Studies On Crack Propagation Under Fatigue-creep Combined Action Of 316L Steel At High Temperature

The engineering structure and mechanical equipment mostly experience hightemperature and cyclic loading. Crack propagation is influenced by material damage,causing leaking out or explosion. All these status will lead to economic losses andpeople casualty. Scholars are engaged in development and research of residual fatiguelife at high temperature, and gain momentous achievement. Crack growth law has notbeen solved well so far under fatigue-creep combined action. So crack propagation rateis studied at high temperature, and fatigue and fatigue-creep crack propagation tests of316L are carried on.The creep damage is analyzed under fatigue-creep combined action. A calculatingmethod of creep crack propagation rate basing on the creep damage and crackpropagation rate under fatigue-creep combined action can be put forward.From the research of fatigue crack propagation tests at the temperature of 673K,823K, 873K, the formula of Paris showing fatigue crack propagation rate can beobtained. The change of the crack propagation rate increases with the increasing ofâ–³K,but the change of the exponent decreases with the increasing of the temperature. Theexponent at the temperature of 823K and 873K is below it at the temperature of 623Kvisibly. The change range of coefficient is also extensive and it increases with theincreasing of temperature. The comparison of the fatigue-creep propagation rate in threetemperature shows that the effect of temperature on the fatigue crack growth is littlewhen the value ofâ–³K between 25 to 45.From the research of fatigue-creep crack propagation tests with hold time of 30second at the temperature of 673K, the change of the crack propagation rate increaseswith the increasing ofâ–³K. And it's higher than crack propagation rate without holdtime. High stress will lead to creep damage and increasement of crack propagation rate.Creep crack propagation rate at this temperature is validated by the calculating method of creep crack propagation rate which is basing on the creep damage.Analyse appearance of fracture using scanning electronic microscope. Followingthe change of the temperature, appearance of fracture is almost invariable, but visibletoughness fatigue striation and weak oxidation are observed. At the temperature of673K, the sample without hold time is more continuous than the sample with hold time.For front part of sample without hold time, the path is smooth. But near the crack tip,there is flexural without secondary crack. Crack propagation is transgranular.Good contact and partially contactless area between layers are considered. FEManalysis and safety assessment are accomplished respectively to obtain the fatigue andfracture performance. It's shown that the flaw fatigue life of partially contactless areabetween layers is lower than good contact between layers evidently.