Materials Flashcards
state hookes law
the extension of an object is directly proportional to the force applied, provided the force does not exceed the elastic limit.
equation relating to hookes law
F = ke
on a graph, at what point will hookes law not be obeyed any more
when the graph starts to curve
this called the limit of propportionality
what is a limitation of a force extension graph
it only applies to metals
define plastic deformation
when an object doesn’t go back to its original shape when the force is removed
what is elastic deformation
when an object does go back to its original shape when the force is removed
what happens to the spring constant for springs in series and why
it gets smaller, by the same rule that resistance does in parallel
In series, the same force will produce double the extension in 2 springs than in one spring.
what happens to the spring constant for springs in parallel and why
it doubles by the same rule as resistors in series
when springs are in parallel the load is distributed along each spring so 2 springs will extend half as much as one spring when the same force is applied
define stress
force per unit area applied to a material
F/A = sigma (stress)
define strain
the ratio of a material’s deformation to its original length.
epsilon(strain) = e/l
define ultimate tensile strength
This is the maximum stress a material can withstand before it breaks.
define breaking stress
The stress level at which a material fractures.
how can you calculate elastic potential energy
the area beneath a force-extension / stress-strain graph
1/2Fe
1/2ke^2
1/2(stress)(strain)
what is young modulus
the ratio of stress to strain
stress / strain
EAe=Fl
young modulus * area * extension = force * original length
describe a brittle material
The material distorts very little even when subject to a large stress and does not exhibit much plastic deformation
describe a ductile material
Materials that have a large plastic region and can therefore be drawn into a wire
define polymeric materials
A material comprised of long chain molecules, for example, rubber, which may show large strains
define tough
Needs a large energy to break and create a new fracture surface
define stiffness
The extent to which a material under stress experiences a strain
stiffer materials experience less strain under stress
define strongness
The extent to which a material resists breaking under stress; a strong material has a high Ultimate Tensile Strength and a high breaking stress
define hardness
A measure of the difficulty of scratching or denting a material
describe the stress/strain graph for ductile materials
An initial straight line portion indicating the material’s obedience to Hooke’s Law.
Beyond the proportional region, the material experiences a large strain for small increases in stress.
Beyond the yield point, the material experiences noticeable deformation.
what will materials of different stiffnesses look like on a stress/strain graph
the stiffer material will have a much steeper gradient and a lower strain at its UTS but a higher UTS
what will a graph look like for a brittle material
there should be a linear relationship for the majority of the graph, with an abrupt plastic region after which the material fractures
describe the difference between energy gained and lost by rubber as the force loads and unloads
the energy gained is greater than the energy lost, indicating that the material has lost some energy as heat
