4.2 - Materials Flashcards
What is Hooke’s Law?
Extension is directly proportional to the Force applied, given that the environmental conditions are kept constant
F=kL
K is the stiffness constant in Nm^-1
What equation us used to calculate density ?
Density = mass/volume
kgm^-3
What is meant by tensile stress?
The force applied per unit cross sectional area
Stress =force/ CSA
Nm^-2
What is tensile strain ?
A measure of how the material stretches : extension divided by original length
What is the difference between elastic and plastic deformation ?
Elastic : when force is removed object will return to original size and shape
Plastic : after load is removed the object will not return to original length
What is breaking stress?
The minimum stress to break a material
What is meant when a material is described as brittle?
It doesn’t deform plastically but breaks when stress reaches a certain value
What is the elastic limit?
The force above which the material will be plastically deformed
What does the area underneath the force-extension graph represent?
The work done to the deformed material
Work done= 1/2 x F x change in length
State the equation to calculate elastic strain energy from the spring constant and extension
E = 1/2 x K x change in length squared
What is Young’s Modulus?
Young’s Modulus (E) = tensile stress / tensile strain
It describes the stiffness of a material
How do you find Young’s Modulus from a stress - strain graph?
Gradient of the line
How can a force extension graph show Hooke’s law is being obeyed?
When it is a straight line through the origin ie force and extension are directly proportional
What is the limit of proportionality and what does it look 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 the 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 plastically deformed material?
The stiffness constant (k) has not changed, the forces between the atoms are the same when loading and unloading
Why isn’t all the work done stored as elastic strain energy when a stretch is plastic?
Work is done to move atoms apart, so work done is not stored as elastic strain energy but are dissipated as heat
How is the dissipation of energy in plastic deformation used to design safer vehicles?
1) crumple zones deform plastically in a crash using the cars kinetic energy so less is transferred to the passengers
2) Seat belts stretch to convert the passengers kinetic energy into elastic strain energy
Outline the energy changes of a fixed spring from the top is pulled down from the top and released?
The work done is pulling it down (stretching it) is stored as elastic strain energy, when the spring is released this is converted to kinetic energy which converts to gravitational potential energy as it rises
Do stress-strain graphs show the behaviour of a material or a specific object?
Material
Where would you find the ultimate tensile stress on a stress-strain graph?
The highest point on a graph it is the maximum stress a material can withstand
What would the stress-strain graph for a ductile material look like?
A ductile material can undergo a large amount of plastic deformation before fracturing
