Yasuo Nagase

Measurement of Variable Amplitude Stress Utilizing the Density of Slip Bands.

In order to evaluate the stress of a machine part subjected to variable stress amplitude, the density of fatigue slip bands produced on aluminum foil is proposed as a measure for this purpose. The fundamental characteristics of slip band formation were examined under both constant and variable amplitude stresses. It was found that the density of slip bands was dependent on both the stress amplitude and the number of stress cycles. The variation of stress amplitude hardly affected the increase of the slip bands in successive stress cycles, irrespectively of the variable mode of stress. The density of slip bands under variable loading, fs was well correlated with the equivalent stress, σeq=(ΣσαiNi/ΣNi)1/α, where σi, Ni and α are stress amplitude, the number of stress cycles at σi, and a material constant determined from an aluminum foil used, respectively. It is concluded that the equivalent stress can be evaluated by the density of slip bands produced on aluminum foil, using the relationship between fs and applied stress amplitude obtained under constant loading. The accuracy of measuring the equivalent stress was within 3%.

On the measurement of second principal stress by the copper electroplating method. Circumferential stress at the bottom of circular groove by bending.

The copper electroplating method of stress analysis utilizes the phenomenon that slip bands appear when the maximum shearing stress in the copper plating attains the proper value to the plating which is a function of the number of stress cycles. Accordingly, in the case of combined stress, it is generally impossible to obtain the second principal stress by the copper electroplating method. In this paper, a method that makes it possible to measure the second principal stress at the bottom of a circular groove subjected to bending is proposed, and measuring accuracy of this method is briefly discussed. Based on this method, stress concentration factors of the second principal stress are obtained. The results are in good agreement with previous results, and it is concluded that the method presented here can be used satisfactorily for the measurement of the second principal stress at the bottom of a groove.

Effects of small prestrain on fatigue strength of eutectoid steel with spheroidal pearlite structure.

In order to clarify the effect of small prestrain on the fatigue strength of carbon steel, fatigue behavior of plain specimens of eutectoid steel subjected to tensile prestrains is investigated, and compared with that of annealed or strain-aged specimens. It is found that small prestrain of a few percent does not decrease either the fatigue limit or the crack initiation and propagation lives. Fatigue slip initiation, however, is accelerated by the small prestrain. It is also found that no cyclic strain aging occurs. From these results, it is concluded that dislocations are unpinned by the small prestrain and this causes fatigue slip to occur easily, but the fatigue behavior after the slip initiation is not affected because spheroidal cementite acts as obstacles to further slip deformation and the stage I crack propagation.