recycling, material failure, surface treatments

hardest materials to recycle

composites - hard to separate

coloured glass - it’s hard to remove impurities from glass + no economic drive to recycle materials whose raw materials are very cheap

recyclable defintion

a material which can reenter the material cycle after its first life application

quality of material drops after each use

biodegradable

a material able to deteriorate back to its original natural constituents once interacting with the environment (photo,chemical, bio, heat)

can glass biodegrade?

no - very inert ceramic

issue with bio-degradable polymers?

more expensive than non-biodegradable counterparts

perofrmance index

material property that we make ourselves depending on the required application - rearrange, sub into eqs and solve for thing you want to minimise like mass - then find the things u can change - i.e. material properties as ur assuming geometry is constant in material selection -

IN ASHBY PLOTS YOU CAN THEN MAKE A LINE EQUATION TO SEE WHAT MATERIALS ARE ABOVE THAT LINE

ONCE DONE CAN EITHER CONTINUE LINE UP TO FIND TOP MATERIAL OR CONSIDER A GROUP OF MATERIALS AND BRING IN ANOTHER VARIABLE: cost! - kinda

with performance indexes why not look at just absolute cost? and what is done instead?

then the performance index wrt cost would vary over time

INSTEAD:

c bar = cost per unit mass of material PER cost per unit mass of low carbon steel - this ratio changes much slower over time than abs cost.

to find new performance index wrt cost just divide by c bar.

corrosion

natural degradation of materials - includes oxidation

oxidation

the corrosion of ferrous metals via reaction with oxygen and water to form oxide layer

• metal loses electrons and gives to oxygen to form full outershell 6-8

• OIL RIG (of e^-)

• cathode = pawsitive = will gain electrons = reduction

can polymers corrode?

No, but they can degrade

how polymers break down

swelling = liquid diffuses into polymer weakening bonds causing it to expand and weaken as a whole

dissolution = for completely soluble polymers - continued swelling

NOTE: can be used as an advantage to smooth surfaces.

bond rupture = breaking down chain bonds caused by heat or radiation, etc.

can ceramics corrode

Most can be thought to be already corroded, so very resistant to corrosion in actual uses

galvanic series key point

order varies depending on environment i.e. water oxygen etc

passivity

When a metal oxidises and in doing so produces a protective layer, preventing the rest of the metal from oxidising

factors effecting corrosion rate

fluid velocity: inc = inc corrosion

temperature: inc = inc corrosion

cold-working: more plastic deformation = inc corrosion

composition: of both metal and environment i.e. salty water or stainless steel

uniform attack

when corrosion/oxidation occurs evenly across entire product surface - most common and can design to prevent

galvanic corrosion/attack

occurs when two different metals are connected in the presence of an electrolyte. - occurs with nuts and bolts and beams they’re on

to solve, insulate areas or add another anode or choose two metals close to each other in the galvanic series

main ways of preventing corrosion

design

environment

surface treatments

cathodic prevention

cathodic protection purpose and how its done x2

prevents corrosion on (buried or submerged surfaces.)

it forces the metal you want to protect to become the cathode in a cell - ensuring no corrosion occurs by supplying electrons:

Either by:

GALVANIC COUPLE: directly connecting it to a more reactive metal which means that when it oxidises, it will pull electrons from the thing u want to protect thus making it a cathode etc.

IMPRESSED CURRENT: power supply negative terminal connected to metal, hence pulling in electrons, and positive end to inert anode.

simple fracture definition

The separation of a body into two or more pieces due to a stress applied at low temps (relative to mp of the material)

two steps:

crack formation

propagation

how to identify ductile rather than brittle fracture?

Lots of plastic deformation in the region, and it originates from an elliptical hole perpendicular to the force applied in the region of plastic deformation.

STABLE CRACK

where do stress concentrations occur?

Anywhere across the material, there are small cracks/flaws that act as stress concentrators

sigma m eq

MAX stress occurs at the point of the elliptical hole. - or if it’s a surface crack, then at the point furthest in the material.

rho t = RADIUS OF CURVATURE

a = length of surface crack OR length/2 of internal crack.

what does stress vs position look like for central hole

1/x² on both sides with a gap in the middle (no stress where theres no material)

stress concentration factor

ratio of sigma m/sigma applied force. = Kt

critical stress

the stress required to propogate a crck - the bigger the a, the smaller the stress required.

y = surface energy

fracture toughness =

= Kc !!!!!!! rememeebr counter intuitive youd think stress concentration would be c and toughness t but other way round.

Fatigue definition

failure of a material caused by cyclical stresses (largest cause of failure)

3 types of cyclical stress

symmetrical - starts at 0 and sin wave

Repeated - starts at non-zero stress and sin wave

random - random (most realistic)

stress ratio and amplitude

in the name

amplitude - height of wave .:. max-min/2

ratio = min/max

what are the main deciders on how long a material doesn’t fail for under cyclical stress?

magnitude of stress - larger = fewer cycles

FATIGUE/ENDURANCE LIMIT = below a certain stress, some materials will never fail

S-N curves

stress and number of cycles

what does the fatigue/endurance limit look like on a SN graph

flat lines at a certain stress (until infinite n of cycles)

otherwise would just continue to curve downwards

safety factor

You never really know the exact stresses the design will go under therefore design with precaution

working stress = yield/safety factor (working always less)

3 main manufacturing industries

primary - natural resources

secondary - converting nr into goods

tertiary - the service industry

2 types of goods

consumer - goods purchased directly by the consumer

capital - goods purchased by companies to make goods/services

2 types of manufacturing techniques

processing (including surface treatments)

assembly

chemical cleaning types and examples

alkaline cleaning - soap

acid cleaning - limescale remover

emulsion cleaning - mixture of two or more liquids that are usually unmixable is created when cleaning

solvent cleaning - whiteboard spray

ultrasonic cleaning

mechanical cleaning types + cons

blast finishing - high pressure/velocity impact media usually sand

just cleans

shot peening - high pressure/velocity impact media ALWAYS cast steel.(shots) + applied to metals only.

cleans and improves fatigue strength!

Tumbling - workpieces and cleaning media put in barrel that is rotated causing landslide like motion used to clean the surface.

LOUD, LONG, & LOTS OF SPACE

electroplating

coating of one metal with another using electrolysis

make object to coat the cathode +ve terminal and vv

current flows

anode surface will dissolve in electrolyte and attach onto the cathode surface

electroforming

same process but rather than plating it onto existing product u plate onto a mould AKA mandrel (JUST LIKE FILAMENT WINDING)

then mandrel is removed leaving hollow metal piece.

what is electroless plating

same concept as electroplating, but without electricity

i.e. use a reducing agent in the solution your workpiece is in in order to cause the same plating effect.

WORKS FOR NON-METALS BUT USUALLY MORE EXPENSIVE AND LESS MATERIALS STILL.

anodizing + its difference to electroplating

producing intentional stable oxide layer surface to act as protection AND aesthetics - can change its colour!

The difference is the oxide layer rather than another metal

fewer metals can be anodised then can be electroplated though.

define organic coatings

Any polymer applied as a liquid then dries on the surface of a material to improve aesthetic or function

organic coating examples

binders - increase the durability of the material

dyes - aesthetic colour (SOLUTION with colour = transparent-ish)

Pigments - aesthetic colour (solid particles = opaque)

Solvents - carry the binder in application (for mobility), then dissolve, leaving the binder to stick strongly to the surface

Additives - anything else, i.e., fungicides, etc.

organic coating advantages

cheap

easy to apply AND REAPPLY
aesthetic AND protective

Transfer efficiency (TE)

ratio between how much paint is used and how much stays on workpiece

application methods of organic coatings

brushing/rolling (TE close to 100)

spray coating (TE close to 30)

electro spray coating (TE close 90)

Immersion, i.e. dip coating (TE low)

Flow coating (shower the workpiece in paint) (TE low)

NOTE: TE is low, but paint can be recycled!!

Drying methods of organic coaitng

RTP curing - chemical reaction causes the solvent to dissolve

Elevated temp cooling - uses heat to speed up ROR/evap it directly

Catalytic curing - resin cures from reactive agents added prior.

Radiation curing - i.e. UV used to cure resin.