Fatigue Flashcards
Cracks can form and grow slowly at low loads if
The stress is cycled and the environment is corrosive
Fatigue=
The process of slow crack growth due to cyclic loading
Key aspects or fatigue of a cracked structure
Component has cracks often due to the fabrication process. These propagate during service life until detected and removed or until failure. Common in large structures
2 types of fatigue in uncracked structures
High cycle fatigue
Low cycle fatigue
Failure is initiation controlled
Aspects of high cycle fatigue
Stresses below the yield strength
More than 10^4 cycles to failure
E.g rotating/ vibrating systems
Aspects of low cycle fatigue
Stresses above yield strength but below tensile strength
Less than 10^4 cycles to failure
E.g components subject to occasional overload
Low amplitudes acoustic vibrations are… so
Common so usually ignored
Stress amplitude=
The difference between the mean strain and maximum strain applied to a material
Basic method of fatigue testing example
Sample pulled apart with an oscillated load of known value and frequency
Fatigue life can be measured by
Carrying out tests on a smooth sample at a given stress amplitude and measure the number of cycles until failure. Plot data on an S-N curve
Features of an S-N curve
S= stress(Max, min, amplitude) (y-axis)
N= number of cycles until failure (x-axis)
N plotted on a log scale
Fatigue limit=
Stress below this value does not cause any crack growth.
Some materials don’t have a fatigue limit
5 ways to improve fatigue life
Surface harden components to increase yield strength
Make design changes to reduce applied tensile stress or put in residual compressive stress
Improve surface finish
Avoid stress concentrations (square corners)
Refine microstructure
Δ K=
Δ σ root(pi *a)
To see how how crack size varies with driving force we plotca graph of
Log driving force against da/dN
There are 3 regimes
