Crack growth in compact specimens of type 304 stainless steel is studied at 538oC. Loading
conditions include pure fatigue loading, static loading and fatigue loading with hold time. Crack
growth rates are correlated with the stress intensity factor. A finite element analysis is performed to
understand the crack tip field under creep-fatigue loading. It is found that fatigue loading interrupts
stress relaxation around the crack tip and cause stress reinstatement, thereby accelerating crack
growth compared with pure static loading. An effort is made to model crack growth rates under
combined influence of creep and fatigue loading. The correlation with the stress intensity factor is
found better when da/dt is used instead of da/dN. Both the linear summation rule and the dominant
damage rule overestimate crack growth rates under creep-fatigue loading. A model is proposed to
better correlate crack growth rates under creep-fatigue loading:
1
c f
da da da
dt dt dt
Ψ −Ψ
=
, where Ψ is an
exponent determined from damage under pure fatigue loading and pure creep loading. This model
correlates crack growth rates for relatively small loads and low stress intensity factors. However,
correlation becomes poor as the crack growth rate becomes large under a high level of load.