L5: Tensile Testing And Material Deformation and L6: hardness, toughness and fracture Flashcards
What is tensile testing designed for?
What is the sample geometry designed for?
Tensile testing- test tensile strength, yield strength, Ym, ductility of materials
Sample geometry- bulletin strength of material tested without regions of high/ non uniform stress being generated
Describe the curve generated from a tensile test
Force- extension curve
Elastic region- peice extends but recovers original length when force removed
Plastic region- deforms and does not recover when tension released- neck forms, then curve ends at failure (snapping)
Why is a stress- strain curve better than a force- extension one?
Removes effect of test price dimensions, makes values a material property
How is the Ym found from a stress- strain graph?
What is it also called?
How does it change with temp?
In the elastic region of the graph- it’s the gradient.
Aka modulus of elasticity/ stiffness
Higher temp, Ym decreases as less energy is required to stretch bonds
What are the 2 methods of determining Ym of materials with no linear elastic region- eg polymers
Secant modulus
Tangent modulus
What is poisson’s ratio?
What are the equations if the material is isotropic?
What is a rough value of the ratio for most metals?
The negative ratio of lateral to axial strain
v= -Ex /Ez
v=- Ey/Ez
Usually- 0.25- 0.35
What is the transition from elastic to plastic deformation called?
How is it and tensile strength determined on a stress- strain graph?
Limit of proportionality/ yielding
Yielding is where the graph starts becoming a curve (stress here is yield stress)
Tensile stress is the maximum of the curve (where neck forms)
What materials display a yield point?
Where do plastic deformation and yield strength occur?
Low C steels especially
Plastic deformation at the upper yield point
Yield strength is the level of the lower yield point
When does dislocation begin?
Which dislocations are the first to slip?
When the metal starts to yield
Those that are aligned in the correct direction- ie with the shear stress
When does yielding occur?
What helps slips to happen more easily?
When the load is large enough to cause dislocations within crystals to slip
Helped by close packing of atoms
Most easily achieved for slip planes at 45degrees to the applied load
Describe yielding in polycrystalline materials?
What does the structure look like after deformation?
Some crystals slip more easily than others as they’re randomly orientated
Grains are restricted in shape by their neighbours
A grain cannot deform until it’s neighbour lay can deform so F must be big enough to cause less favoured slip systems to deform.
After- grains tend to elongate in the direction of applied stress
Describe the properties of ceramics and the strength test which is best suited to them
Very brittle (little/no plastic deformation)
-dislocation slip almost impossible without fracture
- good in compression, weak in tension
- hard to grip for tensile test (must ensure test piece aligned to avoid bending)
Best to use bend test- places bottom of each end under tension- determines flexural strength
What is the glass transition temp Of a thermoplastic polymer
Where it changes from brittle glossy material to a more robbery ductile material
Defamation involves molecule sliding past each other (not dislocation slip)
What are the four main methods of hardness testing
Brinell
Vickers
Knoop
Rockwell
What do you all methods of hardness testing rely on and what makes it difficult to compare them?
All tests rely on pushing a hard object e.g. diamond (vickers) into the material underknown load and measuring size or depth of the indentation
The type and shape of the hard object used varies with each test method
Define hardness
The ability of a material to resist localised plastic deformation (surface indentation or scratching)
Give advantages and disadvantages for hardness testing
It is a convenient and largely nondestructive test method to determine if heat treatment or process has been successful
Hardnessnot a material property and its values very depending on the test method used to measure at different test methods and indent materials are used for evaluating different materials
Give some examples of hard materials
Many ceramics e.g. WC, diamond, Al2O3
Some alloyed and heat treated steels
What types of metals can become brittle and what are the properties of a brittle material
Metals that have become strained hard and all contain impurities that restrict this location movement can become brittle
High hardness values but poor tensile strength
Define toughness
The ability of a material to absorb energy during defamation and fracture an indication of how easy it is to break it
How was toughness determined from a graph and what materials have high toughness
The energy required to break a material as given by the area under the true stress vs true strain graph
Metals with high strength and ductility are tough brittle materials have low toughness.
The toughness of the material depends on the way in which the load is applied- If the load is applied slowly and increases steadily material so increased toughness
What is impact testing used for
To investigate the toughness of materials under sudden loading - materials show reduced toughness
What are the two main methods of impact testing and what do they involve
Charpy
Izod
Both methods used are swinging pendulum to measure the amount of energy absorbed by the test piece during fracture
What affect in general does change in temperature have on metals and polymers
Increasing temperature leads to reduced Young’s modulus reduced yield strength and higher ductility in many metals and polymers a transition from ductile to brittle behaviour can occur at low temperature
What happens to Bcc metals at low temperature and FCC metals
BCC metals become brittle due to the lower rate of dislocation slip however FCC metals tend to remain tough
What are the two steps of the fracture process
Crack initiation and propagation
What is the difference between ductile materials fracturing and brittle materials fracturing
Ductile- Crack propagation proceeds with significant plastic deformation in front of the crack- Get some warning before failure
Brittle- Little deformation and the crack can spread rapidly- Very little or no warning before failure
Why do you cracks appear to tensile strength lower than predicted by theory
Due to microscopic flaws or cracks present in the material as a result of
-crystal defects- Vacancies and dislocations
-Manufacturing defects- porous castings, surface scratchings
What are stressed raisers
Defects and flaws
What areas in courage cracks due to bad design
Areas of the material experiencing high stresses such as sharp curves or notches made in the material
Define fracture toughness
A material property that measures the ability of a material containing a flaw to with stand and applied load
How can I designer reduce the spread of a crack
- Small floors in the material
- Ductile materials
- thinner materials are less rigid
- Low rate of load application dislocations have time to slip
- Increasing temperature dislocations can move rapidly
- Small grain size fewer defects
What does fracture mechanics enable designers to do
Allow for defects in manufactured products
