Scientific Program

 

LIST of ABSTRACTS

 

AERONAUTIC INDUSTRY SESSIONS :

Toward a Unified Damage Mechanics approach for composite laminates sizing 

 Didier  Guédra-Degeorges , Ana-Cristina Galucio, Jean-Mathieu Guimard

EADS Innovation Works, France

Today composite sizing and particularly failure criteria used in industrial environment are mostly based on phenomenological modeling. The implementation of this approach requires to calibrate failure criteria by testing numerous coupons for various local structural details (filled hole compression, open hole tension, stiffener run out, compression after impact...). This process is unfortunately time consuming and costly. The presentation will focus on an innovative approach based on damage mechanics meso-scale modeling developed by LMT Cachan. In this method, a single set of material parameters is identified to characterize the composite ply & interface damage laws evolution. As a consequence, the prediction of the behavior & failure of local composite structural details can be performed in a unified framework which allows describing the various degradation modes (delamination, transvers cracks, splitting, fiber breakage …) during loading and final failure stage.  Illustrations & present limitations of the method will be presented on several application cases

 

Failure prediction models of Laminated Composites for a more efficient design: The link is missing.

 

Thierry Vilain and Dominique Martini

Dassault-Aviation, France

When the composite materials technology started to develop, it was expected that the simple characterization of fiber, matrix and interface would be sufficient to predict the complete behavior and failure of a composite structure. After 40 years, the efforts placed in model developments and the increasing processing power of computers have led to significant achievements in the understanding and simulation of the damage and failure of composite structures. However, this growing expertise in composites is not well transferred into design or sizing tools for a better assessment of the influence of laminate parameters.  These evaluations must be performed through costly tests campaign. This is why current structural solutions are still largely the result of conservative choices, one example being the classical choice of 0°/45°/-45°/90 determined as the only acceptable fibers directions, with limited efforts to explore other lay-up directions. First, Dassault will recall the main aeronautical constraints for the design and sizing of composite structures, the typical design tools and procedures, and their limitations. The difficulties to explore new configurations will be brought to attention and some examples of “unusual” development will be presented. Dassault will end by pointing out its strategy to overcome the limitations of notched laminates failure tool by trying to develop a modelisation with the relevant level of complexity while keeping in mind the final objective of supporting design and sizing methodologies.

 

Composite Structure : Evolution And Future Challenges

Alain Tropis

COO in charge of Development, AEROLIA, AIRBUS GROUP 

To optimize the design of an aircraft, the airframe part (structure plus system installation) represents more than 50% for the total weight.  The use of composite has been progressive in the last 40 years with a significant acceleration in the last 10 years with the introduction of Full Composite Wing and Composite Fuselage to comply with this need of performance (density, fatigue behavior, corrosion, ….). The sizing drivers (modulus/buckling, holes/bolted joints, stifferenrs run out/ply drop off, …) have been improved thanks to different generation of materials (Intermediate Modulus Fiber, Damage Tolerance Resin System, …). The focus in the last 10/20 years have been to improve the mechanical properties in particular for damage tolerance, large damage capability (sizing criteria for a  Composite Fuselage), ….. and the robustness (resistance to high energy/low speed impact). However it can be noticed that the level of performance reached is not sufficient to make a real breakthrough for future composite applications and in particular for fuselage. Over the last 30 years the design of composite airframe came  from a “Load carrying structure approach” i.e fulfilling the sizing requirement to “load carrying structure plus damage tolerance/robustness” i.e capacity in service to be tolerant to high energy/low speed impact to minimize maintenance tasks. On top of this, the next generation of must combine the 2 previous domains plus a “multifunctional capability” i.e electrical conductivity, ….In conclusion to define a fully optimized fuselage, muti functional materials must  be further developed combined with improved damage tolerance/large damage capabilities properties. It  will be the challenge of the next decade.

 

Large Scale models for A350

Michel Mahé, Stéphane Le Drogo, Marion Touboul

 AIRBUS FRANCE

In order to secure the A350 structural test campaign, the Virtual Full Scale Test project (ViFST) was launched to predict more accurately the behaviour of the aircraft, especially to better handle nonlinear effects (large displacements, plasticity, contact ...). The project covers most of the aircraft structure and brings together teams from 5 countries (Germany, England, Spain, India and France).The presentation will focus on the central model, which is composed of the central fuselage and the two wings. This is the largest model of the project (68 millions dof). The presentation will detail the particular challenges overcome by Airbus:

CAD data management / Model built / Model verification / Model runs and calculations management / Methods and tools development to deal with large scale model analysis and secure test campaign.

 

Prediction of composite structures behaviour -  A350 lessons learned

Laurent Risse

Airbus France

Composite structures are widely used on A350 for major structural components. Some specific prediction methodologies were developed or adapted from previous experience to capture the main optimum structure behaviour. The objective of this presentation is to illustrate the key lessons learned for the prediction of the behaviour and failure of composite structures. Some main sizing criteria will be reviewed, like:

  • Stability behaviour and interaction with impact damages
  • Effect of notches on composite panels
  • Global load redistribution effects and interactions at large scale
  • The needs for future challenges and required evolutions will finally be discussed.

 

CONTINUUM DAMAGE MECHANIC SESSIONS:

 

Virtual Testing of Laminated Composites: State-of-the-art and Challenges

Pierre Ladevèze

LMT-Cachan (ENS Cachan / CNRS / UPMC / PRES UniverSud Paris)
France

The field of laminated composite materials is both old and new. It is old in the sense that it was in the early 1960s that scientists and engineers started to study and apply the vast potential of fibrous composite materials seriously. It is new in the sense that the pace of the development of new materials and processes and of the emergence of new applications keeps accelerating, especially in the aeronautical and space industries, as the consequence of considerable research and technological progress. The talk will deal with the following question, which is central and crucial for the design of composite structures: how can one predict the evolution of damage up to—and including—final fracture? Such virtual testing, whose goal is to reduce drastically the huge number of industrial tests involved in current characterization procedures, constitutes one of today’s main industrial challenges in the aerospace industry. First, the talk will emphasize our own modeling answer, and its derivation from today’s understanding of the mechanisms of damage and their evolution on the micro, meso and macro scales. The proposed damage mesomodel could be seen as the homogeneized model of a micromechanics one; it includes a model of the interaction between delamination and ply microcraking which depends on some micromechanical material constants. Last enhancements will be also described. Unfortunately, even with the most advanced numerical methods, the use of these models in structural calculations leads to completely prohibitive computational costs. This is a stumbling block for structural design, which makes extensive use of real-time simulations. The Virtual Chart concept that we introduced should provide a means to overcome this block. This is a reduced model which is based on the fact that, in practice, structures can be divided into “families” which consist of similar structures which differ only in the values given to some parameters.. What are the concepts, tools and challenges which accompany such a Virtual Structural Testing with “work on line” and “work off line” are the questions also discussed here.

 

Simulating progressive in-ply and delamination failure in composite laminates using a combined elastoplastic damage model

E.V. Morozov and J.F. Chen

School of Engineering and Information Technology, The University of New South Wales at the Australian Defence Force Academy, UNSW Canberra, Australia

A combined elastoplastic damage model which accounts for both plasticity and damage effects is used to represent the mechanical response of composite laminates. Plastic behaviour of composite plies and material property degradation caused by the damage initiation and development are taken into account. The model is integrated into a finite element simulation procedure which accounts for in-ply and delamination damage effects. The strain-driven implicit integration procedure is developed using equations of continuum damage mechanics, plasticity theory and includes the return mapping algorithm. A cohesive zone model based on cohesive elements available in Abaqus is employed to simulate delamination behaviour in the adhesive interfaces. Composite and adhesive layers are simulated using continuum shell elements and cohesive elements stacked together in one finite-element model. The results of the progressive failure analyses of AS4/PEEK composite perforated laminates and double-notched AS4/3501-6 carbon-epoxy laminates subjected to in-plane tensile loading are discussed.

 

Damage and Failure of Laminated Composite Structures with Stress Concentrations under Various Mechanical Loads

Christian Hochard

LMA, Aix-Marseille Univ, CNRS 7051, Centrale Marseille, F-13402 Marseille, France

The failure of laminated composite structures is due to many mechanisms acting on various scales. It is possible to observe an early stage of transverse diffuse damage which does not lead to the rupture of the laminate contrary to the rupture in the fibre direction. A model based on the reduction of strength in the fibre direction for high levels of transverse damage was proposed. The influence of various mechanical loads (static-damage, fatigue-damage) on the evolution of the transverse diffuse damage were analysed and a non-linear cumulative damage model has been developed. The influence of loading rates (visco-damage) and constant load (creep-damage) is still under investigation. In addition, an original approach based on a Fracture Characteristic Volume (FCV) has been developed to predict the fibre failure of laminated structures with stress concentrations. The FCV is a cylinder defined at the ply scale on which an average stress is calculated.

ADVANCED NUMERICAL METHODS:

 

Failure analysis of composite laminates on different scales with the extended finite element method

F.P. van der Meer and L.J. Sluys

Delft University of Technology, The Netherlands

In this contribution, computational models for different aspects of laminate failure are presented. The presented models operate on different scales of observation; what they share is their usage of the extended finite element method (XFEM). The first model employs XFEM for transverse matrix cracks in laminates. This approach allows for many discrete cracks in the solution without having to mesh the cracks. Interaction with models for delamination and fiber failure is natural. The second model has been introduced for efficient large scale analysis of delamination. This model uses level sets to describe the location of the crack front and XFEM to create a weak discontinuity in the deformation across the front. Thirdly, results are presented from micromechanical simulation of delamination. A model that combines XFEM with level sets is employed to simulate cusp formation in mode II delamination.

 

 Peridynamics for failure and residual strength prediction of fiber-reinforced composites

Erdogan Madenci and Kyle Colavito

Department of Aerospace and Mechanical Engineering, The University of Arizona

Tucson, AZ 85721 USA

Peridynamics is a reformulation of classical continuum mechanics that utilizes integral equations in place of partial differential equations to remove the difficulty in handling discontinuities, such as cracks or interfaces, within a body. Damage is included within the constitutive model; initiation and propagation can occur without resorting to special crack growth criteria necessary in other commonly utilized approaches. Predicting damage and residual strengths of composite materials involves capturing complex, distinct and progressive failure modes. The peridynamic laminate theory correctly predicts the load redistribution in general laminate layups in the presence of complex failure modes through the use of multiple interaction types. This study also establishes the validity of peridynamics for predicting failure loads and final failure modes by considering composite laminate specimens with various hole diameters subjected to tensile and compressive loads.

Modeling strategies for hydrodynamic ram and blast effects on composite structures

 Jérôme Limido, Lars Olovsson, Jean-Luc Lacome, Arve Grønsund Hanssen

 IMPETUS Afea France, Sweden and Norway

 Modeling interactions between strong shocks in fluids and perforated/cracked composite structure often involves the use of complex numerical approaches limited by weak energy conservation or bad interfaces tracking and contact leakage. We propose a robust Lagrangian monolithic approach based on the coupling of mesh-free methods (corpuscular, γSPH) and a high order iso-geometric finite element approach. First, we make a critical analysis of the proposed approach and its constitutive tools: iso-geometric for very large deformation/perforation phenomena, γSPH for complex free-surface and fluid shock and corpuscular method for mine blast. Then, we will discuss industrial applications within IMPETUS Afea | Solver ®. These studies demonstrate the robustness of the proposed approaches and its effectiveness in terms of computation time with an implementation on GPU workstations.

 

DISCRETE APPROACHES SESSIONS:

 

Size effects on strength of scaled quasi-isotropic specimens with sharp cracks

Michael R. Wisnom, Xiaodong Xu, Stephen R. Hallett

Advanced Composites Centre for Innovation and Science, University of Bristol, UK

Experimental results are presented for centre notched tension (CNT) carbon/epoxy specimens with notch lengths from 3 to 50 mm and all in-plane dimensions scaled. The damage zones are investigated by means of CT scans on interrupted tests. Results are compared with open hole tension tests on laminates with the same specimen dimensions and notch size. Scaled overheight compact tension tests on the same laminates are also presented and compared with the CNT results. Size effects are discussed, and discrete finite element models described that are able to capture the observed phenomena.

 

Simulation of damage in polymer composites at different length scales

Pedro Camaho

University of Porto, Portugal 

The difficulty in the prediction of the inelastic deformation and fracture of fibre-reinforced polymer composites under general thermo-mechanical loading results in the need of expensive, experimentally based certification programs, and in the non-optimal use of these materials. This presentation will describe the efforts that have been conducted to develop enhanced analysis models for composite materials. Different length scales for the representation of the mechanical response of composite materials, from micromechanical to macromechanical models, will be discussed.

 

Discrete ply modelling of impact and compression after impact on composite laminates

C. Bouvet, S. Rivallant, N. Hongkarnjanakul, H. Abdulhamid, B. Ostré

Institut Clément Ader, ISAE Supaéro,  University of Toulouse, France 

Failure of composite laminates is due to a combination of elementary damage types, namely fiber debonding, matrix cracking, delamination and fiber breakage. To simulate damage during impact and compression after impact (CAI), the key points are the interaction between matrix cracking and delamination, and the compressive failure of fibers leading to cracks. One of the most efficient methods to model this complex inter/intra-laminar interaction is the use of interface elements for both delamination and matrix cracking, as it takes into account the discrete character of these phenomena. This is the concept of “dicrete ply model” (DPM). A unique DPM, associated to a specific law for fibers failure in compression, was successfully used to simulate damage developing during impact and its propagation during CAI test - including compression cracks leading to the final collapse. The main advantage of DPM is the low number of material parameters required and their physical signification.

 

Discrete ply modelling of open-hole tensile tests and a route for further experimental validations

B. Castanié, C. Bouvet, V. Achard, J. Serra 

Institut Clément Ader, INSA/ISAE Supaéro,  University of Toulouse, France 

The Discrete Ply Modeling (DPM) method, previously applied with success to out-of-plane loading like impact or pull-through is used to model open hole tensile tests. According to the literature, this kind of test is relevant to assess the efficiency of a modeling strategy. Four different stacking sequences are tested and the failure scenario and patterns are well predicted. The main advantages of DPM are the very few number of parameters required and the robustness. The main drawback is the computation cost. Further validation tests will also be presented: the case of offset failure in filled hole compression tests and complex loadings through the Vertex program.  

 

Semi-continuous approach for the study of impacts on woven composite laminates: modeling interlaminar behaviour with a specific interface element

Florian Pascal, Olivier Dorival, Steven Marguet, Pablo Navarro, Jean-François Ferrero

Institut Clément Ader, UPS,  University of Toulouse, France

This study investigates impacts on sandwich structures made up of a thin woven composite skin and polyethylenimine foam core. A semi-continuous Finite Elements explicit modeling is proposed to represent the response of the woven skin made of plies with different orientations. The plies are modeled with specific woven elements, and are bonded with a new damageable shell-to-shell interface element. The modeling strategy is validated by representing static bending, low velocity impacts and medium velocity oblique impacts. The presented approach is accurate enough to predict the size and the shape of the damage of the woven composite laminate.

 

Damage and failure predictions of composite laminates structure by a multiscale hybrid approach

C. Huchette, F Laurin, N. Carrére, JF Maire

ONERA Chatillon, DMSC, France

This presentation will describe a multiscale hybrid approach for predicting damage and failure of laminated composite structures based on the thermo-mechanical properties (stress/strain behaviour and strength) of the unidirectional plies. This kind of approach is thus predictive for different stacking sequences. The approach introduces viscosity of the matrix in order to obtain an accurate description of the mesoscopic behaviour, especially the non-linearity under shear loading. The failure criterion used is based on physical principles and introduces micromechanical aspects (such as the effect of the local debonding on the non-linear failure behaviour) at the mesoscopic scale. The damage variable of this model are not considered as “effect variables” but could be directly linked to physical observed indicators  such as matrix crack density or the length of local delamination present at the tip of the matrix cracks. Thanks to these damage variables, a coupling between those cracks and delamination (inter-ply damage) has been proposed in order to predict the strength of composite structures for different levels of complexity (unnotched plates, open-hole plates) and subjected to complex loadings (membrane or bending loadings). The identification procedure and the implementation of such model in an implicit finite element code will be also discussed as the possibility to simplify this approach in order to reduce the computation costs.

 Discrete Damage Modeling in Laminated Composites under Static and Fatigue Loading.

Endel Iarve

Air Force research Laboratory,USAF, Dayton, OH, USA 

Composite failure is a complicated process of initiation and interaction of matrix and fiber dominated failure modes. Multiple approach to solution this probleme have been explored over past decades. Computational methodologies such as Mesh Independant Crack (MIC) Modelling have been introduced allowing explicit modeling of large quantities of individual damage events such as matrix cracking and were shown to accurately model complex interactive failure processes including delaminations and fiber failure. Recently, these methods have been extended to fatigue loading and will be discussed in the presentation along with multiscale modeling concepts

 


 

 

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