FATIGUE CRACK-PROPAGATION IN AUSTENITIC AND FERRITIC STAINLESS-STEEL SINGLE-CRYSTALS

The orientation-dependence of fatigue crack propagation rates in single crystals of austenitic and ferritic stainless steels has been investigated at low stress levels on standard fracture mechanics specimens. The stress intensity factor required for a crack growth rate of 10-6 mm/cycle may vary by up to 50% according to the crystal orientation. In general, relatively high crack growth rates are associated with macroscopically plane fracture surfaces for both the f.c.c. and b.c.c. crystal structures. The crystallographic conditions for the formation of macroscopically plane fracture surfaces at low stress levels do not always correspond with the predictions of the original alternating slip model. In particular, for the ferritic steel, only those orientations for which two 〈111〉 slip directions are normal to the crack front and symmetrically inclined to the crack plane allow the formation of macroscopically plane fracture surfaces. The macroscopic appearance of the austenite fracture surfaces depends upon the crystal orientation and the crack growth rate: at low stresses, plane fracture surfaces are developed for most of the orientations tested. A model of crack propagation by alternating shears, in which the shears are accommodated by multiple slip processes at the crack tip is proposed to explain these results. © 1979.