Cyclic Slip Research Articles

Importance of γ′ shearing in low‐cycle fatigue of a wrought superalloy

AbstractThe present study deals with the fatigue behavior of a wrought nickel‐based superalloy René 41. Uniaxial, fully reversed low‐cycle fatigue tests were conducted at room and elevated (800°C) temperatures under total strain control. Cyclic response was analyzed using the statistical theory of the hysteresis loop, and scanning and transmission electron microscopies were employed to study relevant damage mechanisms. At room temperature, cyclic deformation was found to localize to slip bands since the first cycle, and most slip bands were formed very early in the fatigue life. Hence, shearing of the strengthening phase, coherent γ′ precipitates, was found to be significant only in the first few cycles. At 800°C, fatigue life was affected by an environmental attack resulting in a surface oxide layer. Cyclic deformation was less localized; nevertheless, cyclic slip localization in persistent slip bands and resulting γ′ shearing contributed to cyclic softening throughout the fatigue life.

The sources of cyclic slip irreversibility

The sources of cyclic slip irreversibility in cyclic straining are analyzed. Several physically based models proposed to explain the specific character of the surface relief of persistent slip markings (PSMs) consisting of extrusions and intrusions were reviewed. Two random slip models were further developed and the characteristic reliefs were obtained using simulations under different assumptions on the slip and its distribution within the persistent slip band (PSB). In a model based on static extrusion, the kinetics of extrusion formation was derived. For a model based on the dynamic growth of extrusion basic characteristics were introduced. Characteristic experimental data on the shape of extrusions and intrusions in fatigued polycrystals and the kinetics of their formation were presented. The predictions of individual models were compared with the experimental data and it was concluded that neither the random slip mode nor the static extrusion model can explain the shape and the kinetics of PSM formation. The model based on the dynamic growth of PSMs is in good agreement with experiments on copper. Specific features of materials with planar slip and dislocation arrangement of the PSB and the matrix were discussed.

Micro-strain and cyclic slip accumulation in a polycrystalline nickel-based superalloy

This work provides a comprehensive characterization and analysis of deformation and fatigue damage mechanisms in a nickel-based superalloy during ambient temperature fatigue and points to a fundamental deformation mechanism that results in the onset of crack nucleation. Strain and slip irreversibility are investigated at the nanometer scale using high-resolution digital image correlation and high-resolution electron backscatter diffraction, highlighting distinct deformation mechanisms contributing to crack nucleation. It is observed during early fatigue cycling at relatively low applied stress, the formation of intense slip events that induce grain boundary shearing. This results in intense micro-scale strain in the neighboring grains, producing localized plasticity and stresses. Such stresses facilitate fatigue extrusion–intrusion mechanisms during subsequent cycling, resulting in preferred crack nucleation. Finally, the configurations within the microstructure that promote such deformation and damage mechanisms sequence are highlighted.

Partially decoupled polarization demultiplexing and phase noise equalizer for Alamouti code-based simplified coherent systems.

Alamouti space-time block code (STBC) combined with a simple heterodyne coherent receiver can realize polarization-insensitive phase-diversity detection to reduce the cost. In the receiver, a joint equalizer has been used for STBC's polarization demultiplexing and phase tracking. However, the joint equalizer requires two different step size parameters to update the tap weight coefficients for polarization demultiplexing and the phase noise estimation. This leads to the search process being complex so requiring more iterations for convergence. In this paper, we propose a partially decoupled equalizer that consists of a polarization and phase decoupled equalizer (PPDE) and a pilot-aided blind phase search (P-BPS) algorithm to accelerate the convergence and improve the phase noise tolerance. By theoretically calculating the phase noise, the PPDE can achieve polarization demultiplexing with only one single step size parameter, thus suppressing the searching space and greatly reducing the iterations required for convergence. In the carrier phase recovery stage, the P-BPS algorithm can effectively improve the phase noise tolerance and solve the cyclic slip problem of BPS. We conduct numerical simulations and an experiment to transmit a quadrature phase-shift keying (QPSK) signal. The results demonstrate that the number of iterations required for PPDE convergence is only half of that of the joint equalizer while maintaining polarization-insensitive characteristics in large phase noise. Meanwhile, the achievable linewidth tolerance of P-BPS is increased by three times compared with DD-LMS.

Temperature change and fatigue crack initiation associated with local cyclic slip deformation in a single crystal material

AbstractEvaluating the fatigue limit by measuring temperature changes via infrared thermography has attracted significant interest because of its potential for rapid assessments. However, the physical background and validity of this technique are not yet fully understood. In this study, notched specimens of a single crystal Ni‐base superalloy with different crystallographic orientations are subjected to cyclic loading to investigate crack initiation and temperature change. Local slip deformation is quantified using crystal plasticity finite element analyses. It is found from the experimental and analytical results that the distribution of the temperature change and the local slip deformation (a plastic shear strain on the most active slip system) are affected by the crystallographic orientation. However, the correlation between the plastic shear strain and the second harmonic amplitude, as well as the threshold values of these parameters for crack initiation, is independent of the crystallographic orientation.

Frictional shakedown of a coupled continuous contact

The plane elastic problem of a circular disc shrink fitted into a hole in an elastically similar plane, with a frictional interface, is studied. The disc is subject to a radial force and torque both of which vary with time. In the steady-state the loads vary harmonically, and a relative phase shift is considered. Frictional slip is accounted for by edge dislocations distributed along the interface. It is shown that the steady-state response, which might, for example, be cyclic slip or full adhesion (shakedown) for a particular loading regime, depends on the loading history before the steady-state loading regime is entered. This contrasts with Melan’s theorem for elastic–plastic shakedown, and is a consequence of the normal–tangential coupling in this frictional system.

Cyclic friction-slip behaviour of G350-steel bolted connections

In this study, an experimental program was performed to investigate the cyclic friction-slip behaviour of AS/NZS 3678–350 (G350) steel with different surface finishes. Fifteen bolted steel connections consisting of EN 1090–2 slip factor test assemblies with M16 bolts were prepared and tested with five commonly used surface finishes including inorganic zinc silicate (IZS), epoxy zinc primer (EZP), aliphatic acrylic polyurethane (PUR), abrasive blasted (SA2), and clean mill scale (CMS). The first initial force-slip stiffness and the first slip factor are reported for each surface finish and compared with the existing literature and design standards. An empirical model is proposed to predict the initial slip behaviour based on the first slip factor and initial stiffness, and its accuracy is demonstrated for the coated and uncoated steel. The effects of the cyclic load are then considered, and the cyclic slip factor and initial stiffness are evaluated and discussed. Finally, corresponding empirical models are proposed for the cyclic slip factor and friction-slip stiffness as a function of the cumulative displacement.

Strain controlled fatigue behaviour of a wire + arc additive manufactured Ti-6Al-4V

This paper presents the cyclic deformation behaviour and fatigue properties of a wire + arc additive manufactured Ti-6Al-4V alloy in the as built condition under strain-controlled test condition. Higher local energy input used to build the material has resulted in a coarser primary columnar β grain structure along with a coarser α microstructure compared to Ti-6Al-4V alloys produced by other additive manufacture processes. Test specimens were manufactured in horizontal and vertical orientations with respect to the deposited layers. Isotropic fatigue property was observed at lower applied strain values. When the strain amplitude was above 0.6%, the vertical samples, where the loading axis was parallel with the primary columnar β grains, showed marginally higher fatigue life owing to the material being more ductile in this direction. Moreover, higher cyclic softening rate by a factor of two was measured in the vertical samples. No porosity defects were found in the material. Cracks were initiated from either the α laths or α/β interface due to cyclic slip localisation.

Mechanisms of intergranular fatigue crack initiation in polycrystalline materials

Fatigue failure in polycrystalline materials occurs by initiation and propagation of fatigue cracks. Fatigue cracks can initiate transgranularly or intergranularly. While considerable progress has been achieved in the explanation of the mechanism of transgranular fatigue crack initiation intergranular initiation is still the subject of controversy. A recent study of the crack initiation in superaustenitic Sanicro 25 steel [1] revealed the important role of persistent slip markings (PSMs) in the grains adjoining the grain boundary. A novel mechanism of intergranular fatigue crack initiation has been proposed. Provided the cyclic slip cannot proceed from one grain to the neighbour one, extrusions and intrusions arise not only on the surface but also on the grain boundary. Growing intrusions represent void-like defects and growing intrusions push neighbour grain apart. A systematic study of intergranular cracking has been performed on polycrystalline copper cycled with high and low strain amplitudes. SEM observations of the early stages of fatigue crack initiation on the grain boundaries, EBSD study of grains orientation, FIB cutting of the grain boundaries and simultaneous documentation of the effect of extrusions and intrusions on the grain boundary cohesion are reported simultaneously with the conditions favourable for the initiation of the fatigue cracks.

6G Indoor Network Enabled by Photonics- and Electronics-Based sub-THz Technology

We propose an indoor network with a sub-terahertz- band wireless link for 6G applications. In our proposed indoor network, an optical hub unit (OHU) that controls the entire system is optically linked to THz remote nodes (RNs) over optical distribution fibers. The THz RNs communicate with the user equipment through a sub-THz wireless link. The function of the THz RNs is to provide an interface between the optical link and the sub-THz wireless link. For downlink transmission, a photonics-based sub-THz-band signal generation method is adopted to take advantage of the broadband characteristics of the optical components. An electronics-based sub-THz mixer is also used for uplink transmission because of its cost-effectiveness and low energy consumption. A digital signal processor (DSP) is designed to recover the original transmitted baseband signal. The DSP provides frequency offset compensation over a wide frequency range and reduces the probability of cyclic slip. The performance of the proposed system was investigated experimentally with commercially available optical/electrical components. We demonstrate 100 Gb/s 2.5-m wireless transmission with a 16-quadrature amplitude modulation (16-QAM) signal for configuring the downlink. The optical transmission distance was set to 10 km, and the power penalty measured by optical transmission was negligible. We also investigated the scalability and tunability of the photonics-based sub-THz transmitter to confirm the upgradability of our proposed indoor network to consider future capacity expansion. To establish an uplink, a 25 Gb/s 1.5-m wireless transmission with a quadrature phase shift keying (QPSK) signal was employed. A directly modulated laser was used for cost-effective optical transmission. Unlike downstream transmission, a measured bit error rate (BER) penalty caused by the optical transmission was observed. This is due to the interplay between the frequency chirp of the directly modulated laser and the chromatic dispersion in the fiber. Despite this penalty, BERs less than the soft-decision forward error correction (FEC) threshold (2 × 10^−2) with 20% overhead were achieved. We discuss several remaining technical challenges in real-field deployment. These include THz Tx power improvement, photonic integration, reducing form-factor, polarization insensitivity, and automatic beam steering. Our recent efforts to address these issues are also introduced and examined.

Enhancing bending fatigue resistance of the CoCrFeMnNi high-entropy alloy thin foils by Al addition

Fatigue behaviors of the CoCrFeMnNi-based high-entropy alloy thin foils with different Al addition were investigated under dynamical bending fatigue loading. The results reveal that the Al addition can effectively decrease the cyclic strain localization and improve the fatigue resistance, which is associated with the reduced cyclic slip irreversibility. The physical reasons for such reduction can be attributed to the enhanced planar-slip ability induced by Al addition. Furthermore, the formation of BCC phases caused by 10 at.% Al addition also leads to a delay in the fatigue crack propagation.

Literature Review on Fatigue Life Analysis of Railway Track Incorporating Effect of Load Sequence

Abstract: As the railways are the prime mode of transportation, it is extensive to concentrate on railways which increase the economic growth of the country. Considering that the railway traffic system has an enormous influence on the people, the strength reliability of those components is a very important matter related to the safety and safety of the people. As for the strength reliability, one amongst the important study themes is that the strength or fatigue characteristic of every railway component that's subject to repeated loads during use. The reason why it is necessary to consider the fatigue characteristic carefully is that fatigue is a microscopic fracture caused by the initiation and propagation of a crack due to a cyclic slip deformation of the scale of single grain, that fatigue occurs even under a stress smaller than the strength characteristic under a static load (e.g., tensile strength), which it can suddenly result in a fatal fracture without causing any macroscopic plastic deformation and also causes derailment. The literature survey carried out talks about the ideal model for studying dynamic response of railway track, linear and non-linear approaches to find the fatigue life of railway track, crack initiation and propagation, inspection interval for cracks. The literature survey carried out in this paper gives a fair idea about the research gap and thus motivates researchers to carry out future research on the gap found. Keywords: Fatigue life, shear springs, dampers, literature survey, Rain-flow counting method, non-linear damage accumulation, linear mechanics approach, head checks crack and squat crack.

Plastic strain triggers structural instabilities upon cyclic loading in ultrafine-grained nickel

Grain growth accompanied by shear band formation shortens lifetime of nanostructured metals upon cyclic loading. Although the occurrence of structural instabilities was reported frequently, the difficulty to detect and track their initiation and evolution using standard testing routines prevents an in-depth understanding of the underlying mechanisms. Usage of samples from different synthesis routes tested under varying conditions further complicates this issue. Here, cyclic high pressure torsion is presented as an alternative method to mimic low cycle fatigue. It allows to study initiation and evolution of structural instabilities reproducibly up to enormous accumulated strains, not accessible in conventional fatigue tests. It enabled a general understanding of the processes causing structural instabilities in nanostructured nickel tested with different parameters such as strain amplitude or temperature. Grain coarsening starts from the very first cycles and initiates strain localization in shear bands. Accumulation of cyclic strain induces progressive growth of the shear band thickness accompanied by further grain growth within these bands. Clearly, cyclic strain amplifies grain coarsening suggesting that not the applied stress alone forces boundary motion. This is emphasized further as preferential texture components which facilitate cyclic slip evolve. Although the imposed cyclic strain drives grain growth it stagnates at certain grain sizes. Experiments at 77 K revealed identical instabilities, proving that for nickel boundary migration occurred predominantly mechanically driven.

Estimating the Magnitude of Cyclic Slip on Strike‐Slip faults on Europa

AbstractStrike‐slip faults on Europa may slide back and forth in response to diurnal tidal stresses, which could generate significant frictional heating near the surface. Previous shear heating models assumed fault sliding rates a priori, without showing how the sliding rate is connected to the resolved stresses acting on the fault. Here, I calculate the cyclic displacement along tidally driven faults. I use a Mohr‐Coulomb failure criterion to determine the frictional failure depth, which varies throughout the tidal cycle. The displacement on the fault is calculated assuming an elastic broken plate model. The magnitude of cyclic displacements along a fault depends upon the coefficient of friction and the shear modulus of the ice shell. If Europa's ice shell is weak, diurnal tidal stress can cause faults on Europa to slide back and forth by ~0.1 to 2 m each cycle. Such large amounts of cyclic slip may be enough to frictionally heat the ice and potentially produce near‐surface melting. If Europa's ice shell has the strength of intact ice, faults become less responsive to cyclic tidal stresses and would only slide 0.01 to 0.2 m per cycle.

Microstructure and Low Cycle Fatigue Properties of AA5083 H111 Friction Stir Welded Joint.

The present paper aims to analyze the microstructure, microhardness, tensile properties, and low cycle fatigue (LCF) behavior of friction stir welded (FSW) butt joints. The material used in this study was the 5 mm thick 5083 H111 aluminum alloy sheet. Butt joints of AA 5083 H111 were manufactured at different operating parameters of the FSW process. The effect of the welding parameters on microstructure, microhardness, and tensile properties was investigated. Based on microstructure analysis and strength tests, the most favorable parameters of the FSW process were settled on the point of view of weld quality. Then, LCF tests of base material and friction stir welded specimens made of 5083 H111 were carried out for the examined welded samples under selected friction stir welding parameters. The process of low-cycle fatigue of 5083 H111 aluminum alloy was characterized by cyclic hardening for both: base material and FSW joint. It was revealed by a decrease in the width of the hysteresis loop with the simultaneous significant increase in the values of the range of stress. It was determined that fatigue cracks are initiated by cyclic slip deformation due to local stress concentration from the surface in the corner of the samples for the base material and the heat-affected zone for FSW joints. For all tested strain amplitudes, the fatigue crack propagation region is characterized by the presence of fatigue striation with secondary cracks.

Cyclic deformation and cracking behavior of 316LN stainless steel under thermomechanical and isothermal fatigue loadings

Isothermal fatigue (IF) tests at 250 °C, 350 °C and 450 °C, and thermomechanical fatigue (TMF) tests of 316LN stainless steel within the temperature range of 250–450 °C under in-phase (IP) and out-of-phase (OP) mechanical strain-temperature phase angles were carried out. Comparison of the cyclic deformation behavior, microcrack nucleation mechanisms, crack growth and failure behavior were performed between the IF test at 450 °C (IF-450 °C) and TMF tests. Results showed that the cyclic deformation behavior was governed by the combination of thermally activated recovery process, dynamic strain aging (DSA) and nitride precipitations for IF-450 °C and TMF tests. The occurrence of DSA and nitride precipitations including Z-phase and MX phase were found to promote the planar slip of dislocations and enhance the cyclic stress response. The occurrence of recovery process, however, offset the strengthening effects caused by DSA and precipitations, accelerating the softening stage. The intergranular cracking was attributed to the strong impingement of slip bands on severely oxidized grain boundaries, which can be found only on the specimen surface or near-surface regions. The transgranular cracking, resulting from the cyclic slip localization which was characterized by the formation of persistent slip markings (PSMs) in the form of surface intrusions and extrusions was enhanced by the presence of brittle oxide. The ultimate failure was still dominated by the transgranular propagation.

Direct measurements of slip irreversibility in a nickel-based superalloy using high resolution digital image correlation

Fatigue crack nucleation in crystalline materials typically develops due to highly localized cyclic slip. During a fatigue cycle, reverse slip differs locally from slip in the forward direction particularly in precipitate-containing materials such as superalloys. In this paper we report the first direct measurements of irreversibility at the scale of individual slip bands by high-resolution digital image correlation (DIC) in a polycrystalline nickel-based superalloy. Quantitative measurements of the slip irreversibility are challenging for regions of material that have a size that captures the microstructure and its variability. High spatial resolution at the nanometer scale during experimental measurements is needed to observe slip localization during deformation. Moreover, large fields are also needed to obtain the material response over statistically representative populations of microstructural configurations. Recently, high resolution scanning electron microscope (SEM) digital image correlation (DIC) has been extended for quantitative analysis of discontinuities induced by slip events using the Heaviside-DIC method. This novel method provides quantitative measurements of slip localization at the specimen surface. In this paper, the Heaviside-DIC method is used to measure slip irreversibility and plastic strain accumulation in a nickel-based superalloy. The method detects bands with high levels of irreversibility early in cycling that ultimately form fatigue cracks upon further cycling. The local microstructural configurations that induce large amounts of plasticity and slip irreversibility are correlated to crack nucleation locations.

Fatigue Behavior and Fracture Mechanism of a Hot Rolled AA7020 Aluminum Alloy

Fatigue tests of smooth specimens and CT specimens of a hot rolled AA7020 Aluminum alloy have been performed in laboratory at ambient temperature. Fatigue strength and fatigue crack propagation (FCP) characteristic were evaluated and fracture mechanism was discussed on the basis of crack initiation, small crack growth, and fracture surface analysis. The growth behavior of small and large cracks has been investigated on 7020 alloy.The FCP resistance was found significantly lower than that of other aluminum alloys. The crack growth measurements were performed in CT specimens at constant load ratios ranging from R = 0.1–0.5. The fatigue strength at 104 cycles was 110 MPa that led to a considerably fatigue ratio of 0.1, fatigue failure did not occur. There existed two different modes of crack initiation depending on applied stress level. Above 200 MPa, cracks initiated at the specimen surface in transgranular or intergranular manner due to cyclic slip deformation, while below that stress subsurface crack initiation took place. The growth of small cracks initiated at the surface coincided with the FCP characteristic after allowing for crack closure for large cracks, but the operative fracture mechanisms were different between small and large cracks.

Fretting wear modelling incorporating cyclic ratcheting deformations and the debris evolution for Ti-6Al-4V

Abstract In the fretting wear process, wear particles are referred to as the third-body flow in the contact region. During cyclic slip motions, a part of particles are ejected out of the contact region after a certain number of fretting cycles. The rest particles are attached to contact surfaces and consolidated to form a debris layer, which plays an important role in the fretting wear process. In the present work, a fretting wear model incorporating the evolution of the debris layer is proposed within the framework of finite element analysis to predict the wear profile under different fretting conditions. In order to describe the evolution of the debris layer, the dynamic volume fraction of ejected particles is related to the accumulation of trapped debris volume. Furthermore, the Ohno–Wang cyclic plasticity model is employed to describe the cyclic plastic deformation of Ti-6Al-4V alloy. The major contribution of the present work is to study the influence of the accumulative ratcheting deformation and debris layer on the wear profile. Different wear configurations are considered in the simulations to validate the predictive ability of the proposed wear model. The simulation results show that the proposed approach is able to provide a reasonable prediction of the wear profiles in different wear configurations.

Microcrack nucleation and early crack growth of a nuclear grade nitrogen alloyed austenitic stainless steel X2CrNiMo18.12 under thermomechanical fatigue loading

The continuous and interrupted in-phase thermomechanical fatigue (IP-TMF) tests under mechanical strain-controlled mode were conducted to investigate the microcrack nucleation mechanisms and early growth behavior of a nuclear grade nitrogen alloyed austenitic stainless steel (ASS) X2CrNiMo18.12. The temperature interval of 250–450 °C, cycling period of 120s and strain amplitude of 0.8% were used. The principal deformation and damage mechanisms activated in the present work are attributed to dynamic strain aging (DSA), thermal recovery and oxidation. The oxidation-assisted intergranular cracking was attributed primarily to the slip-bands impingement and the thermal mismatch between oxide scales and substrate. The formation of persistent slip markings (PSMs) in the form of extrusions and intrusions due to occurrence of cyclic slip localization also provided a favored site for microcrack nucleation. Additionally, both the oxidized grain boundaries and PSMs acted as the preferential path for the early crack growth leading to the final failure.