Cyclic Strengthening Research Articles

Experimental study on seismic performance of replaceable shear links with low-yield-point steel

Based on the design concept of controllable seismic damage during earthquakes and rapid functional recovery after earthquakes, a replaceable shear link with low-yield-point steel was proposed. Quasi-static cyclic loading tests and numerical analysis were conducted on six specimens. The failure mode, seismic performance, energy dissipation behavior, and shear distribution of each specimen were carefully considered to investigate the effects of web steel, web stiffening configuration and stiffener-plate stiffness ratio. The results showed that the stiffened specimens showed full-sectional shear yielding behavior, while the non-stiffened specimens exhibited coupling behavior of web yielding and buckling. Specimens with reasonable design parameters had excellent ductility and energy dissipation capacity, with the plastic shear angle exceeding 0.157 rad and the maximum equivalent viscous damping coefficients greater than 0.51 (80% of the theoretical maximum value). Owing to the cyclic strengthening of the low-yield-point steel web and the contributions of stiffeners and flanges, the specimens showed obvious overstrength behavior, with the overstrength factor ranging from 1.99 to 2.39. The cross-stiffening configuration was conducive to enhancing the seismic performance of shear links. Smaller web width-to-thickness ratios were recommended to match with smaller stiffener-plate stiffness ratios. Extended cross stiffeners and diagonal stiffeners resulted in earlier cracking of the web welds, which adversely affected the ductility of the specimens.

Strengthening behavior and microstructure of 2195 Al-Cu-Li alloy under cycling loading

Cyclic strengthening process, as a newly developed aluminum alloy strengthening technique, has garnered significant attention due to its advantages of rapidity, efficiency, and superior mechanical properties. This study investigated the effects of strain amplitude and loading frequency on cyclic strengthening, including mechanical properties and microstructural evolution. At room temperature, the cyclic strengthening effect of the 2195 alloy increases with increasing strain amplitude, while the influence of loading frequency is pronounced within the elastic range but nearly negligible within the plastic range. Microstructural analysis reveals that cyclic strengthening behavior within the elastic range is attributed to the precipitation of clusters, whereas within the plastic range, it is attributed to the precipitation of clusters and dislocation multiplication. Unlike other plastic deformations, cyclic plastic loading generates dislocations primarily in the form of dislocation loops. The dynamic precipitation of clusters and the diffuse distribution of dislocation loops significantly enhance the alloy's strength with minimal sacrifice of elongation. Experimental investigations on the influence of loading frequency on these precipitation behaviors demonstrate a positive correlation with dislocation generation and a negative correlation with cluster precipitation. Comparison of specimens with different strain amplitudes reveals that larger strain amplitudes provide more energy for cluster precipitation, thereby promoting precipitation. However, excessively large strain amplitudes lead to the annihilation of vacancies due to the introduction of more dislocations, thus inhibiting cluster precipitation. This indicates that the precipitation of clusters during cyclic deformation is profoundly influenced by dislocation behavior. This study deepens the understanding of cyclic strengthening mechanisms and provides support for subsequent development of cyclic strengthening processes in other materials design.

A novel method simultaneously eliminating Portevin-Le Chatelier effect and enhancing strength in non-heat-treatable Al-Mg alloys

This study investigates the effects of cyclic plasticity on the tensile properties of AA5083 alloys. The low strength and Portevin-Le Chatelier (PLC) effect in Al-Mg alloys means they have found limited use in automotive outer panels. This work shows that by applying cyclic strengthening approach to AA5083, a high density of Mg-Al clusters can be formed in matrix resulting in increased yield and tensile strengths. Moreover, the introduction of cyclic plasticity also effectively suppresses the PLC effect, which is due to the formation of the clusters and the consequent reduction in the number of free Mg atoms in the solid solution. This method introduces the concept of particle strengthening in Al-Mg alloys without altering composition, while simultaneously eliminating the PLC effect. It suggests a potential path for using 5xxx series alloys in applications traditionally dominated by 6xxx series alloys, with added benefits for recycling and sustainability in the automotive industry.

Effect of cluster chemistry on the strengthening of Al alloys

When a supersaturated Al solid solution is held at room temperature, natural ageing occurs through the formation of clusters/zones providing a strengthening of the material. Recently, it was shown that this process can be greatly amplified by applying a cyclic plastic strain to a supersatured Al alloy. The cyclic strengthening also leads to a microstructure dominated by clusters/zones but the strengths are much higher than those obtained by natural aging, and can even exceed the peak aged strength.This work uses a combination of atom probe tomography and small angle x-ray scattering to provide a quantitative comparison of the cluster states in commercial Al alloys AA2024 and AA7075, after both natural aging and cyclic strengthening. We show that the chemistry of the clusters formed by these processes is different, and the clusters formed by cyclic strengthening tend to have chemistries closer to the composition of the relevant local equilibrium precipitate.The average force required to shear an individual cluster/zone is back-calculated from the strengthening observed experimentally and a strong correlation is found between the shearing force and the solute content of the cluster (i.e. 1-XAl, where XAl is the Al content of the cluster). This correlation applies not only for AA2024 and AA7075, but also AA6111 and AA7050 data taken from the literature, as well as some precipitates formed at elevated temperatures. This work shows that the first order effect controlling cluster strengthening is simply the cluster chemistry and its proportional effect on the force required to shear a cluster.

Tailoring precipitate distribution in 2024 aluminum alloy for improving strength and corrosion resistance

For the traditional peak-aged (PA) AA2024 alloy, the formation of large S-phase precipitates within the grains, wide precipitate-free zones (PFZs) near the grain boundaries (GBs), and continuous distribution of grain boundary precipitates (GBPs) can be observed. As a result, the PA alloy exhibits relatively high strength but poor corrosion resistance. However, with the application of cyclic plasticity treatment, high-density 1–2 nm clusters form within the matrix, and no PFZs form near GBs. In this study, this treatment yields the optimal balance between strength–elongation characteristics and corrosion resistance. By combining cyclic plasticity and ageing heat treatment with different heating rates, the nanoscale clusters play a crucial role as heterogeneous nucleation sites, resulting in the formation of finer and higher number density of S precipitates within the matrix. Additionally, the presence of these clusters reduces the formation of GBPs and minimizes the width of PFZs. Consequently, compared to the traditional PA sample, this approach achieves a significantly higher yield strength (increased by 46 %) and ultimate tensile strength (increased by 18 %), along with superior corrosion resistance. Although the influence of ageing heat treatment with different rates on mechanical properties is not significant, it notably affects the formation of GBPs and corrosion resistance. Specifically, a slower heating rate leads to an increase in the spacing between adjacent GBPs, resulting in improved corrosion resistance. In summary, cyclic strengthening, as a novel method for alloy strengthening, when combined with ageing heat treatment, modulates the distribution of S precipitates within the matrix and GBs. This optimization maximizes the effects of precipitation strengthening and breaks the inverse relationship between strength and corrosion resistance.

Towards high strength and ductility of Al–Cu-Mg alloy via cyclic and monotonic pre-straining followed by ageing

This article presents an investigation of the ageing response of modern high-strength aluminium alloy 2519 (Al–5.64Cu-0.33Mn-0.23Mg-0.01Si (wt%)) after cyclic strengthening (CS). The impact of the pre-straining value on the mechanical properties, microstructure and phase composition was elucidated via hardness and tensile testing, analytical transmission and orientation imaging electron microscopy. The results show that cyclic hardening has a significant impact on the ageing behaviour of the alloy. Applying cyclic pre-straining in conjunction with monotonic enhances strength and ductility at once. This novel high-performance treatment results in superior strength-ductility combination. The improved mechanical properties are attributed to the observed changes in microstructure. The introduction of vacancies and dislocations, by repetitive and elastic straining before artificial ageing, leads to great homogeneous structure and an increase in the number density and volume fraction of the main strengthening phases (θ′ and Ω) and clusters/zones. Therefore, this work should suggest a strategy for developing a novel treatment of aluminium alloys with excellent mechanical properties.

Enhancing the strength and sensitization resistance of 5xxx alloys via nanoscale clustering induced by room-temperature cyclic plasticity

Non-heat-treatable 5xxx Al alloys exhibit good corrosion resistance but can be susceptible to intergranular corrosion in service. This study applies a cyclic strengthening method to the AA5083 alloy, resulting in a significant increase in yield strength (∼300 MPa) and tensile strength (∼385 MPa) due to the formation of Mg-Al solute clusters confirmed by scanning transmission electron microscopy and atom probe tomography. After a two-month sensitization treatment at 70 °C, the clusters delay the formation of Mg-rich precipitates at grain boundaries, enhancing the resistance to sensitization. This new approach strengthens Al-Mg alloys and offers potential for manipulating the corrosion resistance and sensitization susceptibility.

Experimental investigation on creep strengthening phenomenon of a Ni-based single crystal superalloy under cyclic loading and unloading conditions

The complex failure behavior of turbine blades under cyclic creep is of great significance to the safety of aeroengines. In this study, the deformation regulation and fracture mechanism of a single-crystal Ni-based superalloy DD6 under cyclic loading at 980 ℃ are investigated, and the strengthened rupture life and fracture deformation are clearly exhibited. Scanning electron microscopy and electron backscatter diffraction observations were conducted to reveal the deformation and fracture mechanisms. A smaller particle spacing of the γ/γ’ interface was observed under cyclic loading/unloading, which restricted dislocation movement and reduced the creep rate. This led to an extended creep life of steady creep stage, and the delay of the accelerated creep stage. Additionally, the creep deformation distribution and slip system activation affected the fracture toughness under the accelerated creep stage, leading to an increase in the creep fracture strain under cyclic loading/unloading. Finally, the fracture mechanism affected by cyclic loading is discussed based on inner and surface cracks. Cyclic loading/unloading promotes the multisource initiation of surface cracks, which improves loading capacity. Furthermore, cyclic loading/unloading promotes the generation of secondary cracks deflected from crack tips. The high residual compressive stress at the crack tip delays creep-crack initiation during unloading, leading to a decrease in the creep-crack propagation rate. These factors led to cyclic strengthening at different scales.

Enhanced mechanical properties in an Al-Mg-Cu alloy processed by the combination of cyclic deformation and aging heat treatment

Aging is a traditional strengthening method used for heat-treatable Al alloys. Cyclic strengthening has recently received widespread attention as a novel method for alloy strengthening. This study applied cyclic deformation to introduce dislocations and promote cluster formation in an Al-Mg-Cu alloy. Cyclically-strengthened (CS) samples have higher strength and ductility than peak-aged samples. The aging behaviors of the CS samples and the effect of pre-aging (artificial aging prior to cyclic deformation) on cyclic strengthening were also studied. Recovery and precipitation promotion were found during the aging of the CS samples regardless of pre-aging for 20 min. The pre-aged CS samples show higher strength than the CS samples without pre-aging. Interestingly, after subsequent aging, the mechanical properties of the CS samples with and without 20 min of pre-aging became the same. In summary, aging and cyclic deformation methods were combined to maximize the effects of precipitation strengthening and work-hardening. The strength limit obtained by individual aging or cyclic deformation methods was successfully overcome. This study provides new insights into alloy strengthening. • Cyclically-strengthened sample had better strength and ductility than the peak-aged sample. • Cyclic deformation promotes precipitation and leads to cluster formation. • Pre-aged samples can be cyclically strengthened due to work-hardening effects. • Enhanced mechanical properties were obtained by combining cyclic deformation and aging.

Behavior of saline ice under cyclic flexural loading

Abstract. New systematic experiments reveal that the flexural strength of saline S2 columnar-grained ice loaded normal to the columns can be increased upon cyclic loading by about a factor of 1.5. The experiments were conducted using reversed cyclic loading over ranges of frequencies from 0.1 to 0.6 Hz and at a temperature of −10 ∘C on saline ice of two salinities: 3.0 ± 0.9 and 5.9 ± 0.6 ‰. Acoustic emission hit rate during cycling increases with an increase in stress amplitude of cycling. Flexural strength of saline ice of 3.0 ± 0.9 ‰ salinity appears to increase linearly with increasing stress amplitude, similar to the behavior of laboratory-grown freshwater ice (Murdza et al., 2020b) and to the behavior of lake ice (Murdza et al., 2021). The flexural strength of saline ice of 5.9 ± 0.6 ‰ depends on the vertical location of the sample within the thickness of an ice puck; i.e., the strength of the upper layers, which have a lower brine content, was found to be as high as 3 times that of lower layers. The fatigue life of saline ice is erratic. Cyclic strengthening is attributed to the development of an internal back stress that opposes the applied stress and possibly originates from dislocation pileups.

Mechanical Properties Test of Butt Welds of Corroded Q690 High Strength Steel under the Coupling of Damp-heat Cycle Dipping

In order to study the mechanical property degradation rule of butt welds of Q690 high strength steel in the marine environment, high strength steel specimens under different corrosion cycles were obtained through indoor artificial acceleration tests, and uniaxial tensile and cyclic loading tests were performed to analyze the different degrees of corrosion affects the Elastic Modulus, yield strength, ductility, and hysteretic energy dissipation performance of the specimen. Based on the secondary plastic flow model and the Ramberg-Osgood model, a constitutive model for butt welds of corroded Q690 high strength steel was established. The results show that with the increase of the corrosion cycle, reddish-brown layered rust products are formed on the surface of the steel, and the metal luster is basically lost. The size of the pits continues to increase, and the maximum depth reaches 440 μm at 100 days of corrosion. Under the action of unidirectional tensile load, the yield strength and elastic modulus of the test specimens decreased by 8.28% and 7.8% after 100 days of corrosion. Based on the quadratic plastic flow model, the stress-strain curve was fitted. The curve shape parameters k1 and k2 were reduced by 26.5% and 19.3%, respectively. Under reciprocating load, the hysteresis energy of the test piece after 100d corrosion was reduced by 10.2%, the cyclic strengthening coefficient K’ and the cyclic strengthening index n’ were reduced by 13.4% and 37.3%, respectively. The research provided basic data and theoretical basis for the mechanical property degradation and reliability evaluation of high strength steel in the low-lying areas of the ocean.

Cyclic strengthening of lake ice

AbstractFurther to systematic experiments on the flexural strength of laboratory-grown, fresh water ice loaded cyclically, this paper describes results from new experiments of the same kind on lake ice harvested in Svalbard. The experiments were conducted at −12 °C, 0.1 Hz frequency and outer-fiber stress in the range from ~ 0.1 to ~ 0.7 MPa. The results suggest that the flexural strength increases linearly with stress amplitude, similar to the behavior of laboratory-grown ice.

Study on the cyclic bending behaviour of CFRP strengthened full-scale CHS members

Circular hollow steel (CHS) members have been regarded as structural elements of choice for use in civil infrastructure. Despite the risk of seismic damage of steel structures during an earthquake due to its cyclic characteristics, studies on the cyclic strengthening of CHSs have been minimal. In the present study, a new finite element (FE) modelling approach is developed and applied to study the cyclic performance of carbon fibre reinforced polymer (CFRP) strengthened CHS specimens. The modelling techniques are first validated using results from the authors’ previous experimental work. A detailed parametric study is then carried out to evaluate the effects of CFRP bond length, CFRP layer numbers, ratio of thickness of CFRP to thickness of CHS, CHS member diameter-to-thickness ratio and steel grade on the cyclic performance of the CFRP strengthened CHS members. Results confirm the significant effects of these parameters on the cyclic performance of CFRP strengthened CHS members. The efficiency of CFRP strengthening increases with an increase in the ratio of the thickness of CFRP to the thickness of CHS and also with an increase in the diameter-to-thickness ratio of CHS specimens. Conversely, the efficiency of CFRP strengthening reduces with an increase in the steel grade. Moreover, the ultimate moment capacities obtained from the FE analyses of the bare and CFRP strengthened CHS specimens agree reasonably well with the theoretically predicted values.

Multiaxial Fatigue Life Prediction of GH4169 Alloy Based on the Critical Plane Method

The multiaxial fatigue life of GH4169 alloy was predicted based on the critical plane method. In this paper, a new critical plane-damage multiaxial fatigue parameter is proposed, in which the maximum shear strain is considered to be the main damage control parameter, and the correction parameter, including the normal stress and strain of the maximum shear strain plane, is defined as the second control parameter. The axis of principle strain rotates under non-proportional loading. Meanwhile, the mechanism of the variation of material microstructure and slip systems leads to an additional hardening phenomenon. The ratio of cyclic yield stress to static yield stress is used to represent cyclic strengthening capacity, and the influence of the phase difference and loading condition on the non-proportional reinforcement effect is considered. It is also proposed that different materials have different influences on the additional hardening phenomenon. Meanwhile, the model revision results in stress under asymmetrical loading. Experimental data of GH4169 alloy show that the proposed model can provide better prediction than the Smith–Watson–Topper (SWT) and Fatemi–Socie (FS) models.

Low-cycle fatigue behaviour and microstructural evolution of pearlitic and bainitic steels

Microstructural analysis of new bainitic steel subject to heat treatment was performed and directly compared with those achieved from the microstructure of R-260 pearlitic rail steel. Mechanical properties, fracture behaviour, and low-cycle fatigue (LCF) tests of the bainitic steel were also evaluated and collated with those achieved from pearlitic rail steel. The LCF tests were performed at room temperature at varying strain amplitudes under total strain control. The basic characteristics of the relationship between the low - cycle fatigue parameters of steels were evaluated based on the Coffin – Manson formula. The obtained test results demonstrate that the two microstructures exhibit different cyclic stress responses, i.e. the cyclic strengthening of pearlitic steel and a cyclic softening in bainitic steel. The variations in the amount of retained austenite in the bainitic structure, before and after LCF, were evaluated by X-ray diffraction and electron transmission microscopy (TEM). The results indicate that during deformation, a partial transformation of the residual austenite into the deformed martensite α' has taken place. Moreover, the quasi-cleavage character of the fracture surface was observed for bainitic steel with a higher deformation degree during the LCF test.

A Pronounced Hardening Response in Non-Heat-Treatable Al-Mg Based 5xxx Aluminum Alloys

The Al-Mg base 5xxx Al alloys are non-heat-treatable. They derive their strength from solid solution and strain hardening. They are, however, supersaturated and this can cause sensitization issues. This work shows that by applying a recently developed cyclic strengthening approach, a high density of Mg-Al clusters can be formed in these materials resulting yield and tensile strengths, and elongations, superior to the precipitation hardened AA6061 (Al-Mg-Si) in the peak aged (T6 state). The implications for categorizations of Al alloys are discussed and future directions of interest are highlighted.

Modeling of Reinforced Concrete Frames with Infill Walls Under Cyclic Loading Strengthening with CFRP

Nowadays, using infill isolators in Reinforced Concrete (RC) structures is very common. In earlier research this infill isolators are usually considered as non-structural elements, but in recent researches it was demonstrated that infill has considerable effects on natural period, stiffness, strength and overall behavior of structure specially when it is subjected to seismic ground motions. In this study a one-bay two-story masonry infilled RC frame is considered for a laboratory sample and various layouts of Carbon Fiber Reinforced Polymer (CFRP) layers have been used to retrofit and strengthen of masonry infill wall. These specimens have been modeled in ABAQUS and analyzed under cyclic loading with Finite Element Method (FEM). FEM analysis have been carried out using ABAQUS/Explicit software and analysis type have been chosen dynamic explicit. Finally, results of Finite Element Analysis have been compared against the experimental results. The effect of different CFRP configurations of retrofitting on ductility, ultimate lateral load capacity, stiffness and energy dissipation capacity of specimens have been discussed. Thereupon the best configuration has been proposed in terms of cost effectiveness and ductility. The results showed that in the frame with infill wall comparing with bare frame, load capacity increased about 277.35% and by comparing infilled frame with or without CFRP, it can be concluded that using CFRP can increase lateral load capacity of masonry infilled RC frames.

On the origin of extraordinary cyclic strengthening of the austenitic stainless steel Sanicro 25 during fatigue at 700°C.

The origin of the extraordinary strengthening of the highly-alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700°C was investigated by use of advanced scanning transmission electron microscopy (STEM). Along with substantial change of dislocation structure, nucleation of two distinct populations of nanoparticles was revealed. Fully coherent Cu-rich nanoparticles were observed homogeneously dispersed with high density along with nanometer-sized incoherent NbC carbides precipitating on dislocations during cyclic loading. Probe-corrected HAADF STEM imaging was used to characterize the atomic structure of nanoparticles. Compositional analysis was conducted using both EELS and high spatial resolution EDS. High temperature exposure induced precipitation of a high density of coherent Cu-rich nanoparticles while strain-induced nucleation of incoherent NbC nanoparticles leads to retardation of dislocation movement. The pinning effects and associated obstacles to dislocation motion prevent recovery and formation of the localized low-energy cellular structures. As a consequence, the alloy exhibits remarkable cyclic hardening at elevated temperature.

Transition and fracture shift behavior in LCF test of dissimilar welded joint at elevated temperature

This work focused on the low-cycle fatigue (LCF) behavior of modified 9Cr/CrMoV dissimilar welded joint at elevated temperature. Narrow gap submerged arc welding (NG-SAW) process via multi-pass and multi-layer techniques was employed to fabricate the welded joint. LCF tests at different strain amplitude range from 0.22% to 0.75% were performed at strain ratio R=−1. The two-slope behavior based on fracture location shift was presented both on the cyclic stress-strain (CSS) curve and Manson-Coffin (M-C) curve, which could be applied to predict the fatigue life more precisely especially at relatively low strain amplitude. The results indicated that the joint failed in CrMoV-base metal (BM) at relatively low strain amplitude below 0.4% while failure shifted to CrMoV-over tempered zone (OTZ) at higher strain amplitude above 0.4%. Fatigue failure occurred in CrMoV-BM at low strain amplitude could be attributed to temperature softening effect in CrMoV-BM combined with cyclic strengthening in CrMoV-OTZ. While CrMoV-OTZ with a comparable number of grain boundaries and much lower hardness than that of CrMoV-BM was deemed to be the weakest zone across the welded joint at higher strain amplitude. EBSD investigations also revealed that CrMoV-BM experienced more fatigue damage at relatively low strain amplitude, while CrMoV-OTZ accumulated more plastic strain at higher strain amplitude.

Applied and engineering versions of the theory of elastoplastic processes of active complex loading part 2: Identification and verification

The deviator constitutive relation of the proposed theory of plasticity has a three-term form (the stress, stress rate, and strain rate vectors formed from the deviators are collinear) and, in the specialized (applied) version, in addition to the simple loading function, contains four dimensionless constants of the material determined from experiments along a two-link strain trajectory with an orthogonal break. The proposed simple mechanism is used to calculate the constants of themodel for four metallic materials that significantly differ in the composition and in the mechanical properties; the obtained constants do not deviate much from their average values (over the four materials). The latter are taken as universal constants in the engineering version of the model, which thus requires only one basic experiment, i. e., a simple loading test. If the material exhibits the strengthening property in cyclic circular deformation, then the model contains an additional constant determined from the experiment along a strain trajectory of this type. (In the engineering version of the model, the cyclic strengthening effect is not taken into account, which imposes a certain upper bound on the difference between the length of the strain trajectory arc and the module of the strain vector.)