Material Flashcards
Elasticity
The property of a body to return to its original shape once the deforming force is removed
Deformation
The change in shape or size of an object
Elastic Deformation
An object that deforms that will return to its original shape once the deforming force is removed
Plastic Deformation
When an object deforms and then when the deforming force is removed the object remains deformed
Tensile force
Creates tension in an object and will result in an extension
Compressive force
A force that causes an object to compress
What a force extension graph will look like for a metal
Will start of straight. Then the limit of proportionality is reached and the gradient will start to change. Then the elastic limit is reached and it will extend more per unit force (gradient decreases) until it reaches the point of fracture
Experiment to observe force-extension of a wire
Clamp a wire on one end of a table and attach a weight on the other end. The weight should dangle off the table and the wire should be placed over a pulley. Attach a marker to the wire and fix a ruler in position. Add more mass to the weight and record how much the wire extends
Hookes Law
The extension of an object is proportional to the force that causes it, provided that the elastic limit is not exceeded
Force constant of a spring
The force required to extend the spring by one unit length
Hookes law in equation form
F=kx
Area under a force-extension graph
Work done to stretch the object
What happens to the work done put into stretching a wire that exceeds its elastic limit
- At first the work done is causing an increase in elastic potential energy
- Once the material has exceeded its elastic limit the rest of the energy that is input will be converted to heat
Stress
Force per unit cross sectional area
Strain
Extension per unit length
Ultimate tensile strength
The maximum stress that an object can endure before breaking
Equation for youngs modulus
Tensile stress / Tensile strain
Young Modulus
A measure of the stiffness of a material
The overall force constant of two springs in series
1/k = 1/(k1) + 1/(k2)
How to work out the ultimate tensile strength from a stress-strain graph
The highest point on the graph
How to work out the ultimate tensile strength from a stress-strain graph
The highest point on the graph
Experiment to determine Young’s Modulus of a wire
Clamp one end of a wire and attach a weight to the other end. Run the wire over a pulley and mark a point on the wire. Measure the distance from the clamp to the marking. Measure the diameter of the wire with a micrometer and calculate its cross sectional area. Increase mass on the weight and at each step record the extension of the wire. Plot the stress against strain and the YM will be the gradient
Stress-Strain graph of a ductile material
At first it is straight and obeys Hooke’s Law.
Then the limit of proportionality is reached and the gradient starts to decrease.
The elastic limit is reached and the gradient continues to decrease.
The yield point is reached and the gradient goes negative for a short while before gradually increasing
Elastic limit
The maximum stress an object can endure while still behaving elastically. After this point it will not return to its original shape
Stress-Strain graph for a brittle material
Straight line then breaks
Stress-Strain graph for a polymeric material
Look in the book its hard to explain
What the area under (or within) a stress strain graph is
The energy per unit volume
