Week 2: Ductility, Brittleness, Hardness & Factor of Safety Flashcards
Yield / Yield Point
Where the behaviour of the material changes from elastic to plastic.
This is indicated on the graph as going from a linear relationship to non-linear. (Point P)
Point of no return
We can find this value by using the 0.2% offset rule
Yield Stress
The measure of resistance to plastic deformation.
Yield Strength
The stress needed in order to reach the yield point.
The value is obtained by the 0.2% (0.002) offset rule and tells us how much the material can safely undergo plastic deformation.
The yield strength and yield stress are the same, but one is a property of the material and the other is a stress induced.
Ductile Materials
Ductile materials are those which undergo plastic deformation before fracture.
Eg: think of a redskin
This is because of their lower stiffness (Young’s Modulus, E)
Brittle Materials
Brittle materials are those which break without strain but rather under stress. This means they don’t experience much plastic deformation before failure occurs.
Eg: diamonds
This is because of their higher stiffness (Young’s Modulus, E)
How do E, yield and tensile strength change in lower temperatures?
E will increase as the material will become stiffer.
Yield and tensile strengths will decrease so the material will fail earlier than usual.
Less plastic deformation can occur.
How do E, yield and tensile strength change in higher temperatures?
E will decrease as the material will become less stiff.
Yield and tensile strengths will increase so the material will fail later than usual.
More plastic deformation can occur.
Hardness
A material’s behaviour to resist localised plastic deformation.
(scratch test)
Proportional Limit
The region in a stress strain curve where the curve is linear. The proportional limit is the stress at which this behaviour stops.
Design Safety Factor
A factor to cover the risk of variability between materials.
