Solids Under Stress

Question 1

Young Modulus?

Answer 1

Stress / Strain

How strong?

Question 2

Spring constant?

Answer 2

How stiff?
= F
——-
X

Question 3

Force extension graph?

Answer 3

Gradient = K
Area under = work done deforming solid
Also 1/2 KX^2

Question 4

Crystalline?

Answer 4

Regular
Repeating 3D arrangement
Salt
Metals etc

Question 5

Amorphous?

Answer 5

No regular long range structure of atoms > no crystal planes to slip over each other > only elastic behaviour > breaking stress low if any imperfections
Glass, ceramics

Question 6

Polymeric?

Answer 6

Large molecules in long chains
Repeating monomers
Eg rubber

Question 7

Yield point?

Answer 7

Crystal planes slide over each other, small increase in force give big extensions

Question 8

Ultimate tensile strength?

Answer 8

UTS- greatest force applied before breaking

Question 9

Fracture?

Answer 9

Material bends, ductile fracture

Question 10

Grains?

Answer 10

Cooling > crystallisation > grains forms > between are grain boundaries > contain impurity atoms forced out of the grains during crystallisation

Question 11

Necking?

Answer 11

Narrowing of tension where it breaks

Question 12

Elastic strain?

Answer 12

Atoms slide and move, but return to original position

Question 13

Plastic strain?

Answer 13

Forced break bonds between atoms of different planes and planes slip over each other
Bonds reform to prevent movement back again

Question 14

Dislocations in crystalline structures?

Answer 14

Defects in otherwise perfect arrangement
Edge dislocation - extra 1/2 plane of atoms
Point dislocation - Latrice ion missing, foreign atom present, additional ion

Question 15

Movement of edge dislocations?

Answer 15

Move through lattice until grain boundary - irreversible - plastic deformation > after process strength increased
Meet up? > tangle up and movement limited > strength increases
Bonds break and remake

Question 16

Strengthening materials ?

Answer 16

Quenching - smaller freaks boundaries > edge dislocations less free to move
More point dislocations - add foreign impurity atoms > edge dislocations less free to move

Question 17

Imperfections?

Answer 17

Before > stress lines uniform

Now > stress lines slip around crack > closer together > increase stress > propagates

Question 18

Support columns?

Answer 18

Compression - top
Tension - bottom
Any cracks propagate if heavy loads > steel fibres added

Question 19

Strengthening of brittle materials

Answer 19

Strong in compression >

Steel stretched > concrete poured > concrete sets, metal cut, contracts > compresses concrete

Question 20

Hookes Law?

Answer 20

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