Solids under stress

Question 1

Hookes law

Answer 1

The tension in a spring or wire is proportional to its extension from its natural length within the limit of proportionality

Question 2

Equation for hookes law

Answer 2

F=k x X

Question 3

Energy stored in stretched springs

Answer 3

E=1/2 x F x X
or
E=1/2 x K x X^2

Question 4

Spring constant if springs are in series

Answer 4

1/ktotal=1/k1+1/k2

Question 5

Spring constant for parallel

Answer 5

extension is halfed and k is doubled

Question 6

Tensile stress equation

Answer 6

force(n)/area(m^2) , area is pir^2

Question 7

Tensile strain equation

Answer 7

extension/length

Question 8

Stress definition

Answer 8

force per unit cross sectional area

Question 9

Strain definition

Answer 9

extension per unit length

Question 10

Equation for young modulus

Answer 10

Stress /strain
or
F x L/A x X

Question 11

Elastic

Answer 11

Describes material that regains its shape after stresses are removed

Question 12

Ductile

Answer 12

This sort of material can be easily stretched or drawn into wire

Question 13

Malleable

Answer 13

Material can be hammered or pressed into shape without breaking or cracking

Question 14

Brittle

Answer 14

Material that would snsap without any yield

Question 15

Stiff

Answer 15

Small strains for large stresses, not stretchy or bendy

Question 16

Plastic

Answer 16

Material that undergoes permanent deformation under large stress rather than cracking

Question 17

Strong

Answer 17

a large stress is needed to break it

Question 18

Hard

Answer 18

Resists indentation on impact

Question 19

Crystalline solids

Answer 19

Crystal lattice consisting of regularly repeating unit cells
exhibit long range order and symmetry e.g Metals Diamonds Graphite and Salt

Question 20

Poly crystalline

Answer 20

Structure is split up into many small crystallites or grains which are randomly arranged thus forming different grain boundaries.

Question 21

Amorphous Solids

Answer 21

Solid structure in which the atoms have no long -range order or symmetry e.g Glasses,Ice, Ceramics

Question 22

Polymeric Solids

Answer 22

Materials comprised of long molecular chains usually containing carbon, e.g Rubber, Cellulose, polyethylene

Question 23

Elastic strain

Answer 23

When a force is applied to a material the atoms are stretched apart by a very small distance, for small strain no bonds are broken

Question 24

Plastic deformation

Answer 24

When the force is removed and the atoms are pulled back into their original positions so that the material regains its original shape.

Question 25

Plastic Strain

Answer 25

Imperfections within a lattice known as edge dislocations causing bonds to break.

Question 26

Work hardening

Answer 26

When lots of dislocations meet within a material then the can impeded each other to stop dislocations propagation. This causes material to become stronger and requires larger stresses to create dislocation.

Question 27

Grain boundaries

Answer 27

Poly crystalline materials have grain boundaries which can form barriers to edge dislocation movement.

Question 28

Necking

Answer 28

The thinning of the cross sectional area of a material at the weakest point. This usually occurs when the material continues to be stretched after it has reached it ultimate tensile stress.

Question 29

Ductile fracture

Answer 29

Product of large stress and necking

Question 30

Why are steel rods used in bridges

Answer 30

Steel rods are placed under tension and stretched
The wet concrete mix is placed around the steel and allowed to set causing it to bond to the steel
the tension is removed from the steel rods allowing it to contract
the concrete is now under permanent compressive forces

Question 31

Creep

Answer 31

Gradual deformity under a constant load

Question 32

Cold working

Answer 32

Repeated bending, stretching of alloy, dislocations become tangled and therefore causing each other to stop moving, therefore inhibiting plastic deformation.

Question 33

Hysteresis

Answer 33

Area under the curve = the work done , more work is done when stretching then when contracting . The extra energy used to stretch is transferred to vibrational energy in the rubber molecules . This is elastic hysteresis