2 Fatigue Mechanisms
Transcript of 2 Fatigue Mechanisms
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FATIGUE MECHANISMS IN MARINE STRUCTURESProf. Ir. Eko Budi Djatmiko, MSc., PhD.
Nur Syahroni, ST., MT.
Fatigue &
Fracture M
echanics (MO
-091334)Te
knik
Kela
uta
n F
TK-
ITS
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OUTLINE
Mechanisms of Fatigue FailureFatigue TerminologyFatigue Testing & AnalysisSN Curve
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Fatigue &
Fracture M
echanics (MO
-091334)
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ILLUSTRATION OF FATIGUE FAILURE
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echanics (MO
-091334)
0
com
pres
sion
LO
AD
(kN
)
tens
ion
cycle
Stress concentrated
Fatigue occurs even though the maximum stress is lower than the yield stress
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FATIGUE COMPRISES OF FOLLOWING STAGES
Crack Initiation (Ni)
Stable Crack Propagation (Np) Unstable Fracture
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echanics (MO
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+
Total Fatigue Life ():
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Fatigue &
Fracture M
echanics (MO
-091334)Fatigue crack initiation in smooth specimen involves the PSB lead to extrusions & intrusions
Crack initiation period is dominant in smooth specimen
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Fatigue &
Fracture M
echanics (MO
-091334)Crack initiation stage in welded joints is almost negligible due to weld defects and other weld discontinuities in which the crack is favorable to initiate
Crack propagation stage becomes a dominant period in welded joint
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Fatigue &
Fracture M
echanics (MO
-091334)Fatigue crack propagation can be microscopically observed by striation and macroscopically by beach mark
• Striation is produced by one cycle load• Beach mark will be form when there is a
variation of load
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TYPICAL FATIGUE FRACTURE SURFACE
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echanics (MO
-091334)
crack initiation
crack growth final fracture
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FINAL FAILURE IS OCCURRED WHEN SIF REACH FRACTURE TOUGHNESS OF MATERIAL
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IC cK Y a
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FATIGUE TERMINOLOGY10
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echanics (MO
-091334)
o Main parameter affect on the fatigue is stress (S) range
Ds = smax - smin
o It is equal to two times of stress amplitude
Ds = 2·sa
o Mean stress gets
sm = (smax + smin) / 2
o Stress ratio (R)
R = smin / smax
0
Str
ess
time
Ds
smax
smin
sa
sm
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FATIGUE TESTING11
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Fracture M
echanics (MO
-091334)
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SN-CURVE (DNV RP C203)12
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echanics (MO
-091334)
log N = log â − m logΔσwhere:N = predicted number of cycles to failure for stress range ΔσΔσ = stress rangem = negative inverse slope of S-N curvelog â = intercept of log N-axis by S-N curve
log â = log a − 2 slogN
where:log a = intercept of mean S-N curve with the log N axisslogN = standard deviation of log N
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SN-CURVE IN AIR13
Fatigue &
Fracture M
echanics (MO
-091334)
DNV RP C203
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SN-CURVE IN AIR (TABLE FORM)14
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echanics (MO
-091334)
Table 2-1 S-N curves in air
S-N curve N ≤ 10 7 cycles N > 10 7 cycleslog a2
m2 = 5.0
Fatigue limit at10 7 cycles *)
Thickness exponent k Structural stress concentration embedded in the detail (S-N class), ref.
also equation (2.3.2)m1 log a
1
B1 4.0 15.117 17.146 106.97 0
B2 4.0 14.885 16.856 93.59 0
C 3.0 12.592 16.320 73.10 0.15
C1 3.0 12.449 16.081 65.50 0.15
C2 3.0 12.301 15.835 58.48 0.15
D 3.0 12.164 15.606 52.63 0.20 1.00
E 3.0 12.010 15.350 46.78 0.20 1.13
F 3.0 11.855 15.091 41.52 0.25 1.27
F1 3.0 11.699 14.832 36.84 0.25 1.43
F3 3.0 11.546 14.576 32.75 0.25 1.61
G 3.0 11.398 14.330 29.24 0.25 1.80
W1 3.0 11.261 14.101 26.32 0.25 2.00
W2 3.0 11.107 13.845 23.39 0.25 2.25
W3 3.0 10.970 13.617 21.05 0.25 2.50
T 3.0 12.164 15.606 52.63 0.25 for SCF ≤ 10.00.30 for SCF >10.0
1.00
*) see also section 2.11
DNV RP C203
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EFFECT OF SEAWATER WITH CP 15
Fatigue &
Fracture M
echanics (MO
-091334)
DNV RP C203
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SN CURVE IN SEAWATER WITH CP(TABLE FORM)
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echanics (MO
-091334)
DNV RP C203
Table 2-2 S-N curves in seawater with cathodic protection
S-N curve N ≤ 10 6 cycles N > 10 6 cycleslog a
m2= 5.0
Fatigue limit at10 7 cycles*)
Thickness exponent k Stress concentration in the S-N detail as derived by the hot
spot methodm1 loga1
B1 4.0 14.917 17.146 106.97 0
B2 4.0 14.685 16.856 93.59 0
C 3.0 12.192 16.320 73.10 0.15
C1 3.0 12.049 16.081 65.50 0.15
C2 3.0 11.901 15.835 58.48 0.15
D 3.0 11.764 15.606 52.63 0.20 1.00
E 3.0 11.610 15.350 46.78 0.20 1.13
F 3.0 11.455 15.091 41.52 0.25 1.27
F1 3.0 11.299 14.832 36.84 0.25 1.43
F3 3.0 11.146 14.576 32.75 0.25 1.61
G 3.0 10.998 14.330 29.24 0.25 1.80
W1 3.0 10.861 14.101 26.32 0.25 2.00
W2 3.0 10.707 13.845 23.39 0.25 2.25
W3 3.0 10.570 13.617 21.05 0.25 2.50
T 3.0 11.764 15.606 52.63 0.25 for SCF ≤ 10.00.30 for SCF >10.0
1.00
*) see also 2.11
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SN CURVE FREE CORROSION (TABLE FORM)17
Fatigue &
Fracture M
echanics (MO
-091334)
DNV RP C203
Table 2-3 S-N curves in seawater for free corrosion
S-N curve log aFor all cycles m = 3.0
Thickness exponent k
B1 12.436 0
B2 12.262 0
C 12.115 0.15
C1 11.972 0.15
C2 11.824 0.15
D 11.687 0.20
E 11.533 0.20
F 11.378 0.25
F1 11.222 0.25
F3 11.068 0.25
G 10.921 0.25
W1 10.784 0.25
W2 10.630 0.25
W3 10.493 0.25
T 11.687 0.25 for SCF ≤ 10.00.30 for SCF >10.0
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THICKNESS EFFECT
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echanics (MO
-091334)
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THICKNESS EFFECT
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Fatigue &
Fracture M
echanics (MO
-091334)
log N = log K − m log(Δσ(t/tref)k)where:tref = reference thickness equal 25 mm for welded
connections other than tubular joints. For tubular joints the reference thickness is 32 mm.
t = thickness through which a crack will most likely grow. t = tref is used for thickness less than tref
k = thickness exponent on fatigue strength as given in Table 2-1, Table 2-2 and Table 2-3
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MEAN STRESS EFFECT
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Fracture M
echanics (MO
-091334)
DNV RP C203