Fracture Flashcards
Contrast a ductile fracture and a brittle fracture
A ductile fracture has:
- > significant plastic deformation in the vicinity of the crack
- > slow crack propagation
- > cracks often require more greater force to propagate
- > one to two pieces
A brittle fracture has:
- > little to no plastic deformation
- > cracks which are unstable
- > rapid crack propagation
- > many pieces (consider PVC vs lead)
Name the steps in the moderately ductile fracture process:
necking -> void nucleation -> void growth -> crack propagation -> fracture
What happens in necking
Necking is the forming of geometrical instability at point M in the stress strain curve (TS)
Due to tensile strength force equation, as the area decreases, the stress on the area increases
What happens at void nucleation
Continued by stress, a lot of dislocations will pile up
The sliding motion of dislocations will create vacancies, which eventually cluster to become noticeable voids
For a sharp crack, compare and contrast why sharp cracks have higher stress than not as sharp cracks. Also what do flaws act as
Sharp cracks often have a smaller curvature, leading to greater stress with greatest stress at the crack tip. Flaws act as stress concentrators. Ductile materials often have plastic deformation which redistributes some of the stress at a crack
Critical stress needs ___ to propogate
surface energy, and elastic energy (released when crack propagates)
Crack propagates when
Applied stress exceeds critical stress (identify the equation on the equation doc)
What is the fracture toughness
The critical stress or stress at which a material will fracture
What is mode 1 of crack propagation dependent on
Temperature, Strain rate, and microstructure
Fatigue is classified as
failure that occurs under applied cyclic (dynamic) stress below Ts or fracture stress over many cycles
Fatigue strength is the
Strength/stress amplitude at which given a number of cycles a metal will fracture
Fatigue life is the
number of cycles at a given stress amplitude which will cause a metal to fracture
Fatigue limit is the
supposedly the stress amplitude (Sfat) in which even with an infinite number of stress cycles, the material will not fracture
What is the idea of creep
method of how to get plastic deformation without the force to drive slip
Molecules move around in “dislocation climb” motion
What are other ways to improve fatigue life
- > have compressive imposed surface stress to prevent crack growth
- > remove stress concentrators the best you can
