Papers by Author: Yuichi Otsuka

Abstract: Total hip replacement is surgical procedure which is widely performed in most of the developed countries due to rapid aging. The extensive application of titanium alloy as hip prosthesis can be seen because of its suitable properties such as good biocompatibility, light weight and high strength. However, coating or bond is required as titanium alloy ineffective to be adhered directly with human bone. Hydroxyapatite (HAp) is common coating material used to bond Ti-6Al-4V hip prosthesis with human bone. HAp-Ti-6Al-4V interface is a possible fretting wear region which is subjected to significant contact pressure. HAp-Ti-6Al-4V interface fretting fatigue delamination leads to contact pressure which can accelerate fretting wear behaviour of HAp coating. Present paper discusses the influence of delamination length and fatigue stress ratio on contact pressure distribution at interface of HAp-Ti-6Al-4V using finite element methodology. A simple two-dimensional finite element contact configuration consisting Ti-6Al-4V substrate, HAp coating and contact pad (representing bone) is employed to examine under static analysis. The finite element predicted results highlighted that contact pressure can be promoted under increased delamination length condition and stress ratio of 0.1 (tension-tension). Contact pressure can accelerate HAp coating fretting wear behaviour.

Abstract: In this study, the effects of curing white Portland cement (WPC) and hydroxyapatite mixed with white Portland cement (HAp/WPC) pastes in water and the in vitro biological environment on the compressive strength and bone-like apatite formation were examined. The compressive strength of both WPC and HAp/WPC pastes increased with longer curing periods in water. The compressive strength of WPC and HAp/WPC pastes was 51.88 and 25.67 MPa, respectively, after curing in water for 28 days. The compressive strength of both samples continuously increased during in vitro testing in a simulated body fluid (SBF). After 4 weeks of immersion in a SBF, the strengths of cured WPC and HAp/WPC samples were 59.01 and 28.06 MPa, respectively. It is due to continued hydration of WPC. The addition of HAp to WPC decreased the compressive strength of the sample. Alternatively, it enhanced bone-like apatite formation on the surface of the samples.

Abstract: This study aims at experimentally revealing the effects of damages/fractures in HAp coating layer of acetabular cups on loosening behavior of the acetabular cups. Aseptic loosening is occurred due to degradation of fixing force of acetabular cups by biological effects or mechanical loading. However, effects of mechanical loading on loosening behaviour have not been observed yet. In order to simulate cyclic loading conditions of gaits, a testing system which can load entire components of joint including acetabular cups and stem parts was designed. Moreover, by applying two positions of AE sensors during fatigue testing, it was possible to observe the damage behavior of HAp coating. AE measurement detected different failure modes of HAp coating, which were locally occurred at an edge part of the acetabular cup due to stress singularity at that region. In the cases of changing fixation angles, even though damages in simulated cancellous bone surrounding acetabular cups were less occurred, extents of rotational displacements were compatible with the one in an original fixation angle.

Abstract: Micro-groove textured surface of alumina-based ceramic coating is successfully fabricated via laser surface texturing (LST) method. The fabrication of textured surface is aimed to improve the resistivity of components against erosive wear. In this study, a slurry pot testing is conducted to investigate erosion damage on the surface of the textured alumina-based coating. The erosive wear rate of the specimens is determined and the surface morphology of the eroded surface is examined using an SEM. From the results, it is found that the erosion damage of textured coatings is dependent on the impact angles of SiO₂ particles. The erosion wear mechanism is found to be varied with the formation of the micro-groove textured coatings. A significant decrease of 45 % in the erosion rate proved that the occurrence of an improved erosion resistance.

Abstract: Biomaterials containing calcium phosphate ceramics have been used as bone substitute materials. In this study, the compressive strength and in vitro bioactivity tests of hydroxyapatite (HAp) from golden apple snail shell mixed with calcined kaolin (CK) were investigated for used as bone substitute materials. Mixed paste samples were cured at 23°C and 60°C for 2 days and curing continued at 23°C for 7 days. The effects of HAp:CK weight ratio on compressive strength and apatite formation in simulated body fluid (SBF) were investigated. The good compressive strength was 32.93 MPa at 25 % hydroxyapatite with 75% calcined kaolin at curing temperature of 60°C. Apatite formation was observed on sample surfaces after soaking in SBF for 28 days using SEM, EDS and XRD analyses. It was found that apatite formation took place on the surface of samples, consisting of HAp, after immersion in SBF.

Abstract: Notch fatigue behavior of Ti-6Al-4V titanium alloy has been investigated under a load-controlled high cycle fatigue test. The S-N curve was kinked at the critical nominal stress amplitude, where the plastic deformation nucleated at notch root. The plastic zone size at the critical nominal stress amplitude was almost equal to the size of 4 grains of the present Ti-6Al-4V alloy, which were the same findings as in the 2024-T4 alloy used in the previous study. Above the critical nominal stress amplitude, the local stress ratio at notch root decreased with increasing nominal stress amplitude. The critical nominal stress amplitudes normalized by the cyclic yield stress were almost the same for Ti-6Al-4V alloy and the 2024-T4 alloy. The significant decrease of the local stress ratio with increasing nominal stress amplitude was observed in the 2024-T4 alloy with low cyclic yield stress and low cyclic hardening coefficient compared to the Ti-6Al-4V alloy with high cyclic yield stress and high cyclic hardening coefficient.

Abstract: CoCrMo alloy are known to be widely applied as biomedical implant materials. They have been practically used for artificial knee joints and hips. This is due to their excellent corrosion and wear resistances as well as good mechanical properties and biocompatibility. This published work is a research to improve the tensile properties of the alloy for load bearing application. This approach is an innovative process that will enhance the mechanical properties of the materials against the conventional processing technique while reducing the number of steps and energy consumption in producing the final parts; hence more economical. Grain refinement is expected to promote significant enhancement in both properties. Preforms are prepared through powder metallurgy route prior to the low strain rate upset forging process. The alloy powder was mixed with zinc stearate as a binder at different milling times. The formulated powder is compacted and then sintered at different temperatures. Characterization of the sintered parts are studied on their microstructure, density, hardness and transverse rupture strength (TRS). Further characterization was done using transmission electron microscopy (TEM) to study the grain refinement in enhancing the properties of the material.

Abstract: Many studies were carried out to investigate the ability of titanium alloys for dental implant. Surface treatment is one of the famous methods to increase the titanium surface properties. The purpose of this paper is to investigate the effects of acid etching on the surface topography and roughness of titanium alloys (Ti6Al4V ASTM 1472-99). Acid etchings were carried out by using different type of acids with same time exposures. All etched surface were characterized by using an X-ray diffraction (XRD), a scanning electron microscope (SEM) and a roughness tester. Acid etched and pure surface were comparatively analysed. Results obtained show that the type of acids influenced the surface topography as well as roughness properties. The microstructure of the surface is highly modified after acid etching. Further we can confirm that, the experimental etched titanium alloys had features of a roughened surface with micro-roughness. In general, the experimental surface (0.137 μm – 3.986 μm) was significantly rougher than control surface (0.124 μm).

Abstract: The bolt load loss behavior of AZ91D magnesium alloy bolted joints with a conventional SCM435 steel bolt and an A5056 aluminum bolt was investigated at elevated temperature. The A5056 bolt could reduce the bolt load loss compared to the SCM435 bolt due to smaller mismatch of thermal expansion between the bolt material and the plates. The mismatch of thermal expansion between bolt material and AZ91D plates was found to induce the compressive creep deformation in the AZ91D plates which performed as the main mechanism of bolt load loss. At higher tightening stress, the bolt load loss could be intensified by additional plastic deformation in bolt occurred during the test. Moreover, it is suggested that the plastic deformation could be reduced by decreasing the friction condition in the bolted joint.

Abstract: Tensile and compressive creep tests of the extruded magnesium alloy AZ91D were carried out in vacuum at 150°C under constant engineering stresses ranged from 60 to 150 MPa. From the test results, the secondary creep rate in tension was found to be significantly higher than that in compression. Moreover, the estimation method of creep curve under a constant true stress was proposed by considering the reduction of cross sectional area during tensile loading where the specimen cross-sectional area and length were measured periodically until the end of creep test. The creep curve under a constant true stress obtained in the present study was still different between tensile and compressive loadings. Therefore, it should be noted that the different creep curves and creep exponents should be used in the creep deformation analysis of structures.