Module 3.4 - Materials Flashcards
Hooke’s Law
Hooke’s law states that the extension of an object is proportional to the force that causes it, provided that the elastic limit is not exceeded
Define elasticity
The property of a body to resume its original shape or size once the deforming force or stress has been removed
Define deformation
The change in shape or size of an object
Define plastic deformation
The material doesn’t return to its original shape/size after deformation
Define elastic deformation
The object does return to its original shape/size after deformation
How will an object get a positive extension?
Object has equal and opposite tensile forces on each end pulling away from one another creating tension, causing it to increase in length
How will an object get compressed?
Object has two equal and opposite compressive forces acting towards each other causing compression, so the object will decrease in length/have a negative extension
Define the limit of proportionality
The point at which the extension is no longer directly proportional to the force/load
The material will still behave elastically after this point, but not for much more load
Define the elastic limit
Beyond this point the material will not show elastic behaviour
Any further load added will led to plastic deformation and the material will not return to its original shape
Define plastic behaviour
Plastic deformation will be seen when the load is removed and the material wont return to its original shape
Define fracture
The point at which the material will break
Before this happens, the material undergoes ‘creep’ where the planes of atoms slide past each other
How would you investigate the stretching of a wire?
Set up a long, thin copper wire held firmly in a clamp at one end, the other end supporting a weight hanger after passing over a pulley (hanger must be just heavy enough to keep the wire taut)
Attach a marker to the wire and place a ruler in a fixed position below it
Add weights progressively to the hanger and record the mass, reading on ruler, wire extension and tension in a table (tension found using T = mg)
Safety points about investigating stretching a wire
Wire may snap
Wear eye protection
Place box/cushion underneath hanger to catch the weight if it falls and to ensure you don’t stand directly underneath it
Define force constant
The constant of proportionality in Hooke’s Law
What is the area under the graph of a force-extension graph?
Work done
What does the work required to stretch a material depend on?
Stretching force used
Distance moved in direction of force (extension)
Define elastic potential energy
The energy stored in a stretched or compressed object
Define stress
Force per unit cross-sectional area
Define strain
Extension per unit length
Two methods of calculating the Young modulus of a material
Gradient of a stress-strain graph
Directly from the equation
Describe an experiment to calculate the Young modulus from a stress-strain graph
Set up Searle’s apparatus, measure the diameter of the wire using a micrometer and measure the original length of the wire using a ruler
Increase tension by adding weights to the right-hand side of the apparatus and measure the extension each time
Using this data, plot a graph of stress against strain and calculate the data to find the Young modulus of the wire
Use a vernier calliper to measure the extension as it’s so small
Define ductile
Can be drawn into wires
Will show plastic deformation under tensile stress before breaking
Define malleable
Can be hammered or beaten into flat sheets
Will show extensive plastic deformation when subjected to compressive forces
Define brittle
Will break with little or no plastic deformation
Define hard
Resist plastic deformation by surface indentation or scratching
Define stiffness
The ability of a material to resist a tensile force
Define polymeric
Made of long chains of molecules (polymers)
Define ultimate tensile strength
Maximum stress it can withstand while being pulled or stretched, before it fails or breaks
Why are polymeric materials (e.g. rubber) difficult to extend after a point, even if they don’t break?
The molecules are arranged in a mass of squashed long chains
When stress is applied, these chains straighten, resulting in a large strain
How have the flexible properties of rubber been harnessed in the making of vehicle tyres?
Natural rubber becomes sticky and weak when warm, so is subjected to vulcanisation
This involves the addition of impurities (e.g. sulphur) to bind the chains of rubber molecules together making them harder and stronger - i.e. less easy to stretch
