lec 3 fatigue failure 031004_for_students
TRANSCRIPT
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
1/32
Chapter 6 Fatigue Failure
Theories
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
2/32
Fatigue Failure
Occurs when stresses are changing
throughout the life of a part.
_______________________ starts with a
crack that propagates until a catastrophic
failure occurs.
This usually begins at a manufacturing defector stress concentration that is subjected to
tensile stresses for part of its load cycle.
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
3/32
Fatigue-Failure Models
Stress-Life - cyclic stresses are kept
below fatigue strength or endurancelimit. Most widely used.
Strain-Life - complicated not often used.
_________________ - sometimesused, gaining favor especially in the
aircraft and aerospace industry.
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
4/32
Fati Str ss
s
-10000
-50000
5000
10000
15000
20000
25000
30000
0 2 4 6 8 10
adC
cl s
A
lidS
tr
ss
-
si
Wa
Wa
Wa
Wm
Wm
Full R
rs
d R
at d Fluctuati
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
5/32
Fully Reversed Failure Criteria
Most data comes from . . Moore
rotating-beam test. Highly polished specimen of . inches
in diameter is subjected to pure bending
stresses that are alternated by rotating
the specimen.
otation is at 7 rpm.
Takes / day to reach 6 cycles.
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
6/32
S-N Curve for Steel with Sut = 100 ksi
10000
100000
1.0E+3 10.0E+3 100.0E+3 1.0E+6 10.0E+6 100.0E+6
Cycles to Failure - N
Fatigue
Strength
-Sf
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
7/32
Endurance Limit and FatigueStrength
Endurance Limit of a Material (Se) -
stress below which fatigue failure doesnot occur regardless of the number of
stress cycles.
____________________Strength of a
Material (Sf) -stress below which fatigue
failure does not occur for a specified
number of stress cycles.
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
8/32
Typical S-N Curve for Non-Ferrous Metals
10000
100000
1.00E+03 1.00E+04 1.00E+05 1.00E+0 1.00E+0 1.00E+08 1.00E+09
Cycles to Failure
Fatigue
Streng
th-
psi
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
9/32
Estimating Se or Sf
For ____________:
Se = .
Sut for
Sut< ksi ( MPa)Se = ksi (7 MPa)
forSut> ksi ( MPa)
For ______________:
Se = . Sut forSut6 ksi ( MPa)
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
10/32
Estimating Se or Sf contd
For __________________:
Sf@ E8 = .
Sut for
Sut ksi ( MPa)
For _____________________:
Se = . Sut forSut< ksi ( 80 MPa)
Se = ksi ( 00 MPa)
forSut> 0 ksi ( 80 MPa)
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
11/32
Correction Factors to EnduranceLimit and Fatigue Strength
Se = load size surf temp reliab Se
Sf= load size surf temp reliab Sf
Se - corrected endurance limit for a part
Sf- corrected fatigue strength for a part
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
12/32
Correction Factors to EnduranceLimit and Fatigue Strength
Cload - load factor see eq. 6.7a pg 8
Csize - size factor see eqs. 6.7a,b,c
pg 8-
Csurf- surface factor see Figs. 6- 6,6. 7, and eq. 6.7e pg -
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
13/32
Correction Factors to EnduranceLimit and Fatigue Strength
Ctemp - temperature factor see eq. 6.7f
pg - Creliab - reliability factor see Table 6.
pg
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
14/32
Stress Concentration Kf= q(Kt+ )
Kt- geometric stress concentrationfactor
Kf-___________________________
q - notch sensitivity factor
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
15/32
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
16/32
Fluctuati
ng Stresses
Modified-Goodman Diagram
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
17/32
Failure Lines for Fluctuating Stress
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
18/32
Augmented Modified-Goodman
Diagram
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
19/32
Stress Concentration
Apply Kfto the ______________
components of stress. For __________ materials apply Kt to
the mean components of stress.
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
20/32
Stress Concentration - contd
IfKf|Wmaxnom| < Sy then: Kfm = Kf
IfKf|Wmaxnom| > Sy then:
Kfm = (Sy - KfWanom)/ |Wmnom|
IfKf|Wmaxnom - Wminnom| < Sy then: Kfm = 0
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
21/32
Factor of Safety for Fluctuating Stress -
Case 1
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
22/32
Factor of Safety for Fluctuating Stress -
Case 2
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
23/32
Factor of Safety for Fluctuating Stress -
Case 3
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
24/32
Factor of Safety for Fluctuating Stress -
Case 3
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
25/32
Factor of Safety for Fluctuating Stress -
Case 4
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
26/32
Factor of Safety for Fluctuating Stress -
Case 4
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
27/32
Fluc
tuatin
g Stresses
Multiaxial Stresses in Fatigue
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
28/32
Fully Reversed SimpleMultiaxial Stresses
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
29/32
Fluctuating Simple Multiaxial Stresses
Von Mises Method
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
30/32
General Approach to High Cycle
Fatigue Design Generate Modified Goodman Diagram
Calculate alternating and mean components
of stress at areas of concern on the part.Include appropriate stress concentrationfactors.
Convert alternating and mean appliedstresses to alternating and mean Von Mises
Stresses. Plot these stresses on the Modified Goodman
Diagram and find the factor of safety.
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
31/32
Example The figure pertains to the shaft of a disk
sander that is made from steel havingSu= 00 MPa, and Sy=7 0 MPa. The most
severe loading occurs when an object is heldnear the periphery of the disk with sufficientforce to develop N-m (which approachesthe stall torque of the motor).
Assume a coefficient of friction between theobject and the disk is 0.6.
What is the factor of safety with respect toeventual fatigue failure of the shaft?
-
8/6/2019 Lec 3 Fatigue Failure 031004_for_students
32/32
Example