3.4 Materials Flashcards
What is a tensile force?
Equal and opposite forces acting on a material to stretch it.
What is a compressive force?
Two or more forces together that reduce the length or volume of an object.
What is Hooke’s law?
Extension of a spring is directly proportional to the force applied.
What equation related force applied to a spring and its extension?
Force (N) = Force Constant (Nm-1) x Extension (m)
What does the spring/force constant tell you about the spring?
It is a measure of the stiffness of a spring.
What is an experiment used to verify Hooke’s law?
- Attach a spring at one end using a clamp stand setup and put a metre rule attached to the same clamp stand parallel next to it.
- Put slotted masses suspended from the other end of the spring, record the total mass and extension.
- Repeat this when unloading, for two results for each mass.
- Plot a graph of force against extension, and if there is a straight line of best fit through the origin, it obeys Hooke’s law.
What does elastic limit mean?
The value of stress/force beyond which elastic deformation becomes plastic deformation, and the material/object will no longer return to its original shape or size when the load is removed.
What does plastic limit mean?
An irreversible change in the shape of an object due to a compressive or tensile force, and the removal of stress or force produces permanent deformation.
How can elastic potential energy be determined from a force-extension graph?
It is determined from the area under the graph.
What are the two equations that can be used to calculate elastic potential energy?
E = 1/2Fx
E = 1/2kx²
F = force (N)
k = force constant (Nm-1)
x = extension (m)
If you double the extension of the spring, what happens to the energy stored?
The energy stored will increase by a factor 4.
What is a hysteresis loop?
A loop shaped plot obtained when loading and unloading a material produces different deformations.
How can you determine the thermal energy lost to the surrounding from a force-extension graph?
The area between the two plotted graph lines/underneath the uppermost line.
What is the molecular structure of a rubber band?
Rubber bands consist of squashed and tangled long chain molecules.
What is the explanation of the shape of the force-extension curve for a rubber band in terms of molecular structure?
The long chain molecules can be easily untangled with small forces but require large forces to go further once fully straightened.
How is tensile stress defined?
The force applied per unit cross sectional area of the material.
What is the equation for tensile stress?
Stress (Pa) = Force (N)/Cross-sectional Area (m2)
How is tensile strain defined?
The fractional change in the original length of the material.
What is the equation for tensile strain?
Tensile Strain = Extension (m)/Original Length (m)
What is meant by ultimate tensile strength?
The maximum stress that a material can withstand before it breaks.
How is Young Modulus defined?
The ratio of tensile stress to tensile strain when these quantities are directly proportional to each other.
What is the equation for Young Modulus?
Young Modulus (Pa or Nm-2) = Tensile Stress (Pa or Nm-2)/Tensile Strain
What is an experiment used to determine the Young Modulus of a metal wire?
- Measure the wire’s diameter multiple times using a micrometer and calculate A using (pi x d²)/4.
- Add masses and each time measure the extension using the metre rule underneath the wire and the marker on the wire, and the force applied.
- Calculate stress and strain, and use these to plot a graph stress against strain, the gradient is the Young modulus.
What is a polymeric material?
A material consisting of long chain molecules that show large strains.
What is meant by the limit of proportionality?
The value of stress or force beyond which stress is no longer directly proportional to strain.
What is a yield point on a stress-strain graph?
A point beyond which the deformation is not entirely elastic.
What is the difference between ultimate tensile strength and breaking strength?
- UTS = the maximum strength a material can withstand.
- Breaking Strength = the value of strain experience at the point of the material breaking.
Which quantity can be determined from the area under a stress-strain curve?
The area determines the strain energy.
