Precipitates formed by aging in deformed Al-Mg2Si alloys were classified using high resolution transmission electron microscopy (HRTEM) in order to understand the effect of excess silicon contents and deformation on precipitation. Two types of precipitates appeared on the dislocation in the deformed Al-Mg2Si alloy including 0.4mass% silicon in excess (excess Si alloy) at early stage during aging at 473K. First one is the string-like precipitate that consisted of a lot of small precipitates linked as a string-beads along the dislocation line. Second one is the elongated type precipitate that is the end-on of the needle- or rod-shaped precipitate having the elongated cross section. There were a lot of the TYPE-C precipitates on the dislocation after a peak aging. There were the random type precipitates and β" phase in the matrix which were independent on the existence of the dislocation. At the early stage during aging, a lot of the random type precipitates appeared in the matrix. Next, the β" phase appeared and the coarse TYPE-A precipitates finally appeared at the over aged condition. It has been clarified in our recent work that the β" phase, the TYPE-A and the TYPE-C precipitates are typical precipitates in the excess Si alloy. It was considered that the string-like precipitate grows to the elongated-type precipitate and then those transforms to the TYPE-C precipitate with progress of aging, because there is similarity of the arrangement of bright dots and orientation relationship in their HRTEM images between the string-like and elongated-type precipitates, and the TYPE-C precipitate. The β' phase increased and the TYPE-C precipitate decreased on the dislocation in the over aged alloy with decreasing of excess silicon contents. There were no TYPE-C precipitates and the β' phase predominantly appeared on the dislocation in the over aged alloy without excess silicon contents. It was found that the TYPE-C precipitate is a typical precipitate which appears inhomogeneously in the deformed excess Si alloy.
The effects of the pre aging and the deformation on the Al-1.0mass%Mg2Si-0.4mass% Mg (excess Mg) alloy were investigated by hardness measurement, tensile test and transmission and scanning electron microscopy to improve its mechanical properties, especially elongation. The time achieving to the maximum hardness (tmax) in the specimen with deformation by 1% prior to final aging at 473K was shortened and the value of maximum hardness (HVmax) was not changed to be compared with that in the specimen without deformation. In the tensile test of this specimen, the value of the elongation was higher, and values of the ultimate tensile strength (U.T.S.) and the 0.2% proof stress (0.2%P.S.) were slightly lower than those of the specimen without deformation. The value of U.T.S., 0.2%P.S. and elongation of the specimen final aged at 473K to HVmax after pre aging at 423K for 6ks were higher than those of the specimen without pre aging. The thermo mechanical treatment (TMT), which means combination of the deformation by 1%, pre aging at 423K for 6ks and final aging at 473K to HVmax, improved U.T.S., 0.2%P.S. and elongation of the specimen and a total time of aging treatment was shortened. The TMT specimen was almost revealed the fracture surface of the transgranular fracture, while fracture surfaces of specimens improved their elongation by pre aging or deformation prior to final aging at 473K to HVmax was mixed feature with the transgranular fracture in the intergranular fracture. The width of the precipitate free zone in the TMT specimen was more narrow than those of specimens either with deformation or with pre aging. When manganese of 0.5mass% was added to the excess Mg alloy, the grain size of this alloy was refined from 550μm to 50μm and its U.T.S., 0.2%P.S. and elongation were further improved. Finally, 290MPa of U.T.S. and 18% of the total elongation were obtained in the excess Mg alloy containing 0.5mass%Mn by TMT.
Longitudinal cracks are found occasionally in the anchorage zone of pretensioned prestressed concrete members with small covering depth. We made an assumption that the crack growth at the time of prestressing was resulted from the flaw in concrete which took place close to the prestressing tendon because of the temperature difference between the tendon and the concrete during steam curing at an early setting period of the concrete. Experimental and analytical studies were made on the effect of temperature difference on the crack growth in hardened concrete and the results approved the above assumption.
The Nueber's rule which is a method widely used for estimating the elasto-plastic stress/strain concentration at notch tends to overestimate the strain concentration in general yield condition. This paper proposes to modify the application procedures of the Nueber's rule to improve the shortcoming. The modified procedures defined the nominal stress and strain by the sectional averaged-Mises equivalent stress and strain in order to take account of the multiaxial stress state in the notched section. This paper also describes the extensional application of the modified Neuber's rule to the creep loading and the deformation-controlled loading conditions which are important in the life evaluation of high temperature components. The effectiveness of the propsoed procedures are verified by the inelastic finite element analyses for notched plates. As the result, it is shown that the change of creep stress/strain concentaration with time can be estiamted by the method proposed. It can be also applied to the deformation-controlled loading based on the nominal equivalent stress estimated by considering the elastic follow-up behavior of the notched members.
Cyclic loading tests are performed by using thin-walled tubular specimens of SUS304 stainless steel at low temperature in liquid nitrogen (77K) and room temperature. Cyclic plastic deformation subsequent to pre-strain at 77K and room temperature is experimentally investigated under torsional, tensile and compressive loading conditions. At small cyclic numbers, the pre-strain perpendicular to cyclic loading direction results in large hardening, and cyclic plastic behavior subsequent to the pre-strain at 77K is more hardened than the pre-strain at room temperature. With increasing cyclic number, the effect of temperature on the hardening behavior becomes small. When the cyclic loadings are conducted in the same direction as the pre-straining direction, the effect of the pre-strain on hardening behavior is not remarkable at room temperature and 77K. The pre-strain at 77K influences the cyclic hardening at room temperature even for large cyclic number.
The relationship between the local deformation within a grain and macroscopic deformation of material was investigated by FEM simulation and tensile tests. Tensile tests for investigating the local deformation within a grain were carried out using on the thin copper specimens in a SEM chamber. From these tests, it was found that the local stress-strain behavior within a grain depends on the orientation of grains and the strain shows distribution within a grain. It was also found that the value of local strain depends mainly on the gauge length used for the calculations of strain. A FEM simulation showed a good agreement with tensile test results by considering the crystallographic orientation of each grain and that of the constraint from circumferential grains.
Inflatable space structures are expected to be a promising candidate for future large-scaled space structures because of the compactness at the launch stage and the simple extension mechanism. In this paper, the feasibility of an inflatable structure was examined utilizing the expansion force of foam materials as the inflation thrust. As a trial inflatable space structure, an extensible mast, which was made of foam material covered with an aluminum sheet or a carbon fiber cloth was chosen. This paper describes foaming behavior of the material in space. As a first report of the study, intensive discussion was placed upon the effect of low pressure environment on the expansion behavior. The expansion behavior of foam material appeared to be controlled mainly by the pressure difference between the inside and outside of foams. It was found that the fracture of foams could be prevented and the density of the foam material could be controlled even in the low pressure environment by controlling the amount of blowing agent. The effect of tube geometry on the expansion behavior was also discussed.
This paper describes the low cycle fatigue criterion of Mod.9Cr-1Mo steel and the fracture mode under multiaxial stress/strain states. Multiaxial tension-torsion low cycle fatigue tests were carried out using plain Mod.9Cr-1Mo tubular specimens at 823K. Plain, surface notched and through notched specimens were also torsionally fatigued at 823K and room temperature to examine the crack propagation direction. LE strain and the equivalent strain based on crack opening displacement were a suitable strain parameter for correlating the multiaxial low cycle fatigue lives with a small scatter. Mises' stress was a suitable stress parameter for the correlation. Main crack of the smooth, surface notched and through notched specimens propagated in the maximum shear strain direction in torsion tests, so there was no influence of the notches on the crack propagation direction. Surface oxidation has no effect on the main crack propagation direction, but it has an influence on the subcrack propagation direction. Oxide films were brittlely broken in the principal direction.
Thickness dependence of bending fracture strength and fracture toughness of WC-Co alloy plates of varying WC particle size, machined by wire electric discharge (WED), was investigated in the thickness range of 0.1 to 3mm. The WED specimens of the alloy with the smaller size WC exhibited a decrease in the bending fracture strength to the extent of one half with the reduction in thickness from 1mm to 0.1-0.3mm range. However, the specimens machined by grinding (GR) showed an opposite trend, as observed by a considerable increase in the bending fracture strength with decrease in the thickness. On the otherhand, the successive microgrinding of 2μm after the WED machining improved the bending fracture strength to the extent of GR specimens. The fracture toughness of the WED specimens were significantly decreased in a thickness range of 0.1-0.3mm, in the same way as the GR specimens. Also, with lowering in the thickness, the chipping around the corner of reground surface became easier. In the WED specimens, coarsening of WC particle improved the bending fracture strength and the fracture toughness, particularly in the ultra-thin specimens.
The diagnosis system is able to observe directly the high temperature pipes of the boiler, recognize automatically creep damage, and determine the residual life on site employing a small laser microscope developed for this purpose. To be concrete, the weld-heat-affected zone is directly observed through the laser microscope, and the data are introduced into the image processing unit and the creep voids are automatically recognized. The area of the recognized voids is calculated in the process. Approximately 50 fields per diagnosis area (1mm2) are automatically observed, without an overlapping at the boundary of images. The void area ratio is then calculated. The residual life is instantly evaluated. With this system, a diagnosis time can be drastically reduced and repairs quickly could be made. This system will contribute to extend the service lives of equipments, which in turn will reduce power generating cost. Moreover, an improved reliability can be expected as a result of the proper preventive maintenance.
The property of many engineering woods depends on the shape and orientation of their elements. This paper describes an attempt to estimate the 3-dimensional fiber orientation in insulation fiberboard (IB). Serial sections of IB were sliced off at every 50μm parallel to the board plane by a sliding microtome. Before sectioning, the specimen was embedded in epoxy resin and cured. Then, it was boiled in water for several hours to make the resin moderately soften. The orientation values were calculated on 2-dimensional Fourier transform image analysis. Analytical value indicates that fibers near the both surfaces orient to machine direction and the orientations decrease on inner plane. Section thickness was shown to affect the orientation values. The value was underestimated on a too thin section, because fibers are fragmented. On the other hand, obscure image of overlapping fibers on a too thick section gave also underestimated value. Some ideas are proposed to search for appropriate thickness.
The reliability-based design methodology for the transmission high-pressure gas pipeline is described. Limit state design criterion is introduced for the lifetime failure assessment in the seismic environment, while the maintenance activities including field patrols, periodic inspections and repair works are also taken into consideration. Pipe damages, which are caused by potential defect, corrosion, earthquake ground displacement, settlement and the third party accident, are classified into three major damage modes which include tension/compression, bending and dent with gouge. The assessment is made for the transmission pipeline system which is surrounded with various hazards spatially distributed along the pipeline route.