1979 Volume 48 Issue 9 Pages 731-736
The plastic zone and the plastic strain behaviours near the propagating fatigue crack for SM41 and HT80 steels were observed continuously on various specimen thicknesses, using optical interference method. The relationship between the shape or size of the plastic zone and stress intensity factor range (ΔK) was clarified. In addition, the effects of plastic zone size, and the slope of plastic strain distribution on crack propagating rate were investigated on each test steel.
Main results obtained are as follows.
1) The necking zone appears periodically ahead of a propagating carck in SM41 steel of which the lower yielding point is clear. The size of necking zone increases with a growth of crack. However, this periodic necking zone is not shown in HT80 steel.
2) The relationship between the length of plastic zone (Rx) and the ratio of ΔK/σY, appeared as follows;
Rx=C1 (ΔK/σY)2 for SM41 steel
Rx=C2 (ΔK/σY)1.5 for HT 80 steel
Here, C1 is affected by specimen thickness, while C2 is almost constant in both specimen thicknesses.
3) The slope of plastic strain distribution ahead of a crack is unvariable during the propagating of the crack.