Creep crack growth and fractal dimension of the grain boundary fracture in austenitic 21Cr-4Ni-9Mn steel

The creep crack growth and the fractal dimension of grain-boundary fracture (D-f or (D) over bar(f)) were examined using surface notched specimens with different fractal dimensions of the grain boundaries (D-gb) in austenitic 21Cr-4Ni-9Mn steel. The value of D-f estimated in the scale range larger than about one grain-boundary length decreased with crack growth and with increasing bulk stress (gross section stress), while the value of D-f was higher in the specimen with higher D-gb value. The value of (D) over bar(f) estimated in the scale range smaller than about one grain-boundary length was close to the value of D-gb in each specimen and did not change with crack growth and with the bulk stress (gross section stress). The growth rate of creep crack was lower and the threshold stress intensity factor for crack growth was higher in the specimen with the higher D-gb value.