Carlos G. Dávila’s research while affiliated with TU Wien and other places

In this paper, we employ the combined Transformation Field Analysis (TEA) method and the Governing Parameter Method (GPM) to predict the overall elastoplastic behavior of multi-phase fibrous composite materials using Gurson-Tvergaard yield criterion in order to account for the effect of void growth in the matrix phase. For the homogenization scheme, we employ the TEA method with concentration factors determined by the Eshelby-Mori-Tanaka (EMT) theory. Regarding to the stress integration of the governing TEA equations, we employ an implicit integration scheme, namely the GPM. Furthermore, a necessary condition for the possible ranges of the governing parameters based on the GPM integration scheme is derived in a more general setting by including the rate of nucleation and coalescence within the context of writing the expression of the rate of change of porosity. To validate our proposed approach, we compare our results to both numerical and experimental results provided in the existing literature.

The present work is concerned with modeling the non-linear behavior of continuous fiber reinforced laminates with a special emphasis on loading conditions that lead to high ply shear stresses. Typically, the modeling of non-linear laminate behavior focuses on damage mechanics approaches and assumes that the non-linearity is caused by brittle matrix cracking. Based on the correlation of experimental data and modeling results, this assumption seems to hold true for load cases in which layers experience mainly tensile stresses. Under shear dominated loads, however, it has been found that the agreement between tests and model predictions is less satisfactory. Additionally, considerable permanent strains develop under such loading conditions that cannot be explained by brittle mechanisms alone. Here, a model is presented that combines damage mechanics with a plasticity law to capture both degradation of stiffness due to cracking and residual strains accumulated under shear loads. It is assumed that damage starts to develop close to the first ply failure load and any non-linear behavior prior to the onset of damage is attributed to plastic shear strains. Predictions of the model are compared to experimental data and are shown to give improved correlation to experiments under shear dominated loading. By taking residual stresses into account, the model is also able to explain discrepancies in the shear behavior derived from two different test methods. Furthermore, the combined damage/plasticity model captures the accumulation of residual strains, the non-linear behavior observed in uniaxial transverse compression tests, and the influence of transverse normal stress on the non-linear shear behavior reported in the literature.

A Domain Superposition Technique (DST) is proposed for the simulation of woven fabric composites. Instead of modelling the tows and the likely degenerated resin pocket regions among tows explicitly, DST separately models the tow domain and the global domain which are both non-degenerated, and can thus be easily discretised using the traditional solid elements. During the solution process, the two domains are superimposed by coupling them together to produce the exact results. Numerical simulation shows that the results of DST correlate very well with the results of conventional finite element analysis.

This contribution starts with a discussion of various phenomena in laminated composite structures that can lead to failure: matrix cracking, delamination between plies, and debonding and subsequent pull-out between fibres and the matrix material. The different scales are discussed at which the effect of these nonlinearities can be analysed. From these scales – the macro, meso and micro-levels – the meso-level is normally used for the analysis of delamination, which is the focus of this contribution. At this level, the plies are modelled as continua and interface elements between them conventionally serve as the framework to model delamination and debonding. After a a derivation of interface elements and a brief discussion of the cohesive–zone concept and its importance for the analysis of delamination, a particular finite element model for the plies is elaborated: the solid–like shell. Next, a more recent method to numerically model delamination is discussed, which exploits the partition–of–unity property of finite element shape functions. This approach offers advantages over interface elements, as will be discussed in detail.

An accurate prediction of the length of the cohesive zone ahead of a crack tip is fundamental for the correct simulation of delamination in composite materials under both quasi-static and fatigue loading. To ensure a correct dissipation of energy during delamination propagation, several cohesive finite elements have to span the cohesive zone. The length of the cohesive zone depends on the material properties, the geometry/size of the structure, and on the loading mode. This chapter presents new expressions to estimate the length of the cohesive zone under general mixed-mode loading conditions and for finite-sized geometries. The analytical model is validated by comparing its predictions with numerical results based on cohesive-zone models. The relevance of the proposed analytical solutions to the effective simulation of delamination is demonstrated by simulating delamination growth under mixed-mode loading using meshes with the length of the elements greater than the cohesive zone length.

A constitutive model is presented for the complete in-plane response of composite materials within the framework of a previously developed continuum damage mechanics model, CODAM. While the previous CODAM formulation was primarily developed to simulate the progression of damage under tensile loading, the proposed extension is guided by a mechanical analogue model that accounts for the initiation and propagation of damage mechanisms under both tension and compression. Calibration of the tensile damage parameters of the model using the over-height compact tension test (OCT) is presented. Simulations of notched panels under quasi-static in-plane tension and compression loading are used to demonstrate the effectiveness of the model in predicting the load-displacement response as well as the overall damage zone size. Finally, limitations of local smeared crack models are discussed and the preliminary results of a non-local approach to simulating damage progression that overcomes such limitations are presented.

This paper describes key features of an advanced, physically-based, numerical material model for predicting the static and dynamic, failure and damage, response of polymer matrix composites with fibrous UD plies. The model has been implemented into the explicit Finite Element code LS-DYNA3D for solid brick elements with one integration point. A comprehensive test programme was conducted for characterising the high velocity impact response of a class of NCF/Epoxy composites. The impact tests were conducted for varying impact conditions and parameters such as: impact angle, coupon thickness, laminate lay-up and projectile material. Data from these tests was reduced in the form of ballistic curves, mass of target debris generated upon complete penetration, and (C-Scan) impact damage areas. This data was used for validation of the proposed model. General conclusions from this work indicate that physically-based modelling approaches can improve considerably the predictive capabilities of current FE codes for structural analysis applications.

In this work, we describe a numerical technique to predict fiber orientation during injection moulding of fiber reinforced polymers, and how the resulting part behaves regarding this process induced orientation. The orientation state of a set of fibers is described by a second order tensor. Its evolution is given by the Folgar and Tucker tensorial hyperbolic equation. Even if this equation contains a fourth order term, it may be expressed as a function of the second order tensor using a closure approximation. The resolution of Folgar and Tucker’s equation is carried out by a continuous approach based on the Standard Galerkin method, with stabilisation. The results are compared with experimental orientation measurements on an injected plate. Once the part solidifies it is considered as a biphasic material, composed by the fibers and the polymer matrix, where each phase has a linear elastic behaviour. The fhermo-elastic properties of the composite material are linked to the fiber orientation and the properties of each phase using a homogenisation technique. Finally, to validate the previous study on the prediction of the thermo-elastic properties at the solid state, a three-dimensional industrial case is deeply analysed.

A mesoscale model for finite element analysis of failure in laminates is presented. The model consists of separate parts for failure inside a ply (intraply) and failure between plies (interply). Both parts offer a description from onset of failure to complete local failure, thus allowing for progressive failure analysis. Intraply failure is simulated with a softening plasticity model based on a Tsai-Wu criterion with viscoplastic regularization. Details are presented on the implementation of the softening law for orthotropic materials in finite element computation. Interply failure is modeled using interface elements with a damage law for mixed mode delamination. The performance of the model is illustrated by means of an analysis of a laminate with a sharp internal notch – a case in which different modes of ply failure successively take place and interact with failure between the plies.

This work presents a new procedure to deal with the delamination problem found in laminated composites, based in a continuum mechanics formulation. The procedure proposed obtains the composite constitutive performance with the Serial/Parallel mixing theory, developed by F. Rastellini. This theory characterizes composite materials by coupling the constitutive behaviour of the composite components, imposing an iso–strain relation among the components in the fibre (or parallel) direction and an iso–stress relation in the remaining directions (serial directions). The proposed procedure also uses a damage formulation to characterize the constitutive behaviour of matrix component in order to obtain the stress–strain performance of this material. With these two formulations, the delamination phenomenon is characterized naturally by the numerical simulation, being unnecessary the definition of special elements or computationally expensive techniques like the definition of contact elements or mesh separation. Matrix failure, as a result of the stress state found in it, leads to a reduction of the stiffness and strength capacity of the composite in its serial direction. This reduction provides a composite performance equivalent to what is found in a delaminated material. To prove the ability of the formulation proposed to solve delamination problems, the End Notch Failure test is numerically simulated and the results obtained are compared with experimental ones. The agreement found in the results with both simulations, numerical and experimental, validate the proposed methodology to solve the delamination problem.