ME 120 - Component
Design
Lecture 4
7-2-2026
Content we missed from Chapter
6
Modifications to life cycle
due to mean stress
Fatigue strength, Sf
Stress life diagram with
changing mean stress
Sut
fSut
Sf
Se
Number of stress cycles, N
Fluctuating stress diagram
Alternating stress, σa
Goodman line for infinite-life
fatigue criterion
Mean stress, σm
Alternating stress, σa
Drawing a load line
Mean stress, σm
Alternating stress, σa
Goodman Failure criteria
Mean stress, σm
Morrow Failure Criteria
a
Se
Sut
m
Gerber failure criteria
Example fatigue failure with
non-zero mean stress
 max  50 kpsi
 min  10 kpsi
Example with Goodman and
Morrow
Example with Gerber
Intro to Chapter 7
• Shafts and Shaft
Components
• Shafts:
• Design Considerations
for shafts
• Rotating member, usually • Fatigue Loading
circular cross section
• Transmits power or motion • Minimize deflections
• Provide an axis of rotation • Longer/thinner rods
for parts like gears and
• Larger bending moment
pulleys
• Larger deflection
• Undergoes fluctuating
loading (fail usually by
• Stress concentrations
fatigue
• Axle:
• Non-rotating member that
supports pulleys, rotating
wheels, etc
• Shafts look like stepped
cylinders to accommodate
other elements like
bearings
Shaft Design with regards to
stress
• Critical locations with
highest stresses
• Outer surface
• Point with larger bending
moment
• Location of torque being
applied
• Stress concentrations
Torque:
Torque due to rotation
Torque comes into the shaft from one
gear, and then leaves at another gear
Highest at outer surface
Bending:
Transverse loads lead to internal
bending moments
Constant loading will give completelyreversed stress cycles
For shafts where the length is much
greater than the diameter, normal
stresses are much greater than
transverse shear stresses
Axial:
Typically negligible
Stress concentrations (3.13)
Stress concentration example
Ways stress can be raised
Examples of stress
concentrations
• Where will material crack first?
Examples of stress
concentrations
Examples of stress
concentrations
Fatigue Stress Concentrations
(6.10)
Notch Sensitivity
Notch Sensitivity in reversed
bending or axial loads
Notch sensitivity in reversed
torsion
Example: Filleted Shaft
Fluctuating stresses in shafts
Combining bending and torsion
into von Mises stress
Example
Machined shaft shoulder
 d = 1.100 in
 D = 1.65 in
 r = 0.11 in
 Material has Sut = 105 kpsi and
Sy = 82 kpsi
 Find the fatigue stressconcentration factors for
bending and shear.

Example Continued
Example
Apply a bending moment M = 1260 lbf in and a steady
torsion T = 1100 lbf in
 Material has Sut = 105 kpsi and Sy = 82 kpsi
 Estimated endurance limit is Se
= 27.1 kpsi
 Find the infinite-life fatigue safety factor with
Goodman

Reminders
• Homework is being assigned TODAY
• Due the following Thursday at 11:59 PM
• Let me know ASAP if there are any issues
• Quiz