corso di laurea in ingegneria gestionale

CORSO DI LAUREA IN INGEGNERIA GESTIONALE Materiali per l’Ingegneria Industriale

Less. 10 – Composite materials Prof. Ing. Raffaele Cioffi Tel. 081-5476732 E-mail: [email protected] 02/12/2013

Anno Accademico 2013-2014

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Composites materials – Introduction

• Combined materials: with the aim of getting a more desirable combination of properties. • Example: get flexibility and weight of a polymer plus the strength of a ceramic • Combined action: mixture gives averaged properties. • Many new technologies require unusual combinations of properties, i.e. aerospace, automotive, underwater, renewable and sustainable etc…

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• Path of nature: Human bones • Bone is a composite material with various different components • Composition of Bone: • ~45 wt% mineral salts (calcium phosphate and calcium carbonate) • ~35 wt% organics (collagen, protein) • ~20 wt% water

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• Composites Design: taking the pros of different materials to overcome the cons. Special applications require high performance materials for specific purpose (lightweight, high density, super insulation, cost reduction, ecc.). • The major issue is how to bond efficiently two intrinsically different materials (interface behavior, intimate mixing, synergic effects). • The connectivity of phases is very important in determining final properties of composite. It depends on phase interaction and physical and structural aspect of employed materials.

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• Composites: Multiphase material with significant proportions of each phase. woven fibers

0.5 mm

• Matrix: The continuous phase with purpose to: - Transfer stress to other phases - Protect phases from environment - Classification: MMC, CMC, PMC (Cross metal ceramic polymer section • Dispersed phase: The dispersed view)

0.5 mm 02/12/2013

phase with purpose to enhance matrix properties.


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• Composites classification

• Based on physical and structural aspect of matrix phase: − MMC − CMC − PMC • Based on physical and structural aspect of dispersed phase: − Particle − Fiber − Structural

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Composites materials – Introduction Composites

Particle-reinforced

Largeparticle

Dispersionstrengthened

Fiber-reinforced

Continuous (aligned)

Structural

Discontinuous (short)

Aligned

Laminates

Sandwich panels

Randomly oriented

• Characteristic dimension of dispersed phase. • Alignment, order of components. • Interface interactions. 02/12/2013


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• Composites are classified according to: – The matrix material (CMC, MMC, PMC) – The reinforcement geometry (particles, fibers, layers). • Composites enhance matrix properties: – MMC: enhance sy, TS, creep performance – CMC: enhance Kc – PMC: enhance E, sy, TS, creep performance • Particulate-reinforced: Elastic modulus can be estimated. Properties are isotropic. • Fiber-reinforced: Elastic modulus and TS can be estimated along fiber dir. Properties can be isotropic or anisotropic. • Structural: Based on build-up of sandwiches in layered form. 02/12/2013


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• Nanocomposites: The dispersed phase consists of nanoscale particles and is distributed in the matrix. Dispersion-strengthened and precipitation-hardened alloys are alloys are examples of traditional nanocomposites, with purpose to enhance matrix properties. • Hybrid nanocomposites: They are made of organic and inorganic materials. These are nanocomposites in which the microstructure of the materials consists on an inorganic part and an organic one. These and other functional composites can provide unusual combinations of electronic, magnetic or optical properties. 02/12/2013


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Snail’s shell is a natural material and can be considered as an hybrid nanocomposite. It is constituted by aragontic platelets (CaCO3 orthorhombic) surronded by a 10 nm thick of proteinaceous organic matrix.

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Composites materials – Particle reinforced composites

• Dispersion-strengthened: Composite materials containing particles of 10 to 250 nm in diameter. The dispersoids, are usually introduced into the matrix by means of phase transformations and diffusion mechanisms. They block dislocations and have a remarkable strengthening effect. • In oppositions to alloys, dispersoids must have low solubility and relative inert chemical behavior.

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• Large particle: They contain some fillers with quite a variety of geometries, but they should be approximately equiaxed to be considered as spherical particles. Characteristic diameters range from 0.1 µm to 1 cm. Fillers modify or improve the properties of the material and/or replace some matrix volume. • Large particle composite are designed to produce isotropic combinations of properties, with limited strength improvement. In fact coarse particle do not block slip and dislocations effectively.

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• Examples: - Spheroidite steel Dispersed

Matrix:

Particles:

ferrite (a)

cementite

(ductile)

( Fe 3 C ) (brittle) 60 mm

- WC/Co Cemented carbide Large

Matrix:

Particles:

cobalt

WC

(ductile)

(brittle, hard)

Vm: 10-15% vol 600 mm

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- Automobile tires

Nano-composite

Matrix:

Particles:

Rubber (compliant)

C (stiffer)

0.75 mm

• Moreover % (v/v) composition, for particle reinforced composites, the fundamental aspect is particles dimension and interface behavior.

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•

Concrete: Gravel + Sand + Cement + Water. It is made up of a binder and fillers wherein the binder (cement paste) "glues" the fillers together to form a cementitious conglomerate. The constituents used for the binder are cement and water, while the filler can be fine or coarse aggregate. • Why sand and gravel? Sand packs into gravel voids • The fillers control the properties of concrete Fine and coarse irregular aggregate.

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Coarse-rounded aggregate


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Microghaph of a concrete with fine and coarse aggregate, irregular and rounded

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• Reinforced concrete: Reinforce with steel rerod or remesh. Increases strength, even if cement matrix is cracked. • Prestressed concrete: Remesh under tension during setting of concrete. Tension release puts concrete under compressive force. Concrete much stronger under compression.

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• Elastic modulus, Ec, of composites: two approaches.

E(GPa) 350 Data: Cu matrix 30 0 w/tungsten 250 particles 20 0 150

upper limit: “rule of mixtures” Ec = VmEm + VpEp

0

(Cu)

lower limit: 1 Vm Vp = + Ec Em Ep 20 4 0 6 0 8 0

10 0 vol% tungsten

(W)

• Application to other properties:

− Electrical conductivity, se: Replace E in equations with se. − Thermal conductivity, k: Replace E in equations with k.

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Composites materials – Fiber reinforced composites

• Fibers: Very strong materials. Provide significant strength improvement to material, like fiber-glass.

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Microghaph of a fiberglass with strong glass fibers in a softer polymer matrix

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• • • •

Continuous glass filaments in a polymer matrix. Strength due to fibers. Polymer simply holds them in place. Fibers have very high specific strength.

•

Fiber Materials – Single crystals, large length to diameter ratio. Ex. graphite, SiN, SiC. High crystal perfection, extremely strong, strongest known, very expensive. – Fibers of polycrystalline or amorphous materials, generally polymers or ceramics. Ex: Carbon, Al2O3, Aramid, Boron, UHMWPE. – Wires of metals. Ex. steel, Mo, W, etc.

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Aligned Random Discontinuous

Aligned continuous

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• Aligned Continuous fibers Examples: Metal: g'(Ni3Al)-a(Mo) by eutectic solidification. matrix:

Ceramic: Glass w/SiC fibers formed by glass slurry. Eglass = 76 GPa; ESiC = 400 GPa.

a (Mo) (ductile) (a)

fracture surface

(b)

2 mm

fibers: 02/12/2013

g ’ (Ni3Al) (brittle) 24


•

Discontinuous, random 2D fibers

Example: Carbon-Carbon - Process: fiber/pitch, then burn out at up to 2500ºC. (b) - Uses: disk brakes, gas turbine exhaust flaps.

C fibers: very stiff very strong

view onto plane

(a)

• Other variations: - Discontinuous, random 3D - Discontinuous, 1D (Aligned) 02/12/2013


C matrix: less stiff less strong fibers lie in plane

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• Stress-Strain: For fiber reinforced composites the stress-strain diagram has intermediate form, between ductile/brittle behavior. Most load is carried by fibers

Fibers start fracturing

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• Critical load: depends on the bonding between fiber and matrix! At the end of the fiber, load is solely carried by matrix!

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• Critical fiber length for effective stiffening & strengthening: fiber strength in tension

sf d fiber length  15 c Shorter, thicker fiber: fiber length  15 s(x)

sf d c

fiber diameter shear strength of fiber-matrix interface

Longer, thinner fiber: sd fiber length  15 f c s(x)

Better fiber efficiency

Poorer fiber efficiency 02/12/2013

s*f d lc  2 c


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• When fiber length is lc, an applied failure load, σ*f, is transferred only to the center.

• If lLc

• Assume continuous fibers to estimate fiber-reinforced composite strength for long continuous fibers in a matrix. • Assume fiber/matrix bond is strong => isostrain • Longitudinal deformation. sc = smVm + sfVf but c = m = f volume fraction Ecl = Em Vm + EfVf

Amount of load carried by fibers vs matrix 02/12/2013

Ff EfVf  Fm EmVm

isostrain longitudinal (extensional) modulus

f = fiber m = matrix Anno Accademico 2013-2014

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• Composite Strength: Longitudinal Loading

For matrix/fiber elastic behavior, from Hooke’s law 02/12/2013


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• Composite Strength: Longitudinal Loading Fiber-to-composite load ratio:

• High

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fiber-to-composite load even for small Vf if Young moduli ratio very high (i.e. very stiff fiber and softer matrix). Anno Accademico 2013-2014 33


• Composite Strength: Longitudinal Loading

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• Composite Strength: Transverse Loading In transverse loading the fibers carry less of the load, isostress BUT sc = sm = sf = s c= mVm + fVf isostress

1 Vm Vf   Ect Em Ef

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Transverse modulus

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• Composite Strength: Transverse Loading

For fiber-matrix in series

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• Estimate of Ec and TS for discontinuous fibers: sf d - Valid when fiber length  15 c

- Elastic modulus in fiber direction: Ec = EmVm + KEfVf Efficiency factor: - aligned 1D: K = 1 (aligned ) - aligned 1D: K = 0 (aligned ) - random 2D: K = 3/8 (2D isotropy) - random 3D: K = 1/5 (3D isotropy)

TS* in fiber direction: (TS)c = (TS)mVm + (TS)fVf 02/12/2013


(*aligned 1D)

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Composites materials – Structural composites

• Stacked and bonded fiberreinforced sheets: − stacking sequence: e.g., 0°/90° − benefit: balanced, inplane stiffness

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Plywood is a laminar composite of layer of wood, compressed and bonded by an ultra thin film of organic adhesive.

[0°/+90]S is a simple cross-ply laminate. 02/12/2013


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• Laminar strength : − Stress-strain: depends on # of layers, ductile-like behavior.

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• Sandwich panels – low density, honeycomb core – benefit: small weight, large bending stiffness face sheet adhesive layer honeycomb

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• Sandwich strength : − Skin layers: bear the major part of applied load − Core: lightweight, thicker to resist to stresses.

• Longitudinal/Transverse: Skin layers, stiffness, high strength • Normal: Core layer, compression strength, high void fraction 02/12/2013


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A complex composite structure is modern ski.

The various components and their functions are shown, as well as, the materials used. 02/12/2013


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Composites materials – The matrix phase

• There are three possible matrix phase for a composite material: – Metal – Polymer – Ceramic • Metals and polymers are used when ductility is required. • In ceramic matrix composites, a reinforcement is added to improve fracture toughness.

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Ag/Cu alloy reinforced with carbon fibers. Fracture surface shows poor bonding between matrix/fibers Anno Accademico 2013-2014

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•

The matrix phase serves several functions: – Binds fibers together – Transmits and distributes external applied stress – Protects fibers from mechanical and chemical attacks – Barrier to crack propagation from fiber to fiber • In most fiber-reinforced composites, the fibers are strong, stiff and resistant, for ex., at high temperatures. • To evaluate fibers performance, important properties are: 02/12/2013


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The highest specific modulus is usually found in materials having a low atomic number and covalent bonding such as carbon and boron

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• It is essential that adhesive bonding forces, between fiber/matrix be high to prevent fiber pullout. • The ultimate strength of composite depends on the magnitude of this bond. • Adequate bonding maximize stress transmittance from weak matrix to strong fibers. • Bonding depends to a large degree on chemical nature, but also on production process.

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Composites materials – Polymer matrix composites

• PCMs consist of a polymer as matrix phase and various type of fibers as reinforcement. • They are used in a large number of applications, as well as in large quantities. • Room temperatures properties, ease of fabrication, low cost. • Mainly, three different type of reinforcement: – Glass Fiber-Reinforced Polymer (GFRP). – Carbon Fiber-Reinforced Polymer (CFRP). – Aramid Fiber-Reinforced Polymer (AFRP)

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• GFRP: a composite consisting of glass fibers, continuous or discontinuous, embedded in a polymer matrix. • The most common glass fiber is E-glass type (with a diameter ϕ=3-20 µm). • Easily drawn in high-strength fibers from molten state. • Available, economic, variety of manufacturing processes. • Composites with high specific strength. E-glass composition Optical-fiber cable, a glass fiber in a polymeric matrix. More layers (Coating, cladding, core) are disposed in a single cable to transport signals made of light. 02/12/2013


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• Surface characteristics of glass are fundamental for strength properties. • Micro-flaws can deleteriously affect tensile strength, fracture toughness and bonding forces. • Atmosphere exposure generally weakens surface, and newly drawn glass fibers must be coated. • A protective thin layer is used as a coupling agent for composite production and adhesive for matrix bonding. • Limitations: – Not very stiff for structural applications. – Low service temperature (200°C), weak to strong acids/bases attack. • Common trade names: Kevlar (29, 49,149), Nomex. • Processed like ordinary textile fibers in epoxies and polyesters composites. Structure of Kevlar.

Structure of Nomex.

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• Glass, carbon and aramid fiber are the most common fiber reinforcements in PMCs polyesters, vinyl esters and epoxies resins. 02/12/2013


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Termosets Structure of a Polyester. (PET) Structure of a Poly-vinylester

Structure of a Epoxy resin 02/12/2013


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Composites materials – Metal matrix composites

• MMCs: The matrix is a ductile metal. They refer to common non-iron alloys and super-alloys. • Higher service temperature, abrasion resistance, nonflammability, very good thermal and electrical properties, but expensive. • Alloys of aluminum, magnesium, titanium, copper with ceramic/refractory (C, SiC, B, Cr-oxides, Al2O3, etc.) particulates or fibers. • High volume fraction of reinforcements 10-60% vol/vol. • Processing: two steps consolidation/synthesis + shaping.

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Composites materials – Ceramic matrix composites

• CMCs: The matrix is a brittle ceramic. Ceramic materials are inherently resistant to oxidation, deterioration and high temperature (refractory). • For high temperature and severe stress applications, CMC requires improvement of fracture properties via reinforcement, with ductile/other ceramic particulate/fibers. • Crack propagation is impeded by particulate thanks to transformation toughening. 02/12/2013


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Composites materials – The matrix phase •

•

CMCs: Increased toughness Force

103

particle-reinf

E(GPa) PMCs 2 10 10

fiber-reinf

1

un-reinf

Elongation/Deformation •

MMCs: Increased creep resistance

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PMCs: Increased E/r

10 -4

6061 Al ss (s-1) 10 -6

10 -8 10 -10

6061 Al w/SiC whiskers

20 30 50

ceramics

metal/ metal alloys

.1 polymers .01 .1 .3 1 3 10 30 Density, r [mg/m3]

s(MPa) 100 200

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Stiffness vs Cost

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Sustainability (in terms of recycle fraction) vs Cost

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Energy (production) vs Cost

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Strength vs Cost

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Strength vs Tmax

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Composites materials – Composites processing

• Pultrusion – Continuous fibers pulled through resin tank, then preforming die & oven to cure

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Composites materials – Composites processing

• Prepreg – Continuous fibers preimpregnated with a polymer resin that is only partially cured (thermoset).

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