The cluster model with f0 =0.0001 gives the best fit both in longitudinal and in transversal direction, see Figure 5. The level of non-metallic inclusions is the same in base metal and HAZ. Consequently the obtained level of f0 could be expected. As for base metal and WM the model tends to overestimate the ductility of the R0.4 specimens and to underestimate the ductility of the smooth specimens. EVALUATION OF THE MODEL The modified Gurson model predicts higher ductility for the sharpest notched (R=0.4 mm) specimens than the R=1.0 mm specimens. This is based on a shift in damage process from stress to strain controlled initiation of fracture [6]. Strain controlled plastic initiation allows more plastic deformation and the ductility level will rise. The experiments show, however, that the model overestimates the critical deformation in the sharpest notched specimens. When evaluating the model one must bear in mind that several simplifications are made. The model is based on the assumption that the material behaves like a continuum, and that the plastic strain happens in a distributed manner. If the plastic strain varies locally caused by defects or weak zones in the matrix, the critical plastic strain will be lower. The ductility level of the R=0.4 mm tensile specimens may well reflect the effect of non-homogeneity in the matrix. It can be noted that the ductility level of the R0.4 specimens in weld metal, which has the highest level of non-metallic inclusions, varies the most. Both the cluster and the continuos void nucleation models are simplifications of the nucleation process. Perhaps does a more detailed void nucleation model better describe the process of void nucleation. The material, as described by the Bridgeman corrected plastic stress strain-curve for smooth specimens, is supposed to represent all geometries. This may not be the best representation of the material behavior for the sharpest notched specimens. One can argue also that the longitudinal stress-strain curves do not represent the correct material behavior in the transversal direction. As presented the model seems well suited to describe the ductility behavior of medium sharp notched tensile specimens in the range ) radius R(notch ) diameter D ( Initial 0 = 2 - 6. ACKNOWLEDGEMENTS The experimental and FE-simulation work presented in this paper, is performed as a part of the ESCS Multinational Research Project PRESS (Prediction of Structural Behavior on the Basis of Small Scale Specimen Testing). The project is partly financed by the Norwegian Research Council. REFERENCES [1] Z. L Zhang, C. Thaulow and J. Ødegård (2000):“A complete Gurson model approach for ductile fracture”, Eng. Fract. Mech., 67, 155 - 168. [2] V. Tvergaard and A. Needleman (1984): "Analysis of the cup-cone fracture in a round tensile bar" Acta Metallurgica. 32, 157-169 [3] P.F. Thomason (1990):"Ductile fracture of metals" Pergamon Press, Oxford [4] Z. Zhang, M. Hauge, C. Thaulow, J. Ødegård (2000): “A notched cross weld tensile testing method for determining true stress strain curves for weldments”, submitted paper to the Engineering fracture Mechanics [5] V. Olden (2000): “Ductile fracture in high strength steel weldments”, Diploma thesis, NTNU (In Norwegian) [6] M.P. Loria (1999):”Determining damage parameters for a X-65 steel”, SINTEF Report STF24 A99281 [7] ”Acceptance criteria and level of safety for high strength steel weldments, Summarizing reports” SINTEF Report STF24 A97210, 1997 [8] AL. Gurson (1975), "Plastic flow and fracture behavior of ductile materials incorporating void nucleation, growth and coalescence, Ph.D. Diss., Brown University.
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