Thermal-Mechanical Fatigue Behavior and Life Analysis of Cast Ni-base Superalloy K417

In-phase (IP) and out-of-phase (OP) thermal-mechanical fatigue (TMF) behavior of cast Ni-base superalloy K417 was studied. All experiments were carried out under total strain control with temperature cycling between 400-850℃. Both in-phase and out-of-phase TMF specimens exhibited cyclic hardening followed by cyclic softening at the minimum temperature. Besides, they cyclically hardened in the early stage of life followed by cyclic softening at the maximum temperature. OP TMF life was longer than that of IP TMF. Various damage mechanisms operating in different controlled strain ranges and phasing were discussed. A few life prediction methods for isothermal fatigue were used to handle TMF fatigue and their applicability to superalloy K417 was evaluated. The SEM analysis of the fracture surface showed that transgranular fracture was the principal cracking mode for both IP and OP TMF. Oxidation was the main damage mechanism in causing shorter fatigue life for IP TMF compared with OP TMF.

Fatigue behaviour of coke drum materials under thermal-mechanical cyclic loading

Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes damage in cracking. There were some studies on coke drums in the form of bulging and the fatigue life estimation for the coke drums, but most of them were based on strain-fatigue life curves at constant temperatures, which do not consider simultaneous cyclic temperature and mechanical loading conditions. In this study, a fatigue testing system is successfully devel- oped to allow performing thermal-mechanical fatigue （TMF） test similar to the coke drum loading condition. Two commonly used base and one clad materials of coke drums are then experimentally investigated. In addition, a comparative study between isothermal and TMF lives of these materials is conducted. The experimental findings lead to better understanding of the damage mechanisms occurring in coke drums and more accurate prediction of fatigue life of coke drum materials.

EFFECTS OF STRAIN RATIO ON THERMAL-MECHANICAL CYCLIC STRESS-STRAIN RESPONSE AND FATIGUE LIFE IN DS SUPERALLOY DZ125

The effects of strain ratio on thermal-mechanical cyclic stress-strain response and fatigue life in DS superalloy DZ125 have been studied by performing tests at various strain ratio experiments, under strain-controlled and temperature cycling from 550 to 1000℃. It is shown that thermal-mechanical cyclic stress-strain response behavior not only depend on magnitude of strain, and temperature-loading phase angle, but also strain ratio. Fatigue life at strain ratio Rε=-0.3 is longer than that of strain ratio Rε=-1.0, under in-phase thermal-mechanical loading. However, Fatigue life at strain ratio Rε=-0.3 is shorter than that of strain ratio Rε=-1.0, under out-of-phase thermal-mechanical loading. The thermal-mechanical fatigue (TMF) damage model was discussed. Results of fractography show that fatigue, creep and oxidation damage always occur during TMF. The main damage mode depends on loading wave, strain ratio and magnitude of strain.

Fatigue behavior of AZ31B magnesium alloy electron beam welded joint based on infrared thermography

Fatigue behavior of AZ31B magnesium alloy electron beam welded joint undergoing cyclic loading was investigated by infrared thermography. Temperature evolution throughout a fatigue process was presented and the mechanism of heat generationwas discussed. Fatigue limit of the welded joint was predicted and the fatigue damage was also assessed based ontheevolution of the temperatureand hotspot zone on the specimen surfaceduring fatigue tests. The presented results show that infrared thermography can not onlyquicklypredict the fatigue behavior of the welded joint, but also qualitatively identify the evolution of fatigue damage in real time. It is found that the predicted fatigue limit agrees well with the conventionalS-Nexperimental results. The evolution of the temperatureand hotspot zone on the specimen surface can be an effectivefatigue damage indicatorfor effectiveevaluationof magnesium alloy electron beam welded joint.

Strength and fatigue fracture behavior of Al-Zn-Mg-Cu-Zr(-Sn) alloys

The strength and fatigue fracture behavior of A1-Zn-Mg-Cu-Zr（-Sn） alloys were studied by performing tensile tests and fatigue crack propagation （FCP） tests. The microstructures of the experimental alloys were further analyzed using optical microscopy （OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）; phase analysis of these alloys was conducted with an X-ray diffraction （XRD）. The results show that when Sn is included, growth of the recrystallization grains in the solution-treated A1-Zn-Mg-Cu-Zr alloy is obstructed, the precipitation-free zone （PFZ） of the overaged A1-Zn-Mg-Cu-Zr-Sn alloy becomes narrow, and the grain boundary precipitates are smaller. Consequently, the FCP resistance is higher. In addition, the overaged Sn-containing alloy has considerably higher tensile strength than the alloy without Sn.

Effect of stress level on fatigue behavior of 2D C/C composites

Laminated carbon fiber clothes were infiltrated to prepare carbon fiber reinforced pyrolytic carbon （C/C） using isothermal chemical vapor infiltration （CVI）. The bending fatigue behavior of the infiltrated C/C composites was tested under two different stress levels. The residual strength and modulus of all fatigued samples were tested to investigate the effect of maximum stress level on fatigue behavior of C/C composites. The microstructure and damage mechanism were also investigated. The results showed that the residual strength and modulus of fatigued samples were improved. High stress level is more effective to increase the modulus. And for the increase of flexural strength, high stress level is more effective only in low cycles. The fatigue loading weakens the bonding between the matrix and fiber, and then affects the damage propagation pathway, and increases the energy consumption. So the properties of C/C composites are improved.

Thermal shock fatigue behavior of TiC/Al_2O_3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al2O3 composites were studied. The effect of TiC and Al2O3 starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference （ΔT） and number of thermal cycles （Ⅳ） on fatigue crack growth （Δa）. The mechanical properties and thermal fatigue resistance of TiC/Al203 composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al2O3 composites is due to a ＂true＂ cycling effect （thermal fatigue）. Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.

Effects of Exercise on Behavior and Peripheral Blood Lymphocyte Apoptosis in a Rat Model of Chronic Fatigue Syndrome

This study examined the effects of exercise on behavior and peripheral blood leukocyte apoptosis in a rat model of chronic fatigue syndrome （CFS）. Thirty-six healthy male Sprague-Dawley rats were equally randomized into 3 groups： the control group, CFS model group and the exercise group in terms of body weight. A total of 25 rats entered the final statistical analysis due to 11 deaths during the study. CFS model was established by subjecting the rats in CFS model group and exercise group to electric shock, chronic restraint stress and cold water swim. Besides, rats in the exercise group took rtmning wheel exercise. After a week of conditioning feeding, model construction and running wheel exercise were performed simultaneously, and lasted for 23 consecutive days. The behavior experiments, including running wheel exercise, open-field test, tail suspension test and Morris water maze test, were conducted, either before or after the model establishment. Rats were sacrificed and peripheral blood was obtained for the assessment of lymphocyte apoptosis index by flow cytometry （FCM）. It was found that as compared with those in the control group, the weight of the rats was decreased obviously （P〈0.01）, the mobility time in the open-field and the tail suspension tests was shortened significantly （P〈0.01）, the time to locate the platform was enhanced （P〈0.01） and the cell apoptosis index was increased substantially （P〈0.01） in the CSF model group. Meanwhile, in comparison to the model group, the behavior in the open-field and the tail suspension tests was improved significantly （P〈0.05）, and the apoptosis index decreased remarkably （P〈0.01） in the exercise group. It is concluded that sport intervention can prevent lymphocyte apoptosis and improve animal behavior rather than the memory.

Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

SEM in-situ investigation on fatigue cracking behavior of P/M Rene95 alloy with surface inclusions

The low-cycle fatigue behavior of powder metallurgy Rene95 alloy containing surface inclusions was investigated by in-situ observation with scanning electron microscopy （SEM）. The process of fatigue crack initiation and early stage of propagation behavior indicates that fatigue crack mainly occurs at the interface between the inclusion and the matrix. The effect of inclusion on the fatigue crack initiation and the early stage of crack growth was very obvious. The fatigue crack growth path in the matrix is similar to the shape of inclusion made on the basis of fatigue fracture image analysis. The empiric relation between the surface and inside crack growth length, near a surface inclusion, can be expressed. Therefore, the fatigue crack growth rate or life of P/M Rene95 alloy including the inclusions can be evaluated on the basis of the measurable surface crack length parameter. In addition, the effect of two inclusions on the fatigue crack initiation behavior was investigated by the in-situ observation with SEM.

Crack growth behavior of SRR99 single crystal superalloy under thermal fatigue

The thermal fatigue behavior of a single crystal superalloy SRR99 was investigated. Specimens with V-type notch were tested at the peak temperatures of 900, 1000, and 1100℃. The crack growth curves as a function of the number of cycles were plotted. With the increase of peak temperature, the crack initiation life was shortened dramatically. Through optical microscopy （OM） and scanning electron microscopy （SEM） observation, it was found that multiple small cracks nucleated at the notch tip region but only one or two of them continued to develop in the following thermal cycles. The primary cracks generally propagated along a preferential direction. Microstructure changes after thermal fatigue were also discussed on the basis of SEM observation.

Effect of electromagnetic bulging on fatigue behavior of 5052 aluminum alloy

The effect of electromagnetic bulging on the fatigue behavior of the5052aluminum alloy was investigated throughtensile-tensile fatigue testing.The intriguing finding is that the bulged specimens exhibited enhanced fatigue strength as depicted bymaximum stress vs the number of cycles until failure(S-N)curves,by comparison with these original aluminum alloys.Althoughthe fatigue process of the original and budged alloys follows the same mechanism with three distinct steps,namely,crack initiation ata corner of the tested samples,stable crack propagation with typical fatigue striations and finally catastrophic fracture with dimplefractographic features.The typical crack propagation rate vs stress intensity factor range(da/dN-ΔK)curves derived from thespacing of striations reveal a lower crack propagation rate in the bulged specimens.The enhancement of fatigue strength inelectromagnetically bulged aluminum alloy is further rationalized in-depth on the basis of strain hardening and dislocation shieldingeffect.

Microstructure and Contact Fatigue Behavior of Nano-SiO_2/Ni Coating Prepared by Electro-Brush Plating

The composite coating nano SiO 2/Ni was prepared by co depositing nano SiO 2 particles with pure nickel through electro brush plating. By taking into account the effect of microstructure, heat treatment and load on the contact fatigue life, the anti contact fatigue behavior of the composite coating was examined and compared with that of nickel coating. As a result, the contact fatigue life decreased with the increasing of load. The contact fatigue lives of nano SiO 2/Ni coating were 16.5% and 45.2% higher than those of nickel coating respectively under the loads of 60 N and 140 N, and 326.3% higher than its counterpart of nickel coating after annealed under the load of 140 N. From the SEM image of fatigue fracture, it has been observed that the fatigue fracture of the composite coating initiated in the sub surface as well as at the track surface due to the huddling of units, and propagated along the interface between grain units.

Effect of Rare Earths on Tensile Behavior of Hot Roller Steel 60CrMnMo and Estimation of Roll′s Fatigue Life

The effect of rare earth(RE) on tensile behavior of hot roller steel 60CrMnMo was investigated at the temperature when roll served The roll′s fatigue at 500 ℃ was estimated The results show that elongation δ of hot roller steel 60CrMnMo can be increased by adding RE It is possible to estimate the fatigue life with tensile behavior of hot roller steel 60CrMnMo

Improved Fatigue Behavior of Pipeline Steel Welded Joint by Surface Mechanical Attrition Treatment(SMAT)

A pipeline steel X80 with welded joint was subjected to surface mechanical attrition treatment （SMAT）. After SMAT, a nanostructure surface layer with an average grain size of about 10 nm was formed in the treated sample, and the fatigue limit of the welded joint was elevated by about 13% relative to the untreated joints. In the low and the high amplitude stress regimes, both fatigue strength and fatigue life were enhanced. Formation of the nanostructured surface layer played more important role in the enhanced fatigue behavior than that of residual stress induced by the SMAT.

Thermal fatigue behavior of niobium microalloyed H13 steel

AISI H13 hot work tool steel is widely used for hot forging, hot-extrusion and die-casting because of its high temperature strength, impact toughness, heat checking resistance and wear resistance, etc. The thermally induced surface damage, i. e., thermal fatigne,is believed to be controlled by the magnitude of the imposed cyclic strain. The thermal fatigue on the surface of hot working die, which is responsible to the initiation of the cracks, is reported to result in more than 80 % of the failure of dies.

Low cycle fatigue behavior of zirconium−titanium−steel composite plate

The low cycle fatigue behavior of zirconium−titanium−steel composite plate under symmetrical and asymmetric stress control was studied.The effects of mean stress and stress amplitude on cyclic deformation,ratcheting effect and damage mechanism were discussed in detail.The results show that under symmetric stress control,the forward ratcheting deformation is observed.Under asymmetric stress control,the ratcheting strain increases rapidly with mean stress and stress amplitude increasing.Under high stress amplitude,the influence of mean stress is more significant.In addition,by studying the variation of strain energy density,it is found that the stress amplitude mainly promotes the fatigue damage,while the mean stress leads to the ratcheting damage.In addition,fractographic observation shows that the crack initiates in the brittle metal compound at the interface,and the steel has higher resistance to crack propagation.Finally,the accuracy of life prediction model considering ratcheting effect is discussed in detail,and a high-precision life prediction model directly based on mean stress and stress amplitude is proposed.

Fatigue Resistance and Failure Behavior of Penetration and Non-Penetration Laser Welded Lap Joints

Penetration and non-penetration lap laser welding is the joining method for assembling side facade panels of railway passenger cars,while their fatigue performances and the difference between them are not completely understood.In this study,the fatigue resistance and failure behavior of penetration 1.5+0.8-P and non-penetration 0.8+1.5-N laser welded lap joints prepared with 0.8 mm and 1.5 mm cold-rolled 301L plates were investigated.The weld beads showed a solidification microstructure of primary ferrite with good thermal cracking resistance,and their hardness was lower than that of the plates.The 1.5+0.8-P joint exhibited better fatigue resistance to low stress amplitudes,whereas the 0.8+1.5-N joint showed greater resistance to high stress amplitudes.The failure modes of 0.8+1.5-N and 1.5+0.8-P joints were 1.5 mm and 0.8 mm lower lap plate fracture,respectively,and the primary cracks were initiated at welding fusion lines on the lap surface.There were long plastic ribs on the penetration plate fracture,but not on the non-penetration plate fracture.The fatigue resistance stresses in the crack initiation area of the penetration and non-penetration plates calculated based on the mean fatigue limits are 408 MPa and 326 MPa,respectively,which can be used as reference stress for the fatigue design of the laser welded structures.The main reason for the difference in fatigue performance between the two laser welded joints was that the asymmetrical heating in the non-penetration plate thickness resulted in higher residual stress near the welding fusion line.

Full－range nonlinear analysis of fatigue behaviors of reinforced concrete structures by finite element method

The offshore reinforced concrete structures are always subject to cyclic load, such as wave load.In this paper a new finite element analysis model is developed to analyze the stress and strain state of reinforced concrete structures including offshore concrete structures, subject to any number of the cyclic load. On the basis of the anal ysis of the experimental data,this model simplifies the number of cycles-total cyclic strain curve of concrete as three straight line segments,and it is assumed that the stress-strain curves of different cycles in each segment are the same, thus the elastoplastic analysis is only needed for the first cycle of each segment, and the stress or strain corresponding to any number of cycles can be obtained by superposition of stress or strain obtained by the above e lastoplastic analysis based on the cyclic numbers in each segment.This model spends less computer time,and can obtain the stress and strain states of the structures after any number of cycles.The endochronic-damage and ideal offshore concrete platform subject to cyclic loading are experimented and analyzed by the finite element method based on the model proposed in this paper. The results between the experiment and the finite element analysis are in good agreement,which demonstrates the validity and accuracy of the proposed model.

GROWTH BEHAVIOR OF SHORT FATIGUE CRACKS FOR ALUMINUM- LITHIUM ALLOY 8090

The growth behaviors of short through cracks (0.2 < △a < 2.2mm) and long cracks are compared using CT type specimens in aluminum-lithium alloy 8090 T651. It is found that the short cracks grow much more than long ones and are observed to grow at the stress intensity ranges far below the long crack threshold. The distinction of growth bahavior between short and long cracks is attributed to the difference of their crack closure effect. The growth behavior of short cracks can be rationalized with that of long ones in terms of effective stress intensity ranges. The upper demarcation value of short through cracks for aluminum-lithium alloy 8090 is presented.