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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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1st CoSACNet Meeting

Design tools for composites

Neil McCartney

NPL Materials Centre

National Physical Laboratory, UK

Southampton University , 30 January 2001

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

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

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 !

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 )

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

A commercially supported package



Component and Composite Design Analysis

Version 3

Graham D Sims and Bill Broughton

NPL Materials Centre

National Physical Laboratory, UK


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


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

  • 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 )

    Designing composites from fibre and matrix level

    Predicting ply properties – validation

    Predicting laminate properties

    Delaying damage formation during loading


    Quasi - isotropic laminates

    Ply geometry and location of ply cracks

    Ply 1

    Ply 2


    Ply 4




    - 45o



    - 45o




    - 45o




    GRP : Predicted from fibre/matrix properties

    Experimental data : Lodiero & Broughton, NPL, 2000

    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

    Degradation of properties of laminates

    Damage parameter :

    Thermo-elastic constants for damaged laminates :

    k, k’ and k1 are easily calculated using CLT

    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





    Unstable growth

    Design limit

    Stable growth

    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 :

    Progressive cracking methodology

    Potential cracking sites

    Ply crack locations

    Potential cracking sites

    are evenly spaced

    Ply cracks are

    non-uniformly spaced

    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 :





    e(w) - e


    A popular approach of damage mechanics

    Degraded properties



    Classical laminate



    damaged ply

    in laminate




    Approach of NPL model





    damaged ply

    in laminate



    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

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






    i = 1

    i = 2




    i = N+1



    - Work in progress to predict ply crack formation -

    GRP laminate laminate

    Ply crack

    90o ply

    0o ply

    0o ply

    90o ply

    0o ply

    Modelling laminate failure laminate

    Physical modelling of damage modes

    Cross-ply laminates subject to biaxial loading

    Prediction of failure strain and strength

    Modelling failure of cross-ply laminates laminate

    • 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




    Biaxial loading

    Thermal stresses

    Multiple plies

    Modelling failure of cross-ply laminates laminate





    failure of


    • fibre/matrix debonding

    • frictional contact

    • shear yielding

    Biaxial loading

    Thermal stresses

    Fibre matrix cell

    Biaxial stresses

    Thermal stresses

    L - b is length of laminate

    debond zone

    Mechanical behaviour

    of fibre / matrix cell

    Could substitute any other model

    for which a look-up table can be constructed

    Monte Carlo model for progressive failure in 0 laminateo plies

    Repeated runs of

    a simulation to

    determine the

    statistical variability

    of performance

    Parameters :

    M, N, L

    Element lengthd

    Critical fibre stress or strain ? laminate

    • 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

    Failure laminate


    The effect of fibre fracture

    on properties


    CFRP (Vf = 0.6)

    Virtual Testing laminate

    • 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

    Virtual testing of composites over the Internet laminate

    Web site address

    Virtual testing : Composite laminates laminate

    • 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

    Composite Laminate Testing laminate

    • The user can generate design data for damaged composite laminates

    Results taken from

    NPL Internet version of Laminate Damage Simulation

    Conclusions laminate

    • 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