Abstract:

Experimental investigation is carried out in this paper on the low cycle fatigue (LCF) behavior of directionally solidified (DS) Ni-based superalloy DZ125 at 850℃ with differential notch geometries and theoretical stress concentration factors (K_t). The stress-strain field at notch roots is obtained based on the elastic-plastic finite element analysis. Life prediction of notched specimens is then performed by the introduction of conventional and K_t-modified theory of critical distance (TCD) to SWT parameters. The results show that, the resistance of LCF has little to do with the geometrical shape of notch specimens but is heavily K_t-dependent. No matter by taking sharp specimens as calibration samples or dealing with critical distances by the statistical averaging method, both point method (PM) and line method (LM) of the conventional TCD exhibit that the life prediction ability related to the stress concentration factor is larger than the five times scatter band. However, a highly improved scatter band within two times can be obtained by both point and line methods of K_t-modified TCD, and the two methods are notch-geometry independent. The SWT parameter may be used to consider the effect of mean stress (stress ratio). Therefore, accurate life prediction, based on the test data of LCF of smooth specimens and just one kind of notched specimens, can be carried out using this modified TCD for other notched specimens at different stress ratios, and its application will be simple and convenient.

Key words: low cycle fatigue, theory of critical distance, SWT parameter, Manson-Coffin method, directionally solidified nickel-based superalloy