Original paper(Vol.45 No.4 pp.417)
Evaluation of fracture strain from measurement of aspect ratio for ferrite grains
Ohashi Makoto
Abstract:A quantitative metallographic study was performed on JIS SS400 steel to estimate the fracture strain from the measurement of aspect ratio for ferrite grains. The circumferentially notched round bar tensile specimens with the notch root radius of 6, 3, 1, 0.5 mm were employed to obtain a wide range of fracture strain.
The relationship between plastic strain and aspect ratio for ferrite grains was obtained from the detailed metallographic observations on the polished and etched longitudinal midsection through the fractured half of smooth round bar tensile specimen. A large number of ferrite grains, plastically deformed to various strains, were examined carefully to determine the dependence of aspect ratio on plastic strain.
The aspect ratio was well expressed by an exponential equation as a function of plastic strain. It was then possible to provide an appropriate estimate of the fracture strain of notched specimens from the above relation. The average aspect ratio of 9 views immediately underneath the fracture surface was used to deduce the fracture strain. The estimated values were found to be in reasonably good agreement with the measured values within an error of about 1 to 8 pct.
Key Words:quantitative metallographic study, fracture strain, aspect ratio, ferrite grain, circumferentially notched round bar tensile specimen