Mechanical Properties Flashcards
What is the equation for stress
Stress = force / area
What is the equation for strain
Strain = change in length / original length
Why do we use stress and strain instead of force and extension
Force and extension look at the dimensions of the sample, where as stress and strain look at the material itself
Draw a ductile stress strain graph
Look at example
Draw a brittle stress strain graph
Look at example
Where is the yield point/strength
Where the straight line begins to curve
Draw a proof stress graph
Look at example
Name the 3 primary bonds
Ionic
Metallic
Covalent
Name a secondary bond
Van der waals
Hydrogen bonding
What is the definition of crystaline?
Atoms that have long range order
What causes plastic deformation
Slip
What allows for slip to occur more easily
Dislocations
What is ductility
Amount of permanent/plastic deformation up till fracture
What unit is stress measured in
Pa
What do u multiple by to get M before a unit
X10^6
What do u multiple by to get G before a unit
X10^9
What section of the graph does stiffness relate to
Elastic region
What is the Young’s modulus equation
E = stress/strain
What are the units of Young’s modulus
Pa
What is the equation for true stress
True stress = load force/instantaneous area
Draw a graph comparing true stress and engineering stress
See example
What equation is preferred when measuring ductility
%AR = (original area - final area)/ original area x100
What is the term used to describe a material with no long range crystalline order
Amorphous
What is the definition of hardness?
How easily a material can be scratched or dented
What is a hardness test? (Steps)
Small indented forced into the surface of a material - size of dent created used to quantify hardness
What does a big intent mean in terms of hardness for a material
Big indent means the material is soft
Why are hardness tests commonly used
Simple, relatively inexpensive, non-destructive, used to infer other mechanical data
What does a hardness test indicate
Resistance to plastic deformation
Name a hardness test
Brinell hardness
Vickers and knoop
What is variability (2)
Material properties are not exact quantities
Even with the same testing machine, variability of results will be achieved using samples cut from the same block of material
What is impact toughness
The ability of a material to absorb kinetic energy up to fracture
What are the four factors affecting toughness
- the material
- geometry (notches make it more brittle)
- rate of loading
- temperature
Conditions of an impact test
Chosen to represent the most servers situation
- low temp
- high strain rate
- presence of a crack
Impact toughness - affects of temperature
A material that is tough at room temp can become brittle at lower temps
How is impact toughness tested (5)
- sample has a notch in it
- weighted pendulum of known mass is released
- hits the sample (will now have known amount of kinetic energy)
- samples breaks which requires energy
- the more energy used the break the sample, lower the pendulum follow through
What is a dipole
An uneven charge distribution
What is poissons ratio
Relationship between how much material contracts in one orientation, while being stretched in the other orientation
How to account for natural variability of mechanical properties in the real world
Use a safety factor or design factor in the designs
How to account for variability of mechanical properties during lab testing
Test multiple identical samples, get an average and standard deviation
What occurs during a bending test
Max compression on top surface
Max tension on bottom surface
What is the neutral axis
Where neither compression or tension happens
Why is %AR the preferred method
Gives more consistent/reliable results compared to %EL
Why are FCC metals packed so closely together?
They are malleable, so it’s easy to change their shape
Because dislocations find it easy to travel in closely packed planes
What happens to bonds during elastic deformation
Bonds stretch but no not break
What are dislocations?
Imperfections in the crystal structure
How do dislocations allow for easier plastic deformation
Fewer bonds need to be broken at once to achieve slip
What is the UTS
The maximum stress before the material fails
How does changing the length of a cable affect its maximum load
Changing the length would no affect the loading capacity
Load relates to stress (stress=force/area)
Length would affect the strain (strain=change in length/original length)
What is engineering stress?
Defined as force/original area of the sample
How will a notch affect a materials toughness
The crack will act as a stress concentrator
The material will show brittle behaviour in the presence of any defect
Why r I beams used?
- beam theory
- compression and tension
- material close to the middle is a waste
Draw and label a SN curve for a ferrous metal and non-ferrous metal
Draw
