Fatigue (Simon Hogg) Flashcards
What is fatigue?
Occurs in dynamic not static stresses over a lengthy time by slow growth of a crack
Largest cause of failure in metals
Particularly problematic as failure results at applied stresses much lower than yield strength
What three factors are required for fatigue failure?
1) Sufficient tensile stress
2) Large fluctuation in applied stress
3) Large enough number of cycles of stress application
What factors can affect fatigue behaviour?
Stress concentration
Temperature
Microstructure
Residual stress
Environment
What is High and Low cycle fatigue?
Low amplitude vibration results in an elastic response (no permanent deformation) - stresses are elastic and well below yield strength so are fine but plastic deformation can occur at crack tip - HIGH CYCLE
Higher amplitude like above the yield strength - material begins to experience permanent damage in the form of fatigue (failure will result in only a few cycles - LOW CYCLE)
The cyclic stress causes dislocation activity which can lead to a crack, which propagates until critical length for fracture
What is an S-N curve?
Stress amplitude against log of the number of cycles to failure Nf for a specified R value
What are the two types of behaviour noticed on a S-N curve?
Some steel and Ti alloys have a ‘fatigue limit’ - stress amplitude about a zero mean stress below which fatigue failure will never occur
Most non-ferrous alloys don’t have a ‘fatigue limit’ - fatigue failure will occur regardless of stress amplitude. Instead, ‘fatigue strength’ gives stress amplitude at which failure will occur after a number of cycles
What is ‘fatigue life’?
Number of cycles to failure at a specified stress amplitude
What would a fatigue limit look like on an S-N curve?
Horizontal line - indicates no matter the amount of cycles, wont fail unless stress is high enough
What would the graph of S-N look like for an aluminium alloy?
No fatigue limit so continuing curve across graph with no stress limit
What’s the difference between fatigue strength and life?
Fatigue strength is stress required for failure at a specified number of cycles
Fatigue life is number of cycles required for failure at a specified stress
What effect does crack propagation have on cyclic stress intensity?
As crack grows, length c increases - stress intensity ^K increases for a fixed stress amplitude
What effect does stress intensity range ^K have on crack propagation?
Rate of crack propagation is proportional to range
For static loading, crack wont propagate until K reaches critical stress intensity K1c
For cyclic loading, crack can propagate stably at stress intensity factors Kmax (can be as low as 1/100 of K1c) - crack grows in small increments over time
Describe the 3 regimes of crack growth on the graph with Ks
Regime A (can last for 1000-10000 cycles):
Stress intensity is low (below Kt) so crack is dormant or undetectable
Over Kt, average crack growth can be as small as one atomic spacing per cycle
Plastic zone at crack tip is very small and constrained with one or two grains - leads to single-crystal shear, cracks propagate along slip planes in each grain - gives a zig zag appearance
Regime B:
Crack growth is transgranular normal to stress axis and forms fatigue striations visible on fracture surface
Plastic zone at crack tip is large enough to encompass many grains
Crack grows by deformation slip along two slip systems (duplex slip)
Regime C:
Crack length is long enough that K-range approaches fracture toughness
Crack growth rate accelerates until fracture toughness is reached - K1c
Final failure occurs in one cycle by tensile overload
