Materials Flashcards
Define density
Mass per unit volume
Density equation
P=m/v
Define Hookes law
The force applied is directly proportional to the extension up to the limit of proportionality
What evidence is on a force against extension graph when Hookes law is obeyed
Straight line
Through origin
Hookes law experiment
Measure original length of spring
Very load between 0 and 20N by attaching masses to the spring
Use at least 8 different masses covering that range
For each load measure extension produced with a ruler
Repeat measurements to spot and remove anomalies
Use set square to reduce parallax errors view at eye level when reading ruler
Plot force against extension graph
Springs in parallel
Force is shared equally
Add spring constants
Eg 3 springs:
K=3k
△L=1/3△L
Springs in series
Do not share force
Elastic potential energy equations
EPE=(1/2)F△L
EPE=(1/2)K(△L)^2
When would you use conservation of energy in an EPE problem?
Dynamic
Stress equation
Stress= F/A
A=cross sectional area
Stress units
Nm^-2
Pa
Strain equation
Strain= △L / L
Strain units
No units
Difference between Young’s modulus and sprint constant
Young’s modulus is a measure of the stiffness of a material
Spring constant is a measure of a particular sample
Young’s modulus equation
E= stress/strain
How to find Young’s modulus from a force against extension graph
F=K△L
K= F / △L
E=FL/A△L
E= K x L/A
Ductile material structure
Atoms can move around and reform bonds
This prevents cracks getting bigger and allows material to stretch
Brittle material structure
Giant rigid structures made up of very strong bonds
Brittle fracture
When stress applied tiny cracks focus the stress and get bigger
Until material breaks completely leaving a flat break
Examples of brittle materials
Glass
China
Examples of ductile materials
Metals
Rubber
Brittle vs ductile
A brittle material will break without warning and leave flat surfaces
A ductile material will deform and stretch slowly before breaking leaving a deformed surface
Define the limit of proportionality
The point beyond which the force applied is no longer directly proportional to the extension produced
What feature of a graph shows the material is elastic?
No permanent extension
Define tensile stress
Force per unit cross sectional area
Define tensile strain
Extension per unit length
Define elastic limit
The maximum force which can be applied without the wire being permanently deformed
Why would an engineer need to consider yield stress when building something
Above the yield stress the building would behave plastically
This means it will deform due to small forced applied
Define yield stress
The force per unit area
At which the material extends considerably
