Strain fields evaluation on aerospace structures using digital image correlation aimed at structural health monitoring

Lightness of structures is a primary requirement in Aerospace Engineering. Therefore the typical structural elements used in this field are thin plates, stiffeners, ribs and fasteners assembled to resist very high loads but weighting as low as possible. Wings, fuselage, tail and control surfaces are built with longitudinal stiffeners and transversal ribs enclosed by a thin shell. These shells guarantee the aerodynamic properties and carry part of the shear stresses while the internal elements satisfy the remaining structural requirements. A similar philosophy is used in spacecraft design. The use of thin-walled reinforced shells is generally very convenient, but it implies a higher difficulty during the design process. Instabilities can arise very easily in these structures, since very thin elements are used, causing undesired consequences, like structural damages or failures. It is also worth noticing that aircrafts and spacecrafts suffer problems of fatigue, because the loads are not static and vary widely and frequently during the lifetime of the vehicle leading to cracks propagation. For these reasons analytical and numerical result obtained during design must be validated by some kind of experiments involving several loading modes. In all of these tests, the experimental measurement of full stress and strain fields would be particularly interesting. These fields often show irregular patterns, seen around cut-outs, structural imperfections, damaged areas including delaminations in composites plates or in case of instabilities. The goal of this paper is to present the employ of a state-of-the-art Digital Image Correlation system to evaluate the full stress and strain fields of a thin-walled structural item under fatigue load cycles, comparing the optical measurements with other inspection techniques. Furthermore a preliminary numerical analysis is presented in order to verify the strains levels and their discontinuity patterns around a simulated delamination when a small composite plate is subjected to different loads conditions; these levels and patterns will be then verified during future experimental activities by DIC. This simple but still effective numerical investigation shows the potentialities of an experimental full strain field availability before and after a delamination occurs for designing a Structural Health Monitoring system in order to monitor, as example, the damage propagation during fatigue tests on composites parts in a laboratory environment, with low time consumption.