Section 4 - Mechanics and Materials: 4.2 - Materials Flashcards
What is Hooke’s law?
Extension (∆L) is directly proportional to force
applied (F), given that the environmental
conditions are kept constant.
F= k∆L k is the stiffness constant in Nm^-1
What equation is used to calculate density?
Density = Mass / Volume
Density units: kgm^-3
Mass: kg
Volume: m^3
What is meant by tensile stress?
The force applied per unit cross sectional area.
Stress = force / CSA
Stress units: Nm-2
Force units: N
Cross sectional area units: m2
What is tensile strain?
A measure of how the material stretches: the
extension (ΔL) divided by the original length (L),
strain has no units.
Strain = ΔL / L
What is the difference between elastic and plastic
deformation?
Elastic deformation: when the force is removed
the object will return to its original shape.
Plastic deformation: after the load is removed the
object will not return to its original shape.
What is breaking stress?
The minimum stress needed to break a material.
Which of these two graphs represents a brittle
material?
(Insert Graphs A and B at home)
Insert Graph A
What is meant when a material is
described as 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 underneath a force - extension
graph represent?
The work done 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 = 1⁄2 kΔL2
What is Young’s modulus?
Young’s modulus (E) = tensile stress/ tensile strain.
E = FL / ΔLA
(by substituting stress and strain equations).
It describes the stiffness of a material.
How do you find the Young’s modulus from a
stress-strain graph?
The gradient of the line.
Which of these graphs would represent a wire which
has plastically deformed?
(Insert pictures of graphs and key)
The unloading
line doesn’t go
through the
origin as the
material is
permanently
extended
(stretched)
(Instert picture of right graph with key)
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 after 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 plastically 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 work done 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 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
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.
(Inset Graph)
