Materials Option

Question 1

Crystal

Answer 1

Solid in which atoms are arranged in a regular array.

Question 2

Crystalline Solid

Answer 2

Solid consisting of a crystal, or of many crystals, usually arranged randomly. The latter is strictly a polycrystalline solid. Metals are polycrystalline.

Question 3

Amorphous solid

Answer 3

A truly amorphous solid would have atoms arranged quite randomly. Examples are rare. In practice we include solids such as glass or brick in which there is no long range order in the way atoms are arranged, though there may be ordered clusters of atoms.

Question 4

Polymeric Solid

Answer 4

Solid made up of chain-like molecules.

Question 5

Hookes Law for wires and rods

Answer 5

The tension in a rod or wire is proportional to its extension from its natural length, provided the extension is not too great.

Question 6

Stress sigma

Answer 6

Stress is the force per unit cross-sectional area when equal opposing forces act on a body.
Unit: pascal (Pa) or Nm-2.

Question 7

Strain epselon

Answer 7

Strain is defined as the extension per unit length due to an applied stress. Unit: none

Question 8

The Youngs Modulus

Answer 8

Young Modulus = Stress / Strain

Unless otherwise indicated this is defined for the Hooke’s Law region. Unit: Pa or Nm-2

Question 9

Ductile Material

Answer 9

A material which can be drawn out into a wire. This implies that plastic strain occurs under enough stress.

Question 10

Elastic Strain

Answer 10

This is strain that disappears when the stress is removed, that is the specimen returns to its original size and shape.

Question 11

Plastic Strain

Answer 11

This is strain that decreases only slightly when the stress is removed. In a metal it arises from the sliding of layers of atoms over each other in the crystals.

Question 12

Elastic Limit

Answer 12

This is the point at which deformation ceases to be elastic. For a specimen it is usually measured by the maximum force, and for a material, by the maximum stress, before the strain ceases to be elastic.

Question 13

Dislocations in crystals

Answer 13

Certain faults in crystals which (if there are not too many) reduce the stress needed for planes of atoms to slide. The easiest dislocation to picture is an edge dislocation: the edge of an intrusive, incomplete plane of atoms.

Question 14

Grain Boundaries

Answer 14

The boundaries between crystals (grains) in a polycrystalline material.

Question 15

Ductile Fracture (necking)

Answer 15

The characteristic fracture process in a ductile material. Fracture of a rod or wire is preceded by local thinning, increasing the stress.

Question 16

Creep

Answer 16

This is an increase of strain with time, which may sometimes occur even if the stress is kept constant.

Question 17

Fatigue Failure

Answer 17

If a metal specimen is subjected to many cycles of increasing and decreasing stress, even if the stress is always below the elastic limit, the metal is likely to develop cracks and eventually break.

Question 18

Work- Hardening (Cold Working)

Answer 18

This is the process of causing inelastic strain in a metal, through at least one application of stress. It raises the elastic limit, but reduces the extent of the ductile region between elastic limit and fracture.

Question 19

Annealing

Answer 19

This is the process of heating a metal, in order to alter its mechanical properties. In the case of copper, it will restore most of the ductility destroyed by any previous work-hardening.

Question 20

Quench- Hardening

Answer 20

This is the heating and rapid cooling of certain steels to produce changes in the crystal structure. The effects include raising of the elastic limit.

Question 21

Brittle Material

Answer 21

Material with no region of plastic flow, which, under tension, fails by brittle fracture.

Question 22

Thermoplastic Polymers

Answer 22

When heated, thermoplastic polymers soften, and at higher temperatures melt. They solidify on cooling. The process can be repeated.

Question 23

Thermosetting Polymers

Answer 23

After one episode of heating and cooling, usually during manufacture of an object, any further heating of a thermosetting polymer may result in cracking and/or charring, but not melting.

Question 24

Elastic hysteresis

Answer 24

When a material such as rubber is put under stress and the stress is then relaxed, the stress-strain graphs for increasing and decreasing stress do not coincide, but form a loop. This is hysteresis (literally, lag).

Question 25

Brittle Fracture

Answer 25

This is the fracture under tension of brittle materials by means of crack propagation.