Fracture Flashcards
(29 cards)
What is fracture defined as
Separation of a solid under stress into 2 or more parts
What are the 2 stages of fracture
Crack Formation and Crack propagation
What are the 2 types of fracture
Brittle fracture and ductile fracture
What type of fracture involves yielding
Ductile
What type of fracture has higher energy absorption
Ductile
Why is ductile fracture more desirable than brittle fracture
Because brittle fracture involves spontaneous catastrophic fast crack growth
How does ductile fracture occur
Specimen forms a neck and cavities form in the neck, The cavities grow and coalesce into a crack. The crack propagates towards the surface in a direction perpendicular to the stress. Fracture occurs leaking to a cup and cone interface
What type of fracture occurs more often
Brittle
What us generally the cause of ductile fracture
Overload of component due to poor design
What increases the likelihood that brittle fracture will occur
Presence of existing defects in the metal formed during manufacturing
Name the two ways a crack can propagate through a polycrystal structure
Transgranular or intergranular
What is transgranular crack propagation
When the crack moves through the interior of the grains causing repeated breaking of atomic bonds along cleavage planes
What is intergranular crack propagation
crack propagates along the grain boundaries
What method of crack propagation is more common, transgranular or intergranular
Transgranular
Describe the texture of the fracture surface for transgranular and intergranular crack propagation
Transgranular- smooth faceted texture
Intergranular- 3d
Under what conditions is a material most likely to experience brittle fracture
At low temperatures
Presence of large cracks
Under high strain rates
In a triaxial stress state
What is the ductile to brittle temperature affected by
Alloy composition
Heat treatment
Processing
Should a material in service should be used above or bellow the ductile to brittle temperature
Above
What are the assumptions made for linear elastic fracture mechanics
Perfectly sharp crack Tin plate of material Tensile stress far from and normal to the crack Material is perfectly linear elastic Fixed boundary displacements
What is the final strain energy of a crack a sum of
The initial energy + (-)Elastic energy reduction + Energy absorbed by crack
If the energy changes by dU, if dU<0 what does this mean
crack propagates spontaneously
What does it mean If the strain energy dU>0
Crack cannot propagate spontaneously
What does it mean If the strain energy dU=0
Unstable condition
What is the critical crack size
The size at which if reached the rack will grow spontaneously
