Modeling of Fatigue Damage and Remaining Lifetime Prediction

In this paper there are some results of the European project AISHA. The project aim is the aircraft health monitoring, using ultrasonic Lamb’s waves technology. The final link in the health assessment process is: using information about the extent of size to predict the remaining strength and lifetime. Fatigue phenomenon is the main cause of growth damage for different kinds of initial damages (stress concentration, corrosion, impact) and different kinds of structures (the rivet-joint of metal sheets, long fiber composite, sandwich composites). Both the classical parameter of initial damage, as well as the new ones where analyzed. The prospective fatigue damage parameters are: density and the average size surface fatigue microcracks. Using such information for construction of monitoring systems of the structural elements of an aircraft has some essential advantages. First, the presence on a surface even the advanced system of microcracks does not create direct danger of canastrophic destruction because their influence on fatigue durability is insignificantly small. Second, using the density and the average size surface fatigue of microcracks it is possible to predict the moment of merge of microcracks group in one crack seen, so that it is possible to predict more precisely the remaining lifetime of a structural element. And lastly, the third very important advantage is the universality of the technique of application irrespective of the concrete geometrical configuration of the surface in the zone of control and the degree of heterogeneity of the stress state connected to it. Really, for the definition of density and the average size surface fatigue microcracks it is enough to allocate the site of the surface, small in comparison with other characteristic sizes. Within the limits of such site the stress state is practically homogeneous in most cases. This and other theretical and experimental fundamentals of the remaining strength and lifetime prediction are the main topics of this report