 0.01 %/s are performed. Based on these tests, the unified Chaboche visco-plasticity model is determined. Then, the TMF test using the V-shaped specimen subjected to the temperature cycle from 250 to 850 – 950 °C with three different dwell times at the maximum temperature is performed. An energy-based model based on the Morrow model is determined using TMF test data by incorporating effects of the dwelling time and maximum temperature.

Equations for efficient cycle-by-cycle computation of fatigue crack retardation and acceleration due to amplitude changes

The paper presents an efficient methodology for computation of the evolution of the fatigue crack retardation and acceleration effects due to crack closure during variable amplitude loading. The presented approach can lead to a very effective tool for estimation of residual fatigue life of engineering structures. Conventionally, consideration of variable-amplitude loading is either computationally too demanding, such as in finite element modelling, or challenging to unify the strip-yield model with another computational code. The simple analytical formulae available in literature are based on residual stress, which does not describe correctly the delayed retardation effect and in some cases it can be non-conservative. This work presents simple analytical equations describing the development of the crack closure based on the results of the strip-yield model for amplitude changes at the load ratio R = 0.1. The parabolic function enables simple computation of the maximum crack closure occurring after an overload without running any simulation. The equations also enable computation of crack closure behaviour following amplitude changes with the consideration of various cyclic material properties reflected in the tuneable ratio between the monotonic and cyclic plastic zone sizes.

Micromechanical modeling of the low-cycle fatigue behavior of additively manufactured AlSi10Mg

Factors such as high degrees of design freedom and flexibility in the production process contribute to a continued interest in the additive manufacturing (AM) process in industrial and academic research. The AM-processed components are widely used in industries; however, their use for production of parts under cyclic loading is still limited due to a significant variance in the cyclic behavior and the effect of the AM-associated defects, like porosity or unmelted powder, on the fatigue resistance. Micromechanical modeling approaches can be used to understand the relationship between the microstructure and the cyclic properties of the AM-processed material and thus expedite its employment in safety-critical applications. By using experiments and microstructure-sensitive models, this study aims to examine and to predict the low-cycle fatigue (LCF) behavior of AlSi10Mg parts produced by laser-based powder bed fusion in both the as-built and the direct-aged condition. Fatigue testing reveals higher stress amplitudes and cyclic hardening capabilities for the as-built condition. Direct-aged specimens demonstrate a higher number of cycles to failure at the highest strain amplitude, albeit at lower stress amplitudes when compared to the as-built condition. Both conditions show significant mean stress relaxation during LCF testing at a load ratio of R = 0. The applied modeling framework consists of a J2-plasticity model for the Si-rich phase and a crystal plasticity model for the Al phase. This hybrid model accurately predicts the LCF behavior under various strain amplitudes and ratios for both the as-built and direct-aged conditions of the AlSi10Mg.

Fatigue Lifetime Analysis of a Bicycle Frame Made by Additive Manufacturing Technology from AlSi10Mg

The development of additive manufactured metals is in the transition phase, from research into the technology of 3D printing and the resulting properties of the metals towards their use in industrial practice. The article analyses the possibilities of producing bicycle frames using 3D printing. The stresses in a bicycle frame are analysed for the measured load spectra and FEM simulation. The approach to the fatigue life assessment of the bicycle frame is based on directly measured load multipliers and detailed FEM simulation with the subsequent calculation of fatigue damage in the individual planes of the critical point of the frame respecting the multiaxial stress state. The scatter of the cyclic properties of the AM material is considered by a statistical approach. The operational fatigue lifetimes of a frame made by 3D printing and one of conventional technology are compared.

Study on mechanical properties of high strength sorbite stainless steel S600E under monotonic and cyclic loadings

A new kind of high strength sorbite stainless steel named S600E was developed recently, which has the 65% corrosion resistance, but three times higher yield strength than commonly used austenitic stainless steel S30408. In addition, the price of S600E is only 70% of S30408 owning to the low nickel content of 1.5–2.5%. Fundamental investigations on the mechanical behaviours of S600E were presented in this study. Two monotonic and seventeen cyclic material tests were conducted subjected to different strain-controlled loading protocols including the constant strain amplitude ranging from 0.25% to 2.5%, as well as the varied strain amplitudes. Monotonic test results show that S600E presents the typical feature of rounded nonlinear strain hardening, but the 0.2% proof stress is extremely high, up to 750 MPa, and the elongation after fracture is over 20%. In the cyclic tests, little cyclic hardening was occurred in the initial few cycles, while cyclic softening tended to appear at the remaining cycles. Based on the test results, the isotropic component in the constitute model was neglected, key material parameters for the nonlinear kinematic hardening model with two pairs of backstresses were obtained and validated using the numerical analysis in ABAQUS. At last, the cyclic material properties were compared with commonly used stainless steel to reveal the advantages of S600E. The results of this study could be used in finite element modeling of members, joints and structures of S600E.

Experimental and modeling study of cyclic plasticity and ductile fracture of thin structural steel sheets

The cyclic material properties of thin steel sheets are critical for accurate prediction of the structural response of the thin-walled steel elements. For the experimental study of the cyclic behaviors of thin steel sheets, we proposed a new anti-buckling device equipped with the disc-springs. This improvement aims to compensate the potential gaps between the fixtures and specimens under large plastic deformation so as to avoid the out-of-plane buckling. Furthermore, the negative influences derived from the transverse pressure and friction can be greatly minimized through the implementation of the disc-springs and the lubricated Teflon sheet, as indicated by the percentage errors of 0.31% and 0.08% for the external load and internal stress respectively. By virtue of this improved device, we perform 8 regular and 1 random cyclic testing with the Q345 steel sheet of the 6 mm thickness. It is noted that the smooth hysteresis loops feature no decrease in force under compression applied. Moreover, the sizes of hysteresis loops are dependent upon the current and maximum strain amplitude. The current strain amplitude controls the re-yielding rate, while the maximum value determines the saturated hardening stress. As for the ductile fracture initiation, the similar nominal stress–strain curves in the final tension stage among all cases and the considerably larger accumulated plastic strain than the one under monotonic loading imply the contribution of negative triaxiality to the decrease in the damage variable. Based on these observations, we develop a new cyclic plasticity and ductile fracture model, which is also programmed into the UMAT subroutine compatible with ABAQUS/Standard. By the comparison with the experimental results, the remarkable agreement of the predicted results demonstrates its validity in terms of the constitutive relationship, deformed shape of necking region and the fracture initiation under regular and particularly random cyclic loading.

Estimation of fatigue life for clinched joints with the Local Strain Approach incorporating the impact of cold forming to cyclic material properties

Joining by forming is a commonly applied technique in the automotive industry to assemble parts of thin metal sheets to meet the demands of lightweight design. The joining operation induces changes in material behaviour due to cold forming, that can be observed in increased hardness in the area close to the joint neck compared to the base material. Complex geometrical features of clinched joints on a small scale and the lack of non-destructive methods to track local stresses and strains require a combined approach utilizing numerical and experimental techniques. Numerical process and loading simulation are performed utilizing commercial finite element software LS-Dyna®. Hardness measurements in the joint are carried out to assess the impact of forming operation. Cyclic material properties are derived from Vickers hardness to estimate fatigue life with the Local Strain Approach using the damage parameter PSWT. Fatigue life estimation with failure criterion crack initiation obtained from simulation results is compared to those from experiments. The results obtained indicate that the Local Strain Approach is suitable for fatigue life estimations of clinched joints under constant amplitude loading as long as the influence of the forming process is considered.

Representative structure elements for the fatigue assessment of additively manufactured components

The fatigue life estimation of additively manufactured structures can be a very challenging task, because the component behaviour will be influenced by many parameters, such as surface roughness, imperfections and inhomogeneous properties. Furthermore, the loading conditions and the component geometry have to be taken into account. The problem of considering the singular influences adequately is intensified by their interactions, which invokes a simultaneous treatment of all relevant influencing factors. Without predefinition of the fatigue approach, properties to describe the cyclic aspects of component behaviour and the fatigue life are required. Even in the case of using small sized specimens, it is not possible to produce a defect-free material for studying the behaviour of sound material in order to derive cyclic material properties as a requirement for the local strain-based fatigue concept, or in order to derive a reference SN-curve and knock-down factors for load-based concepts.Analysing the microstructure offers additional information about the local material state, because, depending on the lightening strategy, the material can be divided into different areas (up-skin, down-skin, contour, core, etc.) with characteristic distributions of imperfections. In order to use a conventional fatigue approach method, this information is not sufficient, because the interaction of statistically distributed imperfections is superimposed on the effect of singular defects. Due to these reasons and after introducing a new interpretation of the measured stress and strain, a fatigue approach will be discussed, which is based on a combined experimental and numerical derivation of the required local properties of the so-called representative structure elements.In addition to the reduced numerical effort compared to the conventional approaches, a combined experimental and numerical derivation of RSE properties is enabled.

Analytical modeling of fatigue crack propagation based on cyclic hardening and a characteristic damage length

An analytical fatigue crack growth model is developed, which combines effectively a cyclic energy dissipation approach and the strain energy density theory. The model uses cyclic material properties and relies on cyclic hardening for fatigue crack growth rate prediction. Fatigue crack propagation occurs incrementally by a characteristic damage length, which is in the length scale of striation spacing in commercial aluminum alloy, supporting the model’s physical background. Analytical crack propagation rates correlate well with existing experimental data, in material with uniform strength and strength gradient, showing the ability of the model to account for strength variation at the crack path.

Monotonic and Fatigue Properties of Steel Material Manufactured by Wire Arc Additive Manufacturing

In this study, the monotonic and cyclic material properties of steel material of medium static strength produced additively in the wire arc additive manufacturing (WAAM) process were investigated. This investigated material is expected to be particularly applicable to the field of mechanical engineering, for which practical applications of the WAAM process are still pending and for which hardly any characteristic values can be found in the literature so far. The focus of the investigation was, on the one hand, to determine how the material characteristics are influenced by the load direction in relation to the layered structure and, on the other hand, how they are affected by different interlayer temperatures. For this purpose, monotonic tensile tests were carried out at room temperature as well as at elevated temperatures, and the cyclic material properties were determined. In addition, the hardness of the material and the residual stresses induced during production were measured and compared. In addition to the provision of characteristic properties for the investigated material, it was aimed to determine the extent to which the interlayer temperature influences the strength characteristics, since this can have a considerable influence on the production times and, thus, the economic efficiency of the process.

Investigation on the static and cyclic anisotropic mechanical behavior of polychloroprene rubber (CR) reinforced with tungsten nano-particles

The research presented in this paper aims to provide an insight of tungsten (W) nano-particles reinforced polychloroprene rubber (CR) material (W-CR material) developed for nuclear decommissioning applications and fabricated in sheets by means of rolling process. Static and cyclic anisotropic mechanical properties have been investigated by means of laboratory experiments and finite element simulations, where a fourth-order anisotropic hyperelastic energy potential function has been defined and employed to describe the mechanical behavior of the W-CR material rolled sheet under different loading directions. From the results of the analysis, it has been identified that in contrast to the previous researches, the transversal direction and not the rolling one presents the highest load-carrying capability. As concerns the cyclic material properties, a fatigue life estimation model based on a modified ε-N equation has been developed to define an orientation-dependent cycles-to-failure curve, comprehensive of the results of the three tested directions (0°, 45°, and 90°), showing a correlation factor equal to 92.9%. To verify the reliability of the developed energy potential function, implemented in the finite element model, and of the defined fatigue life estimation model, complex shape specimens have been cut along 0° (rolling), 45°, and 90° directions. The maximum deviation between experimental and numerical load-stroke curve integrals have been calculated in 2.34% whereas the fatigue life prediction in 17.2%, proving the reliability of the implemented numerical and modeling frame.

Characterization of Low Cycle Fatigue Parameters of Rotor Steel using Sub-sized Specimens

The paper is dealing with strain controlled cyclic testing method employing a novel strain-control technique based on digital image correlation (DIC) in low-cycle fatigue (LCF) region. The cyclic behaviour of 22CrMoNiWV 8-8 rotor steel was investigated on sub-sized round specimens with a diameter of 2 mm in gage length and total length of 20 mm. These results were compared with results obtained using conventional specimens designed in accordance with the ASTM E606 standard. The attention was paid to confirm the suitability of the proposed sub-sized geometry, testing set up and procedure. The test procedure and results obtained enabled to record hysteresis loops, construct Manson-Coffin curves and obtain cyclic material properties in LCF region.

Characterization of the precipitation behavior and resulting mechanical properties of copper-alloyed ferritic steel

From literature, it is well known that Cu precipitates in steels lead to an increase of hardness and quasi-static strength. However, the relation between Cu precipitates and cyclic mechanical properties has been investigated rather limited. Consequently, in the present work the evolution of Cu precipitates in a copper-alloyed low carbon, ferritic steel, aged at 550 °C, 600 °C and 650 °C for various times, and their influence on the resulting static and cyclic material properties were analyzed. Three-dimensional atom probe tomography (3DAPT) provided evidence that the ageing treatment led to the formation of Cu precipitates with different sizes and distribution. The existence of copper precipitates resulted in an increase of the ultimate tensile strength and yield strength of the investigated ferritic steel. Moreover, cyclic indentation tests were performed to investigate the influence of the ageing treatment and, hence, Cu precipitates, on hardness and especially cyclic properties of the material. The present results show a maximum of hardness and cyclic hardening potential at sizes of the precipitates of around 2.2 nm. Additionally, synchrotron X-ray diffraction (SYXRD) measurements were in a first approach proved to be viable for investigating the crystallographic structure of Cu precipitates within ferritic steels. Considering the evolution of Cu precipitates, determined with synchrotron as well as atom probe, a high dependency of quasi-static and cyclic properties on Cu precipitates could be demonstrated.

A test method for the determination of the cyclic material properties of electrical steel strip under strain-controlled loading

Abstract The rotors of electric machines are made of thin steel sheets, called electrical steel. Under service conditions, local strain may exceed the elastic limit i. e. for a lifetime assessment local strain might be of great significance. The main challenge for a reliable determination of the cyclic material properties of 0.3 mm thin electrical steel is to avoid buckling in the area of compression of fully reversed strain-controlled tests. Two approaches have been investigated. First, the test setup was improved and adapted to thin electrical steel specimens for which four different linear supported anti-buckling devices were developed and tested for fully reversed strain-controlled loading with the result that the best solution was a new approach using DLC coating instead of a polytetrafluoroethylene strip for reducing the friction between the specimen and the anti-buckling device. Secondly, the influence of increasing the specimen thickness by stacking single specimens through adhesive bonding is presented here.

Effect of Post Treatment on the Microstructure, Surface Roughness and Residual Stress Regarding the Fatigue Strength of Selectively Laser Melted AlSi10Mg Structures

This paper focusses on the effect of hot isostatic pressing (HIP) and a solution annealing post treatment on the fatigue strength of selectively laser melted (SLM) AlSi10Mg structures. The aim of this work is to assess the effect of the unprocessed (as-built) surface and residual stresses, regarding the fatigue behaviour for each condition. The surface roughness of unprocessed specimens is evaluated based on digital light optical microscopy and subsequent three-dimensional image post processing. To holistically characterize contributing factors to the fatigue strength, the axial surface residual stress of all specimens with unprocessed surfaces is measured using X-ray diffraction. Furthermore, the in-depth residual stress distribution of selected samples is analyzed. The fatigue strength is evaluated by tension-compression high-cycle fatigue tests under a load stress ratio of R = −1. For the machined specimens, intrinsic defects like pores or intermetallic phases are identified as the failure origin. Regarding the unprocessed test series, surface features cause the failures that correspond to significantly reduced cyclic material properties of approximately −60% referring to machined ones. There are beneficial effects on the surface roughness and residual stresses evoked due to the post treatments. Considering the aforementioned influencing factors, this study provides a fatigue assessment of the mentioned conditions of the investigated Al-material.

Determination of the static, dynamic and cyclic properties of the heat affected zone for different steel grades

AbstractThe material properties of the heat affected zone (HAZ) in welds can significantly differ from the material properties of the base material (BM) due to recrystallization effects during heating and cooling. In this work, the microstructure of the heat affected zones of the low- and high strength steels S355J2, S690QL and S960QL was physically simulated by a Gleeble simulator. The temperature profiles for heating and cooling were taken from in-situ temperature measurements of a MAG welding process. The static and dynamic yield strengths of the simulated HAZ were determined by tensile tests under variable strain rates. The cyclic material properties according to Coffin, Manson and Morrow were determined for the HAZ and for the BM of each steel by strain- and stress controlled fatigue tests. Furthermore, the cyclic stress-strain curve of each HAZ was determined according to Ramberg and Osgood. Due to the lack of data for comparison, two approximation methods were used for the determination of the cyclic material properties and were compared to the investigated ones.

Study of Strain-Softening Stage in Structurally Inhomogeneous Materials

The paper considers – in terms of modeling and experimental testing – the softening stage manifested in structurally inhomogeneous materials under quasi-static and cyclic load. The method which is used to build the theoretical basis is typical for mechanics. It starts with defining a new material property – strain-softening phase. The stage is realized in the form of a descending branch of the curve on computer diagrams recorded during simple tension or torsion tests on non-reusable specimens in a reasonably rigid testing machine. The type of such a diagram is determined by the structural inhomogeneity of a material, which is defined by distribution law for strength and plasticity properties of structural elements. Complete stress-strain curves or diagrams (CSSD) with a branch descending to zero have been plotted for materials of various classes. Degradation of parameters of a descending branch on a computer diagram recorded for trained specimens provides a new angle on the subject of relation between static and cyclic material properties. The experiments and modeling which were carried out provide basic understanding of softening in terms of deformation.

Low-cycle fatigue of ship hull damaged in collision

Abstract This work aims to extend the conventional post-accidental structural safety assessment by investigating the possibility of damage propagation in a damaged ship during the salvage period as a low-cycle high-stress fatigue process caused by fluctuating wave loads. Low-cycle loading can cause high-amplitude stresses at the edge of the damage opening, which consequently lead to high-stress concentrations. In the present study, a finite element model is created for an intact ship and 50 random damage scenarios for which the stress concentration factors are calculated. Random damage scenarios are generated using damage parameters obtained from historical data on ship accidents. The number of constant-amplitude wave load cycles to initiate a fatigue crack is calculated using the strain-life method defined according to Det Norske Veritas classification notes. Accumulated low-cycle fatigue damage is then estimated by Monte Carlo simulation, where individual vertical wave bending moment amplitudes are drawn as random numbers according to Weibull distribution. Parametric analysis is also performed in order to investigate the influence of cyclic material properties, sea environment and the number of towing days on the overall low-cycle fatigue damage accumulation. Realistic sea state scenarios are selected in order to investigate the influence of low-cycle fatigue damage accumulation during some specific constant sea states, enabling to determine marginal sea conditions for a safe rescue operation.

On the estimation of cyclic material properties – Part 1: Quality of known estimation methods

AbstractA large variety of methods for estimating cyclic material properties from quasi-static material properties can be found throughout the literature. Therefore, the user usually faces the problem of determining which methods would best suit his purpose. To help answer this question, a massive database containing test results from quasi-static and cyclic tests is gathered and used for a quality rating of the different methods. This rating results in the recommendation to use the uniform material law for steel materials, the modified park-song's method for wrought aluminum and the method of variable slopes 2006 for thin steel sheets. In addition, the accuracy of these estimation methods is compared to experimental results using deviation ranges in the total strain life curve and in the cyclic stress-strain curve.

On the estimation of cyclic material properties – Part 2: Introduction of a new estimation method

AbstractIn the first part of this paper, a large variety of methods for estimating cyclic material properties from quasi-static material properties were rated concerning their accuracy. For this, a massive database containing test results from quasi-static and cyclic tests has been gathered and used to compare estimated and experimental properties. In this second part of the paper, the experience of how different estimation methods work and the database itself are used to develop a new estimation method, the so-called FKM method. This new method is able to estimate cyclic material properties for steel, cast steel and wrought aluminum and thus exceeds the known estimation methods if the quality criteria from part 1 are applied.