3.4.2.1 - Bulk Properties Of Solids Flashcards
Density =
Mass / volume
What does the average density of an object determine?
Whether it floats or sinks - a solid object will float on a fluid if it has a lower density than the fluid.
What is the density of water?
1 g cm-3
What does Hooke’s law say?
The extension of a stretched object is proportional to the load or force.
F =
kΔL - where k is constant
What is the constant k?
The stiffness constant or in springs, the spring constant - it depends on the material being stretched.
On a force extension graph where and what is P?
When the graph starts to curve, it is the limit of proportionality.
What happens after the elastic limit is reached?
The material will be permanently stretched. When the force is removed, the object will not return to its original length.
What does it mean if a deformation is elastic?
The material returns to its original shape and size once the forces are removed.
What happens to the atoms when in an elastic deformation the material is put under tension?
The atoms of the material are pulled apart from one another.
Why in elastic deformation do the atoms return to their original position?
Atoms can move small distances relative to their equilibrium positions, without actually changing position in the material.
What happens to the atoms in elastic deformation when the load is removed?
The atoms return to their equilibrium distance apart.
Under what conditions does elastic deformation occur?
As long as the elastic limit of the object isn’t reached.
Stress =
Force / area
Strain =
Change in length / original length
Cross sectional area =
πd^2 / 4
What are the unit for strain?
It has no units, it is a ratio.
What happens to the stress/strain equations if the forces producing the stress and strain are tensile or compressive?
There is no difference to the equations, but you tend to think of tensile forces as positive and compressive forces as negative.
What happens to atoms as stress increases?
The effect of stress is that the the atoms start to pull apart from one another.
What happens if the stress is too great?
The atoms separate completely, and the material breaks, this is called the breaking stress.
What is the ultimate tensile stress?
The maximum stress that the material can withstand.
Why do engineers have to consider the ultimate tensile stress and breaking stress of materials?
They need to make sure the stress on a material won’t reach the ultimate tensile stress when the conditions change as breaking stress and UTS are dependant on conditions such as temperature.
Where can you find the elastic strain energy?
Under a force-extension graph
What happens to the work done when stretching a material?
All the work done is stored as elastic strain energy in the material.
What is the energy stored in a stretched wire equation?
1/2 kΔL^2
What does plastic deformation mean?
Material is permanently stretched, therefore doesn’t return to its original shape.
What does a force-extension graph look like for brittle objects?
It obeys Hooke’s law for a certain period of time but then it fractures so the line stops, there is no curving as they don’t deform plastically.
What has to be done in order to stretch a material?
Work done
What happens to the work done if the deformation is elastic?
All the work done is stored as elastic strain energy in the material.
What happens to the stored elastic strain energy when the stretching force is removed?
The energy is transferred to other forms.
What happens to energy during plastic deformation?
The work done is to separate atoms and energy is not stored as strain energy.
How is plastic deformation used in transport design?
Crumple zones are designed to deform plastically in a crash. Some energy goes into changing the shape of the vehicles metal body, and so less is transferred to the people inside.
On a stress strain graph what happens at the yield point?
The material suddenly starts to stretch without any entrance load. The yield point is the stress at which a large amount of plastic deformation takes place with a constant or reduced load.
What happens at the elastic limit?
The material starts to behave plastically. From this point on the material would no longer return to its original shape once the stress was removed.
What happens at the point of proportionality?
The graph is no longer a straight line, as the material stops obeying Hooke’s law. However it would still return to its original shape if the stress was removed.
On a stress-strain graph what does a straight line indicate?
The material is obeying Hooke’s law. The gradient of the line is constant - it is the Young modules.
What does the area underneath the straight line on the stress-strain graph indicate?
The energy stored in the material per unit volume.
What do brittle materials look like on a stress-strain graph?
A straight line through the origin and they don’t curve as they fracture.
What is the difference between force-extension graphs and stress-strain graphs?
Force-extension graphs are specific for the tested object and depend on its dimensions. Stress-strain graphs describe the general behaviour of a material because stress and strain are independent of dimensions.
What can you plot unloading on?
A force-extension graph
When unloading, why would the line not go back to the origin?
Because the material as been stretched beyond the its elastic limit and so has deformed plastically.
What does the area between the loading and the unloading lines represent?
The work done to permanently deform the wire.
