Combined Fatigue Loading Mode Basquin curve Finite life

1
Combined Fatigue Loading Mode
Basquin curve

• Finite life found by plotting alternating and
  mean Von Mises stresses and drawing a line from
  the Sut through the plotted point to find sf at
  the intersection of the salt axis.
• From this point, the finite cycle life is found
  by intersection of sf and Basquin line

Finite cycle life
Sut
sf
1/b
a (0.9Sut)
2
( )
Nf sf
Se
a
Se
b _ 1 log(0.9Sut)
3 Se
salt
smean
Sut
3
Infinite life region 1 sa sm
10
Nf
n Se Sut
6
10
Von Mises sa/m (s1 s1 s2 s2 )
Alternating
Mean
1/2
2
2

• Using the combined axial and bending stress and
  shear from torsion/shear, plot Mohrs circle
• Alternating and Mean stress have separate Mohrs
  circles, therefore each has separate principle
  stresses used to calculate Von Mises stresses

sm (bendingaxial) , tm
sa (bendingaxial) , ta
Generic Mohrs Circle using calculated stresses
s1
s2
s2
s1
-tm
-tm
Torsion / Shear K f_ttalt ta tm
Bending K f_bsalt_b sa sm
Axial K f_asalt_a sa sm

Kb
Kb
Kc_axial
Alternating Mean salt_a/b (smax-smin)/2
sm (smaxsmin)/2
Stress calculations done for each plane
Axial/ Bending smax smin
Torsion / Shear tmax tmin
t
sa sb
Uni-axial example of stress element
Given Situation

• Se should be calculated through the Marin
  equation. In this equation surface (Ka),
  temperature (Kd), and misc (Ke) factors are
  accounted for. Fatigue stress concentration (Kf)
  , load (Kc), size (Kb) are excluded in this
  modified endurance limit because there are
  different values for each loading mode. They are
  factored into the alternating and mean stresses
  for each mode.
• Stress concentration factor, Kt (table A-15,16),
  and the notch sensitivity factor, q (figure
  5-16), are entered into Kf 1q(Kt 1)
• Force transmission and stress analysis should be
  completed for the point of interest, giving you
  smax , smin, tmax, and tmin
• All equations and tables found in Mechanical
  Engineering Design 5th Edition by Joseph Edward
  Shigley

By James Watson Ryan Watson Tony Crew