MYP Design Physical and Mechanical Properties Notes

Physical Properties Are What a Material Is, Mechanical Are What It Does Under Load

1. A physical property can be measured without pushing or pulling the material, covering density, conductivity, absorbency and melting point.
2. A mechanical property only shows up when a force acts, which is why you have to load a sample to find it.
3. Properties are the language that connects a specification point to a material, so vague wording here weakens every decision after it.
4. Most products need three or four properties at once, and no material scores top marks on all of them.

Physical Properties: Weight, Heat, Current and Water

1. Density is mass per unit volume, so it tells you how heavy a part will be before you make it.
   1. Steel is around 7.8 grams per cubic centimetre and aluminium around 2.7, which is why aircraft skins are aluminium.
2. Thermal conductivity is how fast heat travels through a material.
   1. A saucepan base is copper or aluminium so heat spreads evenly, while the handle is a thermoset so heat stops before your hand.
3. Electrical conductivity is how easily current flows through it.
   1. A plug uses both ends of the scale in one object: brass pins carry the current, and the urea formaldehyde body stops it reaching you.
4. Absorbency is how much water a material soaks up, and it is the property a tea towel is bought for and a raincoat is rejected for.
   1. Absorbency also predicts trouble, since chipboard swells and crumbles in a bathroom while acrylic ignores water completely.

Strength, Hardness and Toughness Are Three Different Things

1. Strength is the force a material carries before it fails, and it comes in kinds: tensile for pulling, compressive for squashing, shear for sliding.
   1. A lift cable needs tensile strength, a table leg needs compressive strength, and concrete is excellent at one and poor at the other.
2. Hardness is resistance to scratching, denting and wear, and it belongs to the surface rather than the whole part.
   1. A chisel edge is hardened steel so it cuts pine without rolling over; a soft brass edge would fold on the first cut.
3. Toughness is the ability to absorb a sudden shock without cracking, and hard materials are often the least tough.
   1. A hammer head is hard at the face and tough in the body, which is why it dents a nail instead of shattering.
4. Brittleness is the opposite of toughness: the material breaks suddenly with no bending warning first.
   1. Cast iron and acrylic are both brittle, which is why a drill bit grabbing in acrylic cracks the whole sheet.

Ductility, Malleability and Elasticity: How a Material Moves

1. Ductility is being able to stretch into a wire without snapping, which is exactly why copper carries every cable in your house.
2. Malleability is being able to be hammered, rolled or pressed flat without cracking, which is how a car body panel gets pressed from a steel sheet.
3. Elasticity is stretching under load and returning to the original size when the load comes off, which is what a trampoline mat and a spring both rely on.
4. Plasticity is changing shape permanently and staying changed, which is what lets a paperclip hold its new bend.
   1. Most metals are elastic up to a point and plastic beyond it, and that crossover is why a bent bracket springs back a little then stays bent.
5. Fatigue is failure after many small loads that were each far too weak to break the part on their own.
   1. It is why a paperclip snaps on the twelfth bend and why aircraft wings are inspected on a schedule rather than after an obvious problem.

One Property Usually Decides the Choice

1. For most products there is a single property that rules everything else out, and the rest are tie-breakers.
2. Naming that deciding property in your specification turns a personal preference into a design reason.
3. Where two properties fight, say which one wins and why, because that judgement is what the Developing ideas criterion rewards.

Properties Change With Shape and Treatment

1. A property belongs to the material, but the part's performance also depends on its shape, since a steel tube resists bending far better than a solid rod of the same mass.
2. Treatment moves properties too: hardening raises hardness and drops toughness in the same piece of steel.
3. Quote properties for the exact grade and thickness you are using.