Composites: basics and terminology

1. Composites:basics and terminology John Summerscales

2. Reading for a degree Each lecture has: • PowerPoint slides on extranet • these need JS “soundtrack” (i.e. lectures) • individual lecture webpages on extranet • also read these to reinforce your learning … and to really understand the topicfollow up the references and/or review papers

3. Support materials for module • Home page on Extranet • MATS231: http://www.tech.plym.ac.uk/sme/mats231/ • MATS324: http://www.tech.plym.ac.uk/sme/mats324/ • Lecture schedule, notes and PowerPoint: • http://www.tech.plym.ac.uk/sme/mats324/PowerPoint • Home page also includes: • subject index • map of local composites companies • links to ILS Reading Lists • and many other useful resources ;-) • but see Student Portal for assessments

4. Practical • manufacture and test of a composite plate • attendance at Health and Safety lecture is anessential prerequisite for coursework • list of attendees circulated for signature • if your name is not on the list,you will not be allowed to do the practical • if you do not do the practical you will fail the coursework element and hence the module.

5. 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

6. Anisotropy

7. 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.

8. 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

9. 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

10. 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)

11. 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”

12. Basic rule-of-mixtures 4 ηO = fibre orientation distribution factor • a weighted function of fibre alignment, essentially cos4θ: • 1 for unidirectional • 1/2 for biaxial aligned with the stress • 3/8 for random in-plane • 1/4 for biaxial fabric on the bias angle

13. Basic rule-of-mixtures 5 • Vf = fibre volume fraction • 0.1-0.3 for random • 0.3-0.6 for fabrics • 0.5-0.8 for unidirectional • consolidation pressure: • no pressure gives the lower value • Vf increases with pressure

14. Basic rule-of-mixtures 6 • Ef = elastic modulus of fibre • glass = ~70 GPa (equivalent to aluminium) • aramid = ~140 GPa • carbon = ~210 GPa (equivalent to steel) • figures above are lowest values i.e. for standard fibres

15. Transition temperatures in ascending order • Tg = glass transition temperature • Tc = peak crystallisation temperature • Tm = crystalline melting pointtypically Tm = Tg + 200±50°C nb: 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)

16. Glass transition temperature (Tg) • Temperature at whichsegmental motion of the chain is frozen out • below Tg polymer is elastic/brittle • above Tg polymer is viscoelastic/tough • more rigorous than heat distortion temperature • Tg for thermoplastics = Tm - ~200°C • Tg for thermosets follows cure temp.

17. Crystalline melting point (Tm) • all polymers have a Tg • only some polymers have a Tm • they must be able to form crystals • normally a regular repeating structure • rarely 100% crystalline • they might degrade before melting • usually the case for thermoset

18. Composites How fibres can be arrangedin order of increasing stiffness and strength: • 3-D random • e.g. injection moulding grades. • planar random • e.g. moulding compounds, chop strand mat, random swirl. • quasi-isotropic (QI) • e.g. continuous fibres oriented at 0°/-45°/90°/+45° or 0°/60°/120°. • bidirectional • e.g. woven fabrics or cross-plied UD laminates at 0 °/90 °. • unidirectional (UD) • e.g. pultrusions and aligned monolithic fibre composites.

19. Four types offibre-reinforced composite • Monolithic (material) • all layers aligned parallel • laminate (structure - see next slides) • orientation changes between layers • hybrid(structure – MATS324 lecture A6) • more than one type of fibre (e.g. carbon/glass) • Sandwich (structure – MATS320) • composite skins and lightweight core

20. Laminate stacking sequence notation • typical laminate stacking sequence is: • [0º/+45º/-45º/90º]ns • where the subscripts are: • n is the number of repeats of the sequence • Q indicates antisymmetric laminate • s means the laminate is symmetric • T is the total number of plies • overbar denotes that the laminate issymmetric about the mid-plane of the ply • Thus for n = 2 above, the sequence will be: • 0º/+45º/-45º/90º/0º/+45º/-45º/90º*90º/-45º/+45º/0º/90º/-45º/+45º/0º • with * denoting the line of symmetry.

21. I-beam vs stacking sequence Beam stiffness reduces from left to right: Laminated composite plate:0° layer or 90° layer Equivalent beam: high EI vs low EI segments

22. 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