Week 3 Flashcards
(30 cards)
Why do many disasters happen due to brittle failure?
Brittle failure can happen quickly and propagate without warning so many disasters happen due to brittle failure.
What is the process of ductile failure?
- Necking
- Void nucleation
- Coalescence of cavities
- Crack propagation
- Fracture
How can slip result in ductile failure?
A material containing dislocations and precipitates is put under stress, causing dislocations to move to obstacles and are pined. These dislocations build up and coalesce to form voids which join up to form microcracks. These microcracks then join up to form a large crack and the material fails.
What is true stress?
Stress that uses the actual area in the necked down region which continues to rise to the point of fracture in contrast to the engineering stress.
What does the Tresca criterion assume?
That when a material fails, it does so at the maximum shear stress.
What is volumetric strain?
The change in volume of an object without any change in shape due to shear.
What is deviatoric strain?
The distortion and twisting of the material due to shear.
What is the Von Mises criterion?
It builds on the Tresca criterion to take into account that normal stresses do not contribute to failure.
When is the Tresca criterion a good estimate of yield stress?
In orientations when the maximum yield stress is equal or higher than the yield stress.
What is intergranular failure?
Failure that occurs down the grain boundary usually because the GB is weakened by chemical attack or elemental depletion. This is more common for ductile materials.
What is transgranular failure?
The result of cleavage of particular atomic planes, ignoring the plane structure. This is more common in brittle materials.
What is toughness?
The resistance of a material to being broken in two by a crack running across it. This is fracture and absorbs energy.
What does the amount of energy absorbed during facture depend on?
The size of the component which is broken in two.
What does a tough material require to break it?
A lot of energy, usually because the fracture process causes a lot of plastic deformation.
What is the Charpy impact test?
A toughness test in which a hammer pendulum is notched at a height above the sample. By comparing the height of the pendulum before and after hitting the sample, the energy required to break it can be estimated from he change in potential energy.
What does fracture toughness measure?
A material’s resistance to brittle fracture if a flaw is present. This could include cracks, voids, inclusions, weld defects or design discontinuities.
What did Griffith’s glass experiment show?
The smaller the fibre diameter, the higher the tensile strength and if a fibre was acid-etched, the strength would go up. These both reduce the number of surface defects per unit length. He showed that the material strength gets closer to its theoretical value as a glass fibre gets smaller.
What is the Griffith criteria?
A criteria for brittle fracture based on the release of elastic strain energy during a crack
What are Prince Albert drops?
These are tadpole-shaped glass droplets formed wen molten glass is dropped into cold water. The rounded front of the drop is very tough but the thin tail is extremely delicate and if touched causes the whole thing to explode.
What went wrong with the de Havilland Comet?
This jet suffered failure of the airframe from dislocation induced slip bands nucleating cracks. Crystalline defects lead to depressurisation and crashes of 3 planes. The cracks started in rivet holes around the windows.
What went wrong with Aloha Airlines 243?
In 1988, a B737 had the roof tear off in flight due to stress corrosion cracking around the rivets allowing multiple small cracks linking up into a large crack.
What is stress corrosion?
A crack started from a corroded region can propagate through a material under stress. The crack continues to corrode into the material growing and pushing the crack apart.
What is intergranular corrosion?
The macrostructure of the grain boundary is more susceptible to attack and so the corrosion spreads along the edges of the grain.
What are ductile to brittle transition temperatures?
Ductile materials can becomes susceptible to brittle failure at low temperatures.
