The effects of cutting speed, feed rate and cutting depth are investigated on the residual stress in the surfaces, which are machined by a machining center, by the measurements using a X-ray residual stress and the texture. It is found that the pole figures in the surfaces machined by the machining center are different from those by shaping machines and horizontal milling machines. The texture of cut surface by the shaping machines and horizontal milling machines is the rolling texture, on the other hand the texture of cut surface by the machining center is a weak texture similar to that of the initial stages due to rolling contact fatigue. In cutting by the machining center, the residual stress increases as the cutting speed increases and it decreases as the feed rate and cutting depth increase because of the influence of the chips. It is found that the residual stress occurred by the machining center is a half time smaller than that by the horizontal milling machines under the optimal cutting condition. Based on the results obtained from the measurement of texture and X-ray residual stress, it can be concluded that machining with shaping machines and horizontal milling machines shows heavy machining as the feature, while high-speed heavy machining with a machining center indicates easygoing processing from the viewpoint of the crystal deformation behavior, neglecting the damage of tools.
As one of serial studies on the mechanical properties of spider threads, the effect of viscid droplet on their deformation behavior was investigated. The viscid droplets are regularly spaced on the spiral thread building orb web for capturing insects. It was shown that 1) the most abundant amino acid of the droplet is prolyne, while the component of the capture thread is not so different from that of dragline, 2) its surface tension is about 65mN/m which is comparable to that of water, and 3) although the capture thread deforms like rubber due to the existence of the viscid droplet, it behaves like dragline if the droplets are removed from it.
In this paper, the effect of arrangement of inclusions on the effective elastic modulus of composite materials is considered through examining a model, which has two groups of periodically arranged inclusions in a matrix. Here, two groups of inclusions A and B are considered, both having equally shaped equally arranged inclusions, which have the same elastic constant but different from the one of the matrix. Then, the position of group A is fixed and the effect of location of group B on the effective elastic modulus is considered by the application of FEM. The FEM analysis indicates that the effective elastic modulus is almost independent of the location of group B when the projected areas of inclusions of groups A and B are not overlapped. In other words, the volume fraction of inclusion and projected area fraction of inclusions are two major parameters controlling the effective elastic modulus of composites.
Zinc Oxide (ZnO) varistor elements which have non-linear current-voltage characteristic have a number of failure mechanism when ZnO varistor elements absorb surge energy. Failure mode at the edge of the aluminum electrode formed by the thermal spraying process on the ZnO varistor element surface is quite common failure mechanism by high current surges. Such failures take the form of a melt pinhole from the electrode edge to the counter electrode or the edge of the disk element. As a result of the transient finite element analysis for the electric and thermal field distribution and the measurement of the resistively-temperature characteristic at high current region, it was made clear that the failure at the electrode edge was caused by the partial melting by the current concentration and the local thermal runaway at the electrode edge. Moreover, the current concentration was reduced by decreasing of the distance between the electrode edge and the sintered ZnO body edge. We developed the aluminum sprayed electrode manufacturing process which could control the edge margin with accuracy. And using this electrode process, the energy absorption capability of the ZnO elements was improved largely by reducing the current concentration at the electrode edge.
Low cycle fatigue tests were carried out using cruciform specimens of Type 304 stainless steel under fully ranged multiaxial strain states at 823K. Strain paths employed in the tests were not only proportional strain histories but also severe strain histories including out-of-phase strain in order to examine the effect of stain history on multiaxial fatigue life. The crack mode was also discussed by observing the crack direction on specimen surface. For the cruciform specimen, the principal strain axes are always fixed but the maximum principal strain direction is changed into the one of three principal strain axes due to the severe strain histories. The change in direction of the maximum principal strain from one principal axis to the others had no effect on fatigue life because the additional interaction between slip systems did not occur resulting in no large additional hardening. Only the strain multiaxiality influenced the fatigue life which can be predicted as well as the multiaxial fatigue life under proportional strain histories. The strain history also did not influence the mode of crack initiation.
This paper describes the effect of beta grain size on fatigue strength and subsurface crack initiation of solution treated and aged materials (STA) and as-solution treated materials (ST) in beta Ti-22V-4Al alloy. Rotating bending fatigue tests have been conducted at room temperature in laboratory air using smooth specimens and the obtained results were discussed on the basis of crack initiation, small crack growth and fracture surface examination. The fatigue strength of STA materials increased with increasing grain size, reached the maximum at a certain grain size and then decreased with a further increase in grain size, while in ST materials, fine grained material showed a higher fatigue strength than coarse grained material, indicating that the obtained fatigue strength of STA materials depended on both grain size and alpha precipitate distribution. STA materials showed S-N curves composed of two parts which are short life regime less than 105 cycles and long life regime around or more than 107 cycles. In the latter regime, subsurface crack initiation occurred and a relatively flat facet was always seen at the crack origins. No subsurface crack initiation was observed in ST materials. The occurrence of this type of fracture is discussed based on microstructural and mechanical considerations.
The propagation behavior of microstructurally small fatigue cracks was numerically simulated on the basis of the plasticity-induced crack closure model. By assuming that the crack growth rate was controlled by the crack tip openin displacement, ΔCTOD, the simulation of the propagation of a crack nucleated in the weakest grain was conducted. The grain size, the critical value of microscopic stress intensity factor at grain boundary and the frictional stress of dislocation motion were given as random variables following two-parameter Weibull distributions. When the crack approached grains with higher frictional stresses, ΔCTOD decreased, however the crack opening stress, σop, increased. The grain boundary blocking and higher frictional stress act as a resistance of crack propagation. The scatters of ΔCTOD and σop diminished as the crack length becomes longer. When compared at the same stress intensity range, ΔCTOD increased with stress ratio. On the other hand, the relation between ΔCTOD and the effective stress intensity range was unique irrespective of the stress ratio. The crack propagation life was calculated as a function of ΔCTOD. The life of the crack propagation and fatigue limit increased with decreasing grain size and with increasing critical value of microscopic stress intensity factor. The effect of the stress ratio on the fatigue limit was analyzed by the simulation. The fatigue limit as a function of the mean stress follows a modified Goodman relation.
This paper deals with the problem of dynamic stability of angle-ply laminated cylindrical shells subjected to static and periodic external pressure. First, the axially symmetric motion of the shell under loading is determined. Subsequently, certain perturbations are superimposed on this motion, and their behavior in time is investigated. The symmetric state of motion of the shell is called stable if the perturbations remain bounded. The solutions for the prebuckling motion and the perturbated motion are obtained by the use of Galerkin's method. Stability regions are examined by utilizing Mathieu's equation. The inevitability of dynamically unstable behavior is proved analytically and the effects of various factors, such as lamination angle, fundamental natural frequency, amplitude of vibration and dynamic unstable mode, are clarified.
A sintered SiC fiber-bonded ceramic was synthesized by hot-pressing the plied sheets of an amorphous Si-Al-C-O fiber. Here we describe the microscopic structure and mechanical properties of the two-directional sintered SiC fiber-bonded ceramic synthesized from three kinds of starting fiber with different concentration of excess carbon and oxygen and different fiber diameter. The desirable sintered SiC fiber-bonded ceramic has been found to show a perfectly close-packed structure of the hexagonal columnar fibers with a very thin interfacial carbon layer. Furthermore, the interior of the fiber element was composed of sintered β-SiC crystal without an obvious second phase at the grain boundary and its triple points. Its mechanical properties at high temperatures are closely relate to the microscopic structure. The strength and the fracture behavior are strongly dominated by the uniformity of the interfacial carbon layer and a densified structure of the fiber element. A reduction in the excess carbon and oxygen included in the starting fiber resulted in the improved mechanical strength of the fiber-bonded-ceramic. Furthermore, it was found the highest four-point bending strength (-500MPa) could be obtained by the use of a thinner starting fiber (8μm) with low concentration of carbon and oxygen. It was concluded from these facts that the improvement in mechanical strength was strongly related to the formation of the uniform interfacial carbon layer and the most densified structure of the fiber element.
Reinforcing fiber tow can be more widely and thinly opened than its original tow, by using the new pneumatic method previously reported. The opened tow is so wide and thin that it can not be applied to conventional weaving machines. Accordingly, a new weaving method was devised in which the motions of warp yarn supply, shedding, weft insertion, read beat-up and cloth take-up are different from those of conventinal method, and by which not only rectangular (0°/90°) weave, but also bias (0°/α°) weave can be produced. 12K carbon fiber tow was opened over the width of 16mm and then applied to the new weaving machine made for trials. As a result, (0°/30°, 45°, 60°, 90°) weaves were smoothly produced without distortion of weaving yarn, nap and weave opening. Their fabric weight and thickness are lighter and thinner than those of the weave by using the 1K tow. In addition, their weaving crimp angle of warp yarn in these weaves is much smaller than that in conventional weave.
The biodegradable poly (L-lactic acid) (PLLA) stents, made of a cylindrical knitted structure of PLLA monofilament, were implanted into the coronary arteries of 8 pigs by 8 Fr guiding catheters. Follow-up coronary angiography was performed and the standard optical microscopic observations of the cross section of the stented segments were carried out after implantation. Coronary angiographies and the histological examinations revealed that no acute closure was observed immediately after implantation and the patency of stented arteries of PLLA stent segments was wider. The %stenosis was lower than any other biodegradable stents already reported. These results are attributable to the lower degradation rate of PLLA which keeps its strength of the stent to hold the arteries from the inside for a longer period of time. It was further observed that the overlapping portions of the knitted structure tend to press and injure the internal wall of the blood vessel resulting in the growth of neo-intimal hyperplasia. The modified PLLA stents whose overlapping portions were flattened by compression at higher temperature give less %stenosis.
We discussed the influence of sulfate ion on the fluidizing mechanism of polycarboxylate-based superplasticizers (PC) in cement paste by considering adsorption characteristics of PC and steric hindrance effects. The fluidity, amount of adsorbed PC on cement particles and sulfate ion concentration in aqueous phase were measured using cement paste with some dosages of PC and alkali sulfate. The fluidity of cement paste decreased with increasing alkali sulfate. Clear correlation between the amount of PC adsorbed and the fluidity did not obtain. The fluidity decreased linearly with increasing sulfate ion concentration. We had showed that sulfate ion reduced the thickness of adsorbed PC and steric hindrance effects. The results of this study suggested that sulfate ion reduced the fluidity of cement paste mainly because of reducing steric hindrance effects with any dosage of PC.
This is one of the studies aimed at quantitative evaluation of the indices for rock mass classification, which deals with “judgment method by the hammer tapping”. This method includes results from different test operators in order to be expressed qualitatively, for example “clear sound”, “sound slightly muddy”, and “sound remarkably muddy”. The problems of these indices are not quantitative. So, in the previous research we examined specimens (using the artificial rock) with various factors such as shape and dimension, boundary condition, tapping position, and the uneven surface by tapping with steel ball and analyzed the tapping sound. Results showed “response sound pressure pulse inclination” could quantitatively evaluate strength and deformation of a rock. In this report we examined specimens using metal (stainless steel, copper and aluminum) and nonmetal (vinyl chloride and natural rubber). Results confirmed that “response sound pressure pulse inclination” show difference of mechanical and physical properties of the specimens, and could identify parameters like the minimum size of the specimen with fixed tapping conditions.
In order to predict the weathering process of the rock materials for the riprap of the rock fill dam embankment, the authors have been carrying out fundamental studies on the factor of weathering, changes of some properties and the rock deterioration mechanism by the weathering. The freezing and thawing test concerning the Cretaceous Granitic Rock (Kitakyusyu granite) were carried out in this study. The columnar specimens which are used in this test are composed of a series of rocks that includes fresh rocks to weathered rocks. We clarified the weathering properties of the series of rocks by the freezing and thawing, and obtained the regression equations for the weathering process. Using these equations, we can predict the weathering process of the similar series of rocks by the freezing and thawing from the physical and mechanical indices.