EFFECTS OF V-NOTCH DIMENSIONS ON CHARPY IMPACT TEST-RESULTS FOR DIFFERENTLY SIZED MINIATURE SPECIMENS OF FERRITIC STEEL

In order to develop the small specimen technology in Charpy impact tes ting, the effects of V-notch dimensions on the test results were inves tigated for miniaturized specimens of a ferritic steel, Japanese Ferri te/Martensite Dual Phase Steel (JFMS). The miniaturized Charpy specime ns had four different square cross-sections of 3.3, 2, 1.5 and 1 mm, a nd each of them had a variety of V-notch dimensions (notch depth, notc h root radius and notch angle). All of the specimens were subjected to Charpy impact tests between 93 and 373 K using a specially instrument ed impact machine. The fracture surfaces of all tested specimens were examined by scanning electron microscopy. The main results obtained ar e as follows: (1) The ductile-to-brittle transition temperature (DBTT) varied noticeably depending upon the notch dimensions, some of the DB TTs exceeding that of the full size specimens. (2) The DBTTs for the m iniaturized specimens were uniquely defined by the elastic stress conc entration factor, K-t, leading to an important aspect that the DBTT fo r the full size specimens can be directly obtained from the DBTT of th e miniaturized specimens with a V-notch giving a suitable value of K-t . (3) The upper shelf energy (USE) was dependent only on notch depth o r ligament size, indicating that the notch geometry was practically un important. When all of the measured USEs were normalized by Bb(2) or ( Bb)(3/2) (B is the specimen thickness, b the ligament size), the norma lized USEs of the miniaturized specimens agreed with that of the full size specimens within the range of +/-15% except for one specimen whos e notch root radius was as large as 0.25 mm. (4) The observed characte ristics of fracture surface were essentially the same as those of the full size specimens. The measurement of lateral expansion, or ductilit y, was more useful in estimating the impact property of JFMS than that of fracture appearance (fibrous fracture).