Distributions of α' Martensitic Phase Transformed on Fracture Surfaces of SUS 304 Stainless Steel Having Through- and Part-Through-Thickness Fatigue Cracks

The present paper investigates distributions of α’ martensitic phase on fracture surfaces of plate specimens of SUS 304 stainless steel having through- and part-through-thickness fatigue cracks. The volume fraction Vα’ of the α’ phase transformed on the fracture surfaces including the two types of cracks was measured after specimens had been fractured due to fatigue at room temperature in air. The measurements were made with ferrite scope. The contour maps of Vα’ revealed crack shapes imprinted on the fracture surfaces of the specimens; i.e., nearly rectangular shapes on the fracture surfaces of through-thickness-cracked specimens, whereas semi-elliptical shapes on the fracture surfaces of part-through-thickness-cracked specimens. The values of Vα’ measured on crack surfaces can be correlated with the maximum stress intensity factor values Kmax calculated at the points of Vα’ measurements. The resultant Vα’ vs. Kmax plots fall within narrow bands whose upper and lower bounds are expressed by exponential curves. These results imply that quantitative post-fracture analysis can be made by measuring Vα’ values on the fracture surfaces of structural components with ferrite scope.