Materials Flashcards
Hooke’s law
Force is directly proportional to extension until limit of proportionality
Once elastic limit is reached H law is not obeyed.
Force constant
Measure of stiffness only use within elastic limits
Force needed to achieve given extension
Beyond Hooked law
Plastic deformation- permanent deformation / permanent extension
Rubber and meaning of curve
No elastic deformation or Hooke’s law
Area between loading and unloading curves (hysteresis loop) is heat energy and work done
(Area underunder is Ee and can be returned
Total area is total energy store)
YM
Ratio of stress to strain for given material
Measure of stiffness/resistance to deformation
Depends only on material and independent of shape/size
UTS / ultimate tensile strength
Max stress and object can withstand b4 it breaks
Brittle
Shows very little strain before reaching breaking point/stress
No plastic deformation
Loading and unloading curve is same
Ductile
Can undergo large extension without breaking
Can be stretched into wires
Has elastic deformation until elastic limit
Strain
Ration of extension to original length
Stress
Amount of force per unit area
Tensile deformation
Changing an objects shape due to tensile forces
Tension and compression
T causes object to lengthen, acts away from centre
C causes object to shorten, acts toward centre
Polyethene
No hooke’s law but has plastic deformation
So easy to stretch into new shapes
Elastic limit / limit of proportionality
E limit
Furthest amount it can be deformed before it won’t return to original shape
Limit proportionality b4 E limit
Doesn’t obey Hooke’s law
Why is rubber good for plane landing
• Elastic because when load is removed it goes back to original shape
• Area under graph is work done
• Repeated stretching and releasing warms it up
~ because not all stairs energy returned back
• During landing, Ek transferred to thermal energy
~ so it doesn’t
