Effect of thickness and orientation on fatigue crack growth rate in 4340 steel

Fatigue crack growth rate in 4340 is evaluated from the viewpoint of orientation and specimen thickness. It is shown that orientation has little effect on the relation between crack growth rate and stress intensity factor. However, increasing the specimen thickness from 1/16 to 1/2 in. caused a significant increase in the value ofm in the relationda/dN α (ΔK)m, viz. 2.6 to 5.4. This is believed to show an effect of stress state in that, in the thicker specimens, fracture is flat and the stress state tends more toward plane strain. The data show that applying crack growth data from thin specimens to larger structural members can be misleading.