Takahito Goshima

Crack propagation behavior of cermets and cemented carbides under repeated thermal shocks by the improved quench test

In this study, an improved quench testing method for thermal shock resistance has been proposed. Repeated thermal shock tests were performed on cemented carbides to show the advantages of the new proposed method that would enable us to estimate an intrinsic relationship between the crack propagation rate and the stress intensity factor under repeated thermal shocks. The cyclic thermal fatigue crack propagation behavior and fracture toughness values were shown to be independent of the specimen heights and the cooling media employed. We then evaluated the thermal crack propagation behavior for cermets and cemented carbides by using this method, and discussed the differences between both materials in the crack growth behavior on the basis of their microstructures.

On fatigue lifetimes and fatigue crack growth behavior of bone cement.

Bone cement is used to develop a mechanical bond between an artificial joint and the adjacent bone tissue, and any degradation of this bond is of serious concern since it can lead to loosening and eventually malfunction of the artificial joint. In the present study, the fatigue lives and fatigue crack propagation behavior of two bone cements, CMW Type 3 and Zimmer, were investigated, and it was found that the size and distribution of pores played a major role in influencing both the fatigue crack initiation and propagation processes. The fatigue lifetimes of CMW exceeded those of Zimmer because of a lesser density of large pores. When the fatigue lifetimes were plotted as a function of Klimax, the maximum initial stress intensity factor based upon the initiating pore size, the difference in fatigue lifetimes between CMW and Zimmer bone cements was greatly reduced. The fatigue crack growth behavior of both bone cements were similar. This is a further indication that the noted differences in fatigue lifetimes were related to the size of the pore at the crack initiating site.

THERMOMECHANICAL EFFECTS ON CRACK PROPAGATION IN ROLLING CONTACT FATIGUE FAILURE

Three kinds of cracks are considered for the tribological failure process. First, two-dimensional multiple surface cracks are analyzed. The stress intensity factors of the multiple surface cracks are calculated, and the noticeable effects on the crack growth are investigated. Second, as a more realistic model, a three-dimensional analysis is considered for straight growth of the planar surface crack. On the basis of the stress intensity factors at the crack front, and using the fatigue crack growth law for cemented carbide, silicon nitride, and bearing steel, the three-dimensional fatigue crack growth contour and the fatigue crack propagation life are predicted. Finally, corresponding to the curved crack, a two-dimensional multiple kinked crack in a half-space is considered. On the basis of the stress intensity factors, and using the fatigue crack growth law for the bearing steel, the two-dimensional fatigue crack growth path and the fatigue crack propagation life are predicted. In addition, the thermomechanical effects such as the frictional coefficient, sliding/rolling ratio, and the crack-face fluid pressure on the fatigue crack propagation life and induced fatigue pitting failures are considered.

The influence of the stress ratio on fatigue crack growth in a cermet

The fatigue crack growth behavior in a cermet was investigated as a function of the stress ratio, R. At the higher Kmax values the fracture path was through the cermet particles as well as through the binder phase. At low growth rates the fracture path was primarily through the binder phase. As a result the fatigue crack growth process, at a growth rate of l0−7 m/cycle the rate was influenced by Kmax, and to a lesser extent by the R value, whereas at a growth rate of 10−11 m/cycle the growth rate depend upon ΔK as well as the R value.

The static and cyclic strength of a bone-cement bond.

Four-point bending static and fatigue tests were carried out on bone–cement bonds. The effects of the pressurization and the washing of the bone joint face on the bond strength were investigated. The results are summarized as follows. When the bond surface of cancellous bone is washed prior to the application of the bone cement, both the static and fatigue strengths of the bond are increased relative to the corresponding properties of unwashed bone–cement bonds. From observations of bone–cement interfaces as well as the fracture surfaces of bone–cement specimens, it has been determined that bone cement was able to infiltrate into fine holes present in washed cancellous bone. However, such infiltration occurred to a much lesser degree in the case of unwashed cancellous bone. Increasing the molding pressure during the time of cement application to the bone from 39200 to 117600 Pa had a beneficial effect on the bending strength and fatigue properties, particularly in the case of washed bone cement specimens. An increase in molding pressure also resulted in a reduction in the amount of scatter in test results.

An improved method for the determination of the maximum thermal stress induced during a quench test

In the evaluation of the thermal shock resistance of a material, a standard procedure is to heat a number of bend specimens in a furnace to a series of selected temperatures, quench the specimens into a cooling medium, and then determine their bending strengths. The thermal-shock resistance is related to that temperature difference, DTc, between that of the furnace and the cooling medium at which the bending strength is abruptly lowered as the result of the formation of thermal cracks. The larger the value of DTc, the better the thermal shock resistance is considered to be (1–5). However, because the amount of data obtained in this type of testing is limited, it is difficult to investigate the details of the circumstances leading to the development of thermal stresses and the formation of thermal-shock induced cracks by this method. Moreover, uncertainty with respect to the thermal boundary conditions between the specimen surface and the cooling medium inhibits a theoretical analysis because of the dependence of heat transfer on time, specimen size and shape, the characteristics of the cooling medium, and the specimen’s surface condition (6–7). A modified test procedure is needed in order to obtain the data needed for understanding of the circumstances resulting in thermal crack formation. In order to use a material safely under thermal shock loading, the effects of repeated thermal shocks as well as of a single thermal shock on crack growth behavior and fracture toughness are of interest, and a critical parameter is the value of thermal stress developed under thermal shock conditions. Ishihara et al. (7–10) have utilized an improved thermal shock test which involved the determination of the transient thermal stresses immediately following a quench. These thermal stresses were calculated based upon measured temperature distributions which were obtained as a function of time following a quench. In the present paper a more convenient method for obtaining the maximum thermal stress in a thermal shock test without the need for measuring the temperature distributions will be described. In order to obtain experimental data for use in the analysis, thermal shock data using the improved thermal shock test procedure were obtained. Three types of hard, brittle materials were used in this study, a cemented carbide, a cermet and a ceramic (SN-220) which were produced by the Kyocera Corp. In addition tests were also carried out on a medium carbon steel, JIS45C, for purposes of comparison. Pergamon Scripta Materialia, Vol. 41, No. 5, pp. 553–559, 1999 Elsevier Science Ltd Copyright

Transient thermal stresses in a hollow cylinder subjected to γ-ray heating and convective heat losses

Abstract Transient temperature and the thermoelastic problem are analyzed for the transient thermal stresses arising in an infinite long hollow circular cylinder subjected to transient internal heat generation due to γ-ray radiation and cooled by convection on its inner and outer surfaces. Numerical calculations of the tangential stresses are carried out for the case of the heat generation decaying exponentially along the cylinder wall thickness. The influences of a decaying parameter and the Biot number on the results are considered.

On Electrochemical Polarization Curve and Corrosion Fatigue Resistance of the AZ31 Magnesium Alloy

Recently, due to increasing popularity of the magnesium alloys, many studies on corrosion resistance of the magnesium alloys have been reported to improve their corrosion resistance. However, it is not clarified yet whether an electrochemical polarization curve or a corrosion rate is a good measure for the corrosion fatigue resistance or not. In the present study, corrosion fatigue tests, the weight loss test, and measurement of the electrochemical polarization curve were performed in 3% sodium chloride solution using both the extruded and the rolled Mg alloys. It was clarified that there are no differences in the corrosion fatigue lives between the extruded and the rolled Mg alloys, though they have different corrosion resistance. So, it was concluded that the corrosion fatigue characteristic does not correspond well with the trend in corrosion resistance.

Initial Crack Growth Emanating from an Inclusion Due to Rolling/Sliding Contact

The stress intensity factors of a randomly oriented crack emanating from an inclusion in a half space are analyzed under rolling/sliding contact with frictional heat. The complex variable formulation of Muskhelishivili is used to reduce the problem to the simultaneous integral equations. These integral equations are solved numerically thus enabling the numerical calculation of the stress intensity factors. On the basis of these results, the location and direction of initial crack growth emanating from an inclusion are predicted numerically for the case of high carbon-chromium bearing steel. The initial crack growth is assumed to be determined by applying the maximum energy release rate criterion to randomly oriented crack emanating from an inclusion. The effects of frictional coefficient, slide/roll ratio and depth of inclusion on the location and direction of initial crack growth are considered.

TRANSIENT THERMAL STRESSES IN AN INFINITE PLATE WITH A HOLE DUE TO ROTATING HEAT SOURCE

A solution is given for the transient thermal stresses in an infinite plate with a circular hole due to a moving heat source. The heat source suddenly generates at a starting point and rotates around the circular hole at constant angular velocity. The transient temperature distribution is obtained by using the Greens function and by Laplace transformation. The associated transient thermal stresses are analyzed by using the thermoelastic potential and Neuber-Papkovich stress functions.