materials Flashcards
extension
tensile forces act away from the centre of the spring in both directions and will stretch it out causing the spring to extend
compression
forces act towards the centre of the spring in both directions causing compression
what happens to spring when tensile/compressive forces are exerted
the spring undergoes tensile deformation or compressive deformation
hookes law
force applied is directly proportional to the extension in length up to the limit of proportionality
equation for hookes law
f = kx
tensile stress
the force per unit area
tensile strain
measure of how the material stretches, the extension divided by the original length, it has no units
elastic deformation
who force is removed the object will return to its original shape
plastic deformation
after force is removed the object will not return to its original shape - the limit of proportionality has been exceeded
how is energy stored during elastic deformation
work done is transferred and stored as elastic potential energy
describe energy changes that occur during plastic deformation
material is stretched and the energy from the work done is used to break the bonds between the molecules. this causes permanent deformation
breaking stress
amount of stress a material can take without it breaking
brittle material
it does not extend much when a force is applied (tensile strain stays low). the material tends to break rather than stretch under a large force
elastic limit
the point after which plastic deformation occurs. it is sometimes also referred tons the limit of proportionality
area underneath a force extension graph represent
energy stored in the material
equation that calculates elastic strain energy in terms of spring constant and extension
E = 1/2kx^2
Youngs modulus
tensile stress/ tensile strain
how do you find Youngs modulus from a stress strain graph
gradient
one method to determine Youngs models of a material in the form of a wire
- set up wire over a pulley attached to a clamp and attach a mass to the end of the wire
- place a ruler and maker under wire to measure distance travelled
- vary force, by varying mass and measure extension x
- measure the diameter of wire with micrometer
- take readings along he wire and average
- use this to calculate the cross sectional area
- measure original length with ruler
- measure extension x with a marker and ruler or with travelling microscope
- stress = force / cross sectional area
- strain = extension / original length
- Youngs modulus = stress / strain
- this is equal to the gradient from stress strain graph
loading and unloading graph of metal wire
- metal wire obeys hookes law
- exhibits elastic deformation until elastic limit
- up to this point loading curve is the same as unloading
- beyond this point experiences plastic deformation
- unloading curve has the sae gradient as loading
- plastic deformation causes permanent deformation
loading and unloading for rubber
- does not experience plastic deformation
- does not obey hookes law
- area between loading and unloading is the work done in stretching
- energy transferred to thermal when force is removed
loading and unloading graph for polymeric material
- not obey hookes law
- experiences plastic deformation
- make new shapes but difficult to make original
