Wataru Kanematsu

Dependence of high-temperature tensile strength on displacement rate for hot-pressed silicon nitride

Tensile tests of hot-pressed silicon nitride were conducted in a wide range of displacement rates from 0.0005 to 50.0 mm min−1 at 1260 and 1380 ° C, and the dependence of strength on displacement rate was discussed in conjunction with the change of fracture surface features. As displacement rate was lowered, strength was degraded and stress-displacement diagrams altered from linearity to non-linearity, accompanying appearances of yielding phenomena. Strength was largely dependent on displacement rate in the high-rate region and rather independent in the low-rate region. The fracture surface features were drastically changed from brittle fracture to subcritical crack-growth fracture with decreasing displacement rate. A further increase in stress following the lower yielding point in the low-rate region was attributed to increments of fracture resistance due to the pull-out contribution of elongated grains.

Elastic/Plastic Surface Deformation of Porous Composites Subjected to Spherical Nanoindentation

The elastic/plastic parameters of Hydroxyapatite/ Poly (D , L-lactide) porous bio-composites are investigated using two distinct spherical indenta tion methods. A conventional constant-rate indentation method focusing only on the initial loading cur ve is used to determine Young’s modulus and the yield stress. For the evaluation of time-depende nt viscoelastic deformation and flow, a novel constant-penetration-depth (stress relax ation) test is conducted, and the stress relaxation time is successfully determined. Introduction Porous composites consisting of biodegradable materials, i.e. hydroxya patite, HAp, and Poly (D, L-lactide), PLA, are candidate materials for numerious applic ations in bio-material engineering because of their potential to exhibit mechano-compatibility and biocom patibility. Material design of the porous biomaterial requires the knowledge of time independent elas tic/p tic properties and time dependent plastic behavior, as well as fracture parameters. Indenta tion tests are technically simpler than other conventional fracture mechanical testing methods for deter mining these parameters with easier to prepare test specimen During the last two decades, numbers of instrumented indentation system equipped with both load and indentation depth sensors have been developed[1-5]. The indentation load ( P) versus depth (h) curve during loading/unloading cycle gives elastic and/or plastic sur face deformation parameters. Although most of the Young’s modulus evaluation study by the use of Ph curve has been addressed in terms of the initial unloading stiffness, dP/dh, it is expected that polymer based bio-composites will exhibit time-dependence other than elastic behavior. The maj or aim of the present work is the experimental determination of the elastic/plastic properties of HAp/PLA porous composites using two different types of spherical indentation methods. Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 927-930 doi:10.4028/www.scientific.net/KEM.240-242.927

Tensile strength and fracture defects expanded by subcritical crack growth of silicon nitride at high temperatures

On a realise une etude fractographique pour etudier les effets des defauts de rupture developpes par la croissance de fissure sub-critique a 1200 o C sur les proprietes mecaniques de ceramiques de nitrure de silicium

Effect of Machining Conditions on the Strength Distributions of Ground Ceramics

The relationship between strength distribution or ground ceramics and machining condition was examined. The strength of ground silicon carbide ceramics at the failure probability of 1 percent, which is termed 1 % strength, was estimated as a function of the maximum grain depth of cut g, which is used as an index of grinding conditions. It is assumed that the fracture is governed by the competing failure mechanism between pre-existing flaws and machining cracks and that the Weibull modulus of machining cracks was independent of grinding condition. The deviation from estimated value is noticeable in small g region although experimental data of the 1% strength of ground specimen approaches that of polished ones with diminishing g. The observed discrepancy is inferred to be attributed to an increase in stress intensity factor by the interaction between a pre-existing flaw and its adjacent machining crack.Copyright

Estimation for strength of ground ceramics at ambient and elevated temperature

An equation for the strength of ground ceramics has been proposed. The equation is the function of the maximum grain depth of cut g and the fracture toughness of the workpiece material, assuming that the nucleation of median crack during grinding is the same way as that in Vickers indentation test. Experimental results of the flexural strength of sintered silicon carbide ceramics ground with different kinds of grit diamond wheels under various setting conditions showed good agreement with the calculated ones. Assuming that temperature increase does not affect the size and shape of the crack, the strength of ground ceramics at elevated temperature is obtained.

High Temperature Oxidation of Ground Silicon Carbide Ceramics.

The relationship between oxygen penetration depth into ground silicon carbide ceramics during oxidation and the extent of the affected layer has been studied. The penetration depth was deter-mined by measuring oxygen profile using the SIMS technique. The depths of ground ceramics oxidized at 1200 and 1500°C were expressed as the function of the maximum grain depth of cut g which is related to the depth of median crack in grinding. The penetration depth was smaller than the crack size. The increase in oxidation time hardly affected the penetration depth at 1200°C. It is suggested that the oxide film prevents further oxygen penetration and that the oxygen penetration has little effect on the strength at elevated temperatures.