Katsushi Saruki

Fatigue strength at a number of cycles of thin hard coated steels with quench-hardened substrates

Abstract Bending fatigue tests were carried out to clarify the effects of thin hard coatings on the fatigue strength at a number of cycles for steels. Coatings of carbides and nitrides produced by thermoreactive deposition and diffusion, chemical vapour deposition and physical vapour deposition, processes and of electroplated chromium on various steels were tested as well as hardened steels. The results thus obtained revealed that the fatigue strength at a number of cycles highly depended on hardness and residual stress at the substrate surface as in the case of the endurance limit and that the existence of the coatings suppressed the cracking in the substrates to increase the fatigue strength after loading cycles. Effects of coating thickness, surface roughness, notch shape, and post-heat treatment were also examined.

Fatigue strength of steels with thin hard coating

Abstract Bending fatigue tests were made on various steels coated with carbides (VC, NbC or CrC) by the molten salt bath immersion process, with TiC by CVD, with TiN by PVD and also on the same kinds of steels hardened, in order to clarify the effect of coatings. The results indicated that the endurance limits of carbide coated steels were slightly lower than those of hardened steels in some cases. The observations of specimens before and after fracture revealed that the fatigue fracture initiated not in the coatings but in the substrate just adjacent to the coatings. It was concluded from measurements of the residual stress and the hardness that the lower endurance limit of carbide coated steels mentioned above was due to the increase in tensile residual stress and the decrease in hardness of the substrate by consumption of carbon in the substrate for carbide formation. Furthermore, the bending endurance limit was linearly correlated to the residual stress in the substrate on the same hardness level, regardless of the substrate steels used in the tests. Diffusion soaking to recover the consumed carbon was effective in increasing the hardness and decreasing the tensile residual stress of the substrate, leading to an increase of the endurance limit.

Endurance limit of thin hard-coated steels in bending fatigue

Abstract Bending fatigue tests were carried out on steels coated by thermoreactive deposition and diffusion of vanadium carbide (VC) and chromium plating, as well as on ordinarily hardened steels, to clarify the effects of the coatings, VC coating thickness, post-heat treating conditions, surface roughness, and notches. The linear correlation between the endurance limit and the residual stress in the substrates was confirmed in relation to the substrate hardness in hardened and VC-coated steels. The correlation cannot be applied to chromium-plated steel, owing to cracks in the plating. It was recognized that VC coatings as thick as 18 μm, and a very rough surface with 11 μm R max , did not necessarily reduce the endurance limit of VC-coated steels. The endurance limit of notched specimens decreased in proportion to the stress concentration factor in VC-coated steels as well as in hardened steels.

Correlation between Microstructure and Fatigue Behavior of Silicon Nitride Ceramics.

Microstructures of seven kinds of silicon nitride ceramics were quantified by electron cyclotron resonance plasma etching and sequential image analysis. And the correlations between microstructure and fracture toughness, intrinsic bending strength or cyclic fatigue crack growth exponent were estimated by using multiple regression analysis.The characterized microstructure parameters, composed of the mode and distribution of grain size, the fraction of needle-like grains and the amount of additives, were correlated to the experimental mechanical properties with high correlation coefficients. For improving the fracture toughness, intrinsic bending strength and cyclic fatigue crack growth exponent of silicon nitride ceramics at the same time, it may be effective to increase the needle-like grains, according to the weight factors of microstructure constituents.Fatigue crack growth passages in the materials with highest and lowest fatigue crack growth exponents were analyzed. As a result, in the material with a high exponent, crack arrest and bridging were more often caused by the intergranular fracture of needle-like grains. A larger amount of additives and a broader distribution of grain size make crack arrest and bridging tend to occur more often.

Fatigue Lifetime Prediction Method for Ceramics with Defects.

A new method of predicting the fatigue lifetime of ceramics with various sizes of defects has been developed. In this method, first, specimens each of which has an artificial crack of constant size are used to estimate fatigue crack propagation parameters. In the vicinity of the tip of an artificial crack induced by Knoop indentation, a tensile stress field is generated. Then, the stress intensity factor acting as the driving force of fatigue crack growth was considered the sum of two terms associated with the applied stress and residual stress field. By numerical computation inserting the stress intensity factor into the power law of crack propagation, crack propagation parameters are estimated. Subsequently, using the obtained parameters, the fatigue lifetimes of the smooth specimens with fracture originating from a small defect, are predicted.This method was applied to three types of silicon nitride ceramics. As a result, the cyclic fatigue lifetimes of precracked specimens, which were set free from residual stress field by surface removal, were precisely predicted using the obtained parameters. Thus, the values of these parameters were proved to be adequate. Furthermore, the predicted fatigue lifetimes of the smooth specimens were in good agreement with the experimental results, considering the scatter of static strength.

Influences of Defects upon the Fatigue Strength of Si3N4 Ceramics.

Microstructure of engineering ceramics, especially the distribution of defects and the density, sometimes varies with the lot of manufacture. For this reason, it is important to investigate the influences of the microstructure upon the mechanical properties.In this study, the rotating bending fatigue tests were carried out on two lots of Si3N4 ceramics, and the influences of the defects and density upon the fatigue strength were investigated. Bulk densities of lots A and B were 3.02g/cm3 and 3.16g/cm3, respectively. Average static bending strength of lot B was about 20 percent higher than that of lot A due to the differences in defects distribution and fracture toughness. However, the two lots scarcely differed in rotating bending fatigue strength. This is considered to be because the fatigue strength largely depends on the size and kind of defects.Fatigue strength was predicted on the basis of a fatigue datum in the case when the fracture origin was the largest pore defect. Crack propagation rate was assumed to be in proportion to the power of the stress intensity factor. Consequently, when the fracture origin was a pore defect with inclusion or a micropore cluster, the fatigue strength was higher than the predicted value, in most cases. Therefore, the fatigue life prediction based on pore defects leads to reliable estimation.

The development of a resonance type gear fatigue tester.

A resonance type gear fatigue tester has been developed to evaluate tooth root fatigue strengths rapidly and efficiently. The tester operates on the resonance condition. One of a pair of test gears is held stationary to the tester bed and the other, fastened to the vibratory shaft, is oscillated against the stationary gear. The torsional vibration of the vibratory shaft is excited by the rotating eccentric on the end of the lever arm fixed to the shaft. Torque amplitude is controlled by varying the rotational speed of the eccentric, which is rotated with a 1.5 kW induction motor driven by a frequency converter. If the test frequency is slightly below the natural frequency of the vibrating system containing the test gears, sudden overloading occurs with the natural frequency drop due to slip on the tooth face and so on. To prevent overloading, a tester is designed so that the frequency can be set above the natural frequency. The test frequency range of the tester is 1000-3000 cpm and the torque capacity is 1200 N·m.