Fatigue life of post-buckled composite structures

The fatigue life prediction of post-buckled composite structures represents still an unresolved issue due to the complexity of the phenomenon and the high costs of experimental testing. In this paper, a novel numerical approach, called “Min-Max Load Approach” is adopted to analyze the behavior of a single-stringer composite specimen with an initial delamination subjected to post-buckling fatigue compressive load. The proposed approach, based on cohesive zone model technique, is able to evaluate the local stress ratio during the delamination growth, performing, in a single finite element analysis, the simulation of the structure at the maximum and minimum load of the fatigue cycle. The knowledge of the actual value of the local stress ratio is crucial to correctly calculate the crack growth rate. At first, the specimen is analyzed under quasi-static loading conditions, then, the fatigue simulation is performed. The outcomes of the numerical analysis are compared with the data of an experimental campaign previously conducted.Aerospace Structures & Computational Mechanic

Buckling tests of sandwich cylindrical shells with and without cut-outs

The results of buckling tests performed during the project DESICOS funded by the European Commission in the FP7 Programme are here presented. The tested structures are sandwich cylindrical shells that consist of reduced models of a component of the Ariane 5 launcher: the Dual Launch System. In particular, the scaled component is studied with and without the presence of cut-outs. Before performing the tests, the geometric imperfections as well as the thickness variations were measured. The tests were performed using the buckling testing equipment of Politecnico di Milano. The results of the tests contributed to understand the complex phenomenon of buckling of sandwich cylindrical shells, and to study the effect of initial geometric imperfections. They were also used to validate finite element models useful for the design of future launcher structures, and to set-up a probabilistic approach for the buckling analysis of cylindrical shells.Aerospace Structures & Computational Mechanic

Analysis of Local Stress Ratio for Delamination in Composites Under Fatigue Loads

An approach based on the cohesive zone model for analyzing delamination in composite laminates under cyclic fatigue loading is presented. The proposed technique, called “min-max load approach,” is able to dynamically capture the local stress ratio during the progression of delamination. The possibility to know the local stress ratio is relevant in all the situations where its value is different from the applied load ratio and cannot be determined a priori. The methodology analyzes in a single finite element analysis two identical models with two different constant loads, the minimum and the maximum load of the fatigue cycle. The two models interact with each other, exchanging information to calculate the crack growth rate. At first, the approach has been validated in simulations of mode I and mixed-mode propagation using double cantilever beam and mixed-mode bending tests. Then, to prove the effectiveness of the developed methodology, a modified version of the mixed-mode bending test has been analyzed. Mode I and mode II components of the load are decoupled and applied independently, resulting in a local stress ratio different from the applied load ratio. The results obtained from the simulations, compared with the analytical model obtained using the corrected beam theory, show that the proposed approach is able to predict the local stress ratio and thereby to correctly evaluate the crack growth rate during the propagation of the damage.Aerospace Structures & Computational Mechanic

Mechanical Response of Variable and Constant-Stiffness Cylindrical Shells for Launcher Structures

The ability to steer the carbon fibre tape, varying the tow angle can open new designs of cylindrical shells – the main structural component of the space launcher vehicles. This research presents experimental and numerical investigation of two carbon-epoxy cylindrical shells – a cylinder with conventional layup made of unidirectional prepreg and a variable-stiffness cylinder manufactured by applying advanced fibre placement technology. The shells were tested in compression until buckling, measuring load-shortening and capturing the buckling shape by digital image correlation systems. For the purpose of modelling the variable-stiffness cylinder, a simplified stiffness approximation approach was applied. The obtained load-shortening curves and buckling shapes demonstrated good correlation with non-linear numerical models. The results of the investigation contributes to the understanding the phenomenon of buckling of variable-stiffness cylindrical shells, and the influence of initial geometric imperfections and thickness variations.Aerospace Structures & Computational Mechanic

Buckling of Composite Cylindrical Shells with Circular Cutouts

Cylindrical shells are common structural elements in the aerospace sector due to their high load-carrying capacity per unit weight. Cutouts may, however, significantly reduce this load-carrying capacity, especially when cylindrical shells buckle under axial compression. Since the buckling load is often a crucial design parameter, it is important to predict this value efficiently. Hence, a procedure to rapidly calculate the linear buckling load of axially compressed quasi-isotropic composite cylindrical shells with circular cutouts was derived. After minimizing the total potential energy of the structure with the Ritz method, the buckling loads were obtained as the solutions to an eigenvalue problem. Comparing these predictions with the results from linear and nonlinear finite element analyses shows that the analytical buckling loads follow the general trends of the numerical solutions and are calculated orders of magnitude faster. This makes the approach suitable for preliminary design where many design permutations must be evaluated in a short period of time.Aerospace Structures & Computational Mechanic

Multiscale damage in co-cured composites - Perspectives from experiments and modelling

Bonded and co-cured composites are popular alternatives to structures joined with mechanical fasteners in aircraft but the complex and coupled damage mechanisms in the co-cured/bonded region are poorly understood, thus making the evaluation of their strength and durability difficult with current modelling strategies. This study explores the potential of interleaf inclusion in failure-prone, critical regions of co-cured composite specimens in improving the joint strength and interface fracture toughness and strives to advance the understanding of damage initiation in the co-cured region using an atomistic model. A two-pronged approach is pursued here with bench-scale experimental testing and molecular modelling in this study. Experiments are performed for mode I fracture toughness with double cantilever beam (DCB) on composite laminates with an epoxy interleaf layer. Two epoxy resins and three methods for interleaf inclusion are explored in this study; we supplement the results from DCB testing with insights from confocal microscopy on the crack tip and the interleaf layer pre- and post-testing. Molecular dynamic (MD) simulations capture the cohesive interactions at the threephase interface containing the carbon fiber, the prepreg epoxy, and the interleaf epoxy. Results highlight that an interleaf layer made from partially-cured and filmed epoxy, further consolidated in the composite lay-up is the most effective way to suppress void formation, improve dispersion, and maximize cohesive interactions at the interface of co-cured composites.Aerospace Structures & Computational Mechanic

Buckling-driven mechanisms for twisting control in adaptive composite wings

This study aims to design novel tailorable and effective mechanisms by controlling buckling behaviour in structural elements of a composite wing for future morphing application. Instead of the traditional design against buckling, the idea is to embrace this built-in instability by using the nonlinear post-buckling response to control stiffness changes which redistribute the load in the wing structure. To enable desired multi-stable configurations, three buckling-driven mechanisms are investigated by restraining the out-of-plane buckling deformation using point, area and maximum displacement constraints. Numerical studies of the proposed mechanisms are at first conducted on a composite plate and are later integrated to control twisting of a simplified thin-walled composite wing box. The proposed mechanisms offer effective design opportunities of multi-stable configurations and demonstrate the potential to realise morphing of composite wings employing controlled buckling behaviours in structural components.Education AEAerospace Structures & Computational Mechanic

Damage arrest mechanisms in nanoparticle interleaved composite interfaces

The effectiveness of carbonaceous nanoparticles in arresting and delaying damage in nanocomposites has been attributed to multiscale toughening mechanisms. To explore their application in joined interfaces of composites, this study investigates the use of carbon nanotube (CNT) interleaved films for co-cured joining of composite parts and their consequent effects on the interfacial fracture toughness. Carbon nanotubes dispersed in a thermoset resin into thin films of two discrete thicknesses (200 μ and 500 μ) and three concentrations of CNT dispersion were chosen for this study (0.5% wt., 1% wt., and 2% wt.). The films were semi-cured in the oven before being incorporated as interleaves in the composite laminate interface. Fracture toughness of the interface in mode I loading conditions was determined through double cantilever beam (DCB). Micrographs of the fracture surfaces reveal a slip-and-stick based crack jump and arrest phenomena in mode I when nanoparticles are added to the interleaved interface. The thickness of the interleaves has a more significant effect on mode I toughening mechanisms than the concentration of the nanoparticles.Aerospace Structures & Computational Mechanic

Buckling and free vibration study of variable and constant-stiffness cylindrical shells

The ability to steer carbon fibre tapes, varying the tow angle, can widen the designs possibilities of cylindrical shells that are one of the main components of aerospace structures. This research presents experimental and numerical investigation of two carbon fibre reinforced plastic cylindrical shells – a cylinder with conventional layup made of unidirectional prepreg and a variable-stiffness cylinder manufactured by applying fibre placement technology. The shells were tested in compression until buckling and later subjected to a vibration analysis. Load-shortening curves and buckling shapes were acquired during the compression tests, while the natural frequencies and the mode shapes were measured during the vibration tests. Both tests provide a useful data set of the mechanical response of the cylinders which can be applied for further validation of models. The acquired experimental results were compared to a simple, approximated numerical model of the variable-stiffness cylinder showing good correlation with the test results.Aerospace Structures & Computational Mechanic

Fatigue analysis of a post-buckled composite single-stringer specimen taking into account the local stress ratio

The fatigue life prediction of post-buckled composite structures represents still an unresolved issue due to the complexity of the phenomenon and the high costs of experimental testing. In this paper, a novel numerical approach, called “Min-Max Load Approach”, is used to analyze the behavior of a composite single-stringer specimen with an initial skin-stringer delamination subjected to post-buckling fatigue compressive load. The proposed approach, based on cohesive zone model technique, is able to evaluate the local stress ratio during the delamination growth, performing, in a single Finite Element analysis, the simulation of the structure at the maximum and minimum load of the fatigue cycle. The knowledge of the actual value of the local stress ratio is crucial to correctly calculate the crack growth rate. At first, the specimen is analyzed under quasi-static loading conditions, then the fatigue simulation is performed. The results of the numerical analysis are compared with the data of an experimental campaign previously conducted, showing the capabilities of the proposed approach.Aerospace Structures & Computational Mechanic