13th International Conference on Fracture June 16–21, 2013,Beijing, China -4- with D defined in Eq. (4).In expressions above D and χmax have been identified from the phase transformation curve. The plasticity behavior of the material is described by k0, H0, β and Hχ as well as cL and cNL. In comparing with the conventional combined hardening model [11], one more parameter Hc has to be considered from the tensile tests. Note the formulations above are only valid for monotonic loading. 3. Experiments and Results 3.1 Specimens In the present work the austenitic stainless steel 304 is investigated, almost versatile and widely used stainless steel and available in a wider range of products. The austenitic structure gives the steel excellent toughness, even down to cryogenic temperatures. The chemical components are summarized in Table 1. Table 1. Chemical composition of the stainless steel 304 C Cr Ni Mn Si P S Fe 0.04 17.1 8.1 1.05 0.41 0.035 0.003 bal Table 2. Mechanical property of the stainless steel 304 after solution treatment E (GPa) Ultimate Stress (MPa) Yield stress (MPa) Elongation (%) Hardness (MPa) 200 650 240 100 100 The material used in the present work underwent solution treatment (annealing), heated to 1100°C for 1 hour and cooled down to room temperature in air, to eliminate the residual stresses and initial effects from manufacturing. The mechanical property of the heat treated SS304 is listed in Table 2. To study effects of deformations to material property, two kinds of specimens have been tested: (1) The plate specimens were fabricated for learning development of Martensite phase in SS304 under tension and compression. The specimen geometry is given in Fig. 1(a). To study development of the martensite phase under compression, the plate specimens can be pressed in the thickness. (2) The tension specimens were taken for tension and fatigue tests, as shown in Fig. 1(b). (a) Plate specimen (b) Tension specimen Figure 1: Specimen geometries tested in the present work. The plate specimens were fabricated for studying martensite phase transformation and the tension bars for fatigue tests. 3.2 Experimental observation of Martensite phase transformation To quantify evolutions of the martensite phase transformation under compression and it’s effects to mechanical property, the plate specimens were pressed in their thickness direction. With various press forces the thickness of the final plate specimens varies between 6mm (0% deformation) and 2.8mm (-53.3% thickness reduction). Mainly ca. -10%, -20%, -30%, -40% and -50% of reduction in specimen thickness were realized. The stress triaxiality under this loading state is η=-1/3. The tension effects were studies in uniaxial tension specimens after given tensile strains ( η=1/3). Five different deformation grades were examined, 20% - 80%. Note the true strain, i.e. the
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