Materials Flashcards
What is Hooke’s law?
Extension is directly proportional to force applied, given that the environmental conditions are kept constant
F = kΔL
k = stiffness constant in Nm^-1
What is meant by tensile stress?
The force applied per unit cross sectional area.
Stress = F/A
Units: Nm^-2
What is tensile strain?
A measure of how the material stretches: the extension divided by the original length, strain has no units.
Strain = ΔL/L
What is the difference between plastic and elastic deformation?
Elastic deformation: when the force is removed the object will return to its original shape. Atom are pulled apart when under tension, but they can move small distances relative to the equilibrium positions without changing position. Once load is removed they will return to their equilibrium distances apart.
Plastic deformation: after the load is removed the object will not return to its original shape. Some atoms moves position relative to one another and when load is removed atoms don’t return to their original positions.
What is breaking stress?
Minimum stress to break a material
Draw a graph of stress and strain representing brittle material.
Look at cgp/ pmt
What is meant by a material being brittle?
It doesn’t deform plastically but breaks when the stress reaches a certain value.
What is the elastic limit?
The force above which the material will be plastically deformed (permanently stretched).
What does the area under a force extension graph represent?
The work done in order to deform the material
WORK DONE = 1/2 x F x ΔL
State the equation to calculate elastic strain energy from the spring constant and extension.
E =1/2KΔL^2
What is Young’s modulus?
Tensile stress over tensile strain
It describes the stiffness of a material
How do you find the young modulus from a stress strain graph?
The gradient of the line.
Draw a force extension graph which represents a wire being plastically deformed and what does the area represent?
Pmt / cgp
Area - work done to permanently deform the wire.
How can a force extension graph show hooke’s law is being obeyed?
When it’s a straight line through the origin I.e. the force and extension are directly proportional.e
What is the limit of proportionality and what does it look likes like on a force extension graph?
The point at which hooke’s law is no longer obeyed, it is shown by the line beginning to curve on a force extension graph
How is the work done to stretch or compress a material stored?
Elastic strain energy
Why are the loading and unloading lines parallel on a force extension graph for a platically deformed material?
The stiffness constant (k) hasn’t changed, the forces between the atoms are the same when loading and unloading.
Why isn’t all the energy stored as elastic strain energy when a stretch is plastic?
Work is done to move atoms apart, so energy is not stored as elastic strain energy but is dissipated as heat.
How is the dissipation of energy in plastic deformation used to design safer vehicles?
crumple zones deform plastically in a crash using the car’s kinetic energy so less is transferred to the passengers.
Seat belts stretch to convert the passenger’s kinetic energy into elastic strain energy
Outline the energy changes that occur when a spring fixed at the top is pulled down and released
The work done in pulling the spring down (stretching it) is tired as elastic strain energy, when the spring is released this is converted to kinetic energy which is converted to gravitational potential energy as the spring rises.
Do stress- strain graphs show the behaviour of a material or a specific object
Material
Where would you find the maximum tensile stress on an stress-strain graph?
The highest point on a graph, it is the maximum stress a material can withstand.
What would a stress-strain graph for a ductile material look like?
Look at pmt or cgp
How is energy always conserved when stretching? In elastic and plastic
Elastic - all work done is stored as elastic strain energy in the material. When stretching force is removed the stored energy is transferred to other forms such as kinetic
Plastic- work is done to separate atoms and energy is not stored as strain energy (its is mostly dissipated as heat).
What is elastic strain energy? And how can it be calculated from a graph?
the potential energy stored in a material when it is deformed elastically. (Energy stored per unit volume)
Energy per unit volume = 1/2 x stress x strain
What is the yield point on a stress-strain graph
The material suddenly starts to stretch without any extra load. A large amount of plastic deformation takes place with a constant or reduced load.
Why don’t stress-strain graph for a brittle material curve?
It obeys hooke’s law up to a point and then fractures (breaks, does not deform plastically).
