Solids Under Stress Unit 1.5

Question 1

Define Extension?

Answer 1

Increase in length of a object when put under tension

Question 1

Define Tension?

Answer 1

Magnitude of a force that a stretched object exerts on whatever its attached to.

Question 2

Define elastic?

Answer 2

object returns to original size and shape when tension is removed

Question 3

Define Hooke’s Law

Answer 3

Provided the Elastic limit is not exceeded extension of a spring is directly proportional to the tension

Question 4

Define Elastic limit?

Answer 4

point at which deformation ceases o be elastic

Question 5

Hookes equation

Answer 5

Force = Spring Constant x Extention

Question 6

Define Stress?

Answer 6

tension per unit cross sectional area

Question 7

Define strain?

Answer 7

Extension per unit length due to applied stress

Question 8

Define Young Modulus

Answer 8

Ratio of stress to strain for a material in hooke’s law region.

Question 9

Equation for strain

Answer 9

strain = increase in length / original length

Question 10

Equation for Stress?

Answer 10

Stress = Force / AreaEqua

Question 11

Equation for Young Modulus

Answer 11

E = stress/strain (Pa)

Question 12

Define Strain Energy?

Answer 12

energy stored in a object when its deformed elastically.

Question 13

What is the Area under a Force Extension Graph?

Answer 13

Work done

Question 14

Define Crystalline

Answer 14

consists of crystals; regular arrays of particles (ions)

Question 15

Define Polycrystalline

Answer 15

consisting of a large number of interlocking crystals

Question 16

Define a Metal

Answer 16

condensed material in which the atoms have lost more than one electrons to become positive ions which are held together by released delocalised electrons.

Question 17

Properties of crystals

Answer 17

1. Long range order
2. regular arrangement (Lattice)
3. arranged in a hexagonal shape so max no. of neighbours so potential energy is lowest possible

Question 18

Properties of Amorphous materials

Answer 18

1. No long range order
2. if cooled rapidly molecules have no time to make crystal arrangement before they loose ability to move (glass)

Question 19

Properties of Polymeric materials

Answer 19

1. large molecules
2. Large number of repeated sections
3. formed from monomers; double bond opens giving rise to free bonds which link monomers together

Question 20

Define Polymer

Answer 20

Material compromising large molecules which consists of many repeating units

Question 21

Define Monomers

Answer 21

Molecule which can combine with other molecules to form a polymer

Question 22

Define plastic

Answer 22

When the stress is removed the material is permanently deformed.

Question 23

Define Ductile

Answer 23

material which can be drawn into a wire

Question 24

Define elastic limit

Answer 24

stress at which the deformation ceases to be elastic

Question 25

Define yield point

Answer 25

point on a stress-strain graph at which a large increase in strain occurs for a little/no increase in stress

Question 26

Define yield stress

Answer 26

stress at the yield point

Question 27

Learn the graph for stress-strain

Answer 27

!!!

Question 28

What happens when a increase in stress takes place?

Answer 28

increase in separation of lattice ions.

Question 29

Define edge dislocations?

Answer 29

an additional part plane of ions in a crystal

Question 30

How do crystals grow?

Answer 30

Randomly, often have dislocations

Question 31

if stress exceeds yield stress what happens?

Answer 31

dislocation will move irreversibly to direction where force acts, (Permanent deformation)

Question 32

What does movement in edge dislocations produce?

Answer 32

large deformations eg: cause the crystal plane to snap at a weak point eg: missing ion

Question 33

Define Ductile fracture

Answer 33

Breaking that occurs when a ductile material is stressed to breaking point. it involves plastic deformation and necking

Question 34

for a ductile fracture stress increases …

Answer 34

more edge dislocations generated and migrate causing elongation. area decreases increasing stress at the neck.

Question 35

Define Ultimate tensile strength (UTS)

Answer 35

Maximum stress a material can withstand before breaking.

Question 36

In brittle materials how can crack propegation be avoided?

Answer 36

1. concrete designed so material is under compression: cracks don’t open up
2. pre stressed concrete (concrete with bars)
3. prestressed glass (cooling quickly hence tension)

Question 37

Define Elastic Hysteresis

Answer 37

When a material rubber is put under stress and then relaxed the stress strain graph for increasing and decreasing stress do not join but form a loop

Question 38

What is the area between the stress strain graph?

Answer 38

Energy dissipated in vibrational energy of rubber molecules