Materials Flashcards
What is Density
Mass per unit volume
What is the Density Equation
p = m/v
Density (kg m^-3) = Mass (kg) / Volume (m^3)
What does Hooke’s Law state
Force is directly proportional to extension up to the limit of proportionality
What is Directly Proportional
When two things increase by the same scale factor and go through the origin
What is Hooke’s Law Equation
F = k Δl
Force (N) = Spring Constant or Stiffness (N m-1) x Extension (m)
What is a Spring Constant or Force Constant
A measure of how hard it is bend or stretch a spring
Describe the force-extension graph
What does the area under a force-extension graph show
The elastic strain energy of the spring
What is Elastic Strain Energy
Energy stored inside a stretched or compressed material
What is Breaking Stress
The stress required to break a string
What is the Elastic Strain Energy Equation
E = ½ FΔl
Elastic Strain Energy (J) = ½ x Force (N) x Extension (m)
What is the Limit of Proportionality
When a spring no longer follows Hooke’s Law meaning the force is not directly proportional to extension up to the limit of proportionality
What is the Elastic Limit
Just after the limit of proportionality were if you increase the load applied beyond this, the material will deform plastically
What is Elastic Deformation
When a material returns to its original shape when the load is removed
What is Plastic Deformation
When a material is permanently deformed and does not return to its original shape after the load is removed
Describe Brittle and Plastic Materials
Brittle - when a material will extend very little, and therefore is likely to fracture (break apart) at a low extension
Plastic - when a material will experience a large amount of extension as the load is increased, especially beyond the elastic limit
Describe the Force-Extension Graph for Brittle and Plastic Material
Describe the force-extension graph for a streched material and explain why are the loading and unloading lines parallel
Because the material’s stiffness/ spring constant is constant
Explain the Energy Behind Elastic and Plastic Materials
- When a stretch is elastic, all the work done is 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 given off as heat
Describe the Energy within a Spring when it is Stretched Vertically
- When a spring is hung vertically and stretched, kinetic energy is converted into elastic strain energy
- If it the force is removed, the elastic strain energy will be transferred back to kinetic energy (Conservation of energy)
- This kinetic energy is then transferred to gravitational potential energy as it rises
What are some of the energy conservation issues for transport design
- Crash zones are desinged to plastically deform to decrease the car’s kinetic energy
- Seat belts stretch in order to convert some of the passenger’s kinetic energy into elastic strain energy
What is Tensile Stress
Force per unit area of a material
What is Tensile Strain
The ratio of extension to original length
What is the Tensile Stress Equation
Stress = F/ A
Stress (Pa or N m-2) = Force (N) / Area (m2)
What is the Tensile Strain Equation
Strain = Δl / l
Strain = Extension(m) / Length(m)
What is the Young Modulus Equation
Young Modulus = (F/a) / (Δl/l)
Young Modulus (Pa or N m -2) = Stress (Pa or N m-2) / Strain
Why do we use Young Modulus
So that the value will be the same regardless of the dimensions of the material being tested
Describe the Young’s Modulus Graph and all of it’s components
What is UTS (Ultimate Tensile Strength)
The max stress experienced before breaking
Describe the Young’s Modulus Practical
- Set up the apparatus as shown in the diagram
- Measure the initial length of the test wire with the metre ruler
- Add a 1kg mass holder to both wires so they are taut and record the initial scale reading
- Add an additional 1kg mass to the test wire and record the new scale reading. Find the extension by subtracting the initial scale reading from the new scale reading and record it
- Add another 1kg mass and repeat this, adding 1kg each time up to around 8kg
- Repeat the experiment twice more and find and record the mean extension for each
- Measure the diameter of the test wire at various points along it using the micrometer and find and record the mean diameter
- Calculate the cross sectional area of the wire
- Find the force for each mass by using w = mg
- Plot a graph of Force against Extension and the young’s modulus will be the original length multiplied by the gradient and divided by the cross sectional area
Explain why we use a comparison wire in the young’s modulus practical
- The comparison wire provides a reference point against which to measure the extension of the wire
- The comparison wire will be kept taut so by either using a spirit level and micrometer or a sliding vernier scale you can measure the extension more accurately
- The readings also made more accurate by making the original length of the test wire as long as possible, this reduces uncertainty in its measurement
