XIA Yuebo WANG Zhongguang State Key Laboratory for Fatigue and Failure of Materials
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Institute of Metal Research
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Academia Sinica
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Shenyang
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China
金属学报(英文版)
SEM photos for very coarse grained and high ductile polycrystalline pure Al show following characteristics:A few fracture sources are concentrated,the crack propagates in fan-shape near them,and the fatigue striations are cyclic cleavage facets.Each striation consists of a wide cleavage facet and a narrow cleavage stage.Those stages often blunt due to plastic deformation.The fatigue striations are clear and continual,and distribute over whole zone of stage Ⅱ crack propagation.Two kinds of fatigue striations form in different levels,are conjoined by a“twisted baked piece of pastry”band.The secondary crack along the fatigue striations was observed frequently.Some of them have already developed into secondary macrocracks and secondary fatigue striations were found clearly on secondary fracture sur- face.The fracture surface can be divided into two parts,initiation and propagation zone of the crack,however,no statical-fracture zone was observed.The mechanism to form striations was preliminarily discussed.
关键词:
fatigue crack
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JIA Jun CHEN Xianchao Harbin Institute of Technology
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Harbin
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China [Originally published in ACTA METALL SIN(CHINESE EDN)25(3)1989 pp A232—A234
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Associate Professor
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Scientific Research Office
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Harbin Institute of Technology
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Harbin
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China
金属学报(英文版)
The results of the contacting fatigue tests for Cu-22%Pb plain bearing alloy show that the fa- tigue failure behavior of the alloy is related to the distribution of Pb particles.Both the addi- tions of mischmetal and sulphur can refine the Pb particles.However,mischmetal improves and sulphur deteriorates the fatigue property of the alloy.
关键词:
Cu-Pb alloy
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null
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Journal of Materials Science
Thermomechanical fatigue (TMF) is a major life limiting factor for gas turbine blades. In this study, the failure behavior of NiCrAlY overlay coated nickel-based superalloy M963 was investigated under out of phase (OP) TMF. All tests were carried out under mechanical strain control with a cyclic period of 200 s. Results revealed that the fatigue life of high velocity oxygen fuel spraying (HVOF) coated specimen was longer than that of air plasma spraying (APS) coated one, but shorter than that of bare superalloy M963 at a given strain range. It was found that cracking process in the APS coating was different from that in the HVOF coating, which was shown by the sketches to understand the crack initiation and propagation behavior.
关键词:
thermal barrier coatings;low-cycle-fatigue;oxidation behavior;isothermal fatigue;crack-propagation;bond coat;hvof;systems;damage;life
材料科学技术（英文）
The state-of-the-art of fatigue crack closure effect, involving the mechanisms, measurements, models, evaluation methods, influencing factors and existing problems is briefly reviewed. Attention is concentrated on a new consideration of closure effect and a re-definition of the effective driving force for the propagation of a fatigue crack. A theoretical basis for measuring the closure effect is established. The proposed closure concept takes into account the role of the tower portion of loading cycle below the opening load. The conventional closure concept is proven to be correct only for some extreme cases which are practically non-existent for engineering materials. A miniature strain gauge technique is developed to measure the crack closure. Stress-ratio-change tests at threshold level clearly indicate the shortcomings of the conventional closure concept, and the validity of the newly-proposed closure concept. The application of the proposed concept can give rise to a new method for the determination of the effective fatigue threshold of materials. If the fatigue threshold is measured at a sufficiently negative stress ratio (Rfatigue threshold of the material.
关键词:
finite-width;stress-ratio;threshold;growth;load;plate
D.L. Chen
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B. Weiss and R.Stickler(Institute for Physical Chemistry-Materials Science
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University of Vienna
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Waehringer Strasse 42
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A-1090 Vienna
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Austria Zhongguang WANG)(State Key Lab. for Fatigue and Fracture of Materials
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Institute of Metal Research
材料科学技术（英文）
The state-of the-art of fatigue crack closure effect, involving the mechanisms, measurements,models, evaluation methods, innuencing factors and existing problems is briefly reviewed. Attention is concentrated on a new consideration of closure effect and a re-definition of the effective d riving force for the propagation of a fatigue crack. A theoretical basis fOr measuring the closure effect is established. The proposed closure concept takes into account the role of the lower portion of loading cycle below the opening load. The conventionaI closure concept is proven to be correct only for some extreme cases which are practically non-existent fOr engineering materials. A miniature strain gauge tech nique is developed to measure the crack closure. Stressratio-change tests at threshold level clearly indicate the shortcomings of the conventional closure concept, and the validity of the newly-proposed closure concept. The application of the proposed concept can give rise to a new method for the determination of the effective fatigue threshold of materials. If the fatigue threshold is measured at a sufficiently negative stress ratio (Rfatigue threshold of the material.
关键词:
Applied Physics Letters
In this work, fatigue failure mechanisms of single-walled carbon nanotube (SWCNT) bundles embedded in epoxy matrix under repeated tensile load were studied. Observed damage and failure modes include: (1) splitting of SWCNT bundles, (2) kink formation and subsequent failure in SWCNTs, and (3) fracture of SWCNT bundles. Patterns of crack propagation under tension in SWCNTs were studied by molecular mechanics simulations, where defect-free SWCNTs and SWCNTs with two different modes of Stone-Wales defects were studied. It is demonstrated by the results of molecular mechanics simulation that the observed fracture surfaces of SWCNT can be reproduced reasonably well, suggesting possible fatigue failure mechanisms of SWCNT in the composite. (C) 2004 American Institute of Physics.
关键词:
interfacial characteristics;polymer matrix;load-transfer;tensile;load;composites;deformation;fracture;yarns
Philosophical Magazine
The low-cycle fatigue (LCF) cracking behavior in various face-centered-cubic (fcc) crystalline materials, including Cu single crystals, bicrystals and polycrystals, Cu-Al and Cu-Zn alloys, ultrafine-grained (UFG) Al-Cu and Cu-Zn alloys, was systematically investigated and reviewed. In Cu single crystals, fatigue cracking always nucleates along slip bands and deformation bands. The large-angle grain boundary (GB) becomes the preferential site in bicrystals and polycrystals. In addition, fatigue cracking can also nucleate along slip bands and twin boundaries (TBs) in polycrystalline materials. However, shear bands and coarse deformation bands are observed to the preferential sites for fatigue cracking in UFG materials with a large number of GBs. Based on numerous observations on fatigue-cracking behavior, the fatigue-cracking mechanisms along slip bands, GBs, TBs, shear bands and deformation bands were systematically compared and classified into two types, i.e. shear crack and impingement crack. Finally, these fatigue-cracking behaviors are discussed in depth for a better understanding of their physical nature and the transition from intergranular to transgranular cracking in various fcc crystalline materials. These comprehensive results for fatigue damage mechanisms should significantly aid in obtaining the optimum design to further strengthen and toughen metallic materials in practice.
关键词:
fcc crystals;fatigue crack;dislocation;grain boundary;twin boundary;slip band;shear band;deformation band;copper single-crystals;persistent slip bands;perpendicular;grain-boundary;severe plastic-deformation;constant strain amplitude;stage-i propagation;twin boundaries;tensile properties;shear bands;behavior
Jing HAN
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Hongtao CHEN
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Mingyu LI
金属学报(英文版)
A small Pb-free solder joint exhibits an extremely strong anisotropy due to the body-centered tetragonal (BCT) lattice structure of β-Sn. Grain orientations can significantly influence the failure mode of Pb-free solder joints under thermomechanical fatigue (TMF) due to the coefficient of thermal expansion (CTE) mismatch of β-Sn grains. The research work in this paper focused on the microstructure and damage evolution of Sn3.0Ag0.5Cu BGA packages as well as individual Sn3.5Ag solder joints without constraints introduced by the package structure under TMF tests. The microstructure and damage evolution in cross-sections of solder joints under thermomechanical shock tests were characterized using optical microscopy with cross-polarized light and scanning electron microscopy (SEM), and orientations of Sn grains were determined by orientation imaging microscopy (OIM). During TMF, obvious recrystallization regions were observed with different thermomechanical responses depending on Sn grain orientations. It indicates that substantial stresses can build up at grain boundaries, leading to significant grain boundary sliding. The results show that recrystallized grains prefer to nucleate along pre-existing high-angle grain boundaries and fatigue cracks tend to propagate intergranularly in recrystallized regions, leading to an accelerated damage after recrystallization.
关键词:
Pb-free solder
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R. Bidulsky
材料科学技术（英文）
The aim of the work was to find out how the modification of surface treatment and microstructures affect the fatigue characteristics of the considered sintered materials. Two different systems were prepared: as-sintered and shot peened prealloyed sintered (Astaloy CrL based) steels with addition of 0.5% and 0.7% C. Sintering was carried out in laboratory tube furnace in an atmosphere of pure gases 75%N2+25%H2. The sintering temperature was 1180°C and sintering time was 60 min. Heating and cooling rates were 10°C/min. Fatigue tests were carried out in symmetric plane bending at stress ratio R=−1 with frequency of about 24 Hz. The presented experimental results showed that prealloyed water-atomised steels, with surface modification, exhibit positive effects on the fatigue failure resistance, and for that reason are suitable for high-erformance
applications.
关键词:
Sintered steels
