Sotomi Ishihara

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.

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

On the Distinction Between Plasticity- and Roughness-Induced Fatigue Crack Closure

A series of experiments has been carried out to determine why some alloys display plasticity-induced fatigue crack closure (PIFCC), whereas other alloys display roughness-induced crack closure (RIFCC). Two alloys were studied, the aluminum alloy 6061-T6 (PIFCC) and a steel of comparable yield strength, S25C (RIFCC). The experiments included the determination of the crack-opening levels as a function of ΔK, da/dN as a function of ΔKeff – ΔKeffth, removal of the specimen surface layers, removal of the crack wake, the determination of crack front shapes, crack surface roughness profiles, and the degree of lateral contraction in the plastic zone at a crack tip. Based on crack tip opening displacement (CTOD) considerations, it is concluded that PIFCC is favored in alloys of low modulus and relatively low yield strength. In addition, a low strain-hardening rate such as for the 6061 alloy will favor PIFCC. Steels with a higher modulus and a higher strain-hardening rate than 6061 will, in general, exhibit RIFCC, even at comparable yield strength levels. In ferritic steels, the fracture surface roughness and consequently the crack-opening level will increase as the coarseness of the microstructure increases.

Fatigue Lifetime and Crack Growth Behavior of WC-Co Cemented Carbide

It is well known that WC-Co cemented carbides have excellent wear resistance. However, information about their fatigue crack growth behavior and fatigue properties is limited. In the present study, rotating bending fatigue tests were carried out on a fine grained WC-Co cemented carbide to evaluate its fatigue lifetime and crack growth behavior. From observations of the micro-notched specimen surface during the fatigue process, it was revealed that most of the fatigue lifetime of the tested WC-Co cemented carbide is comprised of crack growth cycles. Using the basic equation of fracture mechanics, the relation between the rate of fatigue crack growth da/dN and the maximum stress intensity factor Kmax of the WC-Co cemented carbide was derived. From this relation, both the threshold intensity factor Kth and the fatigue fracture toughness Kfc of the material were determined. Fatigue lifetime of the WC-Co cemented carbide was estimated based on the fatigue crack growth law.

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.

THE INFLUENCE OF CRACK-FACE FLUID PRESSURE ON THE FATIGUE CRACK PROPAGATION DUE TO ROLLING CONTACT WITH FRICTIONAL HEAT

A three-dimensional surface planar crack problem in a half-space is considered under rolling/sliding contact with frictional heat and hydraulic pressure by the entrapped fluid within the crack. Rolling contact is simulated as a line load with both normal and shear components, moving with constant velocity over the surface of the half-space. The body force method for three-dimensional fracture mechanics is utilized to determine the three modes of stress intensity factors along the crack contour. To account for mixed-mode propagation, the modified Paris power law is used. Numerical results for the stress intensity factors and the simulations of fatigue crack propagation are given for 30-degree inclined planar surface cracks of semicircular shape. The effects of the frictional coefficient, sliding/rolling ratio, and the crack-face fluid pressure on the crack propagation life are considered for a high carbon-chromium bearing steel.