Crack Initiation and Propagation in Ferrite/Martensite Dual-Phase Steel with Microscopic Heterogeneity by Fracture Toughness Tests
Masahito MOCHIZUKI, Masaru KOHARA, Hiroshi SHIMANUKI, Yukito HAGIWARA and Masao TOYODA
Abstract:It is important to study effects of microstructure on the characteristics of fracture in steels with microscopic heterogeneity. Crack initiation and propagation behaviors are of course dominant to the characteristics of fracture, and they are studied in two kind of ferrite/martensite dual-phase steels with the same micro-hardness in each microscopic phase. Three-point bending tests are performed to two kind of dual-phase steels, one has 30% volume fraction of ferrite and 70% of martensite (Ferrite 30% steel), and the other is Ferrite 70% steel. Brittle cracks are mainly initiated in the ferrite phase near the fatigue precrack tip in the center of specimen thickness by bending load, regardless of the difference of the volume fraction of microstructure. It is considered that not only the volume fraction of microstructure but also the stress-strain field due to microscopic heterogeneity affects crack generation. Crack mainly propagates along ferritic phase with connection to initial microcracks, and it is little influence on microscopic heterogeneity. Propagated crack width expands in proportion with the increase of CTOD value. Key Words:Microscopic heterogeneity, Crack initiation, Crack propagation, Ferrite/martensite steel, Dual-phase steel, Volume fraction, Brittle crack