Fatigue Prediction Verification
of Fiberglass Hulls
Paul H. Miller
Department of Naval
Architecture and Ocean
Engineering
U. S. Naval Academy
12 April 2001
Northern California SNAME Meeting
1
Why Study Fiberglass Fatigue?
• Approximately
30% of structural
materials now
used in the marine
environment are
fiberglass.
12 April 2001
• Little long-term
fatigue data exists.
• 1998 Coast Guard
data shows 118
fiberglass failures
resulting in 6
fatalities
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This Project’s Goals
• Extend the standard fatigue methods
used for metal vessels to composite
vessels
• Verify the new method by testing
coupons, panels and full-size
vessels.
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Background-Current ABS Composite
Design Methods
• Semi-empirical,
theory and
Factors of Safety
previous vessels (Working Stress Design)
• Quasi-static
• 2.33 for bulkheads
“head”
• 3 for interior decks
• Beam and
• 4 for hull and
isotropic plate
exterior decks
equations
Includes fatigue and
• Conservative
uncertainties in loads
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Simplified Metal Ship Fatigue Design
1. Predict wave encounter ship “history”
2. Find hull pressures and accelerations
using CFD for each condition
3. Find hull stresses using FEA
•
•
Wave pressure and surface elevation
Accelerations
4. Use Miner’s Rule and S/N data to get
fatigue life
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Project Overview
•
•
Material and Application Selection
Testing (Dry, Wet/Dry, Wet)
•
•
•
ASTM Coupons, Panels, Full Size
Static and Fatigue
Analysis
•
•
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Local/Global FEA
Statistical and Probabilistic
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Material & Application Selection
Ideally they should represent a large
fraction of current applications!
•
•
•
•
Polyester Resin (65%)
E-glass (73%)
Balsa Core (30%)
J/24 Class Sailboat
•
•
•
•
5000+ built
Many available locally
Builder support
Small crews
Another day of research…
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Target Structure Analysis
• Hull Shell Design
• 35% of LWL aft of Fwd Perpendicular
• 0 to 1’ off CL
• Determine loss of stiffness vs.
stress cycle history
(microcracking)
• Requires knowing load effects and
test method bias
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Loads on Target Area
• Hydrostatic
• Hydrodynamic
•
•
•
•
Slamming
Wave slap
Motion
Foil lift/drag
• Moisture
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Quantified Material Properties
• Mostly linear stress/strain
• Brittle (0.8-2.7% ultimate strain)
• Stiffness and Strength Properties Needed (ASTM
tests – Wet/Dry)
Tensile
Compressive
Shear
Flex
Fatigue
20000
15000
Stress [psi]
•
•
•
•
•
E-glass Mat/Polyester Sample #1
10000
5000
0
0
0.05
Tensile Test
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Northern California SNAME Meeting
0.1
0.15
0.2
0.25
Extension [in]
10
Moisture Background and Tests
• Porous materials (up to 2% weight)
• Few documented moisture failures
• Test results ambiguous (Stanford vs.
UCSD)
• Test methods suspect (long-term vs.
boiling)
• Fickian Diffusion
• Tested for 1 year
• Dry, 100% relative humidity, submerged
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Moisture Absorption Results
2 .5 0 %
1.8% weight gain
for submerged
We ight G ain
1.3% for 100%
relative humidity
2 .0 0 %
1 .5 0 %
1 .0 0 %
0 .5 0 %
Equilibrium in 4
months
0 .0 0 %
0
33
66
99
132
165
198
231
D a ys
TNR
TNW
S NR
S NW
F ic kia n
These results were used for coupon and vessel test preparation.
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Finite Element Analysis
• Coupon, panel, global
• Element selection
•
•
•
•
Linear/nonlinear
Static/dynamic/quasi-static
CLT shell
Various shear deformation theories used
(Mindlin and DiScuiva)
• COSMOS/M software
• Material property inputs from coupon
tests
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Northern California SNAME Meeting
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Coupon Test Results
• Tensile Mod: 1.2 msi dry, -12% wet, -13% boiled
• Shear Mod: 0.56 msi dry, -11% wet, -16% boiled
• Comp Mod: 0.92 msi dry, -6% wet, -12% boiled
• Tensile Str: 11.3 ksi dry, -20% wet, -24% boiled
• Shear Str: 5.5 ksi dry, -11% wet, -22% boiled
• Comp Str: 25.3 ksi dry, -16% wet, -25% boiled
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Coupon FEA Results
Strains
were within
2%,
strength
within 15%
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Fatigue Analysis for Vessels

E D   T  f 
0
p ( si )ds
N ( si )
E[D] = the expected accumulated damage ratio
T = the time at frequency f
p(si) = the probabilistic distribution of the number of
stress cycles at stress si
N(si) = the number of cycles to failure at stress si

U ( ,U ws )  cos( ) 
T  f  p( )  p(m)  p(U ws )   f (U ws ) 

U
(
U
)

T
(
U
)
w
ws
s
ws 

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Fatigue Testing
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Fatigue Results – S/N Data
Percent of Original Stiffness
Moisture
decreased initial
and final stiffness
but the rate of
loss was the
same.
100%
90%
80%
70%
1
10
100
1,000
10,000
100,000
1,000,000
Stress Cycles
12.5%-Wet
37.5%-Wet
12.5%-Dry
50%-Dry
25%-Wet
50%-Wet
25%-Dry
75%-Dry
37.5%-Dry
75%-Wet
Specimens failed when stiffness dropped 15-25%
No stiffness loss for 12.5% of static failure load specimens
25% load specimens showed gradual stiffness loss
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Northern California SNAME Meeting
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Panel Analysis
• Responds to
•
•
•
•
USCG/SNAME
studies
Solves edge-effect
problems
Hydromat test
system
More expensive
Correlated with FEA
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Panel Test Results
1
0.9
0.8
Deflection (in)
Strain (%)
Wet vs. Dry
results were
similar to those
from coupons;
the one-sided
wet specimens
were
marginally less
stiff.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
Pressure (psi)
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2
4
W et Defl
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8
Dry Defl
10
W et Strain
12
14
Dry Strain
20
Panel FEA Results
1
0 .9
0 .8
De fle ction (in)
0 .7
0 .6
0 .5
0 .4
0 .3
0 .2
0 .1
0
0
2
P re s s u re (p s i)
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4
6
8
W et Deflec tion
Panel Linear
Panel & Frame NonLin
10
12
14
Dry Deflec tion
Panel NonLin
Panel & Frame Linear
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Impact Testing
• The newest boat had the lowest stiffness.
• Did the collision cause significant
microcracking?
Yes, there was
significant
microcracking!
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Global FEA
• Created from plans and boat checks
• Accurately models vessel
• 8424 quad shell elements
• 7940 nodes
• 46728 DOF
• Load balance with accelerations
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Full-Size Testing – Boat History
120
• High Mileage – J6
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J6 Sailing Hours
80
60
40
20
1996
1997
1998
1999
December
November
October
September
August
July
June
May
April
March
February
0
January
• Daily records for 3 years
• Annual records since
new
• NOAA wind records for
the same period
(daylight)
• Course distribution
• Velocity prediction
program for speed
100
Ave
The Bottom Line for J6:
• 11,300 hours sailing
• 10,200,000 wave encounters
• The “low mileage” boat had 740
hours and 600,000 waves
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On-The-Water Testing- Set Up
Instrument Locations for Boat Tests
Instrument
Location
Strain Gage #1 Portside shroud chainplate
Strain Gage #2 Forestay chainplate
Strain Gage #3 Inside hull on centerline
Strain Gage #4 Inside hull off centerline
Strain Gage #5 Outside hull on centerline
Strain Gage #6 Outside hull off centerline
Accelerometer Bulkhead aft of strain gages
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Location of
sink throughhull (behind
fender)
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Location of strain
gauges (under
epoxy)
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29
29
29
29
:5
:5
:5
:5
:5
:4
:4
:4
:4
:3
:3
:3
:3
:2
:2
:2
:2
:2
:1
:1
:1
:1
:0
:0
9
6
4
2
0
7
5
3
1
8
6
4
2
9
7
5
3
0
8
6
4
1
9
7
5
2
0
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
Wind
0 .1 2
0 .1
0 .0 8
-0 .0 6
A ccel
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Inside on CL
Inside off CL
Strains
0 .0 6
0 .0 4
0 .0 2
0
-0 .0 4
Accelerations
26
2:29:59 PM
2:29:56 PM
2:29:54 PM
2:29:52 PM
2:29:50 PM
2:29:47 PM
2:29:45 PM
2:29:43 PM
2:29:41 PM
2:29:38 PM
2:29:36 PM
2:29:34 PM
2:29:32 PM
2:29:29 PM
2:29:27 PM
2:29:25 PM
2:29:23 PM
2:29:20 PM
2:29:18 PM
2:29:16 PM
2:29:14 PM
2:29:11 PM
2:29:09 PM
2:29:07 PM
2:29:05 PM
2:29:02 PM
2:29:00 PM
Imajination
Test
2:
2:
2:
2:
2:
2:
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29
29
29
29
29
29
29
29
29
29
29
29
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29
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29
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:0
:0
:0
J6 Test
2:
2:
2:
2:
2:
2:
2:
2:
2:
2:
2:
2:
2:
2:
2:
2:
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2:
29
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-0 .0 2
2:
2:
2:
Data Records
0.0016
0.0014
0.0012
0.0008
0.001
0.0006
0.0004
0.0002
0
Dockside String-Test FEA
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Slamming FEA
Inner Skin
WS=22.5 knots
Outer Skin
Using measured accelerations and wave heights
from pictures strains were 0.21% for inner and
0.17% for outer. (23% & 18% of ultimate strain)
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Slamming FEA
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Comparison of Results
• Slamming (Low Mileage
Boat- “Imajination”)
• Peak measured 0.136%
• Ave. of measured peaks
0.117%
• FEA prediction 0.125%
With all the fatigue cycles included, the stiffness loss is:
Imajination J6
Predicted Stiff Reduction
-3%
-14%
Measured with Strain Gauges
-4%
-18%
Global "String Test"
-14%
-52%
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The Most Useful Conclusions
• The Metal Ship
• Visual clues for
fatigue failure are
Fatigue Design
evident
Process can be
extended to
• Stiffness loss may
composite vessels
be a better method
of prediction
• Current factors will
lead to fatigue
• Good FEA
lives of 10-30 years
accuracy requires
a lot of work!
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Northern California SNAME Meeting
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Thanks!
•
•
•
•
•
Prof. Bob Bea
Prof. Hari Dharan
Prof. Alaa Mansour
Prof. Ben Gerwick
Mr. Steve
Slaughter
• ABS
• U. S. Naval
•
•
•
•
Academy
Prof. Ron Yeung
Gerald Bellows
Paul Jackson
My wife, Dawn…
Go Bears!
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