In order to examine the behavior of fatigue cracks in pure iron subjected to rotary bending and cyclic torsion, the copper electroplating method of stress analysis and slip phenomena in the material was applied. The charge in metal properties caused by fatigue cracks had no effect on the propagating stress of cracks. The difference of the fatigue-notch factor between bending and torsion was due to that of both the strain-concentration factor and the strain gradient at crack tips. It was found that in torsion the fatigue cracks propagated when the cyclic shear stress over one grain reached cracks depended on the tensile stress exerted over more than one grain size.
By observing slip lines in a copper foil with grown grain structure, the method of measuring cyclic stresses acting on machine elements is studied. The features and distinction of this method in comparison with the copper electroplating method or the method using a recrystallized foil are examined. This method is superior to the method using a recrystallized foil from the standpoint of ease of measurement, and its strain sensitivity is much higher than that of the electroplating method. As an application of this method, the stress concentration factors in various types of circular shafts with a flat part under cyclic torsion are obtained, with the results showing good agreement with those obtained previously.
The dependence of slip line occurrence at the periphery of microcircular holes formed by photoetching in an electrodeposited copper foil upon the number of cycles and cyclic stress magnitude for plane bending and cyclic torsion is examined. The results show that the principal stresses in the element under biaxial stress, which are undetectable by the conventional copper electroplating method, can be evaluated using the equation based on the probability process of slip line occurrence, which takes into account both cyclic stress magnitude and the number of cycles.
The dependency of grown grain density on cyclic stress and temperature was examined by computer image processing. The relation among the increasing rate of grown grain density, cyclic stress and the absolute temperature of atmosphere could be expressed by a rate process equation, and the stress measurement method using an increasing rate of grown grain density was developed. The effect of temperature fluctuation of atmosphere on the accuracy of measurement can be evaluated on the basis of the rate process equation.
The effect of plate thickness on the propagation rate of a mode I surface crack under biaxial stresses by the simultaneous action of plane bending and cyclic torsion was examined, and the crack opening displacement COD was measured by magnifying a scanning electron micrograph 15000 times. The relationship between the propagation rate da/dN and the stress intensity factor rangeΔK depends on both the biaxial stress ratio C and the maximum principal stress amplitudeσ1. COD measured under the same conditions ofσ1 and crack length increases with the decrease in C. A unique relationship is obtained between da/dN and crack tip opening displacement CTOD, irrespective of C and σ1. Thus, it can be concluded that the main factor which controls da/dN is CTOD.
The crack propagation rate of a mode I surface crack in a plate, the crack tip opening stresses or displacements under various biaxial stress ratios by the simultaneous action of plane bending and cyclic torsion are examined. The aspect ratio of a crack is not affected by the biaxial stress ratio, but the relationship between the propagation rate da/dN and the stress intensity factor range ΔK or the effective stress intensity factor range ΔKeff depends on the biaxial stress ratio. This is mainly due to the differences of both CTOD and the scale of the slipped region at the crack tip caused by the difference of the biaxial stress ratio.
The effects of frequencyb and waveform of cyclic stress on the slip initiation are investigated using several kinds of metal foils. On a directional copper foil, the critical stresses of the slip initiation expressed in terms of the loading time agree with one another regardless of the frequency, and they're independent of waveform. On the other hand, an iron foil shows obvious dependence of the slip initiation on both the frequency and waveform of cyclic stress. With an appropriate prestraining, however, these characteristics of the iron foil develop a similar tendency to those of the copper foil. Consequently, a directional copper foil or an appropriately prestrained iron foil is remarkably effective for the practical stress measurements in case that the frequency and waveform of repeated loads acting on a machine element vary in operation.
