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Multi-scale modelling of the structural integrity of composite materials

Scientific meeting

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

Theo Murphy scientific meeting organised by Dr Peter Beaumont and Professor Constantinos Soutis.

Glass thread tuft loop in carbon fibre fabric composite obtained by CT scan, courtesy of Professor Ivana Partridge, University of Bristol

The nature of cracks that threaten the safety of structures is probed. Material weaknesses that compromise strength are identified. Multi-scale phenomena of the structural changes in composites under stress are modelled. Properly interpreted, structural integrity analysis forecasts the limits of performance of the composite and conditions for safe operation of the engineering structure.

Audio recordings of this meeting will become available in the coming weeks.

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Event organisers

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Schedule of talks

25 January

Session 1 09:00-12:30

Multi-scale modelling & analysis: nano, micro and meso mechanics

4 talks Show detail Hide detail

Chairs

Professor James Marrow, University of Oxford, UK

09:05-09:30 Hierarchical analysis of the degradation of FRP under the presence of void imperfections

Professor Karl Schulte, Hamburg University of Technology, Germany

Abstract

The subject of this work is the hierarchical analysis of the degradation of FRP under the presence of void imperfections. To specify the damage accumulation of FRP in the presence of voids, the complex three dimensional structure of the composite inclusive several voids was analysed. Based on this analysis a reduced model composite was derived for mechanical tests. The reduced model composite consists of the matrix and a unique void, which is squeezed between two single fibres, using an injection method. The experimental investigation of the model composite included the description of the stress- and strain behaviour of the matrix using photoelasticity and digital image correlation technology. Additionally, the numerical examination of a parametrical model composite and an analytical study of the stability of a single fibre were conducted. As a result of the experimental investigation of the model composite the failure initiation and propagation could be observed. The failure initiation in the composite is significantly influenced by the stiffness of the matrix and fibres, the fibre foundation and the fibre misalignment. In addition, the aspect ratio of the void leads to stress concentrations in the matrix leading to premature fibre-matrix debonding and to an ongoing loss in stability of the fibre depending on its foundation which finally causes fibre kinking. 

By increasing the number of fibres and utilised rovings, instead of several fibres respectively, the gained experience made on the examination of the model composite can be transferred to real existing composites. Only by taking the essential boundaries into account the hierarchical analysis can be constructive. This allows an answer to the basic question: “why do composites fail in the presence of voids?”, in particular in terms of the damage initiation and the failure propagation due to void imperfections.

In the presentation we span the bow from the experiment (mechanical testing) on void loaded specimens and parts to the numerical interpretation and modelling of its failure behaviour.

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09:45-10:15 The mechanics of reinforcement by nanoplatelets

Professor Robert Young, University of Manchester, UK

Abstract

This presentation will deal with the use of nanoplatelets such as graphene and other 2D materials to reinforce polymers, from both an experimental and theoretical viewpoint. Issues to be covered will include the lateral size of the platelets, number of layers, orientation distribution and surface modification.

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11:00-11:30 Failure of notched composites under multi-axial loading

Professor Norman Fleck FRS, University of Cambridge, UK

Abstract

Characterisation and modelling has now got to the degree of maturity that it is possible to compare damage evolution ply-by-ply and inter-ply in composite components under multi-axial loading.  As an example, we have explored the notch strength of CFRP laminates, including the mechanisms of damage development. The sensitivity of failure envelope to lay-up and notch geometry is explored, and predictions are made using a combination of delamination and damage models. The accuracy of existing models is evaluated, with a view to vector future modelling activity.

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11:45-12:15 Modelling the development of defects during composite reinforcements and prepreg forming

Professor Philippe Boisse, INSA de Lyon, France

Abstract

The deformation modes of composite reinforcement or prepregs with continuous fibers are very specific.  The fibers are very stiff in tension but they can slip over one another and their bending stiffness is very small. In order to decrease the number of trial and error tests used in the process developments, simulation codes allow for a more efficient and cost-effective design approach.

Wrinkling is one of the most common flaws that occur during textile composite reinforcement forming processes. These wrinkles are frequent because of the possible relative motion of fibres making up the reinforcement, leading to a very weak textile bending stiffness. It is necessary to simulate their onset but also their growth and their shape in order to verify that they don’t extend to the useful part of the preform. The simulation of textile composite reinforcement forming and wrinkling is presented. It is based on a simplified form of virtual internal work defined according to tensions, in-plane shear and bending moments on a unit woven cell. The role of the three rigidities (tensile, in-plane shear and bending) in wrinkling simulations will be analyzed.

The onset and development of porosities during the thermoforming of thermoplastic prepregs will also be investigated. The essential role of the reconsolidation stage to remove these defects will be shown. Specific shell elements aiming to model this phenomenon will be presented.

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12:30-13:30

Lunch

Session 2 13:30-17:00

Physical modelling: Impact, fatigue & environmental damage

5 talks Show detail Hide detail

Chairs

Professor Philippe Boisse, INSA de Lyon, France

13:30-14:00 Physical modeling of failure in composites under static and fatigue loading

Professor Ramesh Talreja, Texan A&M University, USA

Abstract

Structural integrity of composite materials is determined by failure mechanisms that initiate at the scale of heterogeneities. The local stress fields and balances between the available and dissipative energy components evolve with the progression of the failure mechanisms. Within the full span from initiation to criticality of the failure mechanisms, the governing length scales in a fiber-reinforced composite structure change from the fiber size to a characteristic fiber-architecture size, and eventually to a structural size, depending on the structural geometry and imposed loading environment. Thus a physical modeling of failure in composites must inherently be of multi-scale nature, although not necessarily with the same hierarchical structure for each failure mode. With this background, the proposed paper will examine the currently available main failure theories of composites to assess their ability to capture the essential features of failure in these material systems. A case will be made for a different approach. An alternative in the form of physical modeling will be presented and its skeleton will be constructed based on systematic observations and considerations of basic failure modes and associated stress fields and energy balances. Both static and time-varying loads will be considered. Some experimental data will be taken to support the proposed approach and methodology.

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14:15-14:45 Environmental degradation of composites for marine structures: new materials and new applications

Dr Peter Davies, IFREMER, France

Abstract

This presentation will describe the influence of seawater aging on composites used in a range of marine structures, from boats to tidal turbines. The traditional approach to account for aging effects, based on testing samples after immersion for different periods, is evolving towards coupled studies involving strong interactions between water diffusion and mechanical loading. These can provide a more realistic estimation of long term behavior but still require some form of acceleration if useful data, for 20 year lifetimes or more, are to be obtained in a reasonable time (a few months). In order to validate extrapolations from short to long times it is essential to understand the degradation mechanisms, so both physico-chemical and mechanical test data are required. Examples of results from some current studies on more environmentally friendly materials including bio-sourced composites will be described first. These materials offer considerable potential for certain applications but few long term data are available. Then case studies for renewable marine energy applications will be discussed, including an investigation of the durability of carbon fibre reinforced tidal turbine blades. The latter have been studied first on coupons at the material level, then during structural testing and analysis of large components, in order to evaluate long term behavior. 

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15:30-16:00 Progressive damage in composites characterised by time lapse x-ray computed tomography

Professor Philip Withers, Regius Professor of Materials, University of Manchester

Abstract

This study investigates the use of time lapse 3D imaging by X-ray micro-computed tomography to follow the evolution of damage as a function of load and time.  Either with our without staining it is possible to detect the morphology, number and progress of cracks and damage in 2D laminated and 3D woven composites under a variety of loading scenarios including fatigue testing.  Aspects of the technique and examples showing its capability will be presented. Furthermore it will be shown that it is possible to use the 3D models as the geometrical basis for image-based finite element models of damage evolution as well as to establish the validity of conventional representative volume element approaches.

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16:15-16:45 From matrix nano-phase tougheners to composite macro-properties

Professor Anthony Kinloch FREng FRS, Imperial College London, UK

Abstract

High-performance fibre-composites typically employ epoxy resins as the matrix. Epoxies are highly-crosslinked thermosetting polymers, which exhibit good elevated temperature resistance and low creep. However, their high crosslink-density causes them to be relatively brittle polymers. This limits their application as structural materials, as they have a poor resistance to the initiation and growth of cracks. The addition of a second particulate-phase, which is well dispersed, can increase the toughness of thermoset polymers. Thus, an important first goal is to achieve a significant increase in toughness in the 'bulk' epoxy polymer and then to translate this to an improved interlaminar fracture energy, GIc, and impact resistance for the fibre-reinforced composite that employs the toughened epoxy-polymeric matrix. Secondly, with the increased demand for relatively inexpensive resin transfer moulding (RTM)  processing -  i.e. 'out-of-the-autoclave processing'   -  the viscosity of the epoxy-resin matrix also needs to be kept to a relatively low value. Thirdly, the toughening second-phase must not be 'strained-out' by the presence of the fibres during RTM processing.
The present paper will demonstrate that a second phase of silica nanoparticles, which is very well dispersed via an in-situ manufacturing route, in the epoxy matrix can achieve all of the above goals. Further, the cyclic-fatigue resistance of both the epoxy polymeric-matrix and the fibre-composite is also significantly improved. The toughening mechanisms will be identified and quantitatively modelled  -  and both continuous glass- and carbon-fibre composites have been studied.
Work currently in progress is studying the combined effect of a second phase of such silica nanoparticles together with a dispersed phase of nano-sized core-shell rubber (CSR) particles, based upon a polysiloxane rubber, to form a 'hybrid' toughened epoxy-matrix. Again, both the 'bulk' epoxy matrices and the fibre-composites employing such matrices will be studied in order to measure and model the translation of the matrix properties to those of the fibre-composites.

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17:50-18:20 String and glue - fifty years in composites

Dr Carl Zweben, Industry Consultant on Composites and Advanced Thermal Materials, USA

Abstract

This paper presents a very personal, half-century view of the history of advanced composites from the early days before carbon fibers were commercial to today, when they are used in a remarkable range of applications from aircraft to serial production automobiles and countless other products. We concentrate on polymer matrix composites, the dominant class of materials, but consider metal matrix composites, ceramic matrix composites and carbon matrix composites, such as carbon/carbon. While outstanding mechanical properties have been the key driving force for composites, physical properties like thermal conductivity and coefficient of thermal expansion have become increasingly important.

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26 January

Session 3 09:00-17:00

Hierarchical analysis: high fidelity simulation; virtual mechanics

4 talks Show detail Hide detail

Chairs

Dr Peter Davies, IFREMER, France

09:00-09:30 Damage in brittle composites: high fidelity simulation of microstructure effects and its experimental validation

Professor James Marrow, University of Oxford, UK

Abstract

Novel analysis methods, such as digital image correlation, provide experimentalists with the tools to detect and quantify damage that can be below the threshold of visible resolution; three dimensional observations within composites may be achieved in suitable microstructures using in situ X-ray computed tomography.  These data may inform and also test high-fidelity simulations that model the development of damage in composites.  The challenge lies in appropriate treatment of the high-resolution characterisation of large representative volumes of microstructure, which can easily lead to inoperable models unless homogenisation is done to reduce the model's computational cost.  Homogenisation, however, results in a loss of detail.

We will present examples of how this challenge may be addressed, using high fidelity modelling and experimental observation of damage in ceramic matrix composites to demonstrate how X-ray tomography and image correlation can be applied to validate computed simulations.  A new approach, 'Cellular-Automa Finite Element - Microstructural Adaptive Meshfree framework' is also presented; this offers significant improvements in computional efficiency, whilst retaining high fidelity to the microstructure.  Its application to a self-healing laminate composite is described.

The aim of this research is to be able to extract critical material parameters from small specimen tests of composite structures.  Applied to monitor the degradation of materials, this approach has the potential to simulate the influence of an aging microstructure on the performance of larger scale components, with applications from aerospace to advanced nuclear energy.

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09:45-10:15 A-FEM for high fidelity multi-scale failure analysis of structural composites

Professor Qingda Yang, University of Miami, USA

Abstract

Advanced composites have been increasingly used as primary load-bearing structures or structural members. While they offer many excellent properties, significant concerns regarding their long term durability and safety remain. One particular unsolved challenge is to quantify the uncertainty associated with the service life. Typical composites exhibit complex, multiple damage events including multiple crack initiation events within and between plies/tows from pristine materials, through their coupled growth, involving multiple bifurcation, coalescence, and ply-jumping, to eventual failure. Numerical and/or analytical methods capable of predicting the progressive damage evolution in composites under general thermal-mechanical loadings and quantifying their effects on structural integrity are in high demand.

High-fidelity thermal-mechanical analyses to composite structures with explicit consideration of arbitrary crack-like damage events are challenging because the heterogeneous nature of composites makes it impossible to know the cracking locations a priori, yet efficient accounting of the nucleation, coalescence, and bifurcation of multiple damage processes is critical to simulation fidelity. There are several methods that can deal with arbitrary cracking in solids such as the extended FEM (X-FEM) and the phantom-node-method (PNM). These methods typically employ crack tip enrichment functions through portioning-of-unity, which are inherently nonlocal and are difficult in handling merging or bifurcating cracks. The need for extra DoFs or nodes for new or propagating cracks also makes these methods inflexible in treating crack interaction. 

In this study, we present a multiscale simulation platform based on our newly developed augmented finite element method (A-FEM). We shall demonstrate that the A-FEM is able to achieve high fidelity simulations of multiple, arbitrary damage evolution processes in composite structures with an improved numerical efficiency of several orders of magnitude. The capability of the new A-FEM will be demonstrated through several case studies on realistic composite systems at various important scales. Important information regarding how the microscopic damage processes and their evolution can impact the macroscopic structural performance/integrity will be discussed.

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11:00-11:30 Multi-scale characterization and representation of composite materials during processing

Professor Anoush Poursartip, University of British Columbia, Canada

Abstract

The worlds of structural simulation and process simulation are rapidly converging as it becomes increasingly clear that failure predictions can be significantly enhanced if process-induced conditions and defects are captured in the structural simulation.  Processing outcomes that affect structural response range from thermal outcomes (e.g. degree of cure or crystallinity, physical morphology), to quality outcomes (e.g. ply thickness, volume fraction, wrinkling and waviness, porosity), or mechanical outcomes (e.g. dimensional change and residual stress).

The state of the art in modelling these outcomes as a function of processing conditions is quite varied in terms of rigour, completeness, breadth, integration, and application.  Broadly speaking, there are three ranges of matrix response where different simulations strategies can be applied: (a) high temperature, low cure/crystallinity where the system is essentially fluid like and viscous; (b) mid-temperature, gelled or crystallized where the system is viscoelastic; and (c) lower temperature, below the glass transition temperature, where the system is essentially elastic.  The behaviour of the system must also be considered at all scales, which in processing includes not only the classical micro-scale, meso-scale, layer/lamina scale, and structural scale, but also the tooling, equipment and other associated manufacturing initial and boundary conditions.  Here we present some of our most recent work in creating a coherent and consistent framework for process modelling, and provide experimental and simulation results to highlight both successes and needs. In particular, we focus on residual stress development under different thermal histories, and show how a multi-scale modelling approach can be exercised to evaluate and predict different aspects of this problem.

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11:45-12:15 Computational micromechanics of composites: a mature discipline?

Professor Carlos González, Polytechnic University of Madrid and IMDEA Materials Institute, Spain

Abstract

Computational micromechanics is emerging as an accurate tool to study the mechanical
behaviour of composites at the micro scale due to the sophistication of the modeling tools and to the ever-increasing power of digital computers [1]. Within this framework, the macroscopic properties of a composite or heterogeneous material can be obtained by means of the numerical simulation of the deformation and failure of a statistically representative volume element of the microstructure. As compared with classic homogenization techniques, computational micromechanics presents two important advantages. Firstly, the influence of the geometry and spatial distribution of the phases (i.e. size, shape, clustering, connectivity, etc.) can be accurately taken into account. Secondly, the details of the stress and strain microfields throughout the microstructure are resolved, leading to precise predictions of the onset and propagation of damage.

Recent advances in this area include the analysis of the effect of different fiber (carbon and glass [2]), interfaces [3], spatial distribution and ply thickness or the influence of damage on the final mechanical performance of fiber-reinforced composites [4,5], as well as the physical determination by detailed experimental techniques based on instrumented nanoindentation [6,7] of the parameters governing the mechanical behavior of the composite at this length scale.

[1] Llorca, J; Gonzalez, C; Molina-Aldareguía, J M; Segurado, J; Seltzer, R; Sket, F; Rodriguez, M; Sadaba, S; Munoz, R; Canal, L P, Multiscale modeling of composite materials: a roadmap towards virtual testing, Advanced materials, 23, 5130-47, 2011, DOI: 10.1002/adma.201101683.
[2] E. Totry, J. M. Molina-Aldareguia, C. González, J. Llorca. Effect of fiber, matrix and interface properties on the inplane shear deformation of carbon-fiber reinforced composites. Composites Science and Technology, 70, 970-980, 2010.
[3] C. González and J. Llorca. Mechanical behaviour of unidirectional fiber-reinforced polymers under transverse compression: Microscopic mechanisms and modelling. Composites Science and Technology, 67, 2795-2806, 2007.
[4] LP. Canal, C. González, J. Segurado, J. LLorca, Intraply fracture of fiber-reinforced composites: Microscopic mechanisms and modeling, Composites Science and Technology, 72, 1223-1232,
DOI:10.1016/j.compscitech.2012.04.008, 2012.
[5] C. González and J. Llorca. Multiscale modelling of fracture in fiber-reinforced composites. Acta Materialia, 54, 4171-4181, 2006.
[6] M. Rodriguez, JM. Molina Aldareguia, C. González, J. LLorca, A methodology to measure the interface shear strength by means of the fiber push-in test, Composite Science and Technology, 72, 1924-1932,
DOI:10.1016/j.compscitech.2012.08.011, 2012.
[7] M. Rodriguez, JM. Molina-Aldareguia, C. González, J. LLorca, Determination of the mechanical properties of amorphous materials through instrumented nanoindentation, Acta Materialia, 60, 3953-3964,
DOI: 10.1016/j.actamat.2012.03.027, 2012.

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12:30-13:30

Lunch

Session 4 13:30-17:00

Science of multi-functional systems: processing, fabrication & QA

4 talks Show detail Hide detail

Chairs

Professor Karl Schulte, Hamburg University of Technology, Germany

13:30-14:00 Progress in the pursuit of reliable bonded composites

Dr Robert Crane, US Air Force Research Lab, USA

Abstract

Reliable and durable adhesive bonding is an indispensable capability that would permit the structural designer to utilize the many inherent capabilities of composites. Until very recently, reliable adhesive bonding depended upon rigid compliance with quality control procedures, which too often did not produce long-term reliable bonds.

Recently two novel approaches that overcome this limitation have been proposed and are under development. One technique can be used to qualify the crucial adherend surface preparation of prior to bond assembly. The second is capable of nondestructively measuring the bond strength in situ after consolidation. Recent research developments with both techniques have been encouraging. These will be discussed, advances highlighted, and future opportunities for bonded composites structures presented.

Initial designs employing these techniques in bonded composite aero structures indicate that the weight saving structural efficiencies associated with bird-inspired designs should allow for a paradigm shift in aircraft structural designs.

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14:15-14:45 Use of microfasteners to produce damage tolerant composite structures

Professor Ivana Partridge, University of Bristol, UK

Abstract

The traditional low delamination resistance of classical laminated composites can be tackled successfully by the insertion of rigid microfasteners along the through-thickness direction, at least as far as resistance to crack growth is concerned. This much has been established for some time now, initially for nails in wood, more recently for the case of thin rigid composite rods (‘Z-pins’) inserted into a prepreg stack prior to cure and for thin threads stitched or tufted into dry fibre preforms prior to resin infusion and cure. Metal to composite bonding can be improved in a similar fashion through the use of metallic pins on the metal adherend. Recent work shows how multi-material microfasteners can be exploited to deliver a mechanical advantage coupled with advantageous changes to thermal and electrical attributes such as self-sensing.

Through-the-thickness reinforcement has been demonstrated to deliver significant increases in delamination resistance under Mode I dominated loading modes in quasi-static as well as impact and fatigue conditions. The balance of out-of-plane and in-plane properties of thus reinforced composite is dependent on the parameters of the reinforcement but also on the stacking sequence and the thickness of the composite.  Parametric studies are therefore best carried out via multi-scale modelling, based on experimental determination of the crack bridging laws from a single microfastener placed in the relevant structural environment.

In practice, the insertion of the microfasteners can be controlled well, but subsequent movements and deformation in the debulking and cure stages of the processing are much more difficult to manage. Modelling can predict the effects of microfastener mis-alignment on mechanical or functional performance and the influences are shown to be considerable. Further progress therefore needs to be made in the control of the processing steps and in QA of such complex structures.

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15:30-16:00 Limits of sustainable damage in filament wound structures under short and long-term loading revealed by Multi-scale modelling

Professor Tony Bunsell, Mines Paris Tech, France

Abstract

Internally pressurised filament wound carbon fibre reinforced composite structures accumulate damage in the form of fibre breaks throughout their use. This raises the possibility of unexpected rupture with possible catastrophic consequences. It has been shown that the viscoelastic nature of the matrix material accounts for the progressive damage of the material. Multi-scale modelling from the level of individual fibres to that of the whole structure is now feasible due to increases in computational power. It will be shown that such an approach models accurately the rate of damage observed both in simple unidirectional composites and filament wound composites and allows damage to be described quantitatively both as a function of applied stress and also time. The ability to determine rates of damage and also critical damage levels, under a variety of loading conditions, which would lead to rupture, allows intrinsic safety factors to be calculated for structures such as pressure vessels used to store gases, including hydrogen and methane, at very high pressures for which failure cannot be envisaged.  It will be shown how multi-scale modelling allows a more detailed understanding of the kinetics of damage accumulation both under monotonic loading but also sustained long-term loading similar to that which would be experience in such structures during their in-service lives. It is expected that this approach should contribute to the redefining of international standards for testing and assessing filament wound composite pressure vessels and also allow them to be designed to be both economical and reliable. 

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16:15-17:00 Panel Discussion

Dr Alastair Johnson, Consultant DLR iR, Germany
Dr Tony Waas, University of Michigan, USA
Dr Carl Zweben, Industry Consultant on Composites and Advanced Thermal Materials, USA
Professor Constantinos Soutis, University of Manchester, UK
Professor Samit Roy, University of Alabama, USA

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Multi-scale modelling of the structural integrity of composite materials

Theo Murphy scientific meeting organised by Dr Peter Beaumont and Professor Constantinos Soutis.

Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ