In order to develop the small specimen technology in Charpy impact testing, the effects of V-notch dimensions on the test results were investigated for miniaturized specimens of a ferritic steel, Japanese Ferrite/Martensite Dual Phase Steel (JFMS). The miniaturized Charpy specimens had four different square cross-sections of 3.3, 2, 1.5 and 1 mm, and each of them had a variety of V-notch dimensions (notch depth, notch root radius and notch angle). All of the specimens were subjected to Charpy impact tests between 93 and 373 K using a specially instrumented impact machine. The fracture surfaces of all tested specimens were examined by scanning electron microscopy. The main results obtained are as follows:
(1) The ductile-to-brittle transition temperature (
DBTT) varied noticeably depending upon the notch dimensions, some of the
DBTTs exceeding that of the full size specimens. (2) The
DBTTs for the miniaturized specimens were uniquely defined by the elastic stress concentration factor,
Kt, leading to an important aspect that the
DBTT for the full size specimens can be directly obtained from the
DBTT of the miniaturized specimens with a V-notch giving a suitable value of
Kt. (3) The upper shelf energy (
USE) was dependent only on notch depth or ligament size, indicating that the notch geometry was practically unimportant. When all of the measured
USEs were normalized by
Bb2 or (
Bb)
3⁄2 (
B is the specimen thickness,
b the ligament size), the normalized
USEs of the miniaturized specimens agreed with that of the full size specimens within the range of ±15% except for one specimen whose notch root radius was as large as 0.25 mm. (4) The observed characteristics of fracture surface were essentially the same as those of the full size specimens. The measurement of lateral expansion, or ductility, was more useful in estimating the impact property of JFMS than that of fracture appearance (fibrous fracture).
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