chap8fatiguepart1
TRANSCRIPT
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Fatigue
• Progressive fracture that begins at small microscopic cracks
• Beach mark- smooth velvety texture- the fatigue zone
• Final rough fracture
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.1 (p. 291)Fatigue failure originating in the fillet of an aircraft crank-shaft [SAE 4340 steel 320-
Bhn].
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Fatigue
• Results from repeated plastic deformation
• Failures occur after 1,000 or more cycles
• Failures can occur at stress levels far below static yield criteria (i.e. Sy)
• Avoid highly localized plastic yielding if loads are cyclic
• Strain strengthening possible if local yielding is small enough; local yielding then ceases– Loss of ductility if local yielding is not sufficiently minute
• Initial fatigue crack produces increase in local stress concentration.– Decrease in cross sectional area for carrying load as crack
propagates
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.2 (p. 292)Enlarged view of a notched region.
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Fatigue Life Models
• Based on empirical data
• 4 pt bending specimen results in pure bending
• Add rotation of specimen to fully reverse and cycle stresses
• R.R. Moore Rotating Beam Fatigue Test
– i.e. σ(t) = σmax sin(ωt)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.3 (p. 293)R.R. Moore rotating-beam fatigue-testing machine.
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.4a (p. 294)Three S-N plots of representative fatigue data for 120 Bhn steel. (Continued on next two slides.)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.4b (cont.)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.4c (cont.)
For a given life, small scatter
in fatigue strength
For a given fatigue strength,
large scatter in life
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.5 (p. 295)Generalized S-N curve for wrought steel with superimposed data points.
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.6 (p. 295)Endurance limit versus hardness for four alloy steels. [From M.F. Garwood et al., Interpretation of Tests and Correlation with Service. American Society of Metals, 1951. p. 13.]
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.7 (p. 296)Representation of maximum bending stress at low fatigue life (1000 cycles). (Note: Calculated maximum stress is used in S-N plots.)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.8 (p. 296)S-N bands for representative aluminum alloys, excluding wrought alloys with Su < 38 ksi.
No knee in diagram!
No infinite life!
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.9 (p. 297)Fatigue strength at 5 x 108 cycles, common wrought-aluminum alloys..
400,000 miles before cylinder fires this
many times
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Rotating Bending Verses Reversed Bending
W
θ = ωt
F(t) = Fmax sin(ωt)
If Fmax = W, which test would result in failure first?
Fatigue strength in reversed bending slightly greater than in rotating bending
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Rotating Bending Verses Reversed Axial Loading
W
θ = ωt
F(t) = Fmax sin(ωt)
If Fmax and W are such that σmax is same, which test would result in failure first?
Fatigue strength in reversed axial loading 10 percent or more lower than in
Reversed bending
Reduce by gradient factor CG = 0.9 for pure axial
loading of precision parts, 0.7 – 0.9 for axial loading of
nonprecision parts
nS′
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Reversed Torsional Loading
Endurance limit in reversed torsion is about 58 percent of endurance limit in
reversed bending
103 cycle strength 0.9 appropriate ultimate strength i.e. ultimate shear strength
Sus = 0.8 Su for steel
Sus = 0.7 Su for other ductile materials
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.12 (p. 300)A σ1–σ2 plot for completely
reversed loading, ductile
materials. [Data from Walter Sawert, Germany, 1943, for annealed mild steel; and H.J. Gough, "Engineering Steels under Combined Cyclic and Static Stresses." J. Appl. Mech., 72: 113–125 (March 1950).]-
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.11 (p. 299)Generalized S-N curves for polished 0.3=in. diameter steel specimens (based
on calculated elastic stresses ignoring possible yielding).
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.13 (p. 301)Reduction in endurance limit owing to surface finish–steel parts.
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.14 (p. 302)Stress gradients versus diameter for bending and torsion.
Parts that are greater than 0.4” diameter and are subjected to reversed bending
or torsion should carry a gradient factor of CG = 0.9, the same as parts subjected
to axial loading
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Summary
n n L G S T RS S C C C C C′=
Load factorGradient
factor Surface
finish factor
Temperature
factor
Reliability
factor
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Table 8.1a (p.
303)Generalized Fatigue
Strength Factors for
Ductile Materials
(S-N curves).
(Continued on next slide.)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Table 8.1b (cont.)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.15 (p. 305)Fluctuating stress notation illustrated with two examples.
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.16 (p. 305)Constant-life fatigue diagram – ductile materials.
Yield free zone
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.17 (p. 306)Fatigue-strength diagram for alloy steel, Su = 125 to 180 ksi, axial loading. Average of test data for polished specimens of AISI 4340 steel (also applicable
to other alloy steels, such as AISI 2330, 4130, 8630). (Courtesy Grumman
Aerospace Corporation.)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.18 (p. 306)Fatigue strength diagram for 2024-T3, 2024-T4, and 2014-T6 aluminum alloys
axial loading. Average of test data for polished specimens (unclad) from rolled and drawn sheet and bar. Static properties for 2024: Su = 72 ksi, Sy = 52 ksi; for 2014, Su = 72 ksi. Sy = 63 ksi. (Courtesy Grumman Aerospace Corporation.)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.19 (p. 307)Fatigue stress diagram for 7075-T6 aluminum alloy, axial loading. Average of test data for polished specimens (unclad) from rolled and drawn sheet and bar.
Static properties: Su = 82 ksi, Sy = 75 ksi. (Courtesy Grummon Aerospace
Corporation.)
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.20 (p. 307)Various fluctuating uniaxial stresses, all of which correspond to equal fatigue life.
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.21 (p. 308)Axial loading of precision steel part.
Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. Marshek
Copyright © 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 8.22 (p. 310)Sample Problem 8.1 – estimate S-N and σm and σa curves for steel, Su = 150 ksi, axial loading, commercially polished surfaces.