Materials Flashcards
what is density
its the compactness of a substance
how much mass per unit volume
what is hookes law
when a force is applied to an object you change its length
change in length is directly proportional to the force applied
(works for stretching and compression)
when do you know hookes law is obeyed on a graph
it will be a straight line that goes through the origin
if it starts curving it no longer obeys the law
what is the limit of proportionality
where an object stops obeying hookes law
what are the key parts of a force against change in length graph
gradient= stiffness constant
area under= work done stretching an object AND elastic potential energy stored
what is the stiffness constant
the force needed to extend an object by 1m
how can springs be arranged and what’s the outcomes of doing this
in series (suspended from one another)
in parallel (placed side by side)
this changes the stiffness constant and therefore extension
what are key points on springs in parallel
they share the force between them meaning the stiffness constant is 2k and extension is only 0.5mm
what are they points on springs in series
they dont share the force between them both carrying the full weight meaning the stiffness constant is 0.5k and extension is 2mm
what is an elastic stretch/deformation
it means object will go back to its original shape once load has been removed
what is a plastic stretch/deformation
it means the object won’t go back to its original shape once load has been removed
this happens when it goes past its elastic limit
what does plastic stretch look like on a graph
on the unloading line it won’t go back to the origin its permanently altered
what is an energy loss in stretching
in an elastic stretch all the work goes into stretching the spring and released when load is removed
in plastic some of that energy is used to rearrange bonds and cant be recovered (heat loss)
what are the areas under a force change in length graph (loading and unloading)
under loading line- work done loading the spring
under unloading line- energy you get back when load is removed
between the 2 lines- energy lost
what are the positive applications of energy loss
crumple zones in a car convert the kinetic energy to heat to reduce impact. they plastically deform
what is stress
the force on an object in comparison to its cross-sectional area
what is strain
when you apply a force to an object it will change in length. its compares the extension by the original length
why is a stress/strain graph similar to a force/change in length graph
because stress in proportional to force
strain is proportional to change in length
what are they key points on a stress strain graph
Gradient= youngs modulus
area=elastic potential per unit volume
differences with stress/strain graph and a force/change in length graph
S/S- used when testing a material
F/L- used when testing an object
what are the 5 key points on the stress/strain line
limit of proportionality- doesn’t obey hookes law
elastic limit- plastically deforms
yield point- deforms more with no extra load
ultimate tensile strain-maximum stress it can take
breaking point- material breaks
what is the Youngs modulus
its a measure of stiffness of a material and doesn’t matter of the dimensions of the object
what sort of values of young modulus
usually giga or mega pascals
how could you find the Youngs modulus from a force against change in length graph
find the gradient and times it by length over cross sectional area
what is a brittle material
will break without warning and leave flat surfaces
what is a ductile material
will deform and stretch slowly leaving deformed surface
how can you identify brittle and ductile materials on a stress/strain graph
brittle materials always obey hookes law. they stretch but dont deform (straight line graph)
ductile materials curve and deform (wavy line)
what makes a material brittle
under a microscope brittle materials have rigid structure with strong bonds. when stress is applied they focus on tiny cracks where they are just ripped apart, leaving a flat break (brittle fracture)
what makes a material ductile
in a ductile material the atoms can move around more and bonds can reform, preventing cracks from getting any bigger and the material stretching
