fatigue Homa Flashcards
what does fatigue life embrace (3)
nucleation of a crack in a previously crack free material
slow growth of the (sub-critical) crack
Rapid growth when the crack attains a critical length.
does fatigue failure require existing defects or cracks for fatigue to occur
no, can be new material but fatigue and fail
when fatigue occurs cracks may form where
on crystallographic slip
what is a crystallographic slip
large displacement of one part of a crystal relative to another part along crystallographic planes and directions
what angle do crystallographic slip occur on and via what stress
45 degrees via shear stress
what is cycle loading
continuous and repeated application of a load, leading to fatgiue
at what RPM and frequency does fatigue crack grow
3000rpm at 50hz
what is the point of S-N curve (2)
It is a measure of the materials performance to resist fatigue under cyclic loading
what are the diffrent regimes of fatigue crack
I, II and III
what are the different regimes of fatigue crack
I, II and III
Crack growth in region I size crack, and atomic
an average crack advance of
less than one atomic spacing per cycle
Crack growth in region II and dection
higher crack growth rate compared to regime I via the paris law. and can be detected via ultrasonic or x rays
Crack growth in region III
crack growth rate will
accelerate and finally fracture will occur
what is the point of using paris law
can predict fatigue life of large cracks
what happens under variable amplitude loading
plastic residual stress fields decrease in crack growth rate
what size of crack propagates faster short or long crack
short cracks propagate faster
in multiaxial loading, most cracks tend to propagate in what region or mode out of mode I, II and III
mode I
low cycle fatigue (3)
stress lvels? high or low
does it exceed elastic limit
max life for cycles
high stress levels
exceeding elastic limit
under 100 cycles life
high cycle fatigue
, stress yield high or low,
lifetime
3 stages of fatigue
low-stress yield
long lifetime
fatigue stages: crack initiation, stable crack growth and unstable failure
what is a Fatigue Striations and its association
an advanced crack - Each striation on the fatigue surface is associated with one
stress cycle for fatigue cracks
what regime does fatigue striation require
paris regime
what can beach marks and striations indicate
direction and type of the damaging stress occuring.
what is LEFM
Linear elastic fracture mechanics
what does LBB stand for
leak before break
what is LBB
a crack of twice the
wall thickness in length should be
stable at a stress equal
design stress.
steps for LBB (5)
- Material characterisation and stress calculation
- Determination of critical through-wall crack lengths
- Calculation of crack opening area
- Evaluation of leak rate
- Assessment of results
what is the plastic zone at crack tip and explain which zone dissipates more heat in plastic zone, and what equation is it based on
the crack tip is the 2 heart circles, which the broken part is called crack zone, inner circle is strain plane and outer being plane stress.
this circle is based on von mises. as stress is larger, it will dissipate more heat than strain
