CAEF [7] Strain Life.pptxThe Strain-Life Method (Local Strain, εε-N, or “Crack Initiation”)
*Relates local strain to fatigue damage at that point
*Useful when cycles have some plastic strain component *Suitable for predicting life in components which are
supposed to be defect free, i.e., not structural joints, sharp features, etc.
(Low Cycle Fatigue – LCF)
Strain-Life (ε-N)
σ 1/2cycle
2Nf
ε-N Fatigue Tests
*Specimens are subjected to constant amplitude strain using an extensometer in the servo loop
*εN test controls plastic strain, the parameter that governs fatigue
*The number of cycles to crack initiation is plotted against the total strain on a log-log plot and the best fit curve computed
( ) ( )cff b
f f
(2Nf)b + εf ’(2Nf)c
The strain concentration factor, and the stress concentration factor,
after plastic yielding. Neither are known but Neuber suggested that the square root of the product of the stress and strain concentration factors was equal to Kt . Hence Neuber’s Rule is simply:
Re-arrangement of this Rule gives a useful equation:
e Ke
( ) σε=SeKT 2
Another re-arrangement gives:
in which the LHS is known. This can be solved with the cyclic stress strain curve equation simultaneously to derive σ and ε Topper simply replaced Kt by Kf to make Neuber’s Rule applicable in fatigue analysis for local stress strain tracking.
Use of Kf in Neuber’s Rule
( ) σε=EeKT 2
1
KfKK
e
ε 2 2 2 =
2 f