Lecture 8 - Interface region

Question 1

Why do we want no-slip?

Answer 1

The fiber achieves its reinforcement effect on the matrix entirely via the particle matrix interface. Load will be taken by the fibers this way.

Question 2

What influence the strength of the interface?

Answer 2

1. Difference in thermal expansivity of fibre and that of matrix -> residual strains and stresses (differential thermal contraction or expansion)
2. Difference in thermal conductivity of fibre and that of matrix -> reduction of the temperature in the matrix during cure -> lower the cross-link density in the vicinity of the fiber
3. Volume contraction on cure (thermosets) can introduce internal stresses -> particularly pronounced close to the fibre
4. Spherulites can nucleate at fibre-fibre contact points; volume contractions accompanying crystallization will set up residual stresses which may affect the behavior
5. Generally different Poisson ́s ratio of the matrix and that of the fibre, can lead to a fibre-matrix mating pressure
6. Water in-take can take place along fibre-matrix interface
7. Presence of fiber surface can influence the crosslink density
8. Size or coating

Question 3

What is a spherulite?

Answer 3

Areas with higher density and hardness but also brittleness and volume contraction.

Question 4

When do spherulites occur?

Answer 4

When crystallization of the matrix occur around fibers. Can lead to voids around the fibers

Question 5

How large can the interface region be for fibers of 17um in diameter and square pattern?

Answer 5

Fibers of 17um in diameter and square pattern can end up with the interface region comprising more than 80% of the volume (maximal packing)

Question 6

What are the different interphases called from the fiber out to the matrix?

Answer 6

Fiber-Modified fiber-coating-modified matrix-matrix

where (Modified fiber-coating-modified matrix) is the interfacial layer/interphase

Question 7

Why is crack deflection good?

Answer 7

Crack deflection can take place if the toughness of the interphase is not too high. Many and repeatedly deflecting cracks will increase the toughness of the composite. If on the other hand, the adhesion strength/toughness at the interface are high, it is likely that a propagating crack will not deflect, but will cut the fibre. Many and repeatedly cut fibres seriously weaken the material.

Question 8

What are the bonding mechanisms in the interface?

Answer 8

1. Wetting
2. Inter-diffusion and chemical reaction
3. Electrostatic attraction
4. Mechanical keying

Question 9

What does L, S and V stand for when talking about wetting?

Answer 9

Liquid, Solid, Vapor

Question 10

what does it mean if γLV is high?

Answer 10

That it inhibits the spreading of the liquid= hinders wetting

Question 11

what should the relation between γSV and γLV be to get good wetting?

Answer 11

γSV>γLV

Question 12

what is good wetting of fibers given by?

Answer 12

1. small contact angle (clean surface)

2. low viscosity of the resin (thermosetting or thermoplastic polymer liquid) 3. no entrapment of air (smooth surface)

Question 13

How can you measure wetting?

Answer 13

Wilhelmi plate principle (dynamic) or contact angle method (static)

Question 14

What is interdiffusion when talking about interfacial bonds?

Answer 14

• diffusion of free ends between a polymer forming e.g. the fibre, and another polymer forming the matrix
• leads to chain entanglements and increases the adhesive strength
• Some coupling agents used on fibres in thermoplastic matrices impart this effect

Question 15

What is chemical reaction when talking about interfacial bonds?

Answer 15

• chemical bonds structure: A-B
• bond types: covalent, ionic etc. depending on the matrix and reinforcement • can be very strong!
• carbon fibers are prone to surface reactions with organic groups
• Oxidative (heat or acid) treatment can raise the bond strength

Question 16

What is electrostatic forces when talking about interfacial bonds?

Answer 16

• adhesive force resulted If the surfaces carry net electrical charges of opposite sign
• used for example in coupling agents on glass fibers
• ! electrostatic forces are unlikely to constitute the major adhesive bond
• can readily be reduced by discharging in the presence of a polar solvent, such as water.

Question 17

What is surface roughness when talking about interfacial bonds?

Answer 17

Fiber and matrix has a rough surface that gets stuck with each other.

• aka mechanical keying
• effects are much more significant under shear loading, than peeling off
• some improved resistance to tensile failure can take place if re-entrant angles are present

Question 18

What can surface roughness also be known as?

Answer 18

Mechanical keying

Question 19

How can you test adhesive interfacial failure?

Answer 19

fibre pull-out,
push-out,
push-down, fragmentation, microbond test and interfacial laminar shear strength (ILSS)

Question 20

How do you do a fiber pull-out and push-out test?

Answer 20

A single fibre, partly embedded within a matrix, is extracted under a tensile load or the fiber is pushed out from the matrix.

Question 21

What are the three stages of fiber pull-out?

Answer 21

1. elastic loading up to debonding

2. propagation of the debonding front 3. pull-out resisted by frictional sliding.

Question 22

How do you do a fragmentation?

Answer 22

Application of the load to a specimen containing a single fibre, where the fibre has a strain to break less than that of the matrix, causes the fibre to fragment. A typical experiment includes a fibre of initial length of few millimetres. The specimen is ground and polished to obtain smooth surfaces. The limiting final fragment length is the critical length, from which the interfacial shear strength may be evaluated. Birefringent resin and a microscope with crossed polarizers are used to efficiently observe the length of fibre fragment.

Question 23

How do you proceed a microbond test?

Answer 23

Fine droplets about 100 μm in diameter are applied to a single fibre, and allowed to cure. Then the fibre is pulled through a gap between knife edges, the separation of which is controlled by a micrometer.

Question 24

How do you do a interlaminar shear stress test?

Answer 24

You do a 3-point bending test on a fiber located in the matrix (UD Beam)

Question 25

what does Size contain ?

Answer 25

• adhesion promoter (AP), called also coupling agent (CA)
• protective polymeric sizing (polyvinyl acetate, epoxy, polyester, polyurethane…)
• polymeric binder (polyvinyl alcohol, polyvinyl acetate…)
• lubricant (reduces abrasion).

Question 26

What does coupling agents generally functional with?

Answer 26

• thermosets
• thermoplastics, to a lower degree
• elastomers

Question 27

What are the generic types of coupling agents?

Answer 27

• organosilanes, functional with glass fibres and
siliceous surfaces
• organotitanates, functional with carbon fibres,
calcium carbonate, Kevlar fibres,
carbon black, titanium dioxide, copper, aluminium, stainless steel.