Issue 16

F. Carta et alii, Frattura ed Integrità Strutturale, 16 (2011) 34-42; DOI: 10.3221/IGF-ESIS.16.04 35 In addition, secondary effects, such as residual stresses generated by the bonding process and bending caused by the eccentricity of the load with respect to the neutral axis of the reinforced panel cross-section, increase the complexity of the phenomenon. Reliable predictions of crack growth and residual strength in bonded structures can be based mainly on empirical considerations. The experimental results which support the numerical analysis reported in this work refer to an experimental investigation carried out by Airbus in a period from 2002 to 2007. Through an extensive campaign of tests, several methods of reinforcement were analyzed, using bonded reinforcements in the fuselage panels. To achieve a quantitative study, in the analysis different types of connection between the reinforcements and the skin were considered. In the literature, numerical studies on FCP (Fatigue Crack Propagation) in reinforced structures are available. However, if the damage tolerance assessments appear to be practicable in integral reinforced structures [5], the same assessment is not straightforward in differential structures with bonded joints between skin and chords [6, 7] due to the complex mechanisms mentioned previously. S TIFFENED DIFFERENTIAL STRUCTURES he stiffened differential structures can actually be reproduced with proper models that allow replicating the stiffener effect using the “crack arrest” philosophy design. In this approach after an initial propagation, the crack arrests due to a stiffener when a given length is reached (see Fig. 1). Figure 1 : Comparison between the several design solutions according the “crack arrest” method. In the differential structures the crack propagates typically only in the skin, then the completely intact stiffeners can control the defect evolution during the propagation, due to the load transfer from the skin to the stiffeners which, in turn, means that the stress intensity factor decreases. This fact underlines the effectiveness design of crack arrest . Several experiments have shown a significant beneficial effect due to the presence of reinforcements in differential structures [1, 2]. In many engineering fields such as aeronautics, automotive, marine or civil engineering, plates and shell made with laminate composite structures are largely used in load-bearing structural members. Important examples of these applications can be observed in aerospace engineering, where thin laminates are reinforced by a certain number of profiles (the so called stringers). Figure 2 : Cross sections of several kinds of stringers. Such structural parts can readily be found in fuselages, tail planes, or wind covers of aircrafts and typically consist of a thin plate or moderately curved thin shell that is stiffened by a certain number of shaped stringers (see Fig. 2). Especially stringers which have a closed-profile cross-section may provide a high torsional stiffness to the plate such that the composite plate itself can be assumed to be elastically restrained to some degrees [8]. T