Lectures 5-6 Flashcards
What is used in the tensile test to measure extension? Why use this piece of equip?
Extensometer, measure extension accurately
Elastic region for the material means?
It can return to its original length, when the force is removed.
Plastic region for the material means?
Test piece cannot return to its original length
What does the elastic region look like on a force extension graph?
Straight line with constant gradient
What does the plastic region look like on a force extension graph?
Straight line ends and it starts to curve (excl. polymers as they do curve from the start)
On a stress strain graph, what is the gradient of the elastic region?
Young’s Modulus (modulus of elasticity)
What does a neck forming in the material look like on a force extension graph?
The highest point of the plastic region curve
σ eng =
Engineering Stress =
F / A (CSA)
Force / cross sectional area
ε eng =
Strain =
change in l / l
change in length (extension) / original length
Alternative units for stress
Pa (Nm-2)
On a stress strain graph, a material that fails in the elastic region means?
It is brittle and no plastic deformation occurs
On a stress strain graph, if the material has a long curve after it enters the plastic region, what does it say about its properties?
Ductile material
Brittle materials show no what before failure?
Plastic deformation
At a higher temperature what happens to the young’s modulus?
It decreases
E =
Young’s Modulus =
𝜎/ε
Stress/strain
Young’s modulus units
Nm-2 or Pa
What does the initial gradient of polymers look like on a stress strain graph?
Curved
Two ways to find the young’s modulus from a curved stress strain graph?
Secant and tangent modulus
How to use the secant modulus?
Origin to the point of stress that the material is under
How to use the tangent modulus?
Draw a tangent at the point of stress that the material is under
What tends to happen to a material which is compressed? And when it is stretched?
Compressed - spread wider/outwards
Stretched - contract in
Poisson’s ratio is represented with the letter…
v
Another word for limit of proportionality
Yielding
In the elastic region two equations for v=
- (ε x / ε z) or -(ε y / ε z)
Tensile strength is where on the graph of stress strain?
At the top
What is tensile strength also known as?
Ultimate tensile strength - UTS
Offset yield strength is?
When a metal does not have a clear point where the elastic and plastic region meet, so 0.02 away from the origin, a line is drawn parallel to the gradient at 0.02 strain.
The line crosses where the offset yield stress is.
When do dislocation slips occur in a metal?
When the metal starts to yield
In a polycrystalline metal, what happens more easily sometimes and why?
Slip can happen more easily as some of the crystals are oriented in the same direction as the shear force applied.
In a polycrystalline material, can a single crystal deform by itself and why?
No, because the neighbour crystals must deform as well
What happens to the crystals in a polycrystalline metal when it is put under a load?
Elongates the grains
What is true stress?
Shows the force divided by the actual CSA of the test piece at each stage.
2 reasons why its difficult to tensile test a ceramic
- hard to grip the specimen
- must ensure it is aligned perfectly, to avoid bending
Ceramics are —- in tension and —- in compression
Weak in tension, stronger in compression
Flexural strength is
ability for the material to withstand bending forces
Glass transition temperature, for polymers, is
the temp where the polymer turns from brittle glassy material to a more rubbery ductile material
Deformation in a polymer involves what sliding past eachother?
Molecules
Hardness is
Ability for a material to resist plastic deformation (surface indentation or scratching)
Four ways to test material hardness
- Brinell test
- Vickers test
- Rockwell test
- Knoop test
Two hardness tests used for polymers
- Rockwell
- Shore
Letters and units in the hardness indentation formula? F, D, d
F = load (force) in kgf
D = ball indenter diameter in mm
d = diameter of indentation in mm
Brittle materials exhibit a sudden what?
Fracture
Ceramic and glass materials are mostly brittle or ductile
brittle
Metals can become brittle after they have undergone what process and why?
Alloyed or heat treated which can restrict dislocation
Toughness is
Ability for the material to absorb energy during deformation and fracture
Toughness is represented where on a stress strain graph?
Area under the graph
Toughness can be considered as the —- required to break the material
Energy or work
Metals with high —- and —- are tough
Strength and ductility
What does toughness of a material depend on?
The speed at which the load is applied
Can a material have a higher toughness?
Yes, if the load is applied slowly
Two methods to test toughness.
- Charpy
- Izod
Both use a swinging pendulum
For many materials, increasing temperature leads to what? 5 points
Reduced;
- Young’s modulus
- Yield strength
- Tensile strength
- Hardness
- Brittleness (increased ductility)
What transition do materials show when going from a high to low temperature?
Ductile to brittle transition
Two steps in fracture
- Crack initiation
- Propagation
In ductile materials, crack propagation proceeds with significant what?
Plastic deformation, warning before failure
In brittle materials, crack propagation happens how?
Very little to no plastic deformation and the crack spreads quickly
Transgranular means
through crystal grains
Cleavage means
along specific crystal directions
Intergranular means
through grain boundaries
Two defects why most materials tensile strengths are lower than predicted theory
- Crystal defects - vacancies and dislocations
- Manufacturing defects - porous castings and surface scratches
Where would a crack be initiated in a material and why?
At the flaw/defect due to a locally increased stress
What are defects and flaws usually referred to as?
Stress raisers
Two example features which are stress raisers in a material?
- Sharp curves
- Notches
What is the alternative test to measure toughness and what does it consider?
Fracture toughness - measures the ability of a material CONTAINING flaws to withstand a an applied load
To obtain a plane strain fracture toughness of a material…
Do the tensile test, with a sample of the material with flaws of known size and shape
Few ways t reduce the spread of a crack?
- Ductile materials
- Thinner materials
- Slow application of loads (dislocations have time to slip)
- Increasing the temperature (dislocations can slip faster)
- Fewer defects
What feature needs to be considered when choosing materials for an engineering project?
The operating temperature
