Chapter 8 - Failure of Materials Flashcards
In this course we will focus solely on failure of?
Metals.
Define: Toughness.
The amount of energy absorbed by plastic deformation before failure.
What are the two major types of fracture?
- Ductile.
2. Brittle.
What is the key feature of a ductile fracture?
Lots of plastic deformation.
What is the key feature of a brittle fracture?
Flat fracture surface. No plastic deformation.
In a ductile fracture what must be continuously supplied for the crack to propagate?
Energy. Because the material plastically deforms all the way up to failure, energy must be continually supplied or the deformation stops. This is referred to as stable crack growth.
What are the different kinds of loading that materials can be put under?
Tension, Torsion, Shear, Bending, Compression. In real life situations multiple can occur simultaneously and through multiple directions.
What is the first stage of tensile fracture of a ductile material?
In the necked region the complex nature of the stress field results in microvoid formation around microscopic particles like oxide inclusions or at grain boundaries.
What is the second stage of tensile fracture of a ductile material?
The microvoids formed in stage one begin to coalesce to form a large central void which then begins to propagate outwards.
What is the third stage of tensile fracture of a ductile material?
When the advancing crack nears the surface final failure occurs along ~45* shear planes to give the typical “cup-and-cone” fracture.
At what point on a stress strain graph does the first stage of tensile fracture of a ductile material occur?
At the UTS (Ultimate Tensile Strength).
At what point on a stress strain graph does the third stage of tensile fracture of a ductile material occur?
At the point of fracture (the end).
What shape does the microvoids in the second stage of tensile fracture of a ductile material form?
An elipse.
Why is the surface of a ductile fracture rough?
In ductile failure each grain undergoes its own microscopic form of failure by extreme plastic deformation and micro-necking (Many small “cups-and-cones”.
How much energy is required to fracture a completely brittle material?
The amount of energy required is however much is needed to create the two new fracture surfaces by breaking bonds.
Why is brittle fracture so dangerous?
Because there is no plastic deformation at all there is no warning of failure.
What is brittle fracture often associated with?
Pre-existing cracks and flaws.
Why does brittle fracture sometimes occur below the material’s yield point?
Local stresses at stress concentrating cracks or flaws may exceed the engineering fracture stress of the material.
Why are brittle fracture surfaces planar/flat/featureless?
This may be due to the crack propagating by the separation of atoms along crystal planes (called cleavage) or along grain boundaries.
True/False: Materials that have undergone brittle fracture can fit together as a matched pair.
True. There is no plastic deformation so the surfaces will fit together.
What situations favor brittle fracture?
- Low temperatures.
- Sudden loading.
- The complex stress state at the tip of cracks, sharp notches, and flaws.
- It may occur in a ductile metal where there is a brittle phase near the grain boundaries of the ductile phase.
How does temperature affect whether a fracture will be brittle or ductile?
At low temperatures, low atomic movement, makes it hard for dislocations to move. No dislocations moving means no plastic deformation, therefore brittle fracture.
Low temps - Brittle.
High temps - Ductile.
How does the speed of loading affect whether a fracture will be brittle or ductile?
If a load is applied too quickly the material will not get the chance to plastically deform. It is therefore more likely to undergo brittle fracture.
Slow loading - Ductile.
Fast loading - Brittle.
True/False: Materials can only fail through either brittle or ductile failure.
False. There is a spectrum of failure between ductile and brittle. Failure can occur in ways that include both some brittle and some ductile failure.
Why is the area under a stress-strain graph only able to give us an approximate indication of toughness and not a reliable indicator of toughness?
Because the information gained through a test is only able to provide information on how a material will react under the same conditions as the test. If the test conditions change at all you can get different outcomes.
What is an impact test?
A hammer attached to a pendulum is allowed to swing down and impact with a sample. The difference in height between the hammers starting position and the position of the hammer at the top of the swing through will be the energy absorbed by the fracture.
In order to compare the toughness of two different materials using an impact test the two specimens must have…?
Identical dimensions and Identical notch geometry.
Why does the specimen in an impact test have a notch built into it?
The purpose of the notch is to create a site on the specimen of high stress concentration. This increases the likelihood of brittle fracture to occur. Therefore if a material shows high toughness/ductility during this test it is likely that it will fare well in real world scenarios.
Why does a notch create a stress concentration?
The stress being transferred from one end of the specimen to the other must move around the notch and begins to “bunch-up” at the edge of the notch.
What is triaxial stress?
The increases stress at the point of the notch makes the metal there want to plastically deform. But the material surrounding it won’t allow it to. This causes transverse tensile stresses perpendicular to the axial stress. This means there is stress in three directions.
Why does triaxial stress increase the likelihood of brittle fracture occuring?
The transverse stresses means that more stress must be applied before the material at the edge of the notch will deform, (Increases the yield stress). Increased yield stress makes the material less ductile and more brittle.
BCC Metals are ____ at low temperatures and _____ at high temperatures.
Brittle at low temperatures, Ductile at higher temperatures.
What is the temperature above which a metal starts to exhibit high ductility?
The ductile-brittle transition temperature Tt.
True/False: FCC metals have a well-defined ductile-brittle transition temperature (Tt).
False. The close-packed slip planes and directions means that dislocation movement is not as reliant on thermal vibration of the atoms. Therefore there is no well-defined Tt.
Polymers also have a ductile-brittle transition temperature (Tt), what is it called?’
The glass transition temperature, (Tg) performs the same function.
How is the ductile-brittle transition temperature (Tt) determined?
Using an impact test. If we plot a graph of the impact energy for fracture against the temperature of the material, for BCC metals there is usually a clear change in ductility over a small range of temperature.
Why is knowledge of the ductile-brittle transition temperature (Tt) so important?
To avoid sudden (brittle) fracture during use a material must have a Tt above it’s minimum operating temperature.
How does the inclusion of carbon change the Tt of steel?
With increasing carbon % the Tt increases, and the maximum ductility will reduce.
How does the inclusion of Manganese change the Tt of steel?
With increasing manganese % the Tt decreases, and the maximum ductility will increase.
What effect does the presence of impurities have on the Tt of steel?
Impurities act as stress raisers and therefore decrease ductility and increases Tt.
What effect does grain size have on the Tt of steel?
Smaller grains mean that cracks travel a smaller distance before encountering a change in the material slowing the crack. This means smaller grains increase ductility and reduce the Tt. Smaller grains also increases strength!!! SMALL GRAINS GOOD!!!
What effect does the rate of application of stress have on the Tt?
Dislocations need time to move and generate plastic deformation. Faster loading increases Tt and decreases ductility.
What effect will the geometry of a crack or flaw have on the Tt?
A sharp crack will have a high stress concentration, making the material more brittle and increase Tt. A blunt crack will have a lower stress concentration, making it less brittle and lowering the Tt. No crack/flaw is best though.
