The high temperature structures suffer from rapid varying loads as well as steady loads. The rapid varying loads induce the fatigue damage to the structures, while the steady loads the creep damage. When a fatigue crack grows through the structure under the situation of creep-fatigue interaction, it joins to microcracks or microvoids. The interaction like that makes the fatigue crack grow fast.
In the present study, the fatigue crack growth behavior and the variation of fracture toughness, $J_c$ were observed with the degree of creep damage with Type 304 stainless steel. At first, various amounts of creep damage were induced to smooth specimens with different creep conditions. The amount of damage was quantified by measuring the variations of mechanical properties such as Young's modulus and hardness. Fatigue crack growth tests and $J_c$ tests were conducted after crack starter notches were machined to the damaged smooth specimens. The fractured surface was observed with a scanning eletron microscope(SEM).
As the result, the fatigue crack growth rate became faster and faster, as the degree of damage got larger and larger. The photographs of the fatigue cracked surfaces by SEM showed the partially intergranular crack growth of damaged specimens, and the wholly transgranular crack growth of undamaged specimen. The larger degree of damage made the value of $J_c$ lower. The photographs by SEM showed that the transgranular dimple fracture was the mechanism of ductile fracture of undamaged specimen, and the intergranular dimple fracture was that of damaged specimens.
