Low-cycle Regime Research Articles

Cyclic stability in NiTi and NiTiCu thin films: Role of precipitates in low- and high-cycle regimes

This study examines the influence of precipitates on the cyclic behavior of submicron-thick NiTi and NiTiCu thin films by conducting tensile and fatigue experiments. X-ray diffraction confirmed that both materials contain austenitic phases at room temperature. NiTi has exhibited superior cyclic stability in the low-cycle regime when compared to NiTiCu, while the cyclic stability of NiTiCu surpasses that of NiTi in the high-cycle fatigue regime. The enhanced cyclic stability of NiTi films in the low-cycle regime is attributed to the homogeneous distribution of semi-coherent Ni4Ti3 precipitates within the grain interior. In contrast, the reduced cyclic stability of NiTiCu films can be directly correlated to the presence of incoherent (Ni,Cu)2Ti precipitates at grain boundaries. Conversely, NiTiCu exhibited enhanced cyclic stability under rising stress ratio, attributable to a notable reduction in hysteresis area.

Small crack growth behavior and correlated microstructure-sensitivity during cyclic deformation of a high-chromium/carbon wrought tool steel

The macro to micro-mechanical mechanisms of cyclic deformation behavior of a carbide-containing ferritic steel was studied, and the exceptional properties obtained in low-cycle regimes were discussed, relying on the sensitivity of the small crack propagation to the microstructural characteristics. In this respect, the evolution of boundaries, sub-boundaries, carbides, and micro-textures in the vicinity of the crack growth path has been characterized in detail. Regarding this matter, the subtle alteration of grain size gradient, resulting in the decrement of incompatibility between mechanical properties of grains, is the main factor that halts the crack growth temporarily. Furthermore, both pre-existed and cyclically developed substructures could tortuous the crack propagation path and induce the crack closure effect, respectively. The advantages and disadvantages introduced by primary carbides, serving as initiators of small cracks, and dispersed high volume fraction of submicron carbides, inducing the formation of an effective strengthening layer (ESL) and acting as deflectors of small cracks, have been investigated, respectively. The {001} <110> oriented grains have been identified as preferred micro-texture contributing to small crack propagation. In total, the small crack sensitivity of the elaborated microstructure has resulted in a prolonged lifetime.

High-cycle fatigue strength of 22Cr-12Ni austenitic stainless steel at 77 K

The high-cycle fatigue strength of 22Cr-12Ni austenitic stainless steel was evaluated at 77 K for three types of materials with partially recrystallized (PR), finely recrystallized (FR), and solution-treated (ST) microstructures. Subsurface crack initiation was detected at the lower stress level and/or higher cycles in the materials, such that the fatigue crack initiation sites were shifted from the specimen surface to the interior of the specimen with increasing cycles. The ST showed a significant decrease in fatigue strength over 106 cycles due to subsurface crack initiation. Both PR and FR showed a significant improvement in their high-cycle fatigue strength in the high-cycle regime, although the increase in fatigue strength in the low-cycle regime was approximately equal to the increase in tensile strength.

Effects of aging and shot peening on surface quality and fatigue properties of material extrusion additive manufactured 17-4PH stainless steel

Material extrusion additive manufacturing (MEX) gains increasing interest due to its cost-effectiveness. However, concerns arise regarding inherent MEX characteristics potentially affecting mechanical properties. This study investigates the effects of shot peening and heat treatment on MEX 17-4PH stainless steel. After shot peening, surface nanocrystalline and compressive residual stress are induced. Aging results in higher hardness and tensile strength with lower elongation, while shot peening results in higher surface hardness with minimal change in elongation. For fatigue properties of as-sintered specimens, Al2O3 particle contamination dominates at high cycle regimes, while other MEX surface inhomogeneities dominate at low cycle regimes. Furthermore, as-aged properties are dominated by all surface inhomogeneities, resulting in the non-improvement of fatigue performance compared to the as-sintered specimens. However, shot peening on the as-sintered specimens significantly enhances fatigue properties through the formation of surface nanocrystalline structures and the induction of compressive residual stress, albeit with the occurrence of wrapped corners acting as fatigue crack initiation sites. Remarkably, the combination of aging and shot peening yields the most improvement in fatigue performance, exhibiting behaviours akin to as-sintered specimens with shot peening but without the generation of wrapped corners. Instead, fatigue crack initiation sites shift to subsurface region’s inhomogeneities.

Influence of the cutting method on the fatigue life and crack initiation of non-oriented electrical steel sheets

The fatigue behavior of thin electrical steel sheets under cyclic loading is investigated in dependence on the edge surface. Therefore, four different edge conditions are compared, whereas the edge is either laser cut, shear cut, wire cut, or polished. Strain- and stress-controlled fatigue tests are performed to determine S-N curves in the low cycle regime as well as in the high cycle regime. Microstructural data is collected by non-contacting (optical) Profilometry, Nanoindentation, X-Ray Diffraction, and Electron Backscatter Diffraction to understand the differences in fatigue life by considering surface roughness, residual stresses, hardness, and microstructure. Shear cut specimens achieve the lowest fatigue life, while the other edge conditions reach relatively similar values in the LCF regime. Crack initiation is mainly intergranular in the case of defect-free edges. This tendency has a considerable influence on the observed fatigue behavior.

A continuum damage model of fatigue and failure in whole bone

Predicting the fatigue failure of whole bone may provide insight into the etiology of stress fractures and lead to new methods for preventing and rehabilitating these injuries. Although finite element (FE) models of whole bone have been used to predict fatigue failure, they often do not consider the cumulative and nonlinear effect of fatigue damage, which causes stress redistribution over many loading cycles. The purpose of this study was to develop and validate a continuum damage mechanics FE model for the prediction of fatigue damage and failure. Sixteen whole rabbit-tibiae were imaged using computed tomography (CT) and then cyclically loaded in uniaxial compression until failure. CT images were used to generate specimen-specific FE models and a custom program was developed to iteratively simulate cyclic loading and progressive modulus degradation associated with mechanical fatigue. Four tibiae from the experimental tests were used to develop a suitable damage model and define a failure criterion; the remaining twelve tibiae were used to test the validity of the continuum damage mechanics model. Fatigue-life predictions explained 71% of the variation in experimental fatigue-life measurements with a directional bias towards over-predicting fatigue-life in the low-cycle regime. These findings demonstrate the efficacy of using FE modeling with continuum damage mechanics to predict damage evolution and fatigue failure of whole bone. Through further refinement and validation, this model may be used to investigate different mechanical factors that influence the risk of stress fractures in humans.

Fatigue-resistant design in low-cycle regime by regulating the micro-structural gradient in a TWIP steel: Modelling and experiment

It remains an intricate problem how to enhance low-cycle fatigue (LCF) performance, in the premise of guaranteeing a relatively high strength (or cyclic stress) which is vital to long-time service, for engineering components. The underlying cause of this puzzle is the lack of practical theory on cracking evolution/life prediction for LCF. In this study, a two-stage constitutive LCF crack growth model was proposed for a homogeneous solid and later developed for graded structures. Based on the model, a material design strategy emphasizing the strength gradient architecture (informally, hard core and soft shell structure) is proposed and analyzed to improve the LCF properties of engineering structural materials, e.g., steels. Experimentally, sharp and gradual strength gradients were carefully introduced into a TWIP steel by a series of well-designed process combinations regarding various (heterogeneous) deformation and annealing treatments. The influences of strength/microstructure and their spatial distributions on the LCF behaviors were systematically investigated. The excellent LCF properties of the TWIP steel with a negative strength gradient are originated from the repeated strengthening and weakening of strain gradient during cyclic loading, which suppresses the fatigue crack initiation and growth. The crack initiation/propagation model developed in this study is conducive and applicable to the LCF life prediction. The theory and process of constructing microstructure gradient, and therefore mechanical property gradient, may provide new and important implications for engineering metals on anti-LCF design.

Stress-controlled fatigue of HfNbTaTiZr high-entropy alloy and associated deformation and fracture mechanisms

The stress-controlled fatigue tests are carried out at a stress ratio of 0.1 and a frequency of 10 Hz, and span both low-cycle and high-cycle regimes by varying the applied stress amplitudes. The high-cycle fatigue regime gives a fatigue strength of 497 MPa and a fatigue ratio of 0.44. At equivalent conditions, the alloy's fatigue strength is greater than all other high-entropy alloys (HEAs) with reported high-cycle fatigue data, dilute body-centered cubic alloys, and many structural alloys such as steels, titanium alloys, and aluminum alloys. Through in-depth analyses of crack-propagation trajectories, fracture-surface morphologies and deformation plasticity by means of various microstructural analysis techniques and theoretical frameworks, the alloy's remarkable fatigue resistance is attributed to delayed crack initiation in the high-cycle regime, which is achieved by retarding the formation of localized persistent slip bands, and its good resistance to crack propagation in the low-cycle regime, which is accomplished by intrinsic toughening backed up by extrinsic toughening. Moreover, the stochastic nature of the fatigue data is neatly captured with a 2-parameter Weibull model.

Fretting fatigue behaviours of SiC reinforced aluminium alloy matrix composite and its monolithic alloy

This study presents the experimental and FEA (finite element analysis) investigations on the fretting fatigue behaviours of the metal matrix composite, 2124 Al–Cu–Mg alloy reinforced with 17 vol% of 3 μm SiC particles by a powder metallurgical route and its equivalent monolithic alloy, 2024. The predicted stress concentration and orientation from the nonlinear FEA stress analyses are aligned with the experimental observation including fretting wear damage and crack initiation and propagation. Compared with its equivalent monolithic alloy, the composite in the naturally aged condition (T4) demonstrates similar subsurface degradation but the crack propagation life is much shorter. In general, the composite has shown superior fretting fatigue performance in the low cycle regime (104 to 4x105 cycles) but not in the high cycle regime (4x105 to 107 cycles). Its fretting fatigue behaviour is interpreted in terms of higher wear resistance, delayed crack initiation, superior tensile strength in the low cycle regime. The reported fretting fatigue performance of the composite suggest the different material behaviours in low and high cycle regimes should be considered when developing a fretting fatigue damage model and assessing an engineering design.

Influence of surface integrity induced by multiple machining processes upon the fatigue performance of a nickel based superalloy

Machining operations are of key importance to the fatigue performance of nickel based superalloys due to the high thermal/mechanical loadings yielded on the machined workpiece which can significantly alter the surface integrity of the components. Therefore, understanding the influence mechanisms of machining induced surface integrity upon fatigue response is vital to determine their manufacturing processes and applications. In this respect, this paper investigates the surface integrity of nickel based superalloy subject to different mechanical and thermal loadings induced by various machining processes including conventional machining (e.g. finish and rough milling) and nonconventional machining (e.g. laser assisted milling and abrasive waterjet cutting) methods, as well as their influences upon fatigue performance and failure mechanisms. In-depth surface metallurgical and crystallographic analysis has been conducted to reveal the surface damage mechanisms, which allows the description of the machining induced mechanical and thermal alterations on the machined workpiece. Furthermore, the examination of the fractography from the fatigue specimen has been conducted, which enables the understanding of the influence mechanism of the corresponding surface defects on the fatigue crack initiation and propagation, subject to a four points bending fatigue test. While the resulted S-N curves indicate that the high cycle fatigue of machined nickel based superalloy is mainly dominated by the machining induced residual stress conditions, the surface defects from different machining processes can particularly influence fatigue crack initiation and propagation mechanisms in both the low and high cycle regimes.

Elucidating the Effect of Additive Friction Stir Deposition on the Resulting Microstructure and Mechanical Properties of Magnesium Alloy WE43

In this work, the effect of processing parameters on the resulting microstructure and mechanical properties of magnesium alloy WE43 processed via Additive Friction Stir Deposition (AFSD), a nascent solid-state additive manufacturing (AM) process, is investigated. In particular, a parameterization study was carried out, using multiple four-layer deposits, to identify a suitable process window for a structural 68-layers bulk WE43 deposition. The parametric study identified an acceptable set of parameters with minimal surface defects and excellent consolidation for the fabrication of a bulk WE43 deposition. Microstructural, tensile, and fatigue life characterization was conducted on the bulk WE43 deposition and compared to commercially available wrought material to elucidate the process-structure-property-performance (PSPP) relationship of the AFSD process. This study shows that the bulk WE43 deposit exhibited a refined homogenous microstructure and a texture shift relative to the wrought material. However, a reduction in hardness and tensile behavior was observed in the as-deposited WE43 compared to the wrought control. Additionally, fatigue specimens extracted from the bulk deposition exhibited a decrease in life in the low-cycle regime but performed comparably to the wrought plate in the high-cycle regime. The outcomes of this study illustrate the potential of the AFSD process in additively manufactured structural load-bearing components made with magnesium alloy WE43 in the as-built condition.

Martensitic transformation within nanotwins enhances fatigue damage resistance of a nanotwinned austenitic stainless steel

We investigated the low-cycle fatigue behavior of a novel nanotwinned 316L stainless steel consisting of nanotwinned austenitic grains embedded in the matrix with characteristic dislocation arrangements. We excitingly found that the α\\prime-martensitic transformation is activated traversing twin lamellae to undertake the localized deformation of nanotwinned grains at high strain amplitudes. These α\\prime-martensite nuclei are mainly originated from the intersections between “zigzag” slip bands and twin lamellae. Thanks to the effective release of stress/strain localization by the α\\prime-martensitic transformation within nanotwins, the fatigue strength and fatigue life in the low-cycle regime are simultaneously enhanced.

Stress analysis and lifetime prediction for Ti–6Al–4V welding joint under fatigue loading

A welded joint has a complex gradient microstructure. The effects of local deformation in the fusion zone, heat-affected zone (HAZ), and other areas, on the macroscopic fatigue properties of an inhomogeneous welded joint, are not fully understood. Thus, it is necessary to correlate the microstructural features and their related local deformation mechanisms with macroscopic fatigue properties. In this study, cyclic stress–strain curves and lifetime model parameters of materials in different HAZs were determined by hardness distribution and base–metal material properties. A user element subroutine was developed in the elastic–plastic finite element analysis to obtain the local stress and strain in the joint. ‘Danger points’ were selected to predict fatigue lifetime with the Manson–Coffin damage equation. The prediction results agreed closely with experimental observations within the low-cycle fatigue lifetime regime. Highlights A new LCF life analysis method, for coated specimens, was developed based on the fracture mechanics. Life prediction method for coated specimens was proposed and verified by LCF tests. Coating-induced lifetime degradation was analysed quantitatively at different temperatures. Effects of stress amplitude on the surface crack initiation were investigated.

Double-stage hardening behavior of a lightweight eutectic high entropy alloy in the course of low cycle fatigue

A unique crack growth behavior was detected during room temperature cyclic loading of a eutectic high entropy alloy, which caused the occurrence of two-stage cyclic hardening under the low cycle regime in tensile and compressive half cycles. This was attributed to the crack trapping between the dendritic branch, which caused retardation of the growth stage until the crack sheared and pass through the dendrite. The mechanical fragmentation of the dendrite and distribution through the microstructure could effectively resist crack propagation. This led to higher stress levels during cyclic loading compared with monotonic loading under both tensile and compressive modes of deformation.

A general phase-field model for fatigue failure in brittle and ductile solids

In this work, the phase-field approach to fracture is extended to model fatigue failure in high- and low-cycle regime. The fracture energy degradation due to the repeated externally applied loads is introduced as a function of a local energy accumulation variable, which takes the structural loading history into account. To this end, a novel definition of the energy accumulation variable is proposed, allowing the fracture analysis at monotonic loading without the interference of the fatigue extension, thus making the framework generalised. Moreover, this definition includes the mean load influence of implicitly. The elastoplastic material model with the combined nonlinear isotropic and nonlinear kinematic hardening is introduced to account for cyclic plasticity. The ability of the proposed phenomenological approach to naturally recover main features of fatigue, including Paris law and Wöhler curve under different load ratios is presented through numerical examples and compared with experimental data from the author’s previous work. Physical interpretation of additional fatigue material parameter is explored through the parametric study.

Fatigue Performance and Strength Assessment of AA2024 Alloy Friction Stir Lap Welds

This work experimentally studied the weld-seam cross section characteristics and the fatigue performance of AA2024 aluminum alloy friction stir linear welded joints. The Vickers hardness values on the cross section of the weld-seam have a tendency of being lower than those of the upper sheet, whereas in the lower sheet there is a significant downward profile across the nugget, like a “V” shape. The grains in the welded nugget are finer than those of the base material. Within low-cycle regime, the fracture always occurred at the hook vertex on the loading-carrying upper sheet, while on the loading-carrying lower sheet within relatively high-cycle regime. The typical SEM fractographs of the upper and lower sheets are observed and compared accordingly. The fatigue lives of these specimens were assessed by fictitious notch radius approach and two local damage equations, Morrow’s modified Manson–Coffin damage equation and the Smith–Watson–Topper damage equation, combined with stress and strain at the weld-seam periphery obtained from finite element analyses. Results showed that both the Morrow’s modified Manson–Coffin damage equation and fictitious notch radius approach can give more reasonable results.

Micromechanics based modelling of fatigue crack initiation of high strength steel

The cyclic behaviour and fatigue crack initiation in low cycle regime of dual phase (DP) steel grade 590 were characterized. Experimental strain-controlled tension–compression tests and corresponding simulations using representative volume elements were carried out. The Chaboche kinematic hardening parameters were determined for each individual phases. The damage model based on accumulated inelastic hysteresis energy was incorporated for describing crack onset mechanism in the microstructure. Damage parameters were calibrated with experimental strain-life curves for crack initiation state. Effects of pre-strain on crack occurrence of steel were studied. Observed crack initiation and its life were well predicted by the proposed modelling scheme.

Gurson-based incremental damage in fatigue life estimate under proportional and non-proportional loading: Constant amplitude and low cycle regime applications

This contribution proposes an incremental approach to fatigue life estimate of ductile materials subjected to low cycle fatigue. In this approach, the life prediction is based on the evolution of the internal damage variable of a proposed extension for the Gurson model capable of describing the gradual degradation of the material in function of the number of loading cycles. The extension of the model consists in the inclusion of the Armstrong-Frederick kinematic hardening law [4] and the reformulation of the damage evolution law proposed by Gurson [22], assuming the Nahshon and Hutchinson shear mechanism [43] and a term based on the normalized third invariant with the role to promote asymmetric damage growth under axial (tension–compression) cyclic conditions. An implicit integration scheme is developed, and numerical analyzes are done through simulations at one Gauss point. In this context, the incremental damage approach has applied to experimental data extracted from the literature for thin-walled tubular specimens made of two different materials, S460N structural and hot-rolled normalized SAE 1045 steels. Finally, the predictive capacity of the proposed approach is evaluated comparing predicted and experimentally observed results considering six different proportional and non-proportional loading paths.

The Comparison between Mechanical Properties of Laser-Welded Ultra-High-Strength Austenitic and Martensitic Steels

The paper investigates experimentally the usability of ultra-high-strength stainless steel and abrasion resistant steel in laser-welded sandwich structures. The fatigue and shear strength of laser joints were investigated using lap joints that were welded using two very different energy inputs. Also the effect of multiple weld tracks was investigated. The properties of separate laser welds were characterized by hardness testing and optical microscopy. Results of the hardness measurements showed that there was softened area at heat-affected-zone and weld metal of the ultra-high-strength stainless steel welds. AR steels weld metal was harder than base metal and there was softened zone in heat-affected-zone of the weld. The shear strength of tested single weld joints of the ultra-high-strength stainless steel was higher compared abrasion resistant steel single weld joints, but stronger joint can be made with multiple weld seams for abrasion resistant steel. Fatigue strength of investigated ultra-high-strength stainless steel lap joint was lower than fatigue strength of abrasion resistant steel lap joint in the low-cycle regime, but there was no practical difference in fatigue limit (10e7 cycles).

Fatigue behavior and microstructural evolution of additively manufactured Inconel 718 under cyclic loading at elevated temperature

This study investigates the fatigue behavior of Inconel 718 (IN718) fabricated through laser beam directed energy deposition (LB-DED) process. Fully-reversed (Rε = −1) strain-controlled fatigue tests are conducted on LB-DED IN718 specimens at 650 °C and the results are compared with those of LB-DED IN718 specimens tested at room temperature as well as the ones obtained from wrought counterparts at both room temperature and 650 °C. The microstructure is characterized after fabrication, heat treatment, and fatigue tests at elevated temperature. Microstructure characterizations reveal that as-built LB-DED IN718 has a dendritic structure. The heat treatment applied could not eliminate the dendritic structure, and needle-like δ phases are formed on the grain boundaries. The comparison of LB-DED and wrought fatigue results show that the LB-DED IN718 possesses slightly lower fatigue resistance at room temperature, while exhibits a comparable fatigue strength at elevated temperature. In the low cycle regime, the fatigue resistance of LB-DED IN718 tested at elevated temperature is somewhat inferior to the ones tested at room temperature, which can be explained by the formation and growth of brittle needle-like δ and Laves phases on the grain boundaries during fatigue testing at elevated temperature and their effects on increasing the fatigue crack growth rate. In the high cycle regime, however, LB-DED IN718 specimens tested at elevated temperature had somewhat similar fatigue lives to the ones tested at room temperature. This may be partially explained by the surface oxidation during fatigue testing at elevated temperature, which can retard the microstructurally short cracks to grow.