Composites:basics and terminology

John Summerscales

Reading for a degree

Each lecture has:•PowerPoint slides on extranet

o these need JS “soundtrack” (i.e. lectures)

•individual lecture webpages on extraneto also read these to reinforce your learning

… and to really understand the topicfollow up the references and/or review papers

Support materials for module

• Home page on Extraneto MATS231: http://www.tech.plym.ac.uk/sme/mats231/o MATS324: http://www.tech.plym.ac.uk/sme/mats324/

• Lecture schedule, notes and PowerPoint:o http://www.tech.plym.ac.uk/sme/mats324/PowerPoint

• Home page also includes:o subject indexo map of local composites companieso links to Library Reading Listso and many other useful resources ;-)

• see MooDLE student portal for assessments

Support materialshttp://www.tech.plym.ac.uk/sme/mats231

Lecture & practical schedule Review papersFree e-books Subject index

Practical

• manufacture and test of a composite plate

• attendance at Health and Safety lecture is an essential prerequisite for courseworko list of attendees circulated for signatureo if your name is not on the list,

you will not be allowed to do the practicalo if you do not do the practical you will fail the

coursework element and hence the module.

Outline of this lecture

• Anisotropy

• Fibre volume fraction (Vf)

• Areal weight of fabric (WF)

• Basic rule-of-mixtures

• Glass transition temperature (Tg)

• Crystalline melting point (Tm)

• Stacking sequence notation

AnisotropyDegree of anisotropy

Principal axes

Properties Example

Isotropic Orthogonal Constant regardless of direction

Metals

Square symmetric

Orthogonal Two different principal axes Unidirectional fibres or woven cloth

Orthotropic

Orthogonal Three different principal axes

Unidirectional weave with light weft

Anisotropic

Any angle Constant relative to axes Filament wound tube or many crystals

Aeolotropic

Any angle May change with position Timber

Fibre volume fraction (Vf)

• n = the number of layers

• AF = the areal weight of the fabric

• ρf = density of the fibre, and

• t = the thickness of the laminate.

t

nAVf

f

F

Basic rule-of-mixtures 1

• Elastic properties (e.g. density or modulus) of composite calculated by rule-of-mixtures

EC = κ . ηd . ηL . ηO . Vf . Ef + Vm . Em

• if the first term of the equation is large,the second term can be neglected

Basic rule-of-mixtures 2a

The parameters are:•EC = modulus of composite

•Vx = volume fraction of component x

•Ex = modulus of component x

•subscripts f and m are fibre and matrix respectively

Basic rule-of-mixtures 2b

• κ = fibre area correction factor*

•ηd = fibre “diameter” distribution factor*

•ηL = fibre length distribution factor

•ηO = fibre orientation distribution factor

* these two factors are set to unity for man-made fibres (but see lecture A9 on natural fibres)

Basic rule-of-mixtures 3

ηL = fibre length distribution factor

• 1 for continuous fibres• fractional for long fibres• 0 if fibre below a “critical length”

Basic rule-of-mixtures 4

ηO = fibre orientation distribution factor

• a weighted function of fibre alignment,

essentially cos4θ:o 1 for unidirectionalo 1/2 for biaxial aligned with the stresso 3/8 for random in-planeo 1/4 for biaxial fabric on the bias angle

Basic rule-of-mixtures 5

•Vf = fibre volume fractiono0.1-0.3 for randomo0.3-0.6 for fabricso0.5-0.8 for unidirectional

•consolidation pressure:ono pressure gives the lower valueoVf increases with pressure

Basic rule-of-mixtures 6

•Ef = elastic modulus of fibreoglass = ~70 GPa

(equivalent to aluminium)oaramid = ~140 GPaocarbon = ~210 GPa

(equivalent to steel)

•figures above are lowest valuesi.e. for standard fibres

Transition temperaturesin ascending order

• Tg = glass transition temperature

• Tc = peak crystallisation temperature

• Tm = crystalline melting pointtypically Tm = Tg + 200±50°Cnb: no melting point in amorphous materials

• Tp = processing temperaturetypically Tp = Tm + ~30°C for “semi”-crystalline polymers

Tg follows cure temperature in thermosets

• Td = degradation/decomposition temperaturemay limit Tp (especially for PVC)

Glass transition temperature (Tg)

• Temperature at whichsegmental motion of the chain is frozen outo below Tg polymer is elastic/brittleo above Tg polymer is viscoelastic/tougho more rigorous than heat distortion

temperature

• Tg for thermoplastics = Tm - ~200°C

• Tg for thermosets follows cure temp.

Crystalline melting point (Tm)• all polymers have a Tg

• only some polymers have a Tm

o they must be able to form crystals normally a regular repeating structure rarely 100% crystalline

• polymers may degrade before melting usually the case for thermoset

CompositesHow fibres can be arrangedin order of increasing stiffness and strength:•3-D random

o e.g. injection moulding grades.

•planar randomo e.g. moulding compounds, chop strand mat, random swirl.

•quasi-isotropic (QI)o e.g. continuous fibres oriented at 0°/-45°/90°/+45° or

0°/60°/120°.

•bidirectionalo e.g. woven fabrics or cross-plied UD laminates at 0 °/90 °.

•unidirectional (UD)o e.g. pultrusions and aligned monolithic fibre composites.

Four types offibre-reinforced composite• Monolithic (material)

o all layers aligned parallel

• laminate (structure - see next slides)o orientation changes between layers

• hybrid (structure – MATS324 lecture A6)o more than one type of fibre (e.g. carbon/glass)

• Sandwich (structure – MATS320)o composite skins and lightweight core

Laminate stacking sequence notation• typical laminate stacking sequence is:

o [0º/+45º/-45º/90º]ns

• where the subscripts are: o n is the number of repeats of the sequenceo Q indicates antisymmetric laminateo s means the laminate is symmetrico T is the total number of plieso overbar denotes that the laminate is

symmetric about the mid-plane of the ply

• Thus for n = 2 above, the sequence will be:o 0º/+45º/-45º/90º/0º/+45º/-45º/90º*90º/-45º/+45º/0º/90º/-45º/+45º/0º

o with * denoting the line of symmetry.

I-beam vs stacking sequenceBeam stiffness reduces from left to right:

Equivalent beam: high EI vs low EI segments

Laminated composite plate: 0° layer or 90° layer

Key points of this lecture

• resources on Student Portal and Extranet• anisotropy• fibre volume fraction (Vf)

• areal weight of fabric (AF … sometimes WF)

• basic rule-of-mixtures• glass transition temperature (Tg)• crystalline melting point (Tm)• stacking sequence notation