Cyclic Deformation Response Research Articles

Low‐Cycle Fatigue Performance of Directed Energy Deposited and Conventionally Manufactured Hybrid Gas Tungsten Arc‐Welded Duplex Stainless Steel Joints

Arc‐directed energy deposition (DED‐Arc) is a technology for the production of complex and large‐scaled components. It benefits from high deposition rates and low investment costs. However, to further expand the applications of additively manufactured parts, the possibility for implementation in conventionally manufactured assemblies is crucial. Therefore, this study focusses on the structural integrity of welded joints of DED‐Arc and as‐rolled duplex stainless steels and the welded and nonwelded as‐rolled benchmark. While the fraction of the constituents is almost balanced in the as‐rolled condition, the amount of austenite in the DED‐Arc specimens exceeds 60%. The similar weldment is predominantly ferritic and the hybrid welded joint contains ≈30% austenite, which is discussed based on the nickel concentration. Results from microstructure characterization are correlated to the superior yield and ultimate tensile strength of the as‐rolled condition. Under total strain‐controlled fatigue experiments, the hybrid welded joints exhibit enhanced fatigue lives at various strain amplitudes while the base material shows lowest fatigue resistance. Computed tomography and fractography conclude that the structural integrity is not governed by inherent defects, but instead the phase ratio and distribution yield significant differences in the cyclic deformation response as well as an asymmetry of the hysteretic stress–strain behavior.

On the cyclic deformation behavior of wire-based directed energy deposited Fe-Ni Invar alloy

Fe-basis alloys with Ni concentrations of approximately 36 wt.-% are widely used for components with high requirements on mechanical properties and dimensional accuracy. Due to the remarkably low coefficient of thermal expansion, these so-called Invar alloys provide for dimensional accuracy over a wide temperature range. Generally, additive manufacturing technologies have already been qualified for processing of Invar alloys in terms of maintaining mechanical as well as functional properties. However, the mechanical behavior under cyclic loading still is not comprehensively understood. Therefore, the present study focuses on the fatigue properties of the Fe-Ni36 Invar alloy manufactured by wire-based directed energy deposition (DED) employing either an electric arc (DED-Arc) or a laser beam (DED-LB) as energy source. Here, a wire-based DED-LB Invar condition is comprehensively studied for the first time in open literature. For initial assessment, specimens were analyzed using optical and scanning electron microscopy as well as computed tomography and tensile testing. Here, the DED-LB material is characterized by a heterogeneous grain size distribution, less pronounced crystallographic texture as well as increased tensile strength and ductility compared to the DED-Arc counterparts. Despite increased strength and ductility, the DED-LB Invar consistently shows inferior fatigue properties. This behavior is discussed based on microstructural heterogeneity, cyclic deformation response, process-induced porosity and the contribution of plastic strain. Furthermore, investigations on the thermal expansion behavior show that the DED Invar retains its outstanding thermal properties, respectively.

Tensile and Cyclic Deformation Response of Aluminum Biphenylene-Based Graphene Composite Laminates: An Atomistic Simulation Study

Tensile and Cyclic Deformation Response of Aluminum Biphenylene-Based Graphene Composite Laminates: An Atomistic Simulation Study

Microstructure and mechanical response of as-built and solution-annealed LPBF Hastelloy X under high-temperature fatigue loading

This study investigates the microstructural characteristics and the high-temperature mechanical behavior of Hastelloy X, fabricated via laser powder-bed fusion (LPBF) technology. Hastelloy X, a solid solution-strengthened nickel-based superalloy known for its high strength and oxidation resistance at elevated temperatures, has gained significant interest for the fabrication of complex aerospace components through LPBF technology. The study initially focuses on the impact of solution annealing heat treatment at 1227 °C on the alloy microstructure, based on scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations. It then explores the fatigue and cyclic deformation response of the alloy at 750 °C across different strain ranges, comparing the as-built and solution-annealed conditions. To understand the observed differences in the cyclic mechanical response of as-built and solution-annealed LPBF HX, for a particular condition, a set of dedicated tests have been performed and interrupted at selected numbers of cycles in the different stages of the mechanical response. At each interruption point, specimens have been examined by TEM to provide an in-depth understanding of the effect of dislocation microstructural evolution on the high-temperature cyclic mechanical response of the alloy.

Cyclic deformation response of annealed low-carbon steel: Insights from ratcheting and LCF experiments

Low cycle fatigue (LCF) and ratcheting experiments were carried out on annealed low-carbon steel at room temperature within a laboratory environment, utilising stress and strain control modes. The annealed low-carbon steel consistently demonstrates a cyclic softening response over its LCF lifespan, across all tested strain amplitudes. Notably, it was observed that ratcheting strain rises while ratcheting life declines with both rising mean stress and stress amplitude. Annealed low-carbon steel, being entirely ferritic and lacking precipitation or substitutional solid solution strengthening or hard phase strengthening, exhibits a restricted ability to withstand or alleviate the accumulation of ratcheting strain, particularly under very low mean stress conditions. In both LCF and ratcheting, significant substructure formation was detected. Nevertheless, there was no discernible difference in substructure formation between LCF and ratcheting when employing electron channelling contrast imaging techniques. The existing mean stress-based fatigue life prediction model has successfully forecasted ratcheting and LCF life within the 102–105 cycles range. A novel approach utilising modulus is introduced to characterise the cyclic hardening/softening behaviour of alloys in stress and strain-controlled experiments. The cyclic hardening model based on modulus effectively captures the responses observed in cyclic hardening/softening during LCF and ratcheting experiments.

Low‐Cycle‐Fatigue Performance of Stress‐Aged EN AW‐7075 Alloy

The effect of a novel heat treatment, that is, aging under superimposed external stress, on the fatigue performance and microstructural evolution of a high‐strength aluminum alloy (EN AW‐7075) is presented. Stress aging, a combination of heat treatment and superimposed external stress, can enhance the mechanical properties of EN AW‐7075 under monotonic loading due to the acceleration of precipitation kinetics. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) reveal that a longer aging time and the presence of superimposed stress both promote the formation and growth of precipitates, that is, the precipitation of strengthening η´ precipitates. This is confirmed by differential scanning calorimetry (DSC) heating experiments of stressless and stress‐aged states. Furthermore, stress aging leads to a reduction of dimensions of precipitate‐free zones near grain boundaries. Cyclic deformation responses (CDRs) and half‐life hysteresis loops are evaluated focusing on the low‐cycle fatigue (LCF) performance of different conditions. A noticeable cyclic hardening seen in case of the specimens aged for a short time indicates the occurrence of dynamic strain aging (DSA). Eventually, stress aging allows for an enhancement of the monotonic mechanical properties of EN AW‐7075 without degrading the cyclic performance in the LCF regime.

Cyclic deformation response and strain-induced martensitic transformation of β single phase Ti-35421 alloy

Compared with bimodal microstructure, the low-cycle fatigue (LCF) behavior and cyclic deformation mechanism of a near β Ti-3Al-5Mo-4Cr-2Zr-1Fe (Ti-35421) alloy with single β were systematically investigated. Lower strength, but higher ductility and longer LCF life were found for Single β. The nanoscale α'' martensite was detected in Single β by selected area electron diffraction pattern. Besides, slips and strain induced martensitic transformation played the major deformation mechanisms in Single β. Moreover, the existence of α'' contributed to the limit of microcracks nucleation in Single β, which resulted in a better fatigue life.

Cyclic deformation response of austenitic Ni-based alloy: Mechanical behavior, internal stress evolution and microstructural feature

Cyclic deformation response of an austenitic Ni-based alloy was investigated by symmetrical/asymmetrical strain cycling tests at room temperature (RT) and 350°C. The effects of strain amplitude/ratio and ambient temperature on the cyclic deformation behavior were discussed in terms of microstructural features and internal stresses. Results indicate that the cyclic hardening/softening at RT is mainly attributed to the increase/decrease in back stress. Whilst at 350°C, the contribution of effective stress to cyclic deformation response becomes important due to the planar slip promoted by the occurrence of dynamic strain aging (DSA) throughout the fatigue process. Accordingly, the continuous cyclic hardening at 350°C comes from the combined action of back stress and effective stress.

Cyclic nanoindentation of ITO/Ag/ITO multilayer films: Hysteresis behavior and cyclic deformation response

The poor evaluation of the cyclic deformation response has been a main challenge for extensive applications and developments of ITO/Ag/ITO (IAI) multilayer films. In this paper, the effects of tri-layer film structure on hysteresis behavior and cyclic deformation response were investigated for IAI multilayer films via nanoindentation. The results indicate that the decreased density of picture-frame cracks and the enhanced mechanical performances for 300°C-IAI multilayer films are compared with those of ITO single-layer films under cyclic nanoindentation. Additionally, according to the energy-based method, the values of fracture toughness for 300°C-IAI multilayer films and ITO single-layer films are 3.49–4.17 MPa m0.5 and 3.04–3.18 MPa m0.5, respectively. Consequently, the structural design of ceramic/metal/ceramic tri-layer films are expected to be a potential method to enhance the resistance of cyclic deformation damage for transparent conducting films.

The effect of adding rubber crumbs on the cyclic permanent deformation of waste mixtures containing coal wash and steel furnace slag

Among the numerous studies into the dynamic loading behaviour of rubber crumbs–soil/waste mixtures, the main focus is on how the content of rubber crumbs ([Formula: see text]) affects the damping ratio, shear modulus and total deformation. However, research into the influence of [Formula: see text] on the permanent strain rate ([Formula: see text]) and the deformation mechanism under repeated loading is very limited. In the current study, the cyclic deformation response for waste mixtures of steel furnace slag (SFS), coal wash (CW) and rubber crumbs (RC) is analysed and the test results reveal that [Formula: see text] has a significant influence on the initial [Formula: see text] and the slope of the permanent axial strain rate line, whereas cyclic deviator stress ([Formula: see text]) mainly affects the initial [Formula: see text]. The influence of [Formula: see text] and [Formula: see text] on the [Formula: see text] value of the waste mixture is incorporated in an empirical model, which enables prediction of the permanent deformation mechanism of SFS + CW + RC mixtures with wider-ranging amounts of RC and higher cyclic deviator stresses.

Grain size effect on cyclic deformation behavior and springback prediction of Ni-based superalloy foil

In order to clarify the influence of grain size on cyclic deformation response of superalloy sheets and springback behavior, cyclic loading–unloading and shearing tests were performed on the superalloy foils with 0.2 mm in thickness and diverse grain sizes. The results show that, the decline ratio of elastic modulus is weakened with increasing grain size, and the Bauschinger effect becomes evident with decreasing grain size. Meanwhile, U-bending test results determine that the springback is diminished with increasing grain size. The Chaboche, Anisotropic Nonlinear Kinematic (ANK) and Yoshida-Uemori (Y-U) models were utilized to fit the shear stress–strain curves of specimens. It is found that Y-U model is sufficient of predicting the springback. However, the prediction accuracy is degraded with increasing grain size.

Transient Phase-Driven Cyclic Deformation in Additively Manufactured 15-5 PH Steel.

The present work extends the examination of selective laser melting (SLM)-fabricated 15-5 PH steel with the 8%-transient-austenite-phase towards fully-reversed strain-controlled low-cycle fatigue (LCF) test. The cyclic-deformation response and microstructural evolution were investigated via in-situ neutron-diffraction measurements. The transient-austenite-phase rapidly transformed into the martensite phase in the initial cyclic-hardening stage, followed by an almost complete martensitic transformation in the cyclic-softening and steady stage. The compressive stress was much greater than the tensile stress at the same strain amplitude. The enhanced martensitic transformation associated with lower dislocation densities under compression predominantly governed such a striking tension-compression asymmetry in the SLM-built 15-5 PH.

Effect of slip to roll ratio on cyclic plastic deformation response at subsurface during rolling contact fatigue

The objective of this investigation is to evaluate the cyclic plastic deformation response at the subsurface during rolling contact fatigue (RCF) under the conditions of pure rolling and rolling-sliding. The finite element simulation results indicate that the slip-to-roll ratio (SRR)has an immense effect on the cyclic plastic deformation evolution at the subsurface during RCF. At SRR of 0% (pure rolling), symmetric shear stress cycling is evident at the subsurface, and as a result plastic shakedown takes place. On the other hand, at SRR of 5% (rolling-sliding), asymmetric shear stress cycling is noticed at the subsurface, and consequently ratcheting i.e. progressive accumulation of shear strain is evident. Rolling-sliding results in the shear flow of the subsurface material along the sliding direction.

A generalised phase field model for fatigue crack growth in elastic–plastic solids with an efficient monolithic solver

We present a generalised phase field-based formulation for predicting fatigue crack growth in metals. The theoretical framework aims at covering a wide range of material behaviour. Different fatigue degradation functions are considered and their influence is benchmarked against experiments. The phase field constitutive theory accommodates the so-called AT1, AT2 and phase field-cohesive zone (PF-CZM) models. In regards to material deformation, both non-linear kinematic and isotropic hardening are considered, as well as the combination of the two. Moreover, a monolithic solution scheme based on quasi-Newton algorithms is presented and shown to significantly outperform staggered approaches. The potential of the computational framework is demonstrated by investigating several 2D and 3D boundary value problems of particular interest. Constitutive and numerical choices are compared and insight is gained into their differences and similarities. The framework enables predicting fatigue crack growth in arbitrary geometries and for materials exhibiting complex (cyclic) deformation and damage responses. The finite element code developed is made freely available at www.empaneda.com/codes.

On the influence of build orientation on properties of friction stir welded Al–Si10Mg parts produced by selective laser melting

Welding and joining of components processed by additive manufacturing (AM) to other AM as well as conventionally produced components is of high importance for industry as this allows to combine advantages of either technique and to produce large-scale structures, respectively. One of the key influencing factors with respect to weldability and mechanical properties of AM components was found to be the inherent microstructural anisotropy of these components. In present work, the precipitation-hardenable Al–Si10Mg was fabricated in different build orientations using selective laser melting (SLM) and subsequently joined by friction stir welding (FSW) in different combinations. Microstructural analysis showed considerable grain refinement in the friction stir zone, however, pronounced softening occurred in this area. The latter can be mainly attributed to changes in the morphology and size of Si particles. Upon combination of different build orientations a remarkable influence on the tensile strength of FSW joints was seen. Cyclic deformation responses of SLM and FSW samples were examined in depth. Fatigue properties of this alloy in the low-cycle fatigue (LCF) regime imply that SLM samples with the building direction parallel to the loading direction show superior performance under cyclic loading as compared to the other conditions and the FSW joints. From results presented solid process-microstructure-property relationships are drawn.

Influence of microstructure on cyclic deformation response and micromechanics of Ti–55531 alloy

Abstract Cyclic deformation response and micromechanics in strain controlled low cycle fatigue of Ti–5Al–5Mo–5V–3Cr–1Zr (Ti–55531) alloy with bimodal microstructure (BM) and lamellar microstructure (LM) were comparatively investigated at ambient temperature. Results showed that cyclic hardening/softening behavior of Ti–55531 alloy was found to be significantly dependent on both the strain amplitude and microstructure. The cyclic softening rate of LM was always larger than that of BM at strain level below 1.0%; however, the opposite result was produced for strain amplitude above 1.0%. Dislocation features revealed that cyclic deformation of the alloy primarily relied on primary α (αp) and secondary α (αs) phases. At low strain levels, lasting hardening of BM was due to the interaction of multiple prismatic and basal slips, and pyramidal slip in αp particles, while slight softening of LM was owing to the activation of a few twins in lamellar αs. However, at high strain levels, dislocation annihilation and numerous twins were detected in both αp and αs phases in BM, which resulted in the larger cyclic softening rate of BM than that of LM. Evidently, besides dislocation annihilation, twinning was found to be another important softening mechanism, which was not revealed by other studies on Ti alloys.

Tensile and cyclic deformation response of friction-stir-welded dissimilar aluminum alloy joints: Strain localization effect

Cyclic deformation behavior of friction-stir-welded dissimilar AA2024-T351 to AA7075-T65 aluminum alloy joints was evaluated via stepwise tests at different strain rates, along with transmission electron microscopy examinations to characterize the precipitates required to assess internal stresses. Electron backscatter diffraction was employed to observe the inhomogeneous microstructures of the FSWed joints. Strain localization appeared in the heat affected zone (HAZ) of AA2024 side. After cyclic deformation of 500 cycles at a total strain amplitude of 0.5 %, the strength of the dissimilar joints resumed basically to that of AA2024 base material. And the AA2024 HAZ was obviously hardened, which should be attributed to the introduced dislocations during cyclic deformation process. Cyclic hardening capacity of the joints increased with decreasing strain rate.

On the structural integrity of Fe-36Ni Invar alloy processed by selective laser melting

Specimens made from pre-alloyed Invar (Fe-36Ni) powder were fabricated by selective laser melting and stress-relief heat treated afterwards. A relative density of the fabricated parts of 99.6 % was determined by computed tomography. The microstructure and mechanical behavior under monotonic and particularly cyclic loading at ambient temperature were investigated. Results reveal a bimodal microstructure containing columnar and equiaxed grains with an average grain size of 75 μm and pronounced texture 〈001〉 || BD. The selective laser melted Invar is characterized by a homogeneous hardness distribution revealing no gradient with respect to the height of the fabricated parts. The mechanical response under monotonic tensile loading is characterized by a fairly ductile behavior. Total strain controlled low-cycle fatigue tests with varying strain amplitudes were performed. The cyclic deformation response is characterized by highly reproducible behavior in terms of stress amplitudes and number of cycles to failure. The corresponding half-life hysteresis loops reveal perfect Masing behavior. Post-mortem fractography revealed crack initiation in direct vicinity of the surfaces. The damage tolerance of Invar is found to be promoted by its high ductility. Finally, a low coefficient of thermal expansion, very similar to conventionally processed Invar, was shown by dilatometer tests in the temperature range of 0 °C–180 °C.

Cyclic deformation responses in α-phase of a lamellar near β-Ti alloy

A lamellar near β-Ti alloy was subjected to cyclic tensile loading at room temperature. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) investigation revealed that the deformation mechanisms of α-phase were found to be changing with increasing loading cycles, from mechanical twinning to martensite transformation. To be specific, at the beginning of straining, the formation of mechanical α-twins was the dominant deformation mechanism to accommodate the applied strain. As the increasing cycles, the secondary twin and hierarchical structure formed in α laths. During further straining, the noticeable groove was observed at the α/β grain boundary. Meanwhile, dispersed nanosized α" martensite phase formed in the α plate and the number of α-twins sharply decreased at this stage. This work will provide a basis for the enhancement of load bearing capabilities of near β-Ti alloys.

Measuring elastoplastic strain loops in the near crack-tip region using a Stereo Microscope DIC system

In this work, DIC analysis is used to assess the cyclic plastic deformation response, Δε, at the near crack-tip region of propagating fatigue cracks and to identify the effects of other phenomena that happen during crack propagation, such as crack closure and partial crack opening on the measured elastoplastic strain loops. The DIC measurements use a stereo microscope and a very refined speckle pattern on the specimen surface. Fatigue tests were performed in constant ΔK and Kmax conditions on aluminum and steel specimens. Such measurements can be used to investigate the actual fatigue crack growth driving force.