The behavior of short fatigue cracks differs from that of long cracks because of greater sensitivity to the microstructure, a greater size of the plastic zone relative to crack length, and a lesser extent of crack closure. Advances have been made in the understanding of the fatigue crack growth process of short cracks, and this understanding has been employed in the development of analytical treatments of short fatigue crack growth. The present paper reviews this progress and also discusses the relevance of short fatigue crack behavior to technologically significant areas.
Rotating bending fatigue tests have been conducted at a room temperature in laboratory air using specimens of medium carbon steel (S45C), low alloy steel (SCM435) and titanium alloy (Ti-6Al-4V) with high velocity oxy-fuel (HVOF) sprayed coating of a cermet (WC-12%Co) and S45C with wire flame spraying (WFS) sprayed coating of a 13Cr steel (SUS420J2), and the fatigue strength and fracture mechanisms were studied. The fatigue strength evaluated by nominal stress was strongly influenced by substrate materials and the thickness of sprayed coatings. Detailed observation of crack initiation on the coating surface and fracture surface revealed that in the WC cermet-sprayed materials, small defects initiated at WC grain boundaries coalesced and then the crack grew rapidly in the coating, while in the 13Cr steel-sprayed material, many microcracks were initiated from defects on the coating and coalesced to be a main crack. Cracks were initiated in the substrate due to the stress concentration of the crack in the coating, which was modeled by finite element analysis. The fatigue strength of the sprayed materials was dominated by that of the sprayed coating. Thus, the fatigue strength could be evaluated uniquely in terms of the true stress on the coating surface.
The initiation and growth of small fatigue crack in a gamma titanium aluminides (TiAl) have been investigated by scanning electron microscope (SEM) in situ observation in vacuum at 750°C and room temperature. The initiation of crack occurs transgranularly at room temperature, but both transgranularly and intergranularly at 750°C. Small cracks grow at stress intensity factor lower than the threshold value for long crack growth. The fatigue life at 750°C is shorter than that at room temperature. When the stress is modified by Young's modulus, fatigue limit at 750°C is the same as that at room temperature. However, fatigue lives at high stresses showed difference between room and elevated temperatures.
In order to investigate the effects of grain bridging degradation on the cyclic fatigue in polycrystalline alumina, crack propagation tests were carried out under cyclic load and static load. Two kinds of small semi-elliptical surface crack were introduced by Knoop indenter. Polishing the surface layer, the residual stress around the Knoop indentation was completely removed. The cyclic loading accelerates crack propagation in small crack as the same as in long crack. On the other hand, measuring and analyzing the crack opening displacement (COD) of 350μm crack, we determined the net stress intensity factor KItip. Under the cyclic load, the reduction of bridging force resulted in an increase in KItip which, in turn, accelerated the crack propagation rate. Furthermore, it is found that bridging degradation mechanism under cyclic loading does occur, but that this mechanism is not sufficient to completely account for "fatigue crack growth in ceramics".
Fatigue crack growth tests were carried out on commercially pure titanium sheets with a drilled hole and a center notch, in order to clarify anisotropy of fatigue crack growth and the influence of pre-straining on the anisotropy. The crack growth resistance in rolling direction was higher than that in a direction transverse to the rolling direction for surface cracks longer than 2.5mm initiated at a drilled hole, and for cracks initiated at a center notch at ΔK higher than 10MPam1/2, in contrast to the growth behavior of various steels. Fractograpy observations showed that the anisotropy became remarkable after transition from cleavage-like facet dominant fracture surface to ductile appearance dominant one. The transition occurred when reversed plastic zone size at the crack tip was comparable to the grain size of the metal during crack growth. Pre-straining had only a little effect on the crack growth anisotropy.
This paper describes the mechanical and thermal properties of a cubic boron nitride (cBN) by molecular orbital and molecular dynamics simulations. The interatomic potential of cBN used for the molecular dynamics simulation was proposed by an ab-initio molecular orbital calculation for a cBN cluster. The elastic stiffness and the bulk modulus of cBN were found to be close to those of diamond by the molecular simulation. The bulk modulus of cBN in the simulation agreed with that in experiment. The equilibrium molecular dynamics simulation estimated the effect of temperature on thermal conductivity and coefficient of thermal expansion of cBN. The thermal conductivity of cBN drastically decreased with increasing temperature above 150K. The coefficient of thermal expansion of cBN was independent of temperature at 50K-900K, but that of cBN increased above 900K with increasing temperature.
Heat conduction phenomena and induced thermal stresses are simulated from a microscopic viewpoint by using a molecular dynamics method. Three dimensional rectangular parallelepiped model composed of 2000 atoms surrounded by periodic boundary is imposed for the simulations. Thermo-physical properties such as melting temperature, specific heat, heat conductivity and latent heat are evaluated at the first step to obtain the fundamental data for the following heat conduction simulation. The central part of the model is heated for two cases with and without melting, and the variation of temperature, potential energy and thermal stresses are simulated. These simulations are carried out with two different potential functions, Lennard-Jones and Morse type, in order to clarify the effects of the interatomic potential, which follows to qualitatively demonstrate similar tendency in spite of remarkable quantitative differences. Then the variation of temperature is compared with the numerical solution of macroscopic heat conduction equation. It is clarified that the results show good agreement with each other if they are plotted against non-dimensional time, or Fourier number. Stresses calculated by molecular dynamics method are also compared with macroscopic thermal stresses.
This paper examines an implementation of a layer-peeling method to inverse scattering problems in the time domain, which is applied here to the case of one-dimensional lossless discontinuous medium. The method is based on the fast Schur recursion applied directly to the discrete problem. The method is tested on several numerical examples in order to compare its performance to the standard invariant imbedding algorithm. The reconstructions of media from both synthetic data and measured data are presented. The layer-peeling method is shown to be considerably faster than the invariant imbedding method without the loss of precision.
Soda lime glass beads were chemically treated by either an aqueous solution of NaOH (3mol/l) or a hydrogen peroxide solution (30vol%) containing TaCl5 in 5×10-3mol/l or by both of them. The treated samples were soaked up to 14 days in a simulated body fluid (SBF): Na+ 142.0, K+ 5.0, Ca2+ 2.5, Mg2+ 1.5, Cl- 147.8, HCO3- 4.2, HPO42- 1.0, SO42- 0.5 (in 10-3mol/l), or in another solution 1.5 times as concentrated as SBF (1.5SBF). The SBFs were kept at 36.5°C and at 7.25 in pH. Apatite was deposited on the samples treated with the NaOH solution, the H2O2/Ta solution, and both of them before soaking in 1.5SBF. Ta(V) ions were present on not only the samples treated with the H2O2/Ta solution but those treated with both NaOH and the H2O2/Ta solutions. It was concluded that Ta(V) ions on the surface layer had ability of inducing apatite deposition, and the ability was enhanced by the coexistence with silanol groups.
A recently developed Zn-Al hot-dip coating on steel has a higher corrosion resistance than that of the Zn hot-dip coating. For estimating the corrosion rate for the hot-dip coating on steel, the polarization resistance (Rp) was measured using a coulostatic method. The cross-section of the Zn-Al hot-dip coating was analyzed by X-ray and EPMA. The results showed a tendency in which the Al concentration increased from the surface of the coating toward the base steel side. Based on the Rp values of five kinds of Zn-Al coatings hot-dip coated in a bath with 5, 10, 15, 20 and 30% Al, the coating obtained in the 15% Al bath showed the highest Rp value. In other words, this indicated that the corrosion resistance of the coating from the 15% Al bath was superior. Electrochemical impedance spectroscopy (EIS) with a frequency response analyzer (FRA) was used to obtain further information about the interface of the Zn-Al hot-dip coating and a 5% NaCl aqueous solution of pH 8. The measurement results showed two capacitive semicircles on the high-frequency side and a diffusion curve on the low-frequency side, which are considered to be formed by a typical Randles-type equivalent circuit. This results the fact that the corrosion reaction is rate limited by both the charge and material transfer processes.