3.4 Materials Flashcards
Define elasticity
The property of a body to resume its original shape or size once the deforming force or stress has been removed
Define plastic deformation
Material does not return to original length when force is removed
Why don’t plastic materials return to their shapes?
Has exceeded its elastic limit
What is A on a force-extension graph?
Limit of proportionality
What is B on a force-extension graph?
Elastic limit
What is C on a force-extension graph?
Plastic behaviour
What is D on a force-extension graph?
Fracture
What is elastic limit?
Beyond this point it will not show elastic behaviour but plastic
What is the limit of proportionality?
The point at which extension is no longer directly proportional to the force
Material behave elastically still
Stops obeying Hookes law
What is fracture?
When the material breaks
Before this happens the material undergoes creep
What type of graph is used to show the difference between plastic and elastic behaviour?
Force against extension
What is Hookes law?
States that extension of an object is proportional to the force that caused it, provided the elastic limit is not exceeded
What is the equation for Hooke’s law?
F=kx
What is the unit of the force constant?
Nm-1
When do you stop using the force constant?
When deformation becomes plastic
Why do we unconventionally plot force on the y axis and extension on the x axis?
Because then the gradient of the line is equal to the force constant
What does the work required to stretch a wire depend on?
Stretching force and extension
How do you find work done from a graph?
Area underneath
Define stress
Force per unit cross sectional area.
Nm-2 or Pa
Define strain
Extension per unit length
No units and is dimensionless
What is young modulus?
Stress/strain
Or Fl/eA
What type of young modulus do stuff materials have?
Large
What are the two ways to calculate you gs modulus?
Gradient of stress/strain graph
Calculate value directly from equation
What is ductile?
Can be drawn into wires and will show plastic deformation under tensile stress before breaking
What is malleable?
Can be hammered or beaten into flat sheets and will show extensive plastic deformation when subjected to compressive forces
What is brittle?
Break with little or no plastic deformation
What is a hard material?
One that will resist plastic deformation by surface indentation
What is stiffness
The ability to resist tensile force
What is a polymeric material?
Made of long chains of polymers and can withstand large strains
What is the ultimate tensile strength?
The maximum stress it can withstand while being pulled or stretched, before it breaks
Precision of micrometer
Micrometer to the nearest 0.01mm
What’s important when plotting the graph?
Through origin
Lobf
How to reduce error in Young’s modulus experiment?
0 error on micrometer or ruler
Remove by adding or subtracting error
Typical Young’s modulus value of a metal?
50-100 *10^9 Pa
What is the unit of Young’s modulus?
Pa
What does a stress strain graph for brittle materials look like?
Doesn’t curve
What does the area under a polymeric material curve?
Amount of energy converted to heat per unit volume
Why are the losing and unloading curves for rubber different?
Energy released when the rubber is unloaded is less than work done to stretch it as some is converted to heat
