動電形引張圧縮ランダム疲労試験装置とこれによる実験例

抄録

A push-pull random load fatigue testing apparatus of electro-dynamic type was newly designed and constructed. In order to conduct random load fatigue tests of both narrow and fairly wide bandwidth, the apparatus was required to keep its frequency response flat from 20cps to 175cps by means of a negative feedback of velocity of the moving coil. The stress on the test specimen was measured by using an extensometer of a semi-conductor strain gage type or of a capacity type. The plastic strain of the test specimen was determined by subtracting the elastic strain from the total strain. The stress and the plastic strain were observed by an oscilloscope, and were counted on line by level crossing counters throughout the random load fatigue test. The drift of average stress amplitude for a long period was controlled by comparing an average voltage of a rectified signal of the load cell output with a pre-setted reference voltage.
The fatigue tests were conducted with unnotched specimens of 0.2 percent carbon steel under the condition of random loading of a narrow band, in addition to the constant stress amplitude test. It was found that a linear cumulative damage assumption expressed in terms of the plastic strain range, gave a good approximation for life prediction.