Specimen size effects and ductile fracture of HY-100 steel
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formerly Graduate Research Assistant with the Department of Materials Science and Engineering, The Pennsylvania State University, is Metallurgist with Applied Technical Services, Marietta, GA. D.A. KOSS, Professor, is with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Contact e-mail: [email protected] R.K. EVERETT, is with the Office of Naval Research, International Field Office, London, England. Manuscript submitted August 2, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
The material was HY-100 steel in the form of a commercial, hot-rolled, quenched and tempered, 25.4-mm-thick steel plate, which was provided by the Naval Surface Warfare Center, Carderock Division. As shown in Figure 1 and described in detail elsewhere,[8,9] the microstructure contains large MnS inclusions that are elongated in the rolling direction and are somewhat lath shaped in the rolling plane (normal to the short-transverse direction). These elongated inclusions, which are, on average, 50 m in length, are present at a level of volume fraction of ⬃0.00015 (⫾0.00005). In addition, small equiaxed submicron MnS particles are also present as are irregular-shaped carbides, typically with a major dimension of 0.3 m or less. Importantly, a chemically banded microstructure is known to exist in the HY-100 steel such that the MnS inclusions tend to locate within elongated “pancake” shaped regions having their major dimension along the rolling direction and minor dimension (i.e., band “thickness”) in the throughthickness direction of the plate.[8,9] The bands that contain the elongated MnS inclusions (that initiate failure) tend to be of the order of about 30 to 200 m in thickness with a mean of roughly 75 m.[10] In order to impose an elevated level of triaxial tension on the failure initiation process, tensile testing was performed on circumferentially notched, round-bar specimens oriented so that the applied tensile loading axis corresponded to the long-transverse plate orientation. The specimens all had similar geometries such that the radius of curvature of the notch ( ) was equal to the minimum specimen diameter (2R) at the notch; thus, R/ ⫽ 1.0 for this “A-notch” specimen geometry. For the HY-100 steel and at failure initiation, this specimen geometry imposes a stress triaxiality of m/eq ⬵ 1.05, where m is the mean stress and eq is the equivalent stress.[4,5] The influence of specimen size was addressed by testing specimens with minimum notch root diameters (2R) ranging from 1.5 to 12 mm (which was the largest that could be machined from 25.4-mm-thick plate). Outside the notch, the specimens had diameters of 15.2 mm, except for the 2R ⫽ 12 mm specimens, which had a “uniform” diameter of 24.8 mm. Specimens of 3 mm and smaller were tested in groups of eight, while at least five tests were performed for a given specimen size of the larger specimen geometries. All tests were performed at 25 ⬚C at an initial equivalent strain rate of approximately 10⫺3/s using the test proce
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