Materials Flashcards
What is deformation? What is required to deform an object?
Deformation is a change in an objects shape or size. At least two forces are required to deform an object (usually equal and opposite forces)
What are the two types of deformation that you have to know? What do these forces do to an object?
Tensile deformation and compressive deformation. Tensile forces act away from the centre of an object and cause extension. Compressive forces act towards the centre of an object and cause compression.
State the definition and formula for Hooke’s law as well as the units for the formula.
For a material within the elastic limit, the extension, x, of an object is proportional to the load of force applied, f. Beyond the elastic limit, this law no longer applies.
F=kx
F= Newtons, N
x = Metres, m
k = Newtons per metre, Nm^-1
What do elastic and plastic deformation mean?
Elastic deformation means that an object that is stretched by a force will return to its original shape when the load is removed. This object obeys Hooke’s law.
Plastic deformation means that an object with a load applied to it will NOT return to its original shape once the load is removed. The material has lots it’s elasticity and is permanently stretched, no longer following Hooke’s law. Stress&strain/Force&extension are NO LONGER proportional.
What is stress? Give the formula and the units
-Force per unit area
Sig. = F/A - Pa (Pascals)
F = force - kgms^-2
A = area - m^2
What is strain? Give the formula and units
Ratio of extension to original length
E = x/L - No units (as units cancel out)
What does it mean when a spring constant is larger?
A larger spring constant means a stiffer spring I.e. spring is more stiff
How do you calculate spring constant in series?
In series = springs hanging off the ends of each other
1/k = 1/k1 + 1/k2…
Reciprocal of combined force constants
How do you calculate spring constant inn parallel?
Parallel = hanging side by side
K = k1 + k2…
-Force is shared between springs
What is the young modulus? Give formula and units
-Ratio of stress/strain an object experiences when being stretched
Y = Stress/Strain
Y = FL/Ax
What is the definition of ultimate tensile strength? What does having a higher ultimate tensile strength say about a material?
The maximum amount of stress an object can withstand before breaking.
A higher UTS means a material is tougher/stronger
Describe an experiment to calculate the young modulus
- Secure a test wire using a g clamp and keep it taut by using a pulley with weights
- Add a marker on the wire and place it above a ruler with mm markings at 0.
- Measure the diameter d of the wire using a micrometer across multiple points. Calculate an average from these values. Calculate the area A using the formula pi x (d/2)^2
- Measure the original length L of the wire.
-Add weights to the end of the wire and note down the weights added and extension x. - Calculate the force F from the weights added by using W=mg.
-Calculate separately stress and strain.
-Plot a stress/strain graph. The point at which the graph produces a straight line is the young modulus. Calculate the young modulus by finding the gradient of the graph.
-Area under the graph is energy stored per unit volume
Describe a stress/strain graph for a ductile material.
-Obeys Hooke’s law before the limit of proportionality. Behaves elastically and will return to original shape once load is removed.
-Beyond the elastic limit, will behave plastically. Will not return to original shape once the load is removed.
-At the yield point, the material will stretch without any extra load.
Describe a stress/strain graph for a brittle material.
- Behaves Hooke’s law until a point where the line stops abruptly. This means it deforms elastically.
-Once stress reaches a certain point, the material will snap without deforming plastically.
