A stress-based fracture criteria validated on mixed microstructures of ferrite and bainite over a range of stress triaxialities

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 674; pp. 232 - 241
• Main Authors: Golling, Stefan, Östlund, Rickad, Oldenburg, Mats
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.09.2016
• Subjects: 22MnB5; Bainite; Cooling rate; Digital image correlation; Ductile fracture; Ferrite; Fracture mechanics; Heat treatment; Homogenizing; Hållfasthetslära; Mean-field homogenization; Microstructure; Solid Mechanics; Triaxiality; Heat treatment; Ductile fracture; 22MnB5; Digital image correlation; Mean-field homogenization
• ISSN: 0921-5093; 1873-4936; 1873-4936
• DOI: 10.1016/j.msea.2016.07.126

Hot stamping is a sequential process for formation and heat-treatment of sheet metal components with superior mechanical properties. By applying different cooling rates, the microstructural composition and thus the material properties of steel can be designed. By controlling the cooling rate in different sections of a blank, the material properties can be tailored depending on the desired toughness. Under continuous cooling, various volume fractions of ferrite and bainite are formed depending on the rate of cooling. This paper focuses on the ductile fracture behavior of a thin sheet metal made of low-alloyed boron steel with varying amounts of ferrite and bainite. An experimental setup was applied in order to produce microstructures with different volume fractions of ferrite and bainite. In total, five different test specimen geometries, representing different stress triaxialities, were heat treated and tensile tested. Through full-field measurements, flow curves extending beyond necking and the equivalent plastic strain to fracture were determined. Experimental results were further investigated using a mean-field homogenization scheme combined with local fracture criteria. The mean-field homogenization scheme comprises the influence of microstructure composition and stress triaxiality with usable accuracy, connoting auspicious possibilities for constitutive modeling of hot-stamped components.