Maurício Vicente Donadon

A Three-Dimensional Ply Failure Model for Composite Structures

A fully 3D failure model to predict damage in composite structures subjected to multiaxial loading is presented in this paper. The formulation incorporates shear nonlinearities effects, irreversible strains, damage and strain rate effects by using a viscoplastic damageable constitutive law. The proposed formulation enables the prediction of failure initiation and failure propagation by combining stress-based, damage mechanics and fracture mechanics approaches within an unified energy based context. An objectivity algorithm has been embedded into the formulation to avoid problems associated with strain localization and mesh dependence. The proposed model has been implemented into ABAQUS/Explicit FE code within brick elements as a userdefined material model. Numerical predictions for standard uniaxial tests at element and coupon levels are presented and discussed.

Stiffening effects on the natural frequencies of laminated plates with piezoelectric actuators

Abstract Piezoelectric actuators are usually mounted to the top and bottom surfaces of plates and may induce in-plane extension, bending and localized shear deformations at the structural element. The in-plane stresses may have a significant influence on the mechanical behavior of thin plates as initial and/or residual stresses affect the flexural stiffness and in turn the dynamic and stability characteristics of plates. In this work, the effect of the in-plane piezoelectric induced stresses on the natural frequencies of composite plates is numerically and experimentally investigated. A finite element formulation is presented for the analysis of laminated plates with an arbitrary number of piezoelectric actuators and/or sensors. Von Karman non-linear strain–displacement relations are used and ideal linear behavior is assumed for the piezoelectric actuation. The problem is decomposed into an in-plane problem where the strain field induced by the piezoelectric actuators is computed. The natural frequencies and vibration modes are then computed taking the stress stiffening effects of these piezoelectric stresses into account. A number of different configurations are numerically and experimentally analyzed to verify the proposed theory. The configurations use eight PZT actuators bonded to three layer glass fiber/epoxy plates. The plates are square and clamped along two opposing edges and free along the other two. Good agreement is obtained between the predicted and measured natural frequencies.

The temperature effects on the fracture toughness of carbon fiber/RTM-6 laminates processed by VARTM:

The increasing use of composite in the aircraft industry has raised the interest for a better understanding of the failure process in these materials, which can be also influenced by the manufacturing process of the laminate. Some materials used in vacuum assisted resin transfer molding process have been studied in the open literature but very few data have been published for resin transfer molding-6 epoxy based laminates, in particular studies showing the influence of the temperature on the interlaminar fracture behavior of this type of laminates. The aim of this article is to investigate the interlaminar fracture behavior of resin transfer molding-6 based carbon composite laminates manufactured by vacuum assisted resin transfer molding subjected to Modes I and II at 25℃ and 80℃. The results show the influence of the temperature on the interlaminar fracture toughness of composites and provide a database to design composite aerostructures subjected to temperatures commonly experienced in civil aviation. The fracture aspects of the tested laminates were also investigated and directly related to the trend in results found for the fracture toughness values.

Prebuckling Enhancement of Imperfect Composite Plates Using Piezoelectric Actuators

The nonlinear response of initially imperfect composite plates with piezoelectric actuators is investigated. The nonlinearity is limited to the prebuckling regime, where higher order terms present in the strain energy expression can be neglected. The advantage of the electromechanical coupling is exploited in two ways. First, the in-plane piezoelectric stress stiffening effect is used to tailor a stress distribution that inherently increases the critical buckling loads of perfect composite plates by posing an optimization problem that efficiently handles eventual uncertainties involved in the application of mechanical loadings. Second, piezoelectric bending moments are applied in order to avoid or ameliorate the undesirable effects of initial imperfections. An actuation strategy, where the piezoelectric membrane forces and bending moments are decomposed via an appropriate selection of voltages applied to piezoelectric patches that are symmetrically bonded to the top and bottom surfaces of the plate, is proposed and shown to be effective.

Acoustic scattering by finite composite plates

Trailing edge scattering is a significant source of sound, and elasticity is known to decrease the radiated sound by a process involving coupled acoustic and bending waves. Most of the analysis available in the literature to deal with this problem is limited to structures of isotropic material. A numerical method is extended, based on the solution of a boundary element method with boundary conditions given by the structural problem, to account for anisotropic composite plates, restricted to symmetric laminates. These conditions are recast in terms of the vibration modes of a rectangular plate. To obtain these modes, the hierarchical finite element method is used to model an elastic flat plate. Expressions for bending waves propagating in such plates are derived, and how the solution of the problem is modified to account for these effects is shown. Results show modifications in the scattered sound as a function of ply orientation and stacking sequence. Composite materials are shown to be advantageous, since laminates lead to lower acoustic scattering when compared to structurally equivalent metallic plates. This is due to a lower specific mass, leading to higher coupling between fluid and solid, and thus to more significant elasticity effects, decreasing substantially the radiated sound.

Active and Passive Control for Acceleration Reduction of an Aeroelastic Typical Wing Section

The main concern related to the flutter phenomenon is predicting and avoiding it. This paper describes the application of a flexural-torsional flutter testbed for acceleration reduction by applying active and passive model-based control. The model consists of the 2D typical section, with aerodynamic loads estimated by an unsteady time-domain formulation based on Wagner’s function. The active control architecture consists of a stability augmentation system with output feedback and gain scheduling via the linear-quadratic regulator theory and actuation by servomechanism. The passive control employs a shape-memory alloy to provide additional torsional stiffness. Experimental results show considerable reduction of oscillations at a relative low cost for both active and passive control strategies, and that the use of shape memory alloys in aeroelastic stability problems is promising.

A damage model for the prediction of static and fatigue-driven delamination in composite laminates

This paper presents a continuum damage mechanics failure model to predict mixed-mode delamination growth in composite laminates subjected to static and high-cycle fatigue loading. The proposed formulation has been developed for robust nonlinear finite element formulations based on explicit direct time integration schemes, particularly the central difference method. The failure model has been implemented as a user-defined material model into ABAQUS/Explicit finite element code within C3D8 hexahedron solid elements. Numerical simulations were performed at coupon level for double cantilever beam, end-notched-flexure, mixed-mode bending and mixed-mode flexure specimens. Predictions obtained using the proposed failure model were compared with experimental results available in the open literature. Good agreement between numerical and experimental results was found.

2.07 – Damage Modeling in Composite Structures

This chapter presents a comprehensive review on existing approaches for modeling damage in composite structures. Special emphasis is given in robust and reliable constitutive models that enable prediction of failure initiation and failure progression in composite laminates within an unified way. Theoretical and numerical issues related to intra and interlaminar failure modeling are also discussed in detail. The constitutive models formulations presented in this chapter are based on the Continuum Damage Mechanics (CDM) approach and enables the control of the energy dissipation associated with each failure mode regardless of mesh refinement and fracture plane orientation. Within the CDM context, internal thermodynamically irreversible damage variables are defined in order to quantify damage concentration associated with each possible failure mode enabling the prediction of the gradual stiffness reduction for each composite ply. Numerical examples are also provided in order to illustrate the models capabilities.

Intralaminar Fracture Toughness Characterization of Composite Laminates

The current emphasis within the composite design community is gradually shifting from achieving minimum weight designs at all costs to more cost-effective and damage tolerant structural designs. A damage tolerant structure must not only be able to effectively absorb energy locally at the point of damage initiation but must also be fail-safe. A very efficient way of designing a composite fail-safe structure is to provide it with the ability to arrest a potentially catastrophic crack by increasing its fracture resistance. The fracture behaviour of composites can be quantified by measuring its toughness and it can be broadly classified into interlaminar and intralaminar fractures. Most of the work reported in the recent literature has focused on the investigation of the interlaminar behaviour of composites with a limited number of works addressing the intralaminar fracture behaviour. The measurement of intralaminar toughness requires a pre-cracked specimen and in most cases, particularly for CFRP materials. Its determination can be based on the Linear Elastic Fracture Mechanics (LEFM) approach. Different types of specimens and crack geometries are currently available in the literature to characterise the intralaminar fracture behaviour of composites under pure mode I (Cowley & Beaumont, 1997; Konstantinos et al., 2005), mode II and mixed-mode loading (Lin & Shetty, 2003), however none of them are standardised. The specimen selection depends on the material system under investigation and expected toughness values range from initiation to propagation. It is also worth mentioning that most of these specimens were originally designed for fracture in isotropic materials. The poor performance of composites in shear and compression loading, compared to tension loading in the fibre direction may lead to failure prior to crack growth. Such restrictions impose limitations on their applicability and alternative specimen designs are needed. For mode-I, the Overheight Compact Tension (OCT) specimen has the advantage of promoting a stable crack growth which eventually enables the evaluation of both initiation and propagation values for the intralaminar toughness. Jose et al. (2001) investigated the mode I intralaminar toughness of carbon/ epoxy cross-ply laminates using overheight compact tension specimens. The experimental results were compared with finite element simulations using a modified crack-closure integral method and a methodology for calculating the stress intensity factor associated with matrix cracking and fibre fracture was presented. Based on the work by Jose et al. (2001), Pinho et al. (2006a) investigated the intralaminar toughness associated with fibre breakage in tension and fibre kinking in compression in unidirectional pre-preg composites using Compact Tension (CT)

A Numerical Model for Post-Buckling Analysis of Composite Shear Webs

This paper presents a detailed numerical investigation of the post-buckling behavior of composite shear webs using the finite element method. The numerical analysis accounts for material and geometric non-linearity effects and has been divided into three steps. The first step consists of computing the critical buckling loads as well as their corresponding buckling modes. Geometric imperfections described approximately in terms of linear combinations of different normal modes are then introduced into the model. Finally a quasi-static analysis is carried out including a progressive failure model. The progressive failure model has been implemented as a user defined material model within shell elements in Abaqus/Explicit® finite element code.