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Brief Review and Reconsideration of Fatigue Crack Closure Effect in Materials

D.L. Chen , B. Weiss and R.Stickler(Institute for Physical Chemistry-Materials Science , University of Vienna , Waehringer Strasse 42 , A-1090 Vienna , Austria Zhongguang WANG)(State Key Lab. for Fatigue and Fracture of Materials , 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.

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INFLUENCE OF TEMPERATURE ON FATIGUE CREEP INTERACTION FRACTURE MAP

ZHANG Hongxue XU Zhichao CHEN Guoliang University of Science and Technology Beijing , Beijing , China Professor , Faculty of Superalloy , University of Science and Technology Beijing , Beijing , China

金属学报(英文版)

The relative ratio of fatigue resistance to creep resistance of materials varies with test temper- ature.As the temperature decreases,the creep resistance,since it is a thermal activation pro- cess,becomes relatively larger than fatigue resistance.Therefore the fatigue damage becomes predominant,and results in expansion of fatigue fracture region(region F),and shrinkage even complete elimination of creep fracture region(region C).A materials parameter Ω can be defined to estimate the temperature at which the creep fracture region is completely de- pressed.This phenomenon could be understood on the basis of the integrated model of compet- itive and cumulative models of fatigue creep interaction.

关键词: creep fracture , null , null , null

The Morphology of Rolling Contact Fatigue Fracture of Hardened Steels

WANG Xu , ZHANG Shouhua , CUI Peiyong Beijing University of Science and Technology , Beijing , China. Central Iron and Steel Research Institute , Beijing , China.

材料科学技术（英文）

Rolling Contact Fatigue(RCF) is a cumulative damage phenomenon when metals are subjected to repeated contact stresses. The fomation of pitting on the contact surface is the result of the rolling contact fatigue. The morphologies of rolling contact fatigue fracture of the har- dened steels (86CrHoV7, 42CrMo) show that strong resemblance in fractuye mechanisms exists between rolling contact fatigue and uni-axial fatigue. Since fatigue striations are hardly observed in hardened steels under uni-axial fatigue, it is interesting to note that the state of stress in rolling contact fatigue is more favor- able to ductile fractures than in uni-axial fatigue.

关键词: rolling contact , null , null , null , null , null

MECHANISM MAP OF TORSIONAL FATIGUE FRACTURE

HU Zhizhong WU Yusheng CAI Heping MA Lihua Xi′an jiaotong University , Xi′an , China Senior Engineer , Res.Inst.for Strength of Metals , Xi′an jiaotong University , Xi′an 710049 , China

金属学报(英文版)

Studies have been made of the torsional fatigue fracture life of notched specimens,the macroscopic fractography and microscopic fracture mechanism of steel 40Cr after various tempering treatments under different stresses,With the increase of stress,the fracture model changes from normal stress fracture to longitudinal shear one,and then transversal shear one. Under same stress,with the increase of strength,the fracture mode transfers from shear to normal stress fracture.The mechanism of normal stress fracture may be:transgranular frac- ture→striation+intergranular fracture→dimple+intergranular fracture,and of shear fracture may be:transgranular fracture→shear trace→dimple.Based on the experimental results,a fracture mechanism map of torsional fatigue has been drawn up.

关键词: mechanism map , null , null

Quantitative study of correlation between fracture surface roughness and fatigue properties of SiC/Al composites

Materials Letters

Quantitative measurements were carried out on the fatigue fracture surface of the SiC/Al composite by a sectioning method. It was shown that the cyclic plastic strain amplitude, SiC volume fraction and particle size have effects on fracture surface roughness R(S). The measured fracture surface roughness R(S) is closely related to the fatigue-crack propagation path and may corresponds to the fatigue life. Moreover, it was found that there is an obvious difference in the R(S) values for fatigue fractures which are due to different fracture mechanisms. These results show that it is possible to reflect the fracture mechanism using fracture surface roughness and relate it to the fracture properties of materials.

关键词: composite;SiC/Al;fatigue fracture;roughness;fractal dimension;sectioning method;general-method;fractography

Fracture mechanisms in bulk metallic glassy materials

Physical Review Letters

We find that the failure of bulk metallic glassy (BMG) materials follows three modes, i.e., shear fracture with a fracture plane significantly deviating from 45degrees to the loading direction, normal tensile fracture with a fracture plane perpendicular to the loading direction, or distensile fracture in a break or splitting mode with a fracture plane parallel to the loading direction. The actually occurring type of failure strongly depends on the applied loading mode and the microstructure of the material. Extensive evidence indicates that the Tresca fracture criterion is invalid, and for the first time, three fracture criteria are developed for isotropic materials with high strength, such as advanced BMGs or the newly developed bulk nanostructural materials.

关键词: amorphous-alloys;enhanced plasticity;zr-ti;flow;deformation;composite;strength;microstructure;ribbons;failure

THERMAL FATIGUE AND FRACTURE MECHANICS ANALYSIS OF GREY CAST IRON

GUO Chengbi ZHOU Weisheng Dalian Institute of Technology , Dalian , Liaoning , China Professor , Dept.of Shipbuilding , Dalian Institute of Technology , Dalian , Liaoning , China

金属学报(英文版)

The in-phase and out-of-phase thermal fatigue,the C-P type and P-C type isothermal fa- tigue of grey cast iron were experimentally studied.The fatigue life was evaluated analytically by using the elastic-plastic fracture mechanics method(mainly J integral).The results of ex- periments and calculations showed that the lifes of in-phase and C-P type fatigue are longer than that of out-of-phase and P-C type fatigue respectively within the same strain range. This is in contrast to the results of other materials such as low carbon steel.On the other hand, the predicted lifes are consistent with experimental results.This suggests that J integral as a mechanics parameter for characterizing the thermal fatigue strength of grey cast iron and the mechanics model and the calculation method developed here are efficient.A parameter ΔW_1 was proposed from energy aspect to characterize the capacity of crack propagation. The isothermal fatigue life is the same as the thermal fatigue life for identical ΔW_1 values.

关键词: grey cast iron , null , null

Fracture toughness and fatigue crack growth characteristics of nanotwinned copper

Acta Materialia

Recent studies have shown that nano twinned copper (NT Cu) exhibits a combination of high strength and moderate ductility. However, most engineering and structural applications would also require materials to have superior fracture toughness and prolonged subcritical fatigue crack growth life. The current study investigates the effect of twin density on the crack initiation toughness and stable fatigue crack propagation characteristics of NT Cu. Specifically, we examine the effects of tailored density of nanotwins, incorporated into a fixed grain size of ultrafine-grained (UFG) copper with an average grain size of 450 nm, on the onset and progression of subcritical fracture under quasi-static and cyclic loading at room temperature. We show here that processing-induced, initially coherent nanoscale twins in UFG copper lead to a noticeable improvement in damage tolerance under conditions of plane stress. This work strongly suggests that an increase in twin density, at a fixed grain size, is beneficial not only for desirable combinations of strength and ductility but also for enhancing damage tolerance characteristics such as fracture toughness, threshold stress intensity factor range for fatigue fracture and subcritical fatigue crack growth life. Possible mechanistic origins of these trends are discussed, along with issues and challenges in the study of damage tolerance in NT Cu. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

关键词: Nanotwins;Fracture toughness;Fatigue crack growth;Damage tolerance;Nanocrystalline copper;strain-rate sensitivity;nanocrystalline metals;mechanical-behavior;opening displacement;grain-size;nickel;deformation;nanoscale;strength;twins

Fatigue and fracture behavior of bulk metallic glass

Metallurgical and Materials Transactions a-Physical Metallurgy and Materials Science

Tensile, compressive, cyclic tension-tension, and cyclic compression-compression tests at room temperature were systematically applied to a Zr52.5CU17.9Al10Ni14.6Ti5 bulk metallic glass for comprehensive understanding of its damage and fracture mechanisms. Under tensile loading, the metallic glass only displays elastic deformation followed by brittle shear fracture. Under compressive loading, after elastic deformadon, obvious plasticity (0.5 to 0.8 pct) can be observed before the final shear fracture. The fracture strength under compression is slightly higher than that under tension. The shear fracture under compression and tension does not occur along the maximum shear stress plane. This indicates that the fracture behavior of the metallic glass does not follow the Tresca criterion. The fracture surfaces show remarkably different features, i.e., a uniform vein structure (compressive fracture) and round cores coexisting with the radiating veins (tensile fracture). Under cyclic tension-tension loading, fatigue cracks are first initiated along localized shear bands on the specimen surface, then propagated along a plane basically perpendicular to the stress axis. A surface damage layer exists under cyclic compression-compression loading. However, the final failure also exhibits a pure shear fracture feature as under uniaxial compression. The cyclic compression-compression fatigue life of the metallic glass is about a factor of 10 higher than the cyclic tension-tension fatigue life at the same stress ratio. Based on these results, the damage and fracture mechanisms of the metallic glass induced by uniaxial and cyclic loading are elucidated.