Chapter 6 - Materials Flashcards
What are tensile forces
They produce extension
They undergo tensile deformation when tensile forces are exerted.
What are compressive forces
They produce compression (shorten an object)
They undergo compressive deformation when compressive forces are exerted.
Explain the force-extension graph for a spring
It is a straight line from the origin to the elastic limit. In this region, the spring undergoes elastic deformation. This means the spring can return to its original length.
Anything after the elastic limit, it will undergo plastic deformation. This is permanent and the spring can’t go back to its original shape when the force is removed.
Hooke’s law
The extension of a spring is directly proportional to the force applied. This i true as the elastic limit of the spring is not exceeded.
Equation for force constant
F= k x
k- force constant, x- extension
What is the force constant
Essentially, it is a measure of the stiffness of the spring. A large force constant is difficult to extend.
How can you investigate Hooke’s Law
Attach a spring to a clamp set up. Set up a metre ruler next to the spring. Keep adding masses from the spring and record the new length of the spring as you add each mass.
How can you improve the accuracy of the length measurements when investigating the Hooke’s law
Using a set square, and by taking readings at eye level to reduce ‘parallax errors’
What happens to the energy put into plastic deformation
If an object is deformed plastically, then the energy in work done can’t be fully recovered. Some has gone into moving the atoms’ arrangement into new, permanent positions. This energy isn’t recoverable
work done equation
work done = force * extension
What is the area underneath a force-extension graph
the work done
What is the work done on a spring transferred to
It is transferred to the elastic potential energy within the spring. This is fully recoverable
Elastic potential energy equation
E= 1/2 f x
E= 1/2 k x2
tensile stress definition
the force applied per unit cross-sectional area
tensile stress equation
force/ cross sectional area
tensile stress unit
Pascals
tensile strain definition
the fractional change in the original length of the wire
tensile strain equation
extension/ original length
Young Modulus definition
Within the limit of proportionality, stress is directly proportional to the strain. The ratio is constant, which is called the Young Modulus (E)
Young Modulus equation
tensile stress/ tensile strain
OR
FL/ AX
Young Modulus unit
Pascals
How can you find E on a graph
On a stress- strain graph, it is the linear gradient of the graph
What does E actually represent
The stiffness of the material
On a stress-strain graph what is UTS
Ultimate Tensile Strength
the maximum stress a material can withstand being stretched before it breaks
Brittle objects
Examples: iron or glass
They reach their elastic limit and will then break.
Polymeric objects
Rubber bands
The loading curve is different from the unloading curve. The middle section is the energy released due to heating on a F-E graph. They don’t obey Hooke’s Law. They will eventually return to their original length.
This is mainly because more work is done when stretching a rubber band than is done when its extension decreases.
What is the shape created by a rubber band called
A hysteresis loop
Plastic Bags
They also don’t follow Hooke’s Law because they suffer plastic deformation.
Metals
The loading curve is linear, and it follows Hooke’s Law, until a little bit past the breaking point.
weighted average
(no. of thing * value of that thing) + (no. of another thing * value of that thing) / total number of things.
what are yield points
points where the material extends rapidly . this type of curve may not happen for other ductile materials, but it does happen to steel wire.
Read a graph of bending steel
Limit of proportionality Elastic limit Yield 1 Yield 2 UTS Breaking point
What does a higher UTS mean
that material is stronger
what type of behaviour does rubber show
it shows elastic behaviour,
whereas polythene shows plastic behaviour
what are polymeric materials
they consist of long molecular chains
describe a graph of force against extension for a metal
If the forces are less than the elastic limit, then the unloading graph is equal to the loading graph. However, beyond the elastic limit, it may be permanently extended after the force is removed. it has suffered plastic deformation.
describe a graph of force against extension for rubber
They don’t obey Hooke’s Law and it will return to its original position after the forces have been removed, but the loading and unloading graphs are different.
describe a graph of force against extension for a polythene
It also doesn’t obey Hooke’s Law. The thin strips of polythene are quite easy to stretch so they can suffer plastic deformation under very little force.
Example of when plastic deformation isn’t bad
Car frames are bent, and we wouldn’t want them to return into their original shape. For example, steel.
