9.1 & 9.2 & 9.3 - Basic Properties Flashcards
What is corrosion?
When metals and alloys are attacked / eaten away by chemical substances
What is degradation?
When non-metals are attacked by chemicals, they are weakened / destroyed
Corrosion example?
Rusting of ferrous metals caused by the action of atmospheric oxygen in the presence of water
Degradation example?
Rubber (if not compounded) is attacked by prolonged oil exposure, and is also damaged by UV rays
What is electrical resistance?
Low resistance results in good conduction whilst high resistance means good insulation (metals are good conductors, whilst non-metals insulators, except for carbon)
What does electrical resistance depend on?
Length (longer means more resistant)
Cross-sectional area (thicker means less resistant)
Temperature (higher temp leads to greater resistance)
Resistivity (resistance measured between opposite faces of a metre cube of a material)
Ferromagnetic material examples?
Iron
Nickel
Cobalt
What are soft magnetic materials?
Materials that can be magnetised via a magnetic field, but cease to be magnetised once the field is removed
How to make soft magnetic materials more efficient?
Adding silicon or nickel to pure iron
Soft magnetic material examples?
Soft iron
Silicon-iron alloys are used for the rotor and stator cores of electric motors and generators, as well as power transformers
What are hard magnetic materials?
Materials that can be magnetised, and retain this property once the field has been removed, becoming permanent magnets
How to make soft magnetic materials more powerful?
Adding cobalt to the steel to make an alloy
Hard magnetic material examples?
High carbon steel that has been hardened by cooling it rapidly (quenching) from red heat
What is strength?
The ability of a material to resist an applied force without fracturing maximum stress it can handle), and to not yield (elongate under pressure, changing shape permanently)
Examples of applied loads?
Tensile load - material is pulled, made longer
Compression load - material is pushed inwards, made shorter
Shear load - material pushed inwards at different points, changed shape (parallelogram)
Tensile elongation?
ε strain = extension/original length (ductility)
Compressive reduction?
ε strain = -extension/original length (malleability)
Shear deflection?
ε strain = extension/original length (length as height)
Equation for stress?
Stress = force/area (cylinder, force active at both ends, area being that of the circle)
What is toughness?
The ability to resist impact loads, amount of energy absorbed before failure
How to test toughness?
Izod test, uses a notched specimen that is hit by a heavy pendulum (test conditions are controlled)
Example of toughness?
Tough steel - a 1.2% carbon steel rod in the annealed (soft) condition will bend under a hammer impact with a tensile strength of 925MN/m2
Hard steel - the same rod, after quench hardening, will fracture when hit, despite having a tensile strength of 1285MN/m2
What is elasticity?
The ability to change shape when subjected to an applied force but spring back to the original size and shape once the force is removed
Opposite of plasticity
What is plasticity?
The ability to flow into a new shape when subjected to an applied force and keep that shape once the force has been removed
Opposite of elasticity
What is ductility?
The ability to change shape via plastic flow when subjected to a tensile force
What is malleability?
The ability to change shape via plastic flow when subjected to a compressive force
What is hardness?
The ability to withstand scratching or indentation by and even harder object
What are the 3 tests for hardness?
Brinell hardness test
Vickers hardness test
Rockwell hardness test
What is the Brinell hardness test?
Hardened steel ball pressed into specimen by controlled load, diameter of indentation measure using a microscope, hardness value obtained via conversion table
What is the Vickers hardness test?
Diamond pyramid pressed into material via controlled force, square indentation is made, diagonal length of square measured to used for conversion value
What is the Rockwell hardness test?
Diamond cone makes small indentation (minor load applied), major load then applied which increases indentation depth, the increase is directly converted into a hardness number to be read from the machine
What is rigidity?
The ability to resist changing shape under a load
Opposite of flexibility
What is flexibility?
The ability to change shape under a load
Opposite of rigidity
Example of rigidity vs flexibility?
Cast iron is more rigid than steel, which is stronger and tougher, but the ability to be rigid makes cast iron a better material for machine frames and beds
What is melting temperature?
The temperature at which a material loses its solid properties, some plastics only become charred and destroyed
What is thermal conductivity?
The ability to conduct heat, metals are typically heat conductors whilst non-metals insulators
What is thermal expansion?
When materials expand then contract due to the application and removal of heat, metals have high coefficients of linear expansion (expand more than non-metals which have a low coefficient)
Ohm’s law?
V = I x R (voltage = current x resistance)
