Low-cycle fatigue behavior of solutionized and aged WE43 magnesium alloys at room temperature

The low-cycle fatigue behavior of solutionized(T4)and aged(T6)WE43 magnesium alloys was studied at room temperature.The total strain amplitudes(△ε_(t)/2)were 0.4%,0.5%,0.6%,0.7%and 1.0%.Detailed microstructure evolution was characterized by scanning electron microscope(SEM),electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM).The results showed that plastic strain amplitude decreased with the increasing cycle number in T4 alloy,which is due to the dense persistent slip bands(PSBs)and dynamic precipitates hinderingdislocation slip.In contrast,the plastic strain amplitude increases gradually in T6 alloy,which is attributed to the enhanced activation of pyramidal slip.The low-cycle fatigue life of T6 alloy with larger fatigue ductility coefficient is longer than that of T4 alloy.The Coffin-Manson model can accurately predict the fatigue life of T4 and T6 alloys compared to Jahed-Varvani(JV)energy model.For T4 alloy,the fatigue damage mechanism was dominated by basal slip.For T6 alloy,the enhanced pyramidal slip plays an important role to accommodate plastic deformation.

Performances of fissured red sandstone after thermal treatment with constant-amplitude and low-cycle impacts

In the engineering practices,it is increasingly common to encounter fractured rocks perturbed by temperatures and frequent dynamic loads.In this paper,the dynamic behaviors and fracture characteristics of red sandstone considering temperatures(25℃,200℃,400℃,600℃,and 800℃)and fissure angles(0°,30°,60°,and 90°)were evaluated under constant-amplitude and low-cycle(CALC)impacts actuated by a modified split Hopkinson pressure bar(SHPB)system.Subsequently,fracture morphology and second-order statistics within the grey-level co-occurrence matrix(GLCM)were examined using scanning electron microscopy(SEM).Meanwhile,the deep analysis and discussion of the mechanical response were conducted through the synchronous thermal analyzer(STA)test,numerical simulations,one-dimensional stress wave theory,and material structure.The multiple regression models between response variables and interactive effects of independent variables were established using the response surface method(RSM).The results demonstrate the fatigue strength and life diminish as temperatures rise and increase with increasing fissure angles,while the strain rate exhibits an inverse behavior.Furthermore,the peak stress intensification and strain rate softening observed during CALC impact exhibit greater prominence at increased fissure angles.The failure is dominated by tensile damage with concise evolution paths and intergranular cracks as well as the compressor-crushed zone which may affect the failure mode after 400℃.The second-order statistics of GLCM in SEM images exhibit a considerable dependence on the temperatures.Also,thermal damage dominated by thermal properties controls the material structure and wave impedance and eventually affects the incident wave intensity.The tensile wave reflected from the fissure surface is the inherent mechanism responsible for the angle effect exhibited by the fatigue strength and life.Ultimately,the peak stress intensification and strain rate softening during impact are determined by both the material structure and compaction governed by thermal damage and tensile wave.

Microstructural characteristics and low-cycle fatigue properties of AZ91 and AZ91-Ca-Y alloys extruded at different temperatures

The commercial AZ91 alloy and nonflammable SEN9(AZ91-0.3Ca-0.2Y,wt%)alloy are extruded at 300°C and 400°C.Their microstructure,tensile and compressive properties,and low-cycle fatigue(LCF)properties are investigated,with particular focus on the influence of the extrusion temperature.In the AZ91 and SEN9 materials extruded at 300°C(300-materials),numerous fine Mg_(17)Al_(12)particles are inhomogeneously distributed owing to localized dynamic precipitation during extrusion,unlike those extruded at 400°C(400-materials).These fine particles suppress the coarsening of recrystallized grains,decreasing the average grain size of 300-materials.Although the four extruded materials have considerably different microstructures,the difference in their tensile yield strengths is insignificant because strong grain-boundary hardening and precipitation hardening effects in 300-materials are offset almost completely by a strong texture hardening effect in 400-materials.However,owing to their finer grains and weaker texture,300-materials have higher compressive yield strengths than400-materials.During the LCF tests,{10-12}twinning is activated at lower stresses in 400-materials than in 300-materials.Because the fatigue damage accumulated per cycle is smaller in 400-materials,they have longer fatigue lives than those of 300-materials.A fatigue life prediction model for the investigated materials is established on the basis of the relationship between the total strain energy density(ΔW_(t))and the number of cycles to fatigue failure(N_(f)),and it is expressed through a simple equation(ΔW_(t)=10·N_(f)-0.59).This model enables fatigue life prediction of both the investigated alloys regardless of the extrusion temperature and strain amplitude.

Experimental and numerical study regarding H-steel all-bolted connection steel frame with composite wall boards

H-steel all-bolted connection steel frame structures with heat preservation and decoration composite wall boards were investigated and the seismic performances of three scaled specimens were studied.The failure modes,hysteresis curves,bearing capacity,ductility,energy dissipation capacity,stiffness degradation and strain distribution were discussed.The calculation method of structural theoretical internal force was presented.The results showed that the overall structural seismic performance was better,and the structural ductility met the demands of elastic-plastic inter-story drift angle for seismic design.The H-steel weak-axis connection structure obtained better energy dissipation capacity,and its bearing capacity and stiffness were slightly different from the strong-axis connection.The heat preservation and decoration performance of composite wallboard and the all-bolted connection of the steel frame realized prefabrication during the whole construction period.The plastic hinge of the steel beam can be moved outwards because of the L-angles,which effectively avoids stress concentration in joint areas and expands the plastic hinge range.The errors between the theoretical structural capacity calculated by the plastic analysis method and the test results were within 2.44%.In addition,structural failure mechanisms and bearing capacities were verified by the finite element(FE)analysis,and the effects of the main parameters on the structures were investigated.The FE verification results were the same as in the test.The research results provide theoretical support and technical guidance for the application of thermal insulation and decorative composite wall panels in H-shaped steel all-bolted steel frames.

Prediction of low-cycle crack initiation life of powder superalloy FGH96 with inclusions based on damage mechanics

The effects of inclusions in powder superalloy FGH96 on low-cycle fatigue life were studied, and a low-cycle crack initiation life prediction model based on the theory of damage mechanics was proposed. The damage characterization parameter was proposed after the construction of damage evolution equations. Fatigue tests of the powder superalloy specimens with and without inclusion were conducted at 530 and 600 ℃, and the model verification was carried out for specimens with elliptical, semi-elliptical, polygon and strip-shaped surface/subsurface inclusion. The stress analysis was performed by finite element simulation and the predicted life was calculated. The results showed a satisfying agreement between predicted and experimental life.

Low-cycle fatigue behavior of permanent mold cast and die-cast Al-Si-Cu-Mg alloys

Fatigue failure is one of the main failure forms of Al-Si-Cu-Mg aluminum alloys. To feature their mechanical aspect of fatigue behavior, the low-cycle fatigue behavior of permanent mold cast and die-cast AI-Si- Cu-Mg alloys at room temperature was investigated. The experimental results show that both permanent mold cast and die-cast AI-Si-Cu-Mg alloys mainly exhibit cyclic strain hardening. At the same total strain amplitude, the diecast AI-Si-Cu-Mg alloy shows higher cyclic deformation resistance and longer fatigue life than does the permanent mold cast AI-Si-Cu-Mg alloy. The relationship between both elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior, and can be described by the Basquin and Coffin-Manson equations, respectively.

Microstructure evolution during heat treatment of Mg-Gd-Y-Zn-Zr alloy and its low-cycle fatigue behavior at 573K

In as-cast Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr(mole fraction,%)alloy,lamellar microstructures that extend from grain boundaries to the interior ofα-Mg grains are identified as clusters ofγ′using a scanning transmission electron microscope equipped with a high-angle annular dark-field detector.Under a total strain-controlled low-cyclic loading at573K,the mechanical response and failure mechanism of Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr alloy(T6peak-aging heat treatment)were investigated.Results show that the alloy exhibits cyclic softening response at diverse total strain amplitudes and573K.The experimental observations using scanning electron microscopy show that the micro-cracks initiate preferentially at the interface between long-period stacking order structures andα-Mg matrix and extend along the basal plane ofα-Mg.The massive long-period stacking order structures distributed at grain boundaries impede the transgranular propagation of cracks.

CYCLIC SOFTENING IN HOT-WORKING DIE STEELS DURING LOW-CYCLE FATIGUE

The characteristics and microstructural changes of cyclic softening in hot-working die steels 5CrNiMo and 5Cr2NiMoVSi were studied under strain controlled low-cycle fatigue.The re- sults show that the cyclic softening is featured in both steels hardened in different conditions under the strain controlled amplitude range of Δε_t/2=0.6-1.8×10^(-2).The softening effect mainly occurs in some initial cycles and the stress amplitude varies slightly in the sequential cycles,i.e.the softening effect is minified.No obvious stress saturation phenomenon was ob- served during the whole cyclic deformation.The TEM analysis shows that the cyclic softening is related to heterogenity of plastic deformation.The softening of the tested steels is caused by the formation of the dislocation cell structure with low density and low internal stress,and by the fragmentation and redissolution of fine carbides into matrix.

Mechanical and low-cycle fatigue behavior of stainless reinforcing steel for earthquake engineering applications

Use of stainless reinforcing steel （SRS） in reinforced concrete （RC） structures is a promising solution to corrosion issues. However, for SRS to be used in seismic applications, several mechanical properties need to be investigated. These include specified and actual yield strengths, tensile strengths, uniform elongations and low-cycle fatigue behavior. Three types of SRSs （Talley S24100, Talley 316LN and Talley 2205） were tested and the results are reported in this paper. They were compared with the properties of A706 carbon reinforcing steel （RS）, which is typical for seismic applications, and MMFX II, which is a high strength, corrosion resistant RS. Low-cycle fatigue tests of the RS coupons were conducted under strain control with constant amplitude to obtain strain life models of the steels. Test results show that the SRSs have slightly lower moduli of elasticity, higher uniform elongations before necking, and better low-cycle fatigue performance than A706 and MMFX II. All five types of RSs tested satisfy the requirements of the ACI 318 code on the lower limit of the tensile to yield strength ratio. Except Talley 2205, the other four types of RSs investigated meet the ACI 318 requirement that the actual yield strength does not exceed the specified yield strength by more than 18 ksi （124 MPa）. Among the three types of SRSs tested, Talley S24100 possesses the highest uniform elongation before necking, and the best low-cycle fatigue performance.

A NEW CYCLIC J-INTEGRAL FOR LOW-CYCLE FATIGUE CRACK GROWTH

The constitutive equation under the low-cycle fatigue （LCF） was discussed, and a two-dimensional （2-D） model for simulating fatigue crack extension was put forward in order to propose a new cyclic J-integral. The definition, primary characteristics, physical interpretations and numerical evaluation of the new parameter were investigated in detail. Moreover, the new cyclic J-integral for LCF behaviors was validated by the compact tension （CT） specimens. Results show that the calculated values of the new parameter can correlate well with LCF crack growth rate, during constant-amplitude loading. In addition, the phenomenon of fatigue retardation was explained through the viewpoint of energy based on the concept of the new parameter.

INVESTIGATION OF THE LOW-CYCLE FATIGUE AND FATIGUE CRACK GROWTH BEHAVIORS OF P91 BASE METAL AND WELD JOINTS

Low cycle fatigue tests and crack growth propagations tests on P91 pipe base metal and its weld joints were conducted at three different temperatures: room temperature, 550℃ and 575℃. The strain-life was analyzed, and the changes in fatigue life behavior and fatigue growth rates with increasing temperature were discussed. The different properties of the base metal and its weld joint have been analyzed.

Experimental study on uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium alloy with different plastic deformation mechanisms

The uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium(Mg)alloy is investigated by uniaxial stress-controlled cyclic tests at room temperature and with addressing the roles of different plastic deformation mechanisms.Different stress levels are prescribed to reflect the cyclic plasticity of the alloy controlled by diverse deformation mechanisms(i.e.,dislocation slipping,deformation twinning and detwinning ones),and then the influences of stress level and stress rate on the ratchetting and fatigue life are discussed.The experimental results demonstrate that different evolution characteristics of whole-life ratchetting and fatigue life presented during cyclic tests with various mean stresses,stress amplitudes and stress rates are determined by the dominated plastic deformation mechanisms.It’s worth noting that the ratchetting can occur in the compressive direction even in the cyclic tests with a positive(tensile)mean stress,and the fatigue life increases first and then decreases with the increase of mean stress on account of the interaction between dislocation slipping and twinning/detwinning mechanisms.Comparing the fatigue lives obtained in the asymmetric stress-controlled and symmetrical strain-controlled cycle tests,it is seen that the ratchetting deformation causes an additional damage,and then leads to a shortening of fatigue life.

Low-Cycle Fatigue Crack Initiation Behavior of Nickel-Based Single Crystal Superalloy

Low-cycle fatigue crack initiation behavior of nickel-based single crystal superalloy at 530℃ was investigated.Results show that the behavior of crack initiation is closely related to the maximum strain.When the maximum strain is 2.0%,the fatigue crack is originated at the position of persistent slip bands on the surface of specimen,which is located on the{111}slip plane.No defects are observed at the crack initiation position.When the maximum strain is lower than 1.6%,the cracks are initiated at the casting defects on sub-surface or at interior of the specimen.The casting defects are located on the{100}slip plane vertical to the axial force.The crack is initiated along the{100}slip plane and then expanded along different{111}slip planes after a short stage of expansion.As the maximum strain decreases,the position of crack initiation gradually changes from the surface to the interior.Moreover,the secondary cracks extending inward along the fracture surface appear in the crack initiation area,and there is obvious stress concentration near the secondary cracks.The dislocation density is high near the fracture surface in the crack initiation zone,where a lot of dislocations cutting into the γ'phase exist.An oxide layer of 50‒100 nm is presented on the fracture surface,and Ni,Al,Cr and Co elements are mainly segregated into the oxide layer of the surface.

反复荷载下型钢再生混凝土柱抗震性能试验研究

为研究型钢再生混凝土柱的破坏形态和抗震性能,对10个不同剪跨比、再生粗骨料取代率、轴压比、体积配箍率的型钢再生混凝土柱进行低周反复荷载试验,观察其受力过程及破坏形态,分析不同设计参数对荷载-位移滞回曲线、骨架曲线、承载能力、延性、耗能能力及刚度退化等力学性能的影响。试验研究结果表明:型钢再生混凝土柱的主要破坏形态为剪切斜压破坏、弯剪破坏以及弯曲型破坏,破坏形态与剪跨比有关;滞回曲线大多呈梭形且较为饱满,说明型钢再生混凝土柱具有较好的延性及耗能能力;取代率对试件承载力影响不明显,延性及耗能能力随取代率的增加而有所降低;随着轴压比的增大,试件承载力有一定提高,但其衰减加快,且延性及耗能能力降低;增大体积配箍率时,试件承载力增大不明显,但延性及耗能能力显著提高。总体上看,型钢再生混凝土柱具有较好的抗震性能,可以通过合理的设计将其应用于抗震结构中。

The Behavior of Rectangular and Circular Reinforced Concrete Columns Under Biaxial Multiple Excitation

The aim of this study is to investigate the dynamic performance of rectangular and circular reinforced concrete(RC)columns considering biaxial multiple excitations.For this purpose,an advanced nonlinear finite element model which can simulate various features of cyclic degradation in material and structural components is used.The implemented nonlinear fiber beam-column model accounts for inelastic buckling and low-cycle fatigue degradation of longitudinal reinforcement and can simulate multiple failure modes of RC columns under dynamic loading.Hypothetical rectangular and circular columns are used to investigate the failure modes of RC columns.A detailed ground motion selection is implemented to generate real mainshock and aftershocks.It was found that multiple excitations due to aftershock has the potential of increasing the damage of the RC columns and longitudinal reinforcements are significantly affected low-cycle fatigue.Also,it was found that rectangular column is more sensitive to accumulative damage due to cyclic fatigue.This study increases the accuracy of structural analysis of RC columns and consequently improves understanding the failure modes of RC columns with different cross-sectional shapes.

Numerical Modeling Strategy for the Simulation of Nonlinear Response of Slender Reinforced Concrete Structural Walls

A three-dimensional nonlinear modeling strategy for simulating the seismic response of slender reinforced concrete structural walls with different cross-sectional shapes is presented in this paper.A combination of nonlinear multi-layer shell elements and displacement-based beam-column elements are used to model the unconfined and confined parts of the walls,respectively.A uniaxial material model for reinforcing steel bars that includes buckling and low-cyclic fatigue effects is used to model the longitudinal steel bars within the structural walls.The material model parameters related to the buckling length are defined based on an analytical expression for reinforcing steel bars embedded in reinforced concrete elements,which are developed based on beam-on-springs model,and validated with experimental tests of boundary elements of structural walls available in the literature.Six experimental case studies of reinforced concrete walls with rectangularshape,T-shape,and U-shape cross-section are used to validate the structural wall numerical modeling strategy.

FRACTAL ANALYSIS OF FRACTURE SURFACE OF WELDED JOINT UNDER LOW CYCLE FATIGUE

Hausdorff dimension of fracture surface roughness of welded joint,both welding metal and heat-affected zone.of pressure vessel steel 16MnR,tested under strain-controlled low-cycle fatigue,was examined with computer vision srstem and by two-dimensional variation method. Results show that it decreases with the increase of cyclic hysteresis energy.The Hausdorff di- mension variation at heat-affected zone is greater than that of weld metal.It is believed that the greater the fractal dimension is,the longer the fatigue life will be.

Fatigue Verification of Class 1 Nuclear Power Piping According to ASME BPV Code

In this paper, fatigue verification of Class 1 nuclear power piping according to ASME Boiler ＆ Pressure Vessel Code, Section III, NB-3600, is addressed. Basic design requirements and relevant verification procedures using Design-By-Analysis are first reviewed in detail. Thereafter, a so-called simplified elastic-plastic discontinuity analysis for further verification when the basic requirements found unsatisfactory is examined and discussed. In addition, necessary computational procedures for evaluating alternating stress intensities and cumulative damage factors are studied in detail. The authors＇ emphasis is placed on alternative verification procedures, which do not violate the general design principles upon which the code is built, for further verification if unsatisfactory results are found in the simplified elastic-plastic analysis. An alternative which employs a non-linear finite element computation and a refined numerical approach for re-evaluating the cumulative damage factors is suggested. Using this alternative, unavoidable plastic strains can be correctly taken into account in a computationally affordable way, and the reliability of the verification will not be affected by uncertainties introduced in the simplified elastic-plastic analysis through the penalty factor Ke and other simplifications.

Influence of Long-term Climate on Fatigue Life of Bridge Pier Concrete and a Reinforcement Method

This paper quantitatively evaluated the fatigue life of concrete around the air-water boundary layer of bridge piers located in inland rivers,considering the long-term climate.The paper suggests a method to predict the low-cycle fatigue life by demonstrating a thermal-fluid-structural analysis of bridge pier concrete according to long-term climate such as temperature,velocity and pressure of air and water in the process of freezing and thawing in winter.In addition,it proposes a reinforcing method to increase the life of damaged piers and proves the feasibility of the proposed method with numerical comparison experiment.

Low-cycle fatigue behavior of DZ125 superalloy under prior exposure conditions

Low-cycle fatigue(LCF) behavior of the directionally solidified(DS) nickel-based DZ125 superalloy was studied at elevated temperature(980 ℃).Specimens were,respectively,exposed for 0,2,25,50,and 100 h in air.The fatigue life of pre-exposed specimens is lower than that of unexposed specimens.The result is closely associated with fatigue crack initiation and propagation due to oxygen embrittlement and cycle loading.Detailed fractographic evaluations indicate the fatigue life is closely related to the surface microstructural modification.The resulting changes in microstructure cause the decrease in the effective area and the increase in actual stress.A methodology based on the continuum damage mechanics is developed to describe the correlation between the residual LCF life and pre-exposed time.