Fatigue oxidation interaction in a superalloy—application to life prediction in high temperature low cycle fatigue

A study of the interaction between fatigue and oxidation has been carried out in the case of a cast cobalt base superalloy MARM 509 tested in laboratory air at 900 °C. The influence of fatigue cycling on oxidation of this alloy has been studied by quantitative metallography on polished specimens exposed to air in a furnace and on strain-cycled low-cycle fatigue specimens. The oxidation kinetics were determined by thickness measurements for matrix oxidation and by oxidized depth measurements for the preferential oxidation of MC carbides. In both cases the oxidation kinetics were found to be dramatically enhanced by cycling for the matrix oxidation according to a linear relationship with plastic strain amplitude and less dramatically for carbides according to an exponential relationship with the maximum cyclic stress. From these observations a damage equation which describes fatigue damage as a crack growth process has been proposed: the elementary crack advance is a summation of a mechanical contribution due to the fatigue process itself which is described by Tomkins’ equation and of an oxidation contribution which has been evaluated from metallographic measurements. Integration of this crack growth equation gives predicted fatigue lives which are in good agreement with experimental results within a factor of two.     

Creep-fatigue life prediction in terms of nucleation and growth of fatigue crack and creep cavities

In low cycle fatigue at elevated temperature, the interaction between fatigue crack and creep damages is known to be responsible for the significant reduction of the fatigue life. In this investigation, a model for the life prediction for low cycle fatigue with hold time at tensile peak strain is suggested for the temperature range of 0.5T _m. This model is formulated on the basis of the assumptions that the creep cavities are formed due to the vacancies generated during fatigue, and are grown during the hold period. The fatigue crack nucleated at the surface due to fatigue loading is affected by the creep damages for its propagation. The model is checked by experimental results with various hold time periods. The predicted creep-fatigue lives are in good agreement with experimentally observed ones for 304 stainless steel and 13CrMo44 steel. Formerly Graduat3e student, Department of Materials Science and Engeneering ,KAIST, Seoul, Korea.     

The strength,fracture toughness,and low cycle fatigue behavior of 17-4 PH stainless steel

The influence of microstructure on the strength, fracture toughness and low cycle fatigue behavior of 17-4 PH stainless steel has been examined. Aging hardening involves initial formation of coherent copper-rich clusters which transform to incoherent fee ∈-copper precipitates upon further aging. The changes in strength level and strain hardening rates observed during aging are consistent with previously suggested models for precipitation hardening based on differing elastic moduli. The fracture toughness and fatigue crack growth rates were shown to be a function of microstructure and environment. At equivalent strength levels overaging resulted in a higher fracture toughness than did underaging. The fatigue crack growth rates increased with increasing strength level and humidity but were not a function of toughness level. Attempts to correlate the fatigue crack growth rates with monotonie tensile properties were unsuccessful. However when final failure obeyed a critical strain criteria, the fracture toughness behavior could be reasonably described and related to preferential void nucleation and growth at δ-ferrite-matrix interfaces.     

Low cycle fatigue,fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables.     

Low cycle fatigue, fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables. AXENA, formerly Graduate Student, Dept. of Materials Science and Metallurgical Engineering, University of Cincinnati is This paper is based on a thesis submitted by Ashok Saxena in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Cincinnati.     

High cycle fatigue life improvement of polycrystalline alpha-iron modified by silver,chromium, aluminium,and yttrium ion implantation

HCF tests on polycrystalline alpha-iron with and without silver, chromium, yttrium, and aluminum ion implantation have been conducted. The stress-life curves obtained were best fit by using the power function. It was shown that the fatigue property had been improved to different extent after ion implantation. Enhanced strengthening of the implanted layer due to radiation induced defects, super solid solution, and formation of intermetallic compounds in implantation was the prominent mechanism for property improvement.     

Tensile,creep, and low-cycle fatigue behavior of a cast γ-TiAl-based alloy for gas turbine applications

The influence of chemical composition on the microstructure of the γ-titanium aluminide alloy Ti-48Al-2W-0.5Si (at. pct) and the accompanying tensile, low-cycle fatigue, and creep properties has been evaluated. The study showed that small variations in chemical composition and casting procedures resulted in considerable variations in the microstructure, yielding vastly different mechanical properties. Low contents of aluminum and tungsten led to a coarse-grained lamellar (γ/α ₂) microstructure with high creep resistance. A composition close to the nominal one produced a duplex (γ+γ/α ₂) structure with favorable strength, ductility, and low-cycle fatigue properties. By controlling the solidification and cooling rates at casting, a pseudoduplex (PS-DP) microstructure with a unique combination of high strength and high fatigue and creep resistance can be obtained. These unique properties can be explained by the diffuse boundaries between the relatively small γ grains and the neighboring lamellar colonies, combined with semicoherent interfaces between the γ and α ₂ phases. At tensile and low-cycle fatigue loading, these boundaries act like high-angle boundaries, producing a virtually fine-grained material promoting strength, whereas at creep loading, grain-boundary sliding is hindered in the semicoherent interfaces leading to high creep resistance.     

连轧钢管限动芯棒温度场及热疲劳有限元分析

针对340MPM机组（Multi-Stand Pipe Mill限动芯棒连轧管机组）芯棒服役过程建立三维有限元模型,研究芯棒在服役过程中温度场变化规律.同时,通过对热应力的研究,分析了芯棒热疲劳裂纹萌生机理及裂纹在芯棒内部的扩展规律.对比实测数据与模拟结果,认为所建立的有限元模型能够反映芯棒温度变化趋势.芯棒首次脱管后表面最高温度为630℃,此后经历三次反复的水冷降温和空冷返温过程,冷却结束后表面最高温度为98℃.脱管后,芯棒表面轴向和环向压缩热应力均达到900 MPa,第三次水冷结束时刻,轴向拉伸热应力达到186 MPa,环向拉伸热应力达到221 MPa.芯棒的拉压交变热应力使其表面出现热疲劳裂纹并逐渐扩展,环向裂纹扩展至距表面17.5mm深、轴向裂纹扩展至距表面20mm深时会显著受阻,热应力对轴向裂纹的促进作用强于环向裂纹.     

Effects of prior cold working on low cycle fatigue behavior of stainless steels, titanium alloy timetal 834 and superalloy IN 718: A review

This paper gives a brief review of the efforts made to study the effects of cold rolling on low cycle fatigue (LCF) behavior of stainless steels, titanium alloy Timetal 834 and Ni-Fe based Super alloy IN 718 at different temperatures and different strain amplitudes. Low Cycle Fatigue tests on cold worked 316L stainless steel at various temperatures from room temperature to 923K have been reported. In all tests the effect of 20% prior cold work(PCW) on LCF behavior of type 316L (N) stainless steel had been studied at 873K under total axial strain controlled mode in air at strain amplitudes from ±0.25% to ±1.0%. Fatigue life in the solution annealed condition was similar to that of the PCW material at higher strain amplitudes (≥0.5%) while at lower strain amplitudes the PCW material exhibited longer life. The influence of PCW on LCF behavior of type 304 and AISI 304LN stainless steel was studied at various temperatures and it was observed that the fatigue life was strongly dependent on prior cold work, temperature, and strain amplitude employed. Cold rolling of the titanium alloy Timetal 834 and age hardenable Ni-Fe-based superalloy IN 718 has been found to cause marked enhancement in LCF life at low strain amplitude and eliminate the bilinearity from the Coffin-Manson(C-M) relationship. Work carried on this aspect, however is very limited particularly in the case of Titanium alloys and Ni-Fe based Superalloy IN 718     

The life cycle of Gorgoderina rochalimai Pereira and Cuocolo, 1940, a Digenea parasite of Bufo marinus in Guadeloupe

The sporocysts of G. rochalimai were found in Guadeloupe, in the bivalve Mollusc Sphaeriidae Eupera viridans. The cystocercous cercariae are ingested by Odonatan larvae in which the encystement occurs. The Toad Bufo marinus is the definitive host of the parasite. The life-cycle of G. rochalimai shows close similarities with the ones already known in many other speicies of Gorgoderidae.     

Components of social support and quality of life in severely mentally ill, low income individuals and a general population group

Satisfaction with social support and quality of life (QOL) were assessed for 60 psychiatric patients, 79 welfare recipients and 266 people of the general population using the Social Provisions Scale (SPS) and the Satisfaction with Life Domains Scale (SLDS). Psychiatric patients reported less satisfaction than the general population on all components of social support. They have a comparable level of satisfaction with welfare recipients on most of the components, with the exception of emotional integration and opportunity for nurturance. However, patients satisfaction with QOL was quite similar to the general population, although higher than welfare recipients on some dimensions. Their lower satisfaction with the personal-intimate dimension suggest deficiencies in social support. All components of social support were consistently related to QOL in the general population. Attachment and reassurance of worth was significant for psychiatric patients but only attachment was related to QOL for the welfare recipients group. The pairing of SPS and SLDS scales seem to be good instruments for discriminating the majority of psychiatric patients.     

Biological effects of a relatively low concentration of 1-beta-D-arabinofuranosylcytosine in K562 cells: alterations of the cell cycle, erythroid-differentiation, and apoptosis

A general procedure is presented for the determination of several N-methyl-D-aspartate (NMDA) receptor open-channel and subtype-selective blockers, which have been evaluated and developed as neuroprotective drugs for the treatment of brain stroke and trauma. The method involves deproteination of plasma with ethanol, or homogenization of brain samples in ethanol, dilution of the supernatant with ammonium acetate and direct injection into an HPLC column-switching system. Although the investigated NMDA receptor blockers are all tertiary amines, they have quite different structures. However, they are all concentrated on the first column (Purospher RP-18, 125 x 4 mm), whereas polar interfering compounds are washed out with 1% ammonium acetate-acetic acid-acetonitrile (100:1:5, v/v/v). Due to the special selectivity of the Purospher RP-18 material, the analytes and the internal standard are then selectively eluted with 25% acetonitrile (without any buffer in the mobile phase) and transferred to the analytical column (Superspher 60 RP-select B, 250 x 4 mm), where they are separated by gradient elution and detected by UV or fluorescence detection. The low degree of interference allowed the development of sensitive methods with quantification limits of 5 ng/ml for animal plasma (0.4 ml used), 0.5 ng/ml for human plasma (1 ml used) and 50 ng/g for brain tissue (200 mg used).     

Fatigue deformation of silver single crystals: STM evidence for crack nucleation,measurements of slip irreversibility and verification of a new scaling relationship for fatigue life

Silver single crystals oriented in the single glide orientation were fatigued in air and vacuum. The surface morphology was observed using a scanning tunneling microscope. A gold film deposited on the silver surface was used as a reference marker so that the same region can be observed upon repeated fatigue cycling. Pit-shaped features were observed to form within a narrow range of cycles and to eventually develop into micro-sized cracks, lending support to the notion that cracks are formed by a nucleation process. In addition, by measuring the slip band height and spacing as a function of cycling, we were able to determine the slip irreversibility and the Coffin-Manson exponents in both environments. With these two sets of data, we were able to verify a new scaling relationship for fatigue life based on our earlier theoretical work.     

A multicenter, uncontrolled clinical investigation of the contraceptive efficacy, cycle control, and safety of a new low dose oral contraceptive containing 20 micrograms ethinyl estradiol and 100 micrograms levonorgestrel over six treatment cycles

The feasibility of using near infrared reflection spectroscopy for non-invasive blood glucose monitoring is discussed. Spectra were obtained using a diode-array spectrometer with a fiberoptic measuring head with a wavelength ranging from 800 nm to 1350 nm. Calibration was performed using partial least-squares regression and radial basis function networks. The results of different methods used to evaluate the quality of the recorded spectra in order to improve the reliability of the calibration models, are presented.