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A. Riemer et alii, Frattura ed Integrità Strutturale, 34 (2015) 437-446; DOI: 10.3221/IGF-ESIS.34.49 442 principal stresses occur, Fig. 6, and the R-ratio of 0.1 are assumed. Furthermore, details regarding the mesh are presented in Fig. 6. Figure 6 : FE-model of the implant with its boundary conditions and meshing details. Fig. 7 presents the crack path, the stress distribution and the crack length in the last simulation step before the unstable crack growth begins. This simulation was performed based on the body weight of 80 kg. The results of crack growth simulation show significant differences in the remaining lifetime, c.f. Fig. 7. In the as-built condition the fracture occurs at 100000 cycles. Following the heat treatment at 800°C the unstable crack growth begins at 2.5 millions of cycles. Consequently, the increase in the lifetime is possible due to a treatment at 800°C. Here, the lifetime extension was found by a factor of about 25. That means that from the fracture mechanical point of view, technical parts consisting of Ti-6-4 require heat treatment aiming at residual stress reduction. Figure 7 : Numerical crack growth simulation depending on the material condition. W EIGHT AND STRENGTH OPTIMISATION OF A BICYCLE CRANK he bicycle crank has been selected to be optimised by numerical studies and to be manufactured by Selective Laser Melting. The optimisation regarding weight and strength is based on biomechanical studies that were carried out at the Institute of Applied Mechanics (FAM). In these studies was found that tall people need long bicycle cranks for better transmission ratios. The optimisation of the structure within this work is based on the consideration of static T