Ch 9 Flashcards
What are the different types of failure?
simple fracture
ductile fracture: some plastic deformation: slow crack propagation
brittle fracture: no plastic deformation - fast crack propagation
fracture d/t crack propagation
flaws are stress concentrators
fracture toughness
fatigue failure
creep
Under what conditions/situations does each type occur?
What is the mechanism associated with each failure type?
What parameter is used to quantify a material’s resistance to fracture?
fracture toughness: measurement of material’s resistance to brittle fracture
What is the role of flaws in promoting brittle fracture?
applied tensile stress amplified at tips of flaws
material eventually fractures when stress at the tip of crack reaches theoretical strength
What measures may be taken to reduce the likelihood of each failure type?
What are the two types of fracture?
Ductile
Brittle
What are the differences between ductile and brittle fracture?
Ductile
-slow crack propagation
-significant plastic deformation
-tearing
-fails with warning
Brittle
-rapid crack propagation
-little or no plastic deformation
-snap deformation
-fails with no warning
-clean break, flat surface
What mechanical property does this correspond to? What does this tell us about energy?
Ductility and toughness (energy absorption)
What are the stages of moderately ductile failure?
- necking: a lot of dislocations are being made and repelling one another
- void nucleation: voids are created at the neck
- void growth and coalescence: the void grows in size and from many small voids, becomes one larger void
- crack propagation: a crack grows
- fracture
How do voids originate?
Sometimes as particles, serve as void nucleation sites. The bonding is different/wrong therefore weaker here
What are stress concentrators?
Voids
What size and shape of cracks/voids are more prone to be stress concentrators
long
sharp
Why does the length and shape of a void/crack matter?
length: the force that is applied to the crack is applied to the closest bond point.
shape: if that bond point is pointed, there is really on a few bond which all of that stress is focused on. if the shape is more rounded, there are more bonds over which to distribute the stress
Define fracture toughness
how resistant a material is to brittle fracture when a crack is present. how much energy can be absorbed by the material (before failure)
Discuss Metals, Ceramics, Polymers and their relative fracture toughness ranges
metals: HIGH fracture toughness
polymers: are ductile but require very little stress to crack
ceramics: LOW fracture toughness
Talk about brittle fracture of ductile materials
temperature affects fracture liklihood
titanic displayed brittle fracture even though it was made out of a ductile material because of the very cold atlantic waters
How else does temperature affect fracture?
Ductile to brittle transition temperature
BCC metals and polymers at low temperatures display brittle fracture
but at high temperatures display ductile fracture
define fatigue
failure under lengthy period of repeated stress or strain cycling
fatigue is responsible for 90 percent of mech. failure
Talk about stress v cycle plots and what they show. specifically talk about steel and aluminum
Steel: at high stresses, very few cycles can occur before failure. however, below a certain amount of stress, cycles can occur almost indefinitely
aluminum: more of a curved downward sloping line . eventually, aluminum will break
define fatigue limit
the stress at a certain number of cycles where the material fails
define fatigue life
total number of stress cycles; to caues fatigue failure at a specified stress amplitude
What are three ways to improve fatigue life
- reduce magnitude of stress
- surface treatment: put surface into a compressive state to push and cracks that do develop together
- remove stress concentrators (round corners)
define creep
deformation over time at a constant stress
