fracture and fatigue from slides Flashcards
(39 cards)
What is the stress approach to failure?
Comparing the yield stress of a components material with the component’s measured stress.
What does G stand for?
Strain Energy Release Rate
What is the strain energy release rate?
The rate that strain energy will be released when a crack grows
(G)
What does Gc or R stand for?
Work of Fracture or Critical Strain Energy Release Rate
What is the critical strain energy release rate?
The energy needed to make a unit area of crack
= work of fracture
(Gc or R)
What is work of fracture
The energy needed to make a unit area of crack
= strain energy release rate
(Gc or R)
Is G or Gc easier to test in reality?
Gc (work of fracture)
Gc can be easy to test (eg Cleavage test/peel test)
Calculating G can be really hard
What happened to the Schenectady ship in 1941?
It broke in the harbour due to fracture. Only plastic deformation was tested for duet to little understanding of fracture and its effects along the plate boundaries.
How did the iceberg cause the titanic to sink?
The titanic was in icey waters which relate to a lower impact energy. So energy wasn’t able to be absorbed by plastic deformation and the iceberg caused a brittle crack that quickly propagated through the steel.
what happened to the De Havilland Comet
Was an aircraft with square windows which failed due to cracks originating at the corners. Several of these planes kept falling from the sky. They cyclically loaded the wreckage to find the corners were stress raisers and fatigue cracks were growing over time
Engineers didn’t realise they’d be a problem as the stresses didn’t seem bad upon initial calculations but skipped on actual testing to save money.
4 limitations of stress-based design
- materials that are relatively brittle may fail by cracking instead of yielding
- triaxial stresses may inhibit yielding (??)
- want to predict if pre-existing cracks will grow under a given load
- must assume invisible flaws are present in safety critical structures
what does kt stand for?
Stress concentration factor
used as a multiplier to estimate stress around a defect; given a known stress in rest of the body
Limitations of the stress concentration equation (Inglis 1913)
- crack tip stress is independent of the crack length and only proportional to a/b ratio (WRONG because this would mean that the stress won’t be impacted if the ratio stays the same)
- stress goes to infinity for a sharp crack with b→0 (WRONG because suggests that the stress→∞ as crack height increases)
Why is energy needed to form new surfaces?
- to pull apart the atoms along the line of the crack
- maybe also Plastic deformation
How does applied load/stress affect the release of stored strain energy?
release of stored strain energy INCREASES with applied load/stress
assumptions for the energy (G) approach
- plastic deformation is confined to a small process zone around the crack tip
- rest of the material remains linearly elastic
when is Gc not constant?
Gc can vary as the crack develops in ductile materials; especially in thin sheets
relationship between G and Gc for stable crack growth
Gc > G
conversely, it will fail catastrophically
PROS of energy approach
- easy to test
- applicable to any material/combo of materials, provided plastic flow is localised
- material property, with a clear physical meaning
CONS of energy approach
- v difficult to calculate G in many cases
- poor for design or practical failure prediction
What is the energy approach to failure?
calculating the energy required for failure as energy is required, as well as a high stress, for plastic failure
PRO for testing for fracture toughness (Kc)
easy to calculate for cracked structures (or those with unknown defects)
CONS for testing for fracture toughness Kc
- harder to test materials
- assumes linear, elastic, homogenous material, which makes it difficult to use for many materials
what is fracture mode 1
Opening - 2 sides of the crack are pulled apart in uniaxial tension => opens the crack face
