13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- m t m ol m hm F F F , , , > + (4) The reason for a stronger hybrid material can be attributed to change of constraint during loading. Figure 10 shows information on the material ductility. Not only does the hybrid material exhibit an increased strength compared to the tube plus open lattice but also a larger strain to failure (here plotted as elongation, which, in this case with equal initial lengths of the different geometries, yields comparable results). The fractographic examination shows that the bulk SLM material is ductile with the expected cup and cone appearance and dimples on the tensile test specimen fracture surface. Open lattice truss structures and lattices in a hybrid specimen tend to fail due to a shear mechanism between struts along preferred planes. The typical fracture appearance is shown above in Figure 14. Tensile testing of components with as-manufactured surfaces reveals that the SLM material has a significant different fracture surface appearance in bulk and surface-near material. During manufacturing, SLM material will have a dense, completely remelted internal bulk. Externally, a thin layer of partly molten material will be present as observed in Figure 16. 5. Conclusions The current paper shows how a selective laser melted material behaves in tensile testing. It is shown that the material is not as ductile as a normal hot-rolled material. However the material strength is good and the yield stress is superior to the hot-rolled material. Ultimate tensile strength is comparable between a selective laser melted material and the corresponding hot-rolled material. The selective laser melted material is highly anisotropic with respect to strength. In light-weight designs, hybrid materials can easily be manufactured with the selective laser melting manufacturing method. It is shown that the yield strength of a hybrid material can be superpositioned by the yield strength of the individual shell and lattice structures. At failure the hybrid material will act stronger compared to the tubular shell and open lattice truss structure components in the hybrid part. The stiffness of the hybrid structure investigated here will mainly be influenced by the tubular shell. 6. References [1] Chu J., Engelbrecht S., Graf G., and Rosen D.W., 2010. “A comparison of synthesis methods for cellular structures with application to additive manufacturing”. Rapid Prototyping Journal, 16, pp. 275-283. [2] Rosen D.W., 2007. “Computer-aided design for additive manufacturing of cellular structures”. Computer-Aided Design and Applications, 4, pp. 585-594. [3] Marchelli G., Prabhakar R., Storti D., and Ganter M., 2011. “The guide to glass 3D printing: developments, methods, diagnostics and results”. Rapid Prototyping Journal, 17(3), pp. 187 – 194. [4] Das S., Beama J.J., Wohlert M., and Bourell D.L., 1998. "Direct laser freeform fabrication of high performance metal components". Rapid Prototyping Journal, 4(3), pp. 112 – 117. [5] Kruth J.P., Froyen L., Van Vaerenberg J., Mercelis P., Rombouts M., and Lauwers B., 2004. “Selective laser melting of iron-based powder”. Journal of Materials Processing Technology, 149, pp. 616-622.
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