Key Engineering Materials Vols. 345-346

Abstract: The topic of plasticity-induced closure and its role in shielding a crack tip from the full range of applied stress intensity factor has provoked considerable controversy over several decades. We are now in an era when full field measurement techniques, e.g. thermoelasticity and photoelasticity, offer a means of directly obtaining the stress field around a crack tip and hence the effective stress intensity factor. Nonetheless, without a clear understanding of the manner in which the development of plasticity around a growing crack affects the applied stress field, it will remain difficult to make crack growth rate predictions except through the use of an often highly conservative upper bound growth rate curve where closure is absent, or through semi-empirical approaches. This paper presents new evidence for an interpretation of plasticity-induced crack tip shielding as arising from two separate effects; a compatibility-induced interfacial shear stress at the elastic-plastic interface along the plastic wake of the crack, and a crack surface contact stress which will vary considerably as a function of stress state, load and material properties.

Abstract: Research on the fatigue resistance of mechanical components/structures has been proceeding for nearly a century and a half. Yet, there is no universally agreed upon theory that can predict most aspects of fatigue failure. The reason is the complexity of phenomenon and its dependence on the microstructure. Here, we present a strain energy based damage parameter which has an underlying microscopic basis. A master life curve is subsequently defined which correlates very well with experimental data.

Abstract: Cracking behavior in low cycle fatigue regime depends on the level and the multiaxiality of the applied stress and also on the microstructure. Such a complex cracking behavior affects failure life significantly. More realistic assessments of failure life and integrity require a new appropriate procedure to analyze the crack growth process in multiaxial fatigue. A model of the fatigue process has been proposed to describe the cracking behavior in biaxial stress state. There is, however, no adequate model to present features of material microstructure. In this work, simulations of crack initiation and propagation based on a previous model were carried out in microstructure modeled by using Voronoi-polygon. In a crack initiation analysis, slip-band crack was modeled for the slip system given randomly in each grain composing the modeled microstructure. In modeling crack growth, a competition model between the coalescence growth and the propagation as a single crack was applied. Simulated cracking morphology and failure life were compared with experimental results observed in biaxial fatigue using circumferentially notched specimens of a pure copper, and the applicability of the proposed model was discussed.

Abstract: Recent interest in bulk-metallic glasses (BMGs) has led to the development of amorphous alloys designed for structural applications in various fields as aircraft frames, rotating equipment, automobiles, and medical implants. Although the mechanical behavior of BMGs is being studied extensively, little attention has been paid to their fatigue behavior. Moreover, early fatigue characteristics have exhibited contradictory results. In the current research, uniaxial tension-tension fatigue experiments were performed on notched Zr52.5Cu17.9Al10Ni14.6Ti5 button-head fatigue specimens with various surface finishes. The fatigue studies were designed to better understand the influence of the average surface roughness and/or critical surface defects on the fatigue behavior of glassy alloys. It was hypothesized that geometric surface flaws would lower the observed life of a BMG sample by shortening the crack initiation phase and providing local stress concentrators. The current studies of surface conditions indicate that fatigue-endurance limits are greatly impacted by the average surface roughness with possible reductions of greater than fifty percent.

Abstract: Rotary bending fatigue tests were conducted in order to investigate the fatigue behaviour and fracture mechanisms of extruded Mg2Si-reinforced magnesium (Mg) alloys. Mg2Si particles were mixed with Mg alloy (AZ31) powder to form billets and then extruded at two different working temperatures of 685K and 646K, where the materials extruded at high and low temperatures were denoted as Mg2Si-H and Mg2Si-L, respectively. The grains of the matrix were finer in Mg2Si-L than in Mg2Si-H. The proof stress and tensile strength were independent of working temperature and increased compared with a conventional extruded AZ31 alloy, with a decreased in ductility. The fatigue strength of Mg2Si-H was almost the same as that of the extruded AZ31 alloy, but the fatigue limit was lower, while the fatigue strength of Mg2Si-L was higher than that of Mg2Si-H and the extruded AZ31 alloy. Both crack initiation and crack growth resistances in Mg2Si-L were improved, which could be attributed to high bonding strength of Mg2Si/Mg interfaces and fine grain refinement of the matrix.

Abstract: Successive observation of transgranular small fatigue crack growth behavior of alpha-brass was performed by means of an atomic force microscope (AFM) equipped with small in-plane bending fatigue testing machine. The fatigue crack deflection behavior, which was observed frequently in the low growth rate region, was investigated by the crystallographic orientation analysis based on the Electron Back Scatter Diffraction (EBSD) technique. The slip factor considering the slip system and singular stress field at the crack tip was introduced in order to evaluate the easiness of slip deformation instead of Schmid factor. The direction of crack deflection was found to be explained well by the slip factor and the geometric relative location between the preferential slip plane and crack front.

Abstract: As a retardation of fatigue crack propagation occur under two step loading such as high to low loading, it is difficult to predictt the fatigue life in variable loading conditions. Therefore, the mechanism of retardation was investigated by changing the ratio of two step-loading in some materials in this paper. It is found in this study that when the ratio of low loading to high loading is less than 50 %, crack arrest occur in agreement with Elber’s formula about crack closure. We believe that crack closure phenomenon is the main problem in considering mechanism of crack propagation in the second stage. In this paper it is estimated that crack closure phenomenon arises because tensile plastic zone at crack tip is compressed by surrounding elastic zone. On the basis of the crack closure phenomenon, the mechanism of crack propagation in the second stage is proposed.

Abstract: The effects of prestrain and strength level on the coaxing behavior were studied in austenitic stainless steels and high strength steels, respectively. The materials used were austenitic stainless steels, SUS304 and SUS316, and high strength steels, SCM435, SNCM439 and SUJ2. Stress incremental fatigue tests were performed using cantilever-type rotating bending fatigue testing machines. It was found that the steels except for SUJ2 showed a marked coaxing effect. Non-propagating cracks were not detected in all the steels examined. Based on hardness test, X-ray diffraction measurement and EBSD analysis, it was indicated that the coaxing effect occurred due to work hardening and strain-induced martensite transformation in austenitic stainless steels and to strain-aging in high strength steels.

Abstract: Low cycle fatigue behavior of Sn-3.8Ag-0.7Cu solder was investigated under fully reversed cyclic loading, with particular emphasis on microstructural effects. The LCF behavior of the solder with equiaxed microstructure was found to differ greatly from that of the solder with a dendrite microstructure. At a given total strain amplitude, the dendrite microstructure exhibited a much longer fatigue life than the equiaxed microstructure. Such a strong microstructural effect on fatigue life arose from the difference in cyclic deformation and fracture mechanisms between the two microstructures. A large number of microcracks along grain boundaries of the equiaxed structure solder developed with increasing cycling, while for the dendrite structure solder, cyclic deformation took place along the direction of the maximal shear stress during fatigue tests and microcracks initiated and propagated along shear deformation bands. Besides, the fatigue behavior of the dendritic microstructure was very sensitive to cyclic frequency whereas the fatigue behavior of the equiaxed microstructure showed less sensitivity to cyclic frequency.

Abstract: Studies on the strength and fatigue life of machines and structures have been conducted in accordance with the development of modern industries. In particular, fine and repetitive cyclic damage occurring in contact regions has been known to have an impact on fretting fatigue fractures. INCONEL alloy 600, 690 and INCOLOY alloy 800 are iron-nickel-chromium alloy having excellent resistance to many corrosive aqueous media and high-temperature atmospheres. These alloy are used extensively in the nuclear power plants industry, the chemical industry, the heat-treating industry and the electronic industry. In this paper, the effect of fretting damage on fatigue behavior for INCONEL alloy 600, 690 and INCOLOY alloy 800 were studied. Also, various kinds of mechanical tests such as tension and plain fatigue tests are performed. The objective of this study is to guarantee reliability of the facility applied in a power plant by comparing three materials in respect to fretting fatigue.