Effects of crack-healing on crack propagation behavior in Si_3N_4/SiC were investigated by in-situ observation. The in-situ observation was conducted for two materials which were sintered with different sintering additives and subjected to raising stress at several temperatures, environment (Air, N_2) and loading rate. As a result, it was revealed that suppression of crack propagation and increase of fracture strength were occurred at high temperature in both Air and N,. These effects in N_2 were caused by release of tensile residual stress at high temperature. Moreover, these effects in Air were affected by crack-healing in addition to release of tensile residual stress. These two factors, crack-healing and release of tensile residual stress were dependent on time. So the effect of these factors on increase of fracture strength was higher, as the loading rate was smaller. From these result, it became clear that release of tensile residual stress as well as crack-healing play an important role in suppression of crack propagation and increase of fracture strength.
The numerical study is made to demonstrate the applicability of the method of decoupling multi-scale analysis to the micro–macro evaluation of the mechanical behavior of fiber-reinforced plastics (FRP) that exhibits inelastic deformations and internal damage of the matrix material. During the course of this demonstration, it is confirmed that the reliability of the decoupling method can be guaranteed if the macroscopic constitutive model is introduced so as to inherit the microscopic material behavior. To this end, with reference to the results of the numerical material testing on the periodic microstructures of FRP, we propose an anisotropic elastoplastic-creep-damage combined constitutive model to represent the macroscopic material behavior and illustrate the characteristics of the inelastic deformations that resemble the material behavior assumed for plastics at micro-scale. With the identified macroscopic material parameters, the macroscopic structural analysis, which is followed by the localization analysis consistently, can be an actual proof of the utility value of the decoupling method in practice.
Whirling, in which a rotating workpiece is machined by rotating tools around the workpiece with an eccentricity between the centers of the workpiece and the tool rotations, has been generally performed in threading. The whiling mechanism is applied to polygon machinings in this study. In proposed machining, the ratio of workpiece rotation rate to tool rotation rate is set to be n : 1 to machine n polygon prism. An analytical model was established to control the machining shape. The cutting tests, then, were conducted to verify the presented machining with the analytical model. Machining examples of triangular, square and pentagonal prisms were demonstrated in whirling. The effect of the tool rotation radius on the curvature of the workpiece surface is discussed in measurement of the machined shapes.
A novel cutting process is presented to machine helical polygon shapes in the through holes with double rotations kinematics. A boring bar and a workpiece are controlled simultaneously to rotate at n : (n-1) rate of the spindles. When the tool is fed with changing the ratio a little, a helical polygon is machined in the hole. A kinematic model is established to simulate and control the polygon shapes in the presented process. The cutting tests were conducted to validate the helical polygon machining with the kinematic simulation. The flank face of the tool was ground to control the clearance of the tool from the inner surface in the cutting tests. In the presented cutting process, the helical polygon shapes are machined in the through holes at high machining rates. The polygon can also be controlled by the ratio of the tool and the workpiece with a boring bar.
The present paper discusses the degradation mechanism for weatherability flexural properties of CFRP subjected to the steady load and unload of the accelerated and the outdoor exposures. One of the specimens is made of epoxy resin and unidirectional carbon fiber prepregs having the fiber orientation angles of 0 and 90 degree, respectively and another specimen is made of the same epoxy resin only as the matrix of the prepregs. In order to clarify the effects of exposure period on the variations of flexural strengths and moduli, matrix volume fractions, thickness and absorption rate of infrared rays, the complex accelerated exposure test was conducted to continue for up 100 cycles and a direct outdoor exposure has been also conducted to continue for 13 years. It is shown that both the weatherability flexural strength can be calculated by the results of epoxy resin and thickness change of CFRP during the exposures, and these calculations are agreed with the experimental ones under the accelerated and the outdoor exposures.
Whirling cut has been applied to manufacturings of worm and ball screws for motion controls. The whirling cut, in which the tool and the workpiece rotate simultaneously with an eccentricity between the centers of the workpiece and the tool rotations, has advantages in the tool life, the surface finish and the chips control compared to the general threading. Although whirling has been used for only machining the screws so far, the several complicated part shapes are machined in control of the ratio of the spindle speeds of tool and workpiece. The paper presented a polygon cutting with combination of the tool and the workpiece rotations. The cutting tests were conducted to verify the presented machining manner. Then, the effect of the tool rotation radius on the curvature of the workpiece surface is discussed in the measurements of the machined shapes..
Disposing conventional FRPs poses environmentally challenging problem. When FRPs are scrapped by combustion, carbon dioxide is discharged in the air because the resin of FRPs is made of fossil fuel. When they are disposed in the ground, FRPs remain semi-permanently without decomposing. In response to these problem, green composites now being developed and are extensively studied as a material of less environmental burden. In this paper, a pultrusion technique was developed for fabricating green composites composed of PLA (Poly Lactic Acid) or PBS (Poly Butylene Succinate) resin as a matrix and kenaf fibers as reinforcement. Their tensile strengths under the different temperatures were examined. These results were compared with those fabricated with a hot press molding. As a result, the performance of pultrusion method which could continuously fabricate green composites was demonstrated here and their tensile strength and Young's modulus of PLA/kenaf and PBS/kenaf composites were larger than those of both resins alone under the different temperatures. This result showed the reinforcement effect of PLA and PBS by kenaf fibers at the room and the higher temperatures. Furthermore, the tensile strength and Young's modulus with the pultrusion method were larger than those with the hot press method, because the tension applied to the fiber bundles during the pultrusion molding. The heat resistance of the PBS/kenaf composite was higher than that of PLA/kenaf composite. Finally, another shape of green composite was fabricated by using the specimen of pultrusion method and its properties were evaluated.
