Papers by Author: Yoshihisa Sakaida

Abstract: A chromiummolybdenum steel composed of 0.20 mass% carbon was used as a starting material. Three kinds of specimens having different case depths were made by carburizing and quenching. Using the scanning electron microscope, the crystallographic information was measured on the cross-sectional hardened layer by EBSD (electron backscattering diffraction) technique. The KAM (kernel average misorientation, Θ) maps were calculated from the carburized surface to the interior below the case depth of each specimen. The area-average, Θ_mean, of the KAM map was compared to the case depth and the cross-sectional residual stress distribution measured by x-ray. As a result, the area-average of the hardened layer was larger than that of the interior of specimen after quenching. The estimated depth of the increment in the Θ_mean was found to accord to the case depth and be proportional to the depth in which large compressive residual stress was distributed on the gradually polished surface. Therefore, both the case depth and eigen strain distribution that induce the compressive residual stress are indirectly estimable by electron backscattering diffraction method.

Abstract: A hollow cylinder specimen of Cr-Mo steel with 0.20 mass% C was used as a model which simplified a motor cycle transmission gear. The specimen was carburized in carrier gas and quenched in oil bath. After quenching, the internal residual strain distributions in the radial, axial and hoop directions were mapped nondestructively by neutron strain scanning, and were compared with results of elastoplastic finite element analysis. The carbon content and hardness gradients were also measured to determine the case depth. Residual strain was calculated from lattice spacing change. In this study, unstressed lattice spacing was determined experimentally by measuring diffraction angle of coupons that were cut from the interior of the same carburized cylinder specimen. As a result, the carbon content and hardness distributions were almost accorded with finite element analysis results. On the other hand, the measured strains in some measuring points discorded with the analyzed weighted average strains. The peak widths in the interior of specimen equaled to that of unstressed coupons. Internal residual strain distributions were complicated and found to be elastic strains that were balanced with equivalent plastic strains due to transformation plasticity of case layer.

Abstract: 800x600 The short and long Japanese swords “WAKIZASHI” (back up sword of the main sword, “KATANA”) were made by one sword craftsman. The short and long swords “WAKIZASHI” were machined by rough grinding and final hand polishing, respectively. The microstructure, carbon content and hardness of short sword were measured experimentally on the cross section of sword blade. The 2θ-sin²ψ diagrams from Fe-211 plane using Cr-Kα radiation on the ground and polished blade were measured. An influence of final hand polishing process on the surface residual stress field of sword blade was examined. As a result, biaxial principal compressive residual stresses were generated and had constant stress gradients in depth on the ground and polished surfaces because the ψ-splitting was not observed and the measured 2θ vs. sin²ψ relations could be approximated as a parabolic curve. Large compressive residual stresses more than −1.0 GPa were distributed on the ground surface from “HASAKI” to “HAMON” of short sword. The surface compressive residual stress and its gradient were diminished gradually from “HAMON” to “MUNE” (the ridge of sword). On the other hand, compressive residual stresses more than −650MPa were distributed on the pol­ished surface from “HASAKI” to “HAMON” of long sword. The surface compressive residual stress and its gradient were also diminished gradually from “HAMON” to “MUNE”, and the residual stress gradient in the transverse direction were greatly degraded in comparison to the short sword. Additional compressive residual stress field induced by rough grinding was superimposed on the residual stress field after tempering process. The residual stress field near the blade surface after rough grinding was released partly by final hand polishing. Normal 0 21 false false false DE X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Century","serif";}

Abstract: The hollow circular cylinder specimen of Cr-Mo steel with 0.20 mass% C was carburized in carrier gas and quenched in oil bath. After quenching, the surface residual stress distributions in the radial, axial and hoop directions of the specimen were measured experimentally by x-ray as a function of the distance from the carburized surface. The case depth of the quenched specimen was about 0.8 mm. Diffraction from Fe-211 by Cr-Kα radiation was used to minimize the effect of carbon content gradient on the nonlinearity of 2θ-sin²ψ, because effective x-ray penetration depth was about 5.8 μm at sin²ψ=0. The hardened case layer was gradually removed by electrolytic polishing. X-ray stress measurement was repeated on the polished surface from the carburized surface to the interior. The ψ-splitting was not observed on the carburized surface. The 2θ-sin²ψ diagrams were found to shift from low to high angles in inverse proportion to carbon content. The residual stresses in the hardened case layer were compressive. The maximum compressive residual stresses on the hollow circle and periphery surfaces were about −559 and −544 MPa at the depth of 0.2 and 0.3 mm, respectively. On the other hand, the tensile residual stress was not observed. The full widths at half-maximum intensity of Fe-211 diffraction peaks in the hardened case layer were wider than that of the interior of the specimen. Martensitic transformation of the carbon-rich layer leaded to the broadening of diffraction peaks. Therefore the interior of carburized specimen were deformed elastically to balance the existence of the surface compressive residual stresses.

Abstract: Three types of polycrystalline alumina, one pressureless and two hot press sintered Al2O3, were used to examine the effects of the characteristics of microstructure and crack face bridging on fracture toughness. The crack opening displacements and microstructures along the pop-in crack of single edge precracked beam (SEPB) specimens were observed in situ at a constant applied stress intensity factor by scanning electron microscopy (SEM). The bridging stress distribution could be determined from the measured crack opening displacement by three-dimensional finite element analysis, and then the stress intensity factor and stress shielding effect at the crack tip could also be determined. Intergranular microcracks of toughened Al2O3 were deflected by a complicated microstructure, and crack closure due to bridging grains was observed near the crack tip. Bridging stress of Al2O3 was compressive perpendicular to the crack face and was distributed behind the crack tip. The maximum bridging stress of two hot press sintered Al2O3 was about twice as large as that of pressureless sintered Al2O3. The fracture toughness of hot press sintered Al2O3 was, therefore, higher than that of pressureless sintered Al2O3, because the total amount of bridging stress and stress shielding effect increased with increasing magnitude of microcrack deflection and the number of interlocking grains.

Abstract: A motorcycle transmission gear of chromium-molybdenum steel with 0.2%C was carburized in carrier gas. Carburizing process including heating, carburizing, diffusing and quenching was simulated using elastoplastic finite element method. The carbon content, hardness, residual strain and residual stress fields of gear were analyzed. The unstressed lattice plane spacing and residual strains of the interior near the internal spline of gear were experimentally measured by synchrotron x-ray and neutron diffraction methods. As a result, the analyzed carbon content and hardness gradients of gear accorded with the experimental results. The radial, hoop and axial directions of cylindrical gear were found to be not always principal axes of residual stress field. On the other hand, the analyzed residual strains in the radial, hoop and axial directions of gear slightly discorded with the experimental results. Although correlation between the measured three strains was similar to that of the weighted average of analyzed strains, residual strain and stress fields of motorcycle transmission gear could not be accurately predicted at the present finite element analysis. It was concluded that carbon diffusion phenomenon and resultant hardening could be analyzed by the finite element method, and the actual interior residual strain and stress fields should be nondestructively measured by neutron diffraction method.

Abstract: In order to understand an effect of crack-face bridging stress field of alumina ceramics on its fracture toughness, local residual stress distribution due to crack face grain bridging behind the crack tip was measured using synchrotron x-ray beam at SPring-8 in Japan. The SEPB (Single Edge Precracked Beam) specimens of two types of polycrystalline Al2O3 were used for stress measurement; one was pressureless sintered Al2O3 (AL1) and the other was hot-press sintered Al2O3 (TAL). Pop-in precracks were introduced by bridge-indentation method. Before residual stress mapping, the SEPB specimens were unloaded from a constant applied load to zero using four points bending device. Two-dimensional residual stress field was mapped by scanning a micro X-ray beam of 50×50 μm2 with the scanning interval of 12.5 or 25 μm. As a result, in the case of AL1 having conventional fracture toughness and strength, the compressive residual stresses due to crack-face bridging were only observed in the close vicinity of crack tip. On the other hand, in the case of TAL having higher fracture toughness and strength, the compressive residual stresses were widely distributed behind the crack tip. Larger compressive stress was locally generated along the crack path at interlocked grains. The compressive bridging stresses distributed behind the crack tip were found to enlarge with a decrease in the crack opening displacement against a constant applied stress intensity factor, Kapp. It was concluded that the difference in residual stress fields behind crack tip was attributed to the differences in its microstructure and microcrack propagation behavior, such as deflections and interlocked grains.