Chiara Bisagni

A fast procedure for the design of composite stiffened panels

This paper describes the analysis and the minimum weight optimization of a fuselage composite stiffened panel made from carbon/epoxy material and stiffened by five omega stringers. The panel investigated inside the European project MAAXIMUS is studied using a fast tool, which relies on a semi-analytical procedure for the analysis and on genetic algorithms for the optimization. The semi-analytical approach is used to compute the buckling load and to study the post-buckling response. Different design variables are considered during the optimization, such as the stacking sequences of the skin and the stiffener, the geometry and the cross-section of the stiffener. The comparison between finite element and fast tool results reveals the ability of the formulation to predict the buckling load and the post-buckling response of the panel. The reduced CPU time necessary for the analysis and the optimization makes the procedure an attractive strategy to improve the effectiveness of the preliminary design phases.

Skin-Stiffener Separation in T-Stiffened Composite Specimens in Postbuckling Condition

An experimental and numerical investigation was conducted to study the skin-stiffener separation of single T-shape stiffener specimens in postbuckling condition. Three specimens were manufactured with a centrally located Teflon insert, and were loaded in compression until collapse. Deformation patterns and separation evolution were monitored during the tests. To measure the full-field displacements and the strain distributions of the specimens, a digital image correlation (DIC) system was used. Skin-stiffener separation was observed and measured with an ultrasound system. Finite-element (FE) analyses were conducted to capture interlaminar damage mechanism based on the virtual crack closure technique. The numerical analysis accurately predicted the postbuckling deformation and the skin-stiffener separation behavior. The close correlation between the experimental and numerical results allows for further exploitation of the strength reserve in the postbuckling region and wider design options for the next generation of composite aircraft designs.

Fatigue life and damage tolerance of postbuckled composite stiffened structures with indentation damage

The fatigue life and damage tolerance of composite stiffened panels with indentation damage are investigated experimentally using single-stringer compression specimens. The indentation damage was induced to one of the two flanges of the stringer of every panel. The advantages of indentation compared to impact are the simplicity of application, less dependence on boundary conditions, better controllability, and repeatability of the imparted damage. The tests were conducted using advanced instrumentation, including digital image correlation, passive thermography, and in situ ultrasonic scanning. Specimens with initial indentation damage ranging between 32 and 56 mm in length were tested quasi-statically and in fatigue, and the effects of cyclic load amplitude and damage size were studied. A means of comparison of the damage propagation rates and collapse loads based on a stress intensity measure and the Paris law is proposed. The stress intensity measure provides the means to compare the collapse loads of specimens with different damage types and damage sizes, while the Paris law is used to compare the damage propagation rates in specimens subjected to different cyclic loads. This approach enables a comparison of different tests and the potential identification of the effects that influence the fatigue lives and damage tolerance of postbuckled structures with defects.

Experimental investigation of reinforced bonded joints for composite laminates

An experimental study has been carried out to investigate the behaviour of co-bonded carbon fibre reinforced plastics joints with a novel design incorporating a through the thickness local reinforcement. Different specimens were manufactured to investigate static and fatigue behaviour, as well as delamination size after impact and damage tolerance characteristics. The mechanical performances of the specimens with local reinforcement, consisting of the insertion of spiked thin metal sheets between co-bonded laminates, were compared with those ones obtained from specimens with purely co-bonded joints. This novel design demonstrated by tests that damage progression under cycling load results significantly delayed by the reinforcements. A significant number of experimental results were obtained that can be used to define preliminary design guidelines.

Scaling Methodology for Buckling of Sandwich Composite Cylindrical Structures [STUB]

The study of the buckling behavior of large shell structures through full-size tests can be complex and expensive. Therefore, scaled structures are often preferred to investigate the buckling behavior efficiently. However, it can be difficult to design scaled structures that are representative of the full-scale structures. Herein, an analytical scaling methodology for compression-loaded sandwich composite cylinders based on the nondimensionalization of the buckling equations is presented. The methodology is used to develop scaled configurations that show a similar buckling response. Both the baseline and the scaled configurations are verified by finite-element analysis. Limitations of the methodology are discussed and are a result of neglecting the flexural anisotropy and the transverse shear compliance.

Fatigue life of postbuckled structures with indentation damage

The fatigue life of composite stiffened panels with indentation damage was investigated experimentally using single stringer compression specimens. Indentation damage was induced on one of the two flanges of the stringer. The experiments were conducted using advanced instrumentation, including digital image correlation, passive thermography, and in-situ ultrasonic scanning. Specimens with initial indentation damage lengths of 37 mm to 56 mm were tested in fatigue and the effects of cyclic load amplitude and damage size were studied. A means of comparison of the damage propagation rates and collapse loads based on a stress intensity measure and the Paris law is proposed.