Page 93 - Biaxial Multiaxial Fatigue and Fracture

78 h! ISOBE AND S. SAKURAI

Mises’ eq. strain range A%q (%)
(a) von Mises’equivalent strain range

I ’ ’ ’I’

0 @=-1.00 -
WCF
Q @=-0.50 -

Maximum principal strain range A&, (“A)
(b) Maximum principal strain range
Fig. 20. Relationship between normalized crack growth rate and strain parameters

stress within the factor of two scatter band, whereas the scatter of data was large when the
von Mises equivalent strain range was employed. Therefore, the maximum principal strain is
a better parameter for evaluating crack growth rates than von Mises’ strain.
There was little difference between cracking in base metal and in weldments. Therefore, the
reduction in fatigue life seen in weldments was not because the base metal and weld metal have
different degrees of resistance to cracks. The initiated length, which is affected by grain
boundary oxidation, is the most important factor in the life reduction of weldments. The scatter
band of the crack growth rate was large in the region where the J-integral range was less than 5
N/m in Fig. 19(b), especially for the torsional fatigue test. Figure 21 shows the relationship
between the maximum principal strain range and failure life. Failure lives in torsional fatigue
tests were larger than those in uniaxial tests although the principal stress and