Materials Flashcards
Define density, giving units
mass of the object per volume it takes up
ρ = m/v
kgm^-3
Explain density qualitatively
Measure of how compact a substance is. Therefore, doesn’t vary with size or shape, just depends on what the object is made of
(kind of like resistivity)
What determines whether an object sinks or floats
The average density of the object
A solid object will float on a fluid if it has a lower density than the fluid
What is the density of water
ρ = 1 gcm^-3
so 1 cm^3 of water has a mass of 1g
Why do objects deform ie stretch, twist, bend etc
Opposite forces acting on the object
e.g. force down on a spring and reaction force up from the support
State Hooke’s law qualitatively
The extension of a stretched object is proportional to the load or force, given that environmental factor i.e. temperature remain constant
What is the equation for Hooke’s law
F = k∆L
where k is a constant, being the stiffness, which depends on the object being stretched
What is the difference between tensile and compressive forces
Tensile forces stretch the object
Compressive forces squash the object
Why are springs a special case of Hooke’s law
The value of k, known as the spring constant for springs, is the same for both tensile and compressive forces
How is Hooke’s law shown on a graph
Plot a graph of force against extension and will be a straight line
Why does Hooke’s law stop applying
Hooke’s law assumes reversibility from the deformation so will not apply if the deformation begins to be permanent due to too much force being applied
What is the point on the force/extension graph where Hooke’s law no longer applies
Up to the limit of proportionality, where the graph starts becoming curved instead of a straight line
What is the elastic limit
The point on the graph (after the limit of proportionality) where any more force would cause permanent deformation
Define elastic deformation
Deformation in which the material returns back to its original shape and size once all forces are removed
Explain why elastic deformation occurs
Tension on a material pulls atoms apart. Atoms have an equilibrium point so they can move small distances and then return back to the equilibrium point after the force is removed
Define plastic deformation
Deformation in which the material is permanently stretched
Explain why plastic deformation occurs
When a material is stretched past its elastic limit, the atoms move too far from their equilibrium points so they do not return once the force is removed
Explain the energy transfer during elastic deformation, giving example
Work is done when stretching the material, which is stored as elastic strain energy
When the force is removed, this energy is transferred to other forms
e.g. elastic band flies across room when released from stretch
Explain the energy transfer during plastic deformation
Work is done to separate atoms and this energy is not stored as strain energy, it is mainly just dissipated as heat
Explain how the energy transfers during plastic deformation are helpful in the real world
Crumple zones in cars are designed to deform plastically in a crash
This means energy goes in to changing the shape of the car and then dissipated as heat instead of being stored and transferred to passengers.
How can you calculate the elastic strain energy that a material stores
Area under a force/extension graph = 1/2FΔL
Derive the formula for elastic strain energy
Elastic strain energy stored = work done on material = force x extension
As the force is not constant and rises from 0 to F, the average force is 1/2F
Therefore ESE = 1/2FΔL
Since ESE is only stored when obeying Hooke’s law, F = kΔL
Therefore, ESE = 1/2kΔL^2
What does unloading look like on a force extension graph
The unloading graph is a straight line parallel to the loading one (because value of k is the same)
However, it is shifted to the right as when the force is 0, there is still extension due to plastic deformation
What is the area between the loading and unloading curves
Work done in permanently deforming the metal
What is the difference between a stress strain and force extension graph
Force extension graphs describe the behaviour of a specific object
Stress strain graphs describe the behaviour of the material
Explain why stress and strain may be used instead of force and extension
Stress and strain takes into account the area and length of a material, which would affect how much force it can withstand. This makes it easier to compare how different materials behave under similar loading conditions
Define tensile stress, giving units
Force applied/ cross-sectional area
Units are N/m^2 or Pa
Define tensile strain, giving units
The extension/ original length of the material
Strain has no units, can be written as number or percentage
How do the equations for stress and strain compare for tensile and compressive forces
Same equation but force and extension is negative for compressive
Define the breaking stress of a material
The stress at which the atoms in the material separate completely and the material breaks, depends on conditions e.g. temperature
Define the ultimate tensile stress of a material
The maximum stress that the material can withstand, depends on conditions e.g. temperature
Define the yield point
Point on a stress strain graph after the elastic limit, where plastic deformation actually becomes noticeable. Mainly seen in ductile materials.
Define the Young’s Modulus of a material
A measure of the stiffness of a material
Stress/Strain when Hooke’s law is being obeyed = FL/ΔLA
What are the units of young’s modulus
N/m^2 (same as stress as strain doesn’t have units)
How can you find Young’s modulus from a stress/strain graph
The gradient
What does the area under a Stress/Strain graph give
The strain energy = energy stored per unit volume
Therefore, 1/2 x stress x strain = energy stored per unit volume
Explain the stress strain and force extension graph for a brittle material
The material doesn’t behave plastically at all, it first behaves elastically and then just breaks when the stress or force reaches a certain point
Explain the stress strain and force extension graph for a ductile material
Can’t be shown on force extension graph
On stress strain graph, it has limit of proportionality, elastic limit, yield point, ultimate tensile strength and breaking point
Explain the stress strain and force extension graph for a plastic material
Same for both graphs, limit of proportionality, elastic limit, breaking point, no yield point
