Flashcards in Unit 2 Section 2 Materials Deck (33)
What does Hooke's law say?
Extension is proportional to force.
What does k mean in Hooke's law?
The stiffness constant of the spring.
What does Hooke's law apply to?
Springs and stretched wire.
What is a tensile force?
When something is stretched.
In springs, what is the K value for compression equal to?
The value when stretching. This isn't the same for all materials.
What is the elastic limit?
If the load is increased past the elastic limit, the material will be permanently stretched. Plastic stretching will occur.
What are the two types of stretch?
Elastic or plastic.
What is an elastic deformation? Explain it at an atomic level.
The material returns to the original shape when the forces are removed.
When the material is put under tension, the atoms are pulled apart.
They can move small distances relative to the equilibrium positions, without actually changing position in the material.
When the load is removed the atoms return to their equilibrium distance apart.
In a metal when Hooke's law is obeyed, what type of deformation is it?
What is plastic deformation?
The material will be permanently stretched.
The atoms in the material move position relative to each other and when the load is lifted they don't return to the original positions.
When a metal is stretched past its elastic limit, what deformation does it show?
How is the young modulus found?
Tensile strength ÷ tensile strain.
What are units for the young modulus?
What type of wire should be used when testing the young modulus of a material?
The thinnest wire will stretch the most.
However it is more likely to snap.
If you play a graph of stress against startin, how do you find the young modulus?
It is the gradient.
How is tensile stress found?
force applied ÷ cross sectional area.
What are the units for stress?
Nm^-2 or pascals
Define tensile strain.
The change in length divided by the original length of the material.
What happens when a material feels too much force and breaks?
The stress increases and the atoms are pulled away from each other. Eventually the stress becomes so strong the atoms fall apart completely and it breaks.
The stress where this happens is the braking stress.
What is the UTS?
Ultimate tensile stress. The maximum stress a material can withstand.
What is the elastic strain energy?
The energy stored in a stretched material. Before the elastic limit, all the work done to the material becomes energy stored in it.
How can elastic strain energy be found with a graph?
Draw a graph of force against extension. The stored energy is the area under the graph.
How can the energy stored in a material be calculated?
Work done=0.5 X force X extension.
word done= 0.5 X spring constant X extension²
Why do the equations for finding the work done to a wire only work when the force is below the elastic limit?
If the limit is exceeded then work is done to move the atoms about.
How do you calculate the strain energy per unit volume of a loaded wire?
0.5 X stress X strain.
Or 0.5 X force X extension
Or E= 0.5 X K X extension²
What is a brittle material?
A material that won't deform plastically when a force is applied but will snap suddenly when the force gets big enough.
What is the structure of a rigid material?
They are crystalline or polycrystalline. There are many different grains that all line up in different directions.All the atoms are tightly bonded to one another meaning they are stiff. It is a giant rigid structure, thus brittle.
What happens when a brittle structure has a force applied?
Tiny cracks will appear in the materials surface that get bigger and bigger until the material breaks.
They grow because they have rigid structures.
How do you find the area under a curve?
Count the squares.