An alternative presentation of the effects of the stress-ratio on the fatigue threshold

The effect of the stress ratio on the fatigue-crack-growth threshold value was investigated for pure Cu, an age-hardened aluminum alloy Al-2024, and an ultrafine-grain mechanically alloyed IN-905XL alloy, with particular consideration given to the effect of the compressive portion of the loading cycle on the closure behavior. Experimental results indicated that, to achieve a complete closure of a crack, different materials require the application of different compressive stresses. On the other hand, it was found that the effective threshold value is not affected by mean stress, and that it is characteristic for the materials tested. (I.S.)