Materials Flashcards
Hookes law
Extension is directly proportional to force applied, given that conditions are kept constant
(F=kl)
Density equation
Density = mass / volume
What is meant by tensile stress
The force applied per unit cross sectional area
Stress = force / CSA
What is meant by tensile strain
A measure of how the material stretches : the extension divided by the original length (no units)
Strain = L/L
Difference between plastic and elastic deformation
Plastic : after the load is removed the object will not return to its original shape
Elastic : when the force is removed the object will return to original shape
Breaking stress
The minimum stress needed to break a material
Brittle
It doesn’t deform plastically but breaks when stress reaches a certain value
Elastic limit
The force above which the material will be plastically deformed
Elastic limit
The force above which the material will be plastically deformed
Area underneath a force
The work done to deform a material
Work done = 1/2 x F x L
State the equation to calculate the elastic strain energy from the spring constant and extension
E = 1/2 kL^2
Young’s Modulus
tensile stress / tensile strain
E = FL / LA
Describes stiffness
How to find Young’s Modulus from a stress-strain graph
the gradient of the line
Graphically represent plastic deformation
Unloading line does not go through origin as the material is plastically derformed
How can a force-extension graph show Hookes law is being obeyed
When it is a straight line through the origin
What is the limit of proportionality and what does it look like on a force-extension graph
The point after which Hookes 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 plastically deformed material
The stiffness constant hasnt changed, the forces between the atoms are the same when loading and unloading
Why isnt all work done stored as elastic strain energy when a stretch is plastic
Work us 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 cars kinetic energy so less is transferred to the passengers
¬ Seat belts stretch to convert the passengers 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 is stored 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
Where would you find the ultimate tensile stress a material can withstand
The highest point on a graph, it is the maximum stress a material can withstand
