Issue 33

J.M. Ayllon et alii, Frattura ed Integrità Strutturale, 33 (2015) 415-426; DOI: 10.3221/IGF-ESIS.33.46 415 Focussed on multiaxial fatigue Comparison of two multiaxial fatigue models applied to dental implants JM. Ayllon, C Navarro, J. Vázquez, J. Domínguez Departamento de Ingeniería Mecánica y de los Materiales, Escuela Superior de Ingenieros. Camino de los Descubrimientos s/n, 41092, Sevilla, Spain cnp@us.es A BSTRACT . This paper presents two multiaxial fatigue life prediction models applied to a commercial dental implant. One model is called Variable Initiation Length Model and takes into account both the crack initiation and propagation phases. The second model combines the Theory of Critical Distance with a critical plane damage model to characterise the initiation and initial propagation of micro/meso cracks in the material. This paper discusses which material properties are necessary for the implementation of these models and how to obtain them in the laboratory from simple test specimens. It also describes the FE models developed for the stress/strain and stress intensity factor characterisation in the implant. The results of applying both life prediction models are compared with experimental results arising from the application of ISO-14801 standard to a commercial dental implant. K EYWORDS . Titanium; Dental implants; Fatigue testing; Life estimation. I NTRODUCTION esigning real mechanical components subjected to cyclic loading is a traditionally complex matter due to the numerous variables affecting fatigue life. In addition to the material properties, the stress concentration, the component size and its surface characteristics as well as the multiaxiality of the stress state induced, influence its fatigue response. The use of prediction models allows studying this response but, taking into account the randomness of the process and that the effect of these variables on the fatigue life of different materials is not perfectly known, experimental analysis is often needed. Numerous methods have been proposed to carry out the fatigue design of components in presence of stress risers and multiaxial stress states. Some of these methods consider only the crack initiation phase [1], assuming that the length of the propagation phase is negligible compared to that of initiation. This approach usually provides good results for large notches and low stress levels. However, it is difficult to know a priori if, for a given combination of geometry, materials and load state, the above assumption is correct. In general, these methods tend to produce more or less conservative results, depending on the actual importance of the propagation phase with respect to the total life. A common aspect to these methods is the definition of the point or depth from the surface where the stresses have to be evaluated in order to apply the corresponding initiation criteria. Other methods only consider the propagation phase assuming that the duration of the initiation is small, either by the prior existence of small defects (microcracks) in the material [2], or by the assumption that these microcracks are quickly initiated during the first cycles, due to the high stress levels produced by a stress riser. This stress concentration can be D