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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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slide1

1st CoSACNet Meeting

Design tools for composites

Neil McCartney

NPL Materials Centre

National Physical Laboratory, UK

Southampton University , 30 January 2001

slide2

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
slide3

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
slide4

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 !

slide5

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 )
slide6

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

slide7

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

slide13

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
slide14

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 )

slide15

Designing composites from fibre and matrix level

Predicting ply properties – validation

Predicting laminate properties

Delaying damage formation during loading

PREDICT

slide16

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

slide17

45o

- 45o

90o

0o

Plycrack

slide19

GRP : Predicted from fibre/matrix properties

Experimental data : Lodiero & Broughton, NPL, 2000

slide20

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

slide21

Degradation of properties of laminates

Damage parameter :

Thermo-elastic constants for damaged laminates :

k, k’ and k1 are easily calculated using CLT

slide22

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

slide24

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

slide25

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 :

slide26

Progressive cracking methodology

Potential cracking sites

Ply crack locations

Potential cracking sites

are evenly spaced

Ply cracks are

non-uniformly spaced

slide27

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

slide28

A popular approach of damage mechanics

Degraded properties

modelled

semi-empirically

Classical laminate

analysis

Homogeneous

damaged ply

in laminate

Homogenised

Cracked

laminate

slide29

Approach of NPL model

Homogenised

Cracked

laminate

Homogeneous

damaged ply

in laminate

Cracked

laminate

slide30

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

slide31

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 -

slide32

GRP laminate

Ply crack

90o ply

0o ply

0o ply

90o ply

0o ply

slide33

Modelling laminate failure

Physical modelling of damage modes

Cross-ply laminates subject to biaxial loading

Prediction of failure strain and strength

slide34

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

slide35

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

slide36

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

slide37

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

slide38

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
slide39

Failure

Failure

The effect of fibre fracture

on properties

Failure

CFRP (Vf = 0.6)

slide40

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
slide41

Virtual testing of composites over the Internet

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

slide42

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

slide43

Composite Laminate Testing

  • The user can generate design data for damaged composite laminates

Results taken from

NPL Internet version of Laminate Damage Simulation

slide45

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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