Low Cycle Fatigue Regime Research Articles

Characterization and performance of high strength steel 51CrV4 under cyclic loading

The high alloyed spring steel 51CrV4 is widely used in engineering applications with high requirements on strength, durability, and safety. The present paper contains experimental results regarding its microstructural characterization and the mechanical behaviour, especially the fatigue life of 51CrV4. Fundamental S-N curves at fully reversed tension-compression and rotating bending have been determined covering the whole area of practical interest, from the Low Cycle Fatigue regime until the engineering endurance limit. These S-N curves can be directly used for the design as well as for the fatigue assessment of engineering components made of this material. The international literature contains very few and isolated data regarding the mechanical behaviour of 51CrV4, while corresponding material properties are totally missing. The material data and properties provided here may be implemented in international cyclic material databases for further exploitation in Industry and Academia.

PREDICTION OF FIBER BREAKAGE AND MATRIX CRACKING IN POLYMERIC COMPOSITES UNDER LOW-CYCLE FATIGUE REGIMES BY FUZZY AND WAVELET CLUSTERING OF ACOUSTIC EMISSION SIGNALS

PREDICTION OF FIBER BREAKAGE AND MATRIX CRACKING IN POLYMERIC COMPOSITES UNDER LOW-CYCLE FATIGUE REGIMES BY FUZZY AND WAVELET CLUSTERING OF ACOUSTIC EMISSION SIGNALS

Role of mean stress on fatigue behavior of a 316L austenitic stainless steel in LWR and air environments

Mean stress effects on fatigue life of a stainless steel in air and boiling water reactor (BWR) environments were evaluated by load-controlled tests. In stress-life, non-zero mean stresses increase life in low cycle fatigue regime in both environments and in high cycle regime in air. In high cycle fatigue regime in BWR environment, higher fatigue life was observed with negative mean stress while a decrease of the fatigue limit was found for positive one. In strain-life, the mean stress effects are insignificant. All data are well correlated with a Smith-Watson-Topper parameter showing no interaction between mean stress and BWR environment.

Fourier transform fatigue analysis of the stress in tension/tension of HDPE and PA6

AbstractMechanical fatigue under strain controlled tension/tension (T/T) of rectangular un‐notched and notched specimens of high density polyethylene and polyamide 6 was performed. Under large amplitude oscillatory elongation, the stress response is nonlinear asymmetric due to a different stress response during loading and unloading and was analyzed via discrete Fourier transform. Dynamic strain (ε0) sweep tests revealed odd (I3/1 α ε02) and even (I2/1 α ε01) harmonics in the stress, well described by the Neo‐Hooke's law. Before crack onset for the Nc specimens, the storage and loss moduli and I2/1 are constant, but I3/1 increases with fatigue. The first derivative of I3/1 (dI3/1/dN) and the cumulative nonlinearity Qf parameter (integral of I3/1/ε02) were found to be strong criteria predicting fatigue. To break the unnotched specimens in the low and high cycle fatigue regime, large strain amplitudes were applied, resulting for both materials in initial plastic deformation followed by buckling when a critical compression force is exceeded. This results in a different time evolution of the linear and nonlinear parameters, especially as a minimum in the I2/1 curve. Finally, a model is proposed to describe the nonlinear viscoelastic specimen response under T/T and the effect of buckling.

Cyclic plastic zone modified critical distance theory for notch fatigue analysis of metals

Notch fatigue strength analysis has decisive role in safe structural design and operational reliability of key components. For notched structures, the presence of the cyclic plastic zone (CPZ) is generally cause for crack initiation and growth at the notch tip, which alters notch geometries and acts as if it is wider than its physical size. Particularly, the influence of the CPZ is more significant in a low cycle fatigue regime. Accordingly, a novel approach for notch fatigue analysis is proposed by combining theory of critical distance (TCD) with CPZ concepts. Experimental data of En3B low carbon steel and 6082 aluminum alloy are introduced for model affirmation and correlation. Results indicate that the critical distance increases with the size of CPZ instead of a material constant during low cycle fatigue analysis, and the proposed model provides better correlations of predicted fatigue lives with tested results than classic TCD and Susmel and Taylor models.

Strain-based fatigue reliability assessment of welded joints in ship structures

A strain-based fatigue reliability assessment of welded ship structural joints is presented. To obtain a complete description of the fatigue stress acting on a butt-welded joint in a ship, a probabilistic distribution for the low cycle fatigue stress range is developed considering the uncertainty in the static and dynamic stresses. A model considering the effect of a secondary notch embedded into the primary notch is developed considering the low and high cycle fatigue regimes. The misalignment, secondary notch, residual stress and fatigue limit effects are incorporated into the notch strain approach on which the fatigue reliability assessment is based in the present study. The efficient Advanced Mean Value method is used to derive the distribution of the fatigue crack initiation life where a detectable crack is formed. The influence of different modelling options is analysed, and the importance of accounting for the misalignment, residual stress and especially the proposed secondary notch effect is discussed. The sensitivity of the reliability estimates to the variables’ uncertainty is investigated based on the First Order Reliability Method, and the dominant ones are identified.

Integral treatment of butt joints for the fatigue life assessment in the low cycle fatigue regime

The framework for a fatigue assessment of welded joints under service loading conditions of crane structures from the low cycle to the high cycle fatigue regime includes the consideration of elastic-plastic material behavior, variable amplitude loading, and acceptable calculation times. Therefore, an integral treatment of butt joints has been developed for fatigue life estimation. The butt weld is considered in its entirety, so that it can be described by its cyclic behavior. The evaluation of the cyclic stress-strain behavior and tri-linear strain-life curves of butt joints for different high-strength, fine-grained structural steels, derived by strain-controlled fatigue tests, is the basis for this description. This procedure is not limited to conventionally applied gas metal arc welding only, but also the fatigue assessment of laser beam welding is possible, for example. Cyclic transient effects have been analyzed and a distinctive cyclic softening is described by linearization of Ramberg-Osgood parameters, depending on the damage content of each cycle derived from constant amplitude, strain-controlled tests. On the basis of the cyclic behavior in combination with memory and Masing behavior, a simulation of the stress-strain paths of investigated butt welds, under constant and variable amplitude loading, has been performed. Damage parameters are used to accumulate the damage cycle by cycle in order to derive the fatigue lifetime. Finally, calculated fatigue lives were compared with experimentally determined lives, showing the impact of this procedure.

Stress-dependent opening- and shear-mode propagation behavior of fatigue cracks in ultrafine-grained Cu fabricated by equal channel angular pressing

Constant and two-step stress amplitude tests were conducted to study the surface crack growth mechanism of ultrafine-grained copper processed by equal channel angular pressing (ECAP) in high- and low-cycle fatigue regimes. The crack growth direction varied depending on the location on the circumference of the round bar specimen and applied stress amplitude. On the surface where an intersection between the shear plane of final pressing and the specimen’s surface made an angle of 45° with respect to the loading axis, the crack grew with a 45° inclination to the loading axis at stress amplitudes ≥ 180 MPa. At stress amplitudes < 160 MPa, the crack grew nearly perpendicular to the loading axis. The physical background of various crack growth directions at high and low stresses is discussed based on the microstructural evolution caused by cyclic stressing and mixed-mode deformation at the crack tip.

Improved effective notch strain approach for fatigue reliability assessment of load-carrying fillet welded cruciform joints in low and high cycle fatigue

An improved effective notch strain (ENSN) approach is developed for the fatigue reliability assessment of load-carrying fillet welded cruciform joints in low and high cycle fatigue regions. The ENSN approach is improved in the ENSN estimation using an analytical approach, and the improvement is verified using some published experiments performed in the low cycle fatigue regime. The cruciform joint is modelled as a correlated two-component series-system, and various sources of uncertainties are considered. The fillet weld geometry uncertainty is modelled by correlated position variations of geometrical control points. To avoid iterative calls of the finite element analysis in the fatigue reliability assessment, response surface models are constructed, and they are used to approximate the influence of position changes. A critical fatigue notch factor is proposed to represent the fatigue resistance for a given fatigue loading and material, which allows a direct reliability approximation. The critical fatigue notch factor for an ice-induced fatigue loading is derived. The ENSN approach is more reasonable to be used for this type of fatigue loading than the high cycle fatigue approach. A cruciform joint subjected to ice-induced fatigue loading is assessed to demonstrate the application of the fatigue reliability assessment procedures.

Cyclic Deformation Behavior of A Heat-Treated Die-Cast Al-Mg-Si-Based Aluminum Alloy.

The purpose of this investigation was to study the low-cycle fatigue (LCF) behavior of a newly developed high-pressure die-cast (HPDC) Al-5.5Mg-2.5Si-0.6Mn-0.2Fe (AlMgSiMnFe) alloy. The effect of heat-treatment in comparison with its as-cast counterpart was also identified. The layered (α-Al + Mg2Si) eutectic structure plus a small amount of Al8(Fe,Mn)2Si phase in the as-cast condition became an in-situ Mg2Si particulate-reinforced aluminum composite with spherical Mg2Si particles uniformly distributed in the α-Al matrix after heat treatment. Due to the spheroidization of intermetallic phases including both Mg2Si and Al8(Fe,Mn)2Si, the ductility and hardening capacity increased while the yield stress (YS) and ultimate tensile strength (UTS) decreased. Portevin–Le Chatelier effect (or serrated flow) was observed in both tensile stress–strain curves and initial hysteresis loops during cyclic deformation because of dynamic strain aging caused by strong dislocation–precipitate interactions. The alloy exhibited cyclic hardening in both as-cast and heat-treated conditions when the applied total strain amplitude was above 0.4%, below which cyclic stabilization was sustained. The heat-treated alloy displayed a larger plastic strain amplitude and a lower stress amplitude at a given total strain amplitude, demonstrating a superior fatigue resistance in the LCF regime. A simple equation based on the stress amplitude of the first and mid-life cycles (, ) was proposed to characterize the degree of cyclic hardening/softening (D): where the positive sign “+” represents cyclic hardening and the negative sign “−“ reflects cyclic softening.

Low-cycle fatigue performance of remelted laser powder bed fusion (L-PBF) biomedical Ti25Ta

In this study, the fatigue performance of additively manufactured Ti25Ta, produced by laser powder bed fusion (L-PBF) using pre-mixed powder is investigated. Ti25Ta shows promise as a biomedical implant alloy, due to its high strength to elastic modulus ratio. However, the fatigue response of L-PBF Ti25Ta is yet unknown and understanding fatigue behaviour is crucial for cyclically loaded implants.The Ti25Ta alloy was produced employing single melt and remelt scanning strategies. It was shown that the remelt strategy had a positive effect on reducing the amount of remaining partially melted Ta particles from 2.07 ± 0.01 vol % to 0.22 ± 0.01 vol % while only slightly increasing the porosity from 0.15 ± 0.01 vol % to 0.37 ± 0.01 vol %. Furthermore, it was found that the remelt strategy resulted in alloy strengthening and a randomised orientation of the α′ lath microstructure.Machined fatigue samples were tested in the low-cycle fatigue regime under strain-controlled conditions. The alloy demonstrated a superior yield stress normalised fatigue performance compared with commercially pure (CP) Ti, and Ti–6Al–4V ELI, and was second only to pure Ta. However, the Ti25Ta L-PBF material retains less than half the elastic modulus of all the compared materials. The remelt samples showed an increased stress response due to their higher strength and an increased elastic modulus, however a reduced number of cycles to failure. This was attributed to reduced ductility and increased crack propagation rate. It is believed that remelt scan parameter optimisation can further enhance the performance of this alloy.

Effect of Thermomechanical Processing on Fatigue Behavior in Solid-State Additive Manufacturing of Al-Mg-Si Alloy

This work presents, for the first time, an in-depth investigation of the structure–property–fatigue relationships of an Al-Mg-Si alloy (AA6061) processed via additive friction stir-deposition (AFS-D). As industry focus continues to shift for more efficient and lightweight structures, quantitative studies on the cyclic performance of additively manufactured materials are needed. In this study, the AFS-D processed AA6061-T6 was machined into specimens in two orthogonal orientations and subjected to monotonic and strain-controlled fatigue testing. The microstructural features of as-deposited AA6061 exhibited evidence of dynamic recrystallization and grain refinement. In addition, significant reduction in the intermetallic particles was observed after AFS-D processing. The fatigue results demonstrate that the as-deposited material, particularly the longitudinal direction, exhibited similar fatigue performance to wrought AA6061-T6 in both low-cycle and high-cycle fatigue regimes, which is a promising result for additively manufactured material in the as-deposited condition. By contrast, the as-deposited build direction orientation possessed slightly lower fatigue resistance than the wrought feedstock material. The AFS-D material was observed to exhibit different damage mechanisms from porosity-based damage mechanisms observed in fusion-based additively manufactured materials. Lastly, a microstructure-sensitive fatigue model was employed to capture the fatigue effects of the AFS-D processing on the AA6061.

Derivation and analysis of a combined fretting wear-fatigue model based on the asymptotic equivalence between perfect plasticity and friction

Fretting analysis may be of critical importance in the design of mechanical structural components when the contact interfaces are subjected to cyclic tangential loading. Indeed, fretting damage may induce various mechanisms of failure, such as wear, but may also lead to fretting fatigue through the early development of embryo cracks than can result in a significant decrease in fatigue performance. In the present paper, an original theoretical model encompassing both phenomena is proposed, as derived from the proven asymptotic equivalence between perfect plasticity and friction and using a continuum damage approach in the intermediate perfectly plastic volume of small thickness. This asymptotic process enables to obtain a law for sliding wear rate evolution and a fretting-fatigue law in the low-cycle fatigue regime based on a unified approach. Both parts of the resulting model are discussed and analyzed in detail by comparison or analogy with existing approaches from the literature on the topic. A detailed strategy for numerical implementation is finally addressed at the end of the paper.

Effect of Work‐Hardening Mechanisms in Asymmetrically Cyclic‐Loaded Austenitic Stainless Steels on Low‐Cycle and High‐Cycle Fatigue Behavior

Dislocation characteristics and deformation‐induced martensite (DIM) transformation in an asymmetrically cyclic‐loaded metastable type 304 stainless steel (SS) are investigated compared with a stable type 316SS using EBSD and X‐ray diffraction line‐profile analysis. In low‐cycle fatigue (LCF) regime, differences between dislocation densities of type 304SS and type 316SS are increased with decreasing fatigue life, and contribution of α′‐martensite to work hardening of cyclically loaded type 304SS increased with decreasing fatigue life. However, the contribution of austenite slightly changed. Higher work hardening via DIM transformation improves the fatigue life of type 304SS than that of type 316SS in LCF regime in the same fatigue stress amplitude. Thus, the contribution of α′‐martensite to work hardening is a determinant parameter, affecting the fatigue behavior of type 304SS. In HCF regime when the α′‐martensite had a small contribution to the work hardening, the fatigue life of type 304SS is slightly higher than that of type 316SS.

Performance-Related Characterization of Forming-Induced Initial Damage in 16MnCrS5 Steel under a Torsional Forward-Reverse Loading Path at LCF Regime.

Forming technology and in particular cold forward rod extrusion is one of the key manufacturing technologies with regard to the production of shafts. The selection of process parameters determines the global and local material properties. This particularly implies forming-induced initial damage in representation of pores. On this background, this study aims on describing the influence of these pores in the performance of the material 16MnCrS5 (DIN 1.7139, AISI/SAE 5115) under a torsional load path in the low cycle fatigue regime, which is highly relevant for shafts under operation conditions. For this purpose, the method of cyclic forward-reverse torsional testing was applied. Additionally, intermittent testing method and the characterization of the state of crack growth using selective electron microscopy analysis of the surface were combined. A first attempt was made to describe the influence of forming-induced initial damage on the fatigue performance and the crack growth mechanisms. The correlation of fatigue performance and initial damage was contiguous in the sense that the initial damage corresponds with a decrease of material performance. It was concluded that the focus of further investigations must be on small crack growth and the related material changes to identify the role of initial damage under cyclic loads.

A calibrated weakest-link model for probabilistic assessment of LCF life considering notch size effects

This study proposes a calibrated weakest-link model for probabilistic life in low cycle fatigue (LCF) regime considering the geometric size effect and the statistical size effect. Weakest-link model is first extended to LCF regime by using damage parameter, and then calibrated based on crack propagation process. Meanwhile, experiments on standard smooth specimens and central hole plate (CHP) specimens are conducted. To improve model accuracy, parameters in the proposed model are calibrated by using Bayesian inference method. Finally, probabilistic LCF life is predicted for CHP specimens with different scales, which shows good agreement with test data.

Fatigue Examination of HSLA Steel with Yield Strength of 960 MPa and Its Welded Joints under Strain Mode

The full benefits of application the high strength low alloyed steels HSLA can be achieved if the structures will be able to carry the alternate loads and fatigue cracks will not be formed, even in the vicinity of welded joints. For this reason the purpose of this study is to find and to explain the influence of different factors on fatigue crack initiation and the nature of crack propagation in HSLA steel and its welded joints. The S960QL steel and two types of welded joints were subjected to low cycle fatigue (LCF) tests at a strain mode and the received surfaces of fractures were analyzed using SEM microscope. Additionally, the microhardness measurements and the residual stress analyze in a cross-section of the joint were conducted. The maximum hardness was determined on the fusion line and more favorable hardness distribution was in the square joints than in single-V. Compiled maps of residual stresses have shown that the local orientation and values of the principal stress vector near the fusion line can influence negative the fatigue life. Finally, the square joints tested in the low cycle fatigue regime have shown a slightly higher fatigue life in comparison with single-V.

Low and ultra-low-cycle fatigue behavior of X52 piping steel based on theory of critical distances

The cyclic failure observed in structural components such as pipelines subjected to extreme loading conditions highlights some limitations concerning the application of existing fatigue damage models. The evaluation and prediction of this type of failure in these steel components under large-scale plastic yielding associated with high levels of stress triaxiality are not sufficiently known nor explored. This fatigue domain is conventionally called ultra-low-cycle fatigue (ULCF) and damage features are representative of both low-cycle fatigue (LCF) and monotonic ductile fracture. Thus, in order to understand the ULCF damage mechanisms both monotonic and LCF tests are required to get representative bounding damage information to model the material damage behaviour under such extreme loading conditions. This paper aims at exploring the Theory of Critical Distances (TCD) in the LCF and ULCF fatigue regimes, including the application of the point, line and area methods. The application of the TCD theories has not been explored so far in the ULCF fatigue regimes, despite its promising results in the LCF and high-cycle fatigue. An experimental program was carried out on several specimens’ geometries made of X52 piping steel. In detail, smooth plane specimens and notched plane specimens were cyclic loaded under tension/compression loading in order to obtain fatigue lives within the range of 101–104 cycles. In addition, cyclic bending tests on notched plane specimens were also incorporated in this study. Finite element simulations of all small-scale tests were conducted allowing to derive elastoplastic stress/strain fields along the potential crack paths. The numerical data were subjected to a post-processing in order to find characteristic lengths that can be treated as a fatigue property according to the TCD. A unified strain-life relation is proposed for the X52 piping steel together with a characteristic material length, consisting of a practical relation for pipeline strain-based design under extreme cyclic loading conditions.

On the strain-life fatigue parameters of additive manufactured plastic materials through fused filament fabrication process

In this paper, the effects of part build directions or raster orientations have been studied on the strain-life fatigue parameters of a wide range of 3D printed plastic materials. These materials have been manufactured through Fused Filament Fabrication (FFF), also known under its trademarked name Fused Deposition Modeling (FDM). To do so, precise analyses of fatigue data with the Ramberg-Osgood form of stress-strain curves were utilized through a strain-based approach to fatigue. The effects of different load ratios and types of loading on fatigue parameters have also been studied. Materials considered in this study were Ultem 9085, Polycarbonate (PC), and Polylactic Acid (PLA). Additive manufactured plastic parts that are FDM-processed exhibited large anisotropy of strain-life fatigue parameters. Hence, the upper and lower bounds for fatigue life prediction were introduced based on the strongest and weakest part build directions or raster orientations of 3D printed materials. For all materials studied in the present paper, fill densities, which seem to have significant impact on fatigue strength of 3D printed parts, have been selected based on the maximum fatigue strength of each part. Results showed that, in some build orientations, the transition fatigue life does not exist. In other orientations, in which the plastic strain components are high enough, transition fatigue lives vary roughly between 20–400 cycles. This means that if the part design in very low cycle fatigue regime is of interest, plastic strains and more complicated plasticity analysis are needed. Results show that the load ratio has no major impact on the fatigue parameters of 3D printed PC parts. In addition, changing in the loading type from tensile fatigue to rotating bending fatigue can significantly impact the fatigue strength coefficient of 3D printed PLA specimens however, it does not noticeably alter the fatigue strength exponents.

Torsional Fatigue Failure of Additively Manufactured Stainless Steel of Reduced Specimen Size

The torsional fatigue response of as-built direct metal laser sintered (DMLS) stainless steel (SS) GP1 of reduced specimen size and horizontal build orientation subjected to cyclic loading at a highly plastic shear strain has been investigated. The average shear modulus, G, was found to be 55.36 GPa, with shear stress history revealing cyclic hardening to stabilization and softening to fracture. Findings suggest that stainless steel GP1 of reduced specimen size experiences fatigue crack initiation at voids near the surface, resulting in a star-spline brittle fracture response, and premature torsional fatigue failure within the low cycle fatigue regime. A noninteraction deformation constitutive model through finite element simulations has been used to successfully simulate the deformation response of the gage section under torsional fatigue conditions.