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1 st CoSACNet Meeting. Design tools for composites. Neil McCartney NPL Materials Centre National Physical Laboratory, UK. Southampton University , 30 January 2001. Definition of design ?.

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1 st cosacnet meeting

1st CoSACNet Meeting

Design tools for composites

Neil McCartney

NPL Materials Centre

National Physical Laboratory, UK

Southampton University , 30 January 2001


1 st cosacnet meeting

Definition of design ?

  • Selection of materials, geometry, loading modes and limits so that products meet specified performance criteria e.g.

    • deflections within specification

    • failure loads in excess of maximum expected load during service

    • avoidance of microstructural damage ( ply cracks / delaminations )

    • lifetimes ( cycles / time ) in excess of specification

  • Design is quantitative and based on mathematical models that adequately represent behaviour

    • semi-empirical / phenomenological relationships

    • analytical formulae ( fromHooke’s law to complex analyses )

    • finite element or boundary element analysis


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Modelling issues for composites

  • Reliable design procedures will be based on physical modelling

  • The availability of high performance computers will revolutionise the design of composite structures

  • Realistic complex models can be used for design of materials (Virtual Testing) and components

  • Models must be thoroughly validated and incorporated into easy-to-use design procedures

  • Life prediction, durability are exceedingly complex phenomena that are very difficult to model physically

  • Phenomenological approaches can be useful but are not usually reliable – e.g. failure criteria for composites


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Conventional failure criteria

Tsai - Wu (1971)

  • phenomenological in nature - no physics !

    • based on invariance requirements

  • applied to stress states for composite structures where no damage has been allowed for

  • not easily applied to environmentally or fatigue damaged composites

Physically based models are needed !


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Design issues for composites

  • Materials design

    • Fibre / matrix / volume fraction selection for UD laminates

    • Orientation / thickness selection for plies in a laminate

  • Prediction of elastic constants

  • Prediction of expansion coefficients ( thermal / moisture )

  • Types of loading

    • In-plane biaxial - through-thickness - shear

    • Out-of-plane bending ( anticlastic bending )

  • Damage growth and property degradation

    • Ply cracking – delamination – fibre fracture – interface debonding

  • Strength predictions

  • Durability issues – fatigue – environmental exposure

  • Delivery of design methods to users ( Software – Web )


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Delivering design tools to users

Commercial systems LAP and CoDA

UK Composite Design Toolset

( DERA, AEAT, NPL, SER Systems Ltd )

Web-based design tools – E-mail communication

Smart design manuals


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A commercially supported package

CoDA

for

Component and Composite Design Analysis

Version 3

Graham D Sims and Bill Broughton

NPL Materials Centre

National Physical Laboratory, UK

CoDA


What does coda do

What does CoDA do ?

  • CoDA has four independent, but integrated, modules that have been validated experimentally

    • Panels, Beams, Laminates, Materials Synthesiser

    • Pre-preg laminates, chopped strand mat, sandwich panels

    • Implementation of failure criteria

  • CoDA can be used to undertake preliminary analysis of sub-components with Plate or Beam geometries

  • CoDA can also synthesise the properties of composite materials, laminates and sandwich structures, which can be used in a seamless manner within the design modules

CoDA


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CoDA


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CoDA


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CoDA


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CoDA


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UK Composite Design Toolset

  • A collaboration between DERA, UKAEA & NPL

  • Integrated toolbox comprising modules that can exchange data & results

    • PC008A/15A – DERA – Micromechanics, LPT, 2D/3D

    • GENLAM – DERA – Non-linear LPT – thermal stresses

      – scissoring

    • CCSM – Cambridge, IC, DERA – Micromechanics + LPT

      – unnotched & notched failure

    • PREDICT – NPL – progressive damage modelling in laminates

    • LAMFAIL – UMIST, DERA progressive damage with empirical

      model – nonlinear scissoring – complex load histories

  • A global data base of materials properties – links to other systems


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    PREDICT - Design objectives

    • Predict properties of UD composites from properties of fibre and matrix

    • Predict in-plane properties of general symmetric laminates

    • Predict initial formation of fully developed ply cracks in a general symmetric laminates subject to general in-plane loading and thermal residual stresses

      ( in fatigue loading designers will want to avoid damage )

    • Predict progressive degradation of thermo-elastic constants as a function of applied stress or strain ( strain softening rules needed for FEA analyses )

    • Predict effects on damage resistance of varying orientations and thicknesses of plies in a laminate

    • Predict effects of temperature changes on ply crack formation

      ( investigate thermal cracking during manufacture, or cooling )


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    Designing composites from fibre and matrix level

    Predicting ply properties – validation

    Predicting laminate properties

    Delaying damage formation during loading

    PREDICT


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    Quasi - isotropic laminates

    Ply geometry and location of ply cracks

    Ply 1

    Ply 2

    Ply3

    Ply 4

    2L

    0o

    90o

    - 45o

    45o

    45o

    - 45o

    90o

    0o


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    45o

    - 45o

    90o

    0o

    Plycrack


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    GRP : Predicted from fibre/matrix properties

    Experimental data : Lodiero & Broughton, NPL, 2000


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    Stress - strain relations for damaged laminate

    w is a label denoting the presence of some form of damage in the

    laminate defined by a set of other parameters

    Same form as those for an undamaged laminate

    Validity confirmed by accurate stress analysis


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    Degradation of properties of laminates

    Damage parameter :

    Thermo-elastic constants for damaged laminates :

    k, k’ and k1 are easily calculated using CLT


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    Continuum damage mechanics (CDM)

    Stress - strain relation e = Sswhere

    Damage parameters d1, d2and d3 are such that


    Face view of crack growth using continuum model

    Face view of crack growth using continuum model

    Master curve for ply cracking

    s

    s

    s

    s


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    • Design limit is derived for long cracks

    • Design limit is exact if growth is stable

    • Design limit is a lower bound if growth is unstable

      • Predictions are pessimistic

      • Designs will be safe

    Unstable growth

    Design limit

    Stable growth


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    Criteria for ply crack formation

    Ply crack initiation criterion :

    s0 is value of s at ply crack closure

    Criterion for progressive discrete ply crack formation :


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    Progressive cracking methodology

    Potential cracking sites

    Ply crack locations

    Potential cracking sites

    are evenly spaced

    Ply cracks are

    non-uniformly spaced


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    Master curve for triaxial loading

    Key features :

    s - s0

    • Damage initiation stress

    • Gradient of unloading line

    • Enclosed area

    • = 0

    si- s0

    Apply directly to other stress

    and temperature states

    for which g = 0 :

    Area

    G(w)

    Inelastic

    strain

    e(w) - e

    0


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    A popular approach of damage mechanics

    Degraded properties

    modelled

    semi-empirically

    Classical laminate

    analysis

    Homogeneous

    damaged ply

    in laminate

    Homogenised

    Cracked

    laminate


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    Approach of NPL model

    Homogenised

    Cracked

    laminate

    Homogeneous

    damaged ply

    in laminate

    Cracked

    laminate


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    Out - of - plane bending

    • Non-symmetrical laminates

    • Through-thickness thermal gradients

    • Major problem is dealing with anti-clastic bending

    • Model already exists for ply cracks subject to plane strain bending

    Anti-clastic bending of UD ply


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    Plane strain bending of cracked [ 0 / 90 / 0 / 90 / 0 ] laminate

    M

    M

    y

    0

    0

    i = 1

    i = 2

    90

    0

    90

    i = N+1

    0

    x

    - Work in progress to predict ply crack formation -


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    GRP laminate

    Ply crack

    90o ply

    0o ply

    0o ply

    90o ply

    0o ply


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    Modelling laminate failure

    Physical modelling of damage modes

    Cross-ply laminates subject to biaxial loading

    Prediction of failure strain and strength


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    Modelling failure of cross-ply laminates

    • Effects of ply cracking alone on laminate properties are well understood

    • Ply cracking affects thermo-elastic properties (strain softening) but we need to address laminate failure issues

    • Tensile failure is determined principally by fibre fracture

    • Statistical nature of fibre failure must be included

    • Predicting the failure of cross-ply laminates is the first step

    0o

    90o

    0o

    Biaxial loading

    Thermal stresses

    Multiple plies


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    Modelling failure of cross-ply laminates

    0o

    90o

    0o

    Static

    failure of

    fibres

    • fibre/matrix debonding

    • frictional contact

    • shear yielding

    Biaxial loading

    Thermal stresses

    Fibre matrix cell

    Biaxial stresses

    Thermal stresses


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    L - b is length of

    debond zone

    Mechanical behaviour

    of fibre / matrix cell

    Could substitute any other model

    for which a look-up table can be constructed


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    Monte Carlo model for progressive failure in 0o plies

    Repeated runs of

    a simulation to

    determine the

    statistical variability

    of performance

    Parameters :

    M, N, L

    Element lengthd


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    Critical fibre stress or strain ?

    • In fibre tests performance of fibre in tension can be characterised by :

      • axial fibre stress at failure

      • axial strain at failuresf = Efef

    • In a composite fibre subject to triaxial loading

      • there are both loading & Poisson ratio effects on axial fibre strain

    • Assume fibre strength in a composite is governed by axial fibre failure strain

      • consistent with concept of fibre axial strain controlling the stability of fibre defects initiating tensile failure


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    Failure

    Failure

    The effect of fibre fracture

    on properties

    Failure

    CFRP (Vf = 0.6)


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    Virtual Testing

    • Virtual testing is defined as the combination of high quality models, high performance computing and a user - friendly interface

      • Will replicate many aspects of physical mechanical testing so that engineers do not need to learn a new vocabulary

      • Will allow material properties to be derived from more fundamental properties, leading to inventive materials design

      • It will be more than just a simulation, because extensive validation and testing will have taken place, resulting in a reliable replacement for some physical testing


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    Virtual testing of composites over the Internet

    Web site address http://materials.npl.co.uk/


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    Virtual testing : Composite laminates

    • Developed at NPL, the Internet based system enables a materials designer to ‘create’ an entirely new material and to test it

    The system simulates the damage caused by cracking

    as load increases and predicts the subsequent degrading of material properties

    Image taken from NPL Internet Laminate Damage Simulation


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    Composite Laminate Testing

    • The user can generate design data for damaged composite laminates

    Results taken from

    NPL Internet version of Laminate Damage Simulation


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    The Future : The era of simulation


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    Conclusions

    • Reliable design methods for composite materials will be based on physical models of behaviour. Key to reliability is rigorous validation of design methods

    • Physical models are complex in nature and not usually amenable to simple design rules ( sometimes there is no alternative )

    • Damage models have good potential for application in construction sector

      ( e.g. bridge strengthening with CFRP )

    • The implementation of physically based design methods in design offices will usually involve the use of computer based techniques :

      • Specific software packages – LAP, CoDA, Toolset

      • Web-based access to specific design packages, NPL demonstrator

      • Web-based access to distributed software – networking ?

      • Integration of design, optimisation, prototype and production simulation in virtual manufacturing


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