Lecture 5 - Composites

Question 1

What is a composite?

Answer 1

A material made from two or more distinct materials that are combined to create a new material with properties that are superior to those of the individual components.

Question 2

What are the two main components of composites?

Answer 2

Matrix + reinforcement

Question 3

What historical perspective is noted about gluing wood?

Answer 3

Gluing wood at angles produced better properties than single-ply wood.

Question 4

What are examples of natural composites?

Answer 4

• Bone
• Wood

Question 5

What are examples of synthetic composites?

Answer 5

• Fuselage
• Appliances and parts
• Skis

Question 6

What are examples of industrial composites?

Answer 6

• Concrete/reinforced concrete
• Fiber-glass
• Glass reinforced aluminium

Question 7

What are the classifications of composite materials?

Answer 7

• Particle reinforced
• Fiber reinforced
• Structural
• Nano-Composites

Question 8

What are the two types of particle-reinforced composites?

Answer 8

• Large particle
• Dispersion strengthened

Question 9

What defines large particle-reinforced composites?

Answer 9

Large particles are added to a softer matrix. Because they’re too big for atomic-level modeling, continuum mechanics is used. These particles are harder and stiffer, restrict matrix deformation, and help carry load when strongly bonded to the matrix.

Question 10

What is the effect of large particles in a composite?

Answer 10

Large particles block matrix movement and help share the load. This improves strength and stiffness — but only if the particles are well bonded to the matrix.

Question 11

What is a characteristic of dispersion-strengthened composites?

Answer 11

Particles are normally much smaller, with diameters between 10 and 100 nm.

Question 12

What do small dispersed particles do in dispersion-strengthened composites?

Answer 12

They hinder or impede the motion of dislocations, restricting plastic deformation.

Question 13

What is specific strength?

Answer 13

The ratio of tensile strength to specific gravity.

Question 14

What is specific modulus?

Answer 14

The ratio of modulus of elasticity to specific gravity.

Question 15

What is critical fiber length (lc)?

Answer 15

The necessary length for effective strengthening and stiffening of the composite material.

The minimum length needed for effective load transfer from matrix to fiber

Question 16

What happens when fiber length (L) is equal to critical fiber length (lc)?

Answer 16

The maximum fiber load is achieved only at the axial center of the fiber.

Question 17

What is the outcome when fiber length (L) is greater than critical fiber length (lc)?

Answer 17

The fiber reinforcement becomes more effective, reaching full strength over a central region.

Question 18

What occurs when fiber length (L) is less than critical fiber length (lc)?

Answer 18

The matrix deforms around the fiber, minimizing stress transfer and providing little reinforcement.

Question 19

What factors influence the mechanics of continuous and aligned fiber composites?

Answer 19

• Stress-strain behaviors
• Phase volume fractions
• Direction of applied stress
• Direction of applied load

Question 20

What is the difference between aligned and randomly oriented discontinuous fiber composites?

Answer 20

Aligned composites are anisotropic, while randomly oriented composites are isotropic.

Question 21

What are whiskers in fiber-reinforced composites?

Answer 21

Thin single crystals with high crystalline perfection and strength.

Question 22

What types of matrices are used in fiber-reinforced composites?

Answer 22

• Polymer
• Ceramic
• Metal

Question 23

What is a structural composite?

Answer 23

A multilayered/hierarchical and normally low-density composite used in applications requiring structural integrity.

Question 24

What is a sandwich panel?

Answer 24

A lightweight beam or panel having relatively high stiffnesses and strengths, consisting of two outer sheets and a thicker core.

Question 25

What are nanocomposites composed of?

Answer 25

Nanosized particles embedded in a matrix material.

Question 26

What properties can nanocomposites have?

Answer 26

* Mechanical * Electrical * Magnetic * Optical * Thermal * Biological * Transport

Question 27

What is the significance of the ratio of particle surface area to volume in nanoparticles?

Answer 27

As the size of a particle decreases, the relative ratio of surface atoms to bulk atoms increases. ## Footnote As a particle gets smaller, more of its atoms are on the surface compared to inside it

Question 28

What application uses graphene nanocomposites?

Answer 28

Anodes for lithium-ion rechargeable batteries.

Question 29

What are the types of dispersion of clay layers in a polymer matrix?

Answer 29

* Phase separated * Intercalated nanocomposite * Exfoliated nanocomposite

Question 30

What defines dispersion-strengthened composites?

Answer 30

Very small particles (10–100 nm) are dispersed in the matrix. They hinder dislocation motion on the atomic level, which strengthens the composite and restricts plastic deformation.

Question 31

What is critical fiber length (lc)?

Answer 31

Minimum length a fiber needs to be in order to fully carry the stress applied to it. If it’s shorter than this, it won’t reinforce the composite effectively. ## Footnote It depends on how strong the fiber is, how thick it is, and how well it sticks to the matrix.

Question 32

What happens when fiber length l > lc?

Answer 32

The fiber reaches full strength over a larger region, making reinforcement more effective.

Question 33

What happens when fiber length l < lc?

Answer 33

The fiber cannot carry the full load. Most stress is taken by the matrix, so reinforcement is minimal.

Question 34

What is the difference between continuous and discontinuous fibers?

Answer 34

Continuous fibers are long and effective at carrying load. Discontinuous fibers are short — aligned ones give some reinforcement, but random ones are weaker and isotropic.