Fatigue Flashcards
Define fatigue
Progressive localised damage to a material under repeated or oscillating loads
Define low-cycle fatigue
Fatigue in which there are few cycles to failure - plastic deformation occurs in each cycle and there is hysteresis.
Define high-cycle fatigue
Fatigue wherein there is no macroscopic plasticity and the stress-strain curve is linear - there are many cycles to failure
Give three methods for analyzing fatigue
Fracture mechanics - consider the behaviour of existing cracks
Stress-based - relates stress to lifespan, empirical
Strain-based - relates strain during low-cycle fatigue to lifespan
Define fatigue limit
A threshold stress below which fatigue is never seen in a material - typically caused by interstitial atoms that pin dislocations and prevent localised plasticity
What is the stress range, Δσ? How does it relate to stress amplitude, σa?
The difference between the maximum and minimum stress in a cycle. Stress amplitude is a half the value of stress range.
Does mean stress alter a component’s lifetime?
YES
What is the definition of th R-value?
σmin / σmax
What is the Basquin model?
A mathematical model of stress-lifespan for a given component of the form
σar = AN^B
Where σa is the stress amplitude, N is the number of cycles to failure, and A, B are constants
What is the fatigue strength coefficient, σf’?
σf’ = A / 2^B
Corresponds to the projected failure stress at half a cycle [one reversal]
How are fatigue safety factors defined?
In terms of stress of lifetime: they are the ratio of the design stress/lifetime to the expected stress/lifetime
What are typical values for Xs and Xn, safety factors for fatigue stress and lifetime respectively?
1.5 and 15 respectively
What is difference between a gross and net section?
A gross section is equivalent to the area described by the outer boundaries, the net section is equivalent to the true area of the section
When designing a component to withstand fatigue, what should you be asking?
Have I ensured the maximum stress is less than the fatigue limit/endurance limit of the component’s material?
What is the endurance limit?
A stress at which a material will last some large number of cycles (between 10 and 100 million)
How is a material’s endurance limit estimated?
σ_{e} = m σ_{ts}
Where m is in the range 0.4 - 0.6 for most metals.
Why is the approximation for a material’s endurance limit, σ_{e} = m σ_{ts}, not valid for highly strengthened materials?
Strengthened materials aren’t plastic at the tips of cracks (unlike their unstrengthened counterparts) - this is essential to limit crack propagation
What factors influence a material’s endurance limit?
Material factor (empirical, assume 0.5 w/out other info)
Surface quality
Stress concentration factor (mk = 1/K)
Reliability (mr)
Size effect (bigger components are more likely to have defects)
Loading type (mt)
How does a tensile mean stress affect a component’s lifetime?
Typically reduces lifetime
How does a compressive mean stress affect a component’s lifetime?
Usually increases lifetime
What is σar?
Stress amplitude for fully reversed loading (i.e. the equivalent stress were the mean stress zero)
What generic expression relates stress amplitude with a mean stress present to equivalent stress amplitude w/ zero mean stress? What is the name of this expression?
σa = σar [ 1 - (σm/σu)^x ]
> The constant life equation
Give four constant-life equation variables
Goodman: x = 1, σu»_space; σts
Modified Goodman: x = 1, σu»_space; σf’
Gerber: x = 2, σu»_space; σts
Soderberg: x = 1, σu»_space; σy
You are analyzing a notched component that has a mean stress and is being periodically loaded. What must you account for when applying the constant life equation?
The stress concentration factor attributable to the notch - ensure the σm and σa used in the CLE encompass this.
Do you need to worry about stress concentration of mean stresses in a notched component fabricated from a ductile material?
No - the material’s ductility means that high mean stresses cause plastic flow which mitigates the effects of the notch.
What is the Palmgren-Miner rule, and how can it be applied?
Assume that a component has a certain amount of fatigue life available - loading at different stresses for given numbers of cycles consumes certain fractions of this lifespan. (Nb. pertains to variable loading scenarios)
