Issue 22

D. Gentile et alii, Frattura ed Integrità Strutturale, 22 (2012) 85-92; DOI: 10.3221/IGF-ESIS.22.09 86 configuration can be adapted easily in order to test more samples together. Prot [4] developed a machine able to test up to 30 samples in the same time. Basically a simple bending moment is applied to a specimen gripped at both ends to a rotating shafts, Fig. 1. This configuration has been modified every time following the needed. Recently Irfan et al. [5], has been realized a rotating bending fatigue machine able to test at the same time up to 16 laminated composite samples. New machines, D. Brandolisio et al. [6], realized recently show that design process requires a specific analysis of the new mechanical components available commercially. A previous analysis on the materials used to realize the shaft has been conducted following a methodology proposed from Bonora et al. [7-10]. Today the use of parametric CAD models plays an essential role in the product design process, mainly, but not only [11], in the industrial field [12, 13]. Having the parametric CAD model of the product, it is possible to make any virtual simulations on it, also in terms of fatigue lifetime [14]. Although the fatigue virtual simulations are very useful, the experimental tests are fundamental to validate the results. In the paper the analysis have been carried out using CAD parametric models and electronic calculus sheets formatted following the project desiderata. Figure 1 : basic design of a R.R. Moore fatigue testing machine. G EOMETRY AND COMPONENTS DEFINITION n order to take into account all the specifications required has been realized an electronic work sheet containing all the calculations and all the characteristic of the components adopted for the design. In this way every time there was a needed of a change of component in order to, for example, minimize cost, all the analysis come out automatically. In order to calculate the bending moment and rotation, the bearing life, the spring requirements and the chassis design, has been used the classic mechanical laws such as:   1 2 2 3 3 16 4 3 y S M T d n    (1) where M is the bending moment, T is the torsional moment, S y is the yield stress of the material, n is the safety factor and d is the minimum diameter. Or the more useful following equation that allows to estimate an initial shaft size early in the design process: 1 3 2 32 3 4 f e yt k M n T d S S                          (2) where S e is the material ultimate endurance limit, S yt is the yield stress to torsion and k f is the stress intensity factor under cyclic load. This equation gives the minimum diameter shaft that will result in infinite fatigue life, and appears in the ANSI Standard. First question was to ensure the constant bending load. This specification can be met if the specimen grips can avoid inflection and rotation of the specimen. For this reason commercial grips have been adopted These grips are produced