Takehiro Morita

Experimental and Fracture Mechanics Study of the Pit Formation Mechanism Under Repeated Lubricated Rolling-Sliding Contact: Effects of Reversal of Rotation and Change of the Driving Roller

Five rolling contact fatigue tests, Tests (1)-(5) have been conducted. In Tests (1)-(3), when a fatigue crack was initiated on the surface of a follower, the test was halted. Then, in Test (1) the rotating direction was reversed. In Test (2) the follower and driver were interchanged, and in Test (3) the test was continued unchanged. In Test (3) the original crack grew to a pit. In Tests (1) and (2) the original crack immediately stopped propagating. In Tests (4) and (5), mating with a harder roller, a softer roller was used as the follower in Test (4) and as the driver in Test (5). A typical pit occurred in Test (4). In Test (5), surface damage substantially different from a typical pit was generated. Based on these experimental results, a 3-D crack analysis including the effect of frictional force on the contact surface and oil hydraulic pressure on crack surfaces, was conducted to elucidate the mechanisms of pit formation and surface damage in contact fatigue.

Observation and numerical simulation of oil-film formation under reciprocating rolling point contact

A basic study on oil-film formation in reciprocating pure rolling point contact was conducted by optical interferometry observation and numerical analysis. In the analysis the program code published by Venner and Lubrecht was extended for time dependent and starved problems. The observations showed that the oil-film thickness in reciprocating rolling was less than that in unidirectional rolling; this because of the starved condition that was induced by cavitation generated in the previous passing of the contacting region and which survived in the inlet region for a while after reversal. The Elrod algorithm was used in the numerical analysis, and the amount of oil on the trailing edge of the calculation domain was stored and used on the leading edge of the calculation domain when it returned after reversal. In this calculation model, the oil-film thickness decreased with cycle repetition, while in the fully flooded condition the oil-film almost recovered when the contacting area returned to the stroke center, and nearly the same oil-film thickness trajectory was traced along with changes of the entrainment velocity.

Estimation of Cavity Length in EHL Rolling Point Contact

Reciprocating lubricated contacts sometimes suffer from oil starvation due to cavitation at the reversal of motion. However, the behavior of cavities is not well understood such that starvation can be theoretically predicted. In this study, the length of cavity in a steady state elastohydrodynamic lubricated point contact was calculated. For numerical simulation, a modified Elrod algorithm was used. An equation was obtained for the cavity enclosed in the oil meniscus. The equation was constructed with Moes dimensionless parameters M and L, assumed pressure of cavity, and viscosity pressure index of the lubricant. The dimensionless cavity length (or the ratio of cavity length by Hertzian contact radius) is proportional to the product of M−a and Lb. Careful examination of the equation elucidated that the cavity length is dominated by the viscosity, sum velocity, cavity pressure, and geometry of the contact. Experimental measurements with a ball-on-disk apparatus have shown good agreement. The validity of the equation suggests that the algorithm is applicable for complete transient simulations. In practice, the estimated cavity length can be a parameter for starvation.

The effective design of zirconia coping on titanium base in dental implant superstructure

Zirconia exhibits good tissue compatibility and nontoxicity, making it a widely used esthetic replacement material for implant abutments. To avoid abutment-fracture, the parts composed of zirconia with a bonded metal component connected to the implant can be used. The purpose of this study was to design titanium and zirconia components with high fracture resistance at the zirconia components edge line. Three edge line designs of the titanium base and zirconia sleeve were made: chamfer, shoulder, and back-taper. To assess the strength of the abutment design, static loads were applied vertically and 30 degrees from the vertical axis. A test of tensile strength was also performed after chewing simulation. Conventional zirconia components mounted on a chamfer-type titanium base showed significantly lower fracture resistance than shoulder and back-taper types. This study suggests that to improve the durability of zirconia abutments with a titanium base, a back-tapered edge design is recommended.

Relationship between dynamic stress field and ECM production in regenerated cartilage tissue

Mechanical stresses exerted in articular cartilage during daily joint movements can stimulate the metabolism of chondrocytes in cartilage. Upregulative effects of mechanical stimuli on extracellular matrix (ECM) biosynthesis of chondrocytes have been utilized in the cartilage tissue for regenerative tissue engineering. However, dynamic functionalities of these regenerated cartilages were lacked compared with the natural arthrodial cartilage. Natural synovial joint surfaces are contacting and sliding each other. As a result, the loading condition applied to the cartilage tissue is not simple. Under the contact between surfaces, non-uniform stress distribution is exerted in the cartilage tissue. The shear stress at contact area is highly depended on the distance from the surface. Moreover, the surface region is exposed to larger shear stress compared with the deep region. We hypothesized that high stress in surface region may stimulate chondrocytes selectively and establish anisotropic structure in elaborated tissue. In this study, the relative motion between cartilage surfaces in a synovial joint is simulated by the rolling-sliding motion of the roller on the cultured chondrocyte-agarose construct. Then, we investigated effects on the ECM distribution and morphology of regenerated cartilage tissue.

A deterministic model of mixed EHL in line contact considering the three-dimensional surface roughness

In mixed elastohydrodynamic lubrication, it is supposed that asperity conjunctions should interrupt oil flow in advance of the occurrence of a direct contact. The local transient Reynolds equation, which is expressed in the coordinate system travelling with a rough surface, will serve to simulate the interruption process of oil flow. In this paper, we propose a new model for mixed EHL for the rolling-sliding contact of cylindrical bodies with 3-dimensional single-sided roughness considering a periodical boundary condition and oil mass conservation. Under the conditions of pure rolling, as the local Reynolds equation does not include the wedge-film term, direct contact is predicted to occur easily, especially in the case of a single bump. On the other hand, in the case of a sinusoidal wavy surface, oil entrapped in the space between asperities at the entrance of the Hertz conjunction will be compressed up to a considerable pressure in the Hertz region so that direct contact will be prevented unless there is a large amplitude of roughness.