Unit 3 - Failure Modes in Materials Flashcards
What is a Brittle Failure?
Seperation normal to the tensile stress. Outwardly no evidence of deformation, with x-ray diffraction it is possible to detect a thin layer of deformed metal at the fracture surface.
Brittle fractures occur in BCC and HCP metals, but not in FCC unless there are factors contributing to grain boundary movement.
What is Ductile Failure?
Single crystals of HCP metals may slip on successive basal planes until finally the crystal separates by shear.
Polycrystalline specimens may be drawn down to a point before they fracture, plastic deformation eventually produces a necked region. Fracture begins at the centre of the specimen and then extends by a shear separation along the dashed lines. Cup and Cone.
What is Transgranular, and inter granular?
Transgranular - Crack propagates through the grains
Intergranular - Crack propagares along the grain boundaries
What are defects real materials contain?
1- Crystallographic point or line defects
2- Grain and low angle boundaries
3- 2nd phase constituents (precipitates, toughness, ductility ect)
4- Processing defects (pores, inclusion, lack of weld fusion)
What are the fracture temperature regimes?
Low temperature regime - Plasticity not affected by time on load
High temperature regime - Additional time dependent creep effects and environmental (oxidisation) effects.
Brittle/Ductile behaviour is controlled by plasticity, but can occur in either low or high temperature regimes.
Draw the Diagram for failure classifications under monotonic load
PP3 - P9
Describe and draw the idealised ductile fracture
In a polycrystalline fracture - 100% reduction in area and slip occurs on several planes (reduced to a point)
In a single crystal failure - Failure on a single slip plane occurs
Draw a ductile stress/strain curve
P12
What happens in a single, and double cup and cone fracture
SCUC - Holes form due to decohesion
DCUC - Hole forms within a fractured particle
What are characteristics of brittle fractures?
Catastrophic, rapid event
Minimal or no plastic deformation preceding the failure ( no necking or ductility)
Brittle fractures can be either inter or transgranular.
What are transgranular brittle fractures accocisated with
Associated with Cleavage - low energy crystallographic planes
Common with BCC, HCP, Ionic Crystals, and covalent bonded materials
FCC only prone under extreme enviromentatl conditions
Draw a brittle fracture stress strain graph
P23
What is impact testing?
Toughness - Absorption of energy during fracture
2 standards IZOD or Charpy
Measure difference in potential energy of pendulum before and after fracture.
Qualitative - Used to rank materials
Results do not quantify the fracture toughness K1c.
Good to define ductile to brittle transitions in materials and temperature range for transitions.
Characteristics of Ductile to Brittle Transition?
High energy = Ductile Mechanism
Low Energy = Brittle Mechanism
