Issue34

A. Riemer et alii, Frattura ed Integrità Strutturale, 34 (2015) 437-446; DOI: 10.3221/IGF-ESIS.34.49 443 loading defined in the German standard DIN EN ISO 4210-8 and on the cyclic loading that was deduced from [11]. In the last step, the final geometry was selected under consideration of the balance between low deformation and weight. Crank and section geometry The standard crank lengths available on the market range from 165 mm to 180 mm. The length used for analyses and fabrication in this work was 200 mm, cf. Fig. 8a. The area of modification was defined in order to vary the cross-sectional geometry. In regions outside the modification area bulk material was defined due to high local loadings that occur in these regions. Figure 8 : Dimensions of the personalised bicycle crank a) and variants of geometry for the cross section in the area of modification b) . The variants used to figure out the most suited cross section are illustrated in Fig. 8b. The cross section consisting of solid material is used as reference. In this case, the highest weight as well as lowest local stresses and displacements occur. The both variants with horizontal and vertical walls contain framework that stabilises the walls. The last variant represents a hollow cross section. This geometry is well suitable for bending and torsion. Determination of the most suitable cross section In a first step a loading of 1500 N was applied to the FE-Models of the various variants presented in Fig. 8. The selected boundary conditions correspond to DIN EN ISO 4210-8. The material used in this study is Ti-6-4. The results for the maximum equivalent stress are presented in Fig. 9b. In these analyses, three different crank angles were taken into account. Furthermore, the limitation (456 MPa) of the stresses that occur in the bicycle crank is drawn in the diagram in Fig. 9b. For that reason a safety factor of 2 and the Yield strength of 912 MPa (cf. [7]) was used. The solid variant shows the lowest values of maximum stresses that occur in the structure. In addition, the stresses here are in the same range for the three considered angles, cf. Fig. 9a and b. The huge difference between the limitation and loading occurring show the large potential for optimisation. The displacements for this cross section are the lowest within the considered structures, Fig. 9c. The value of 470 g for the weight, Fig. 9d, is high compared to the non-solid designs. In case of the variant “Framework with horizontal walls”, the high displacement, Fig. 9c, was not acceptable. Consequently, this cross-sectional geometry was rejected. On closer examination of the designs “Framework with vertical walls” and