Contribution of Aircraft Gear Loads to Reflective Cracking in Airport Asphalt Overlays
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Transcript of Contribution of Aircraft Gear Loads to Reflective Cracking in Airport Asphalt Overlays
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Contribution of Aircraft Gear Loads to
Reflective Cracking in Airport Asphalt Overlays
January 30th, 2007
FAA COE Project Review and Project Proposal Meeting
William G. Buttlar, Associate Professor
Hyunwook Kim, Research Assistant
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Outline
Discuss Research Ideas
Review State-of-the-Art in RC Analysis Overview
J-Contour (LEFM) Approach
Cohesive Zone Model Approach
Summary and Discussion
RC Research Ideas
3Some Open Asphalt Research Areas
Reflective Cracking Analysis and Design Analysis
Supporting Lab and Field Data
State-of-the Art on AC Stiffness Prediction Models (Witczak, Hirsch, DiBenedetto, You, Yin, etc.)
Thermal Cracking Test for Airfield Pavements
Performance Testing Suite for High Traffic HMA Designs
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Reflective Cracking
Reflective Cracking
Reflective cracking is very complex
airfield distress mechanism; likely a
result of combined environmental and
gear loading effects.
Cracks can begin to appear as soon as
the first winter after construction.
Can cause the acceleration of other
pavement distresses through water
infiltration, stripping in HMA layers, and
loss of subgrade support.
The key challenge is the geometric
discontinuity which makes modeling
much more complex and tedious.* Kim and Buttlar (2002)
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RC Model Evolution
Empirical Models
“Simple” FEM Models – Strength of Materials Approach
LEFM Approach
Cohesive Zone Modeling Approach
Reduced Physics (Surrogate Model)
New Design Method
More Physics, More Complexity, More Difficult to Implement
Understand Physics of Problem (Distress Mechanism)
Simplify Problem
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J-Contour Approach (LEFM)
Cohesive Zone Model Approach
Can study mode-I and mixed-mode loading conditions.
Can model crack nucleation, initiation, and propagation.
Compatible with elastic and viscoelastic bulk material behavior.
Can be used in conjunction with realistic thermal gradients.
More realistic approach but needs more computational effort.
Recent Fracture Modeling Approaches
Can be used to study the mixed-mode loading condition (combined
opening and shearing).
Provides reliable, numerical estimates of stress concentration at the
crack tip.
Can not be used directly with viscoelastic material.
Does not directly predict crack propagation.
Simplified approach requiring less computational effort.
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#1 - J-Contour Approach
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Stress Intensity Factor (SIF) - LEFM
fij () = Dimensionless function of
K = Stress intensity factor
Where, (= I, II, or III): Mode of loading
)(2
lim )()(
0
ijijrf
r
K
The stress fields ahead of a crack tip for each mode in an isotropic linear elastic material are:
A stress singularity occurs at the crack tip as r approaches zero.
Traffic-induced Unstable subgrade presents Less common cause of RC
Traffic & thermalMost common mode
Three Failure Modes
Mode II (In-Plane Shear)Mode I (Opening) Mode III (Out-of-Plane Shear)
Where,
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J-Integral: Path Independence
1
2
3
4
nj
y
x
nj
mj
Crack faces
4321
043211
1 JJJJdsnx
uWnJ jij
i
For any closed contour, J = 0. The J-integral is a conservation integral. J-integral is independent of the contour taken around the crack tip
Rice (1968) showed a mathematical presentation to prove the path-independency of the J-integral.
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2
'
2
'
2
IIIIII K
E
K
E
KGJ
For a linear elastic, isotropic material
10Airport Overlay System
A typical pavement section of an
airport that serves Boeing 777
aircraft was studied
The selected model geometry
and pavement cross sections
are based on the structure and
geometric information of un-
doweled sections of runway
34R-16L at Denver international
airport (DIA) in Colorado
Concrete Slabs
ECTB = 2,000 ksi; CTB = 0.20
k = 300 pciSubgrade
CTB
18 in
8 in
AC Overlay 5 in EAC = 200 ksi; AC = 0.350.5 in0.2 in
EPCC = 4,000 ksiPCC = 0.15
Cross Section
Transverse Joint = 0.5in
Longitudinal Joint = 0.5in
240 in
225 in
Top View CL
Traffic Direction
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36 ft (10.97 m)
Boeing 777-200
57 in 57 in
21.82 in
13.64 in
55in
One Boeing-777-200 aircraft: 2 dual-tridem main gears Gear width = 36 ft Main gear (6 wheels; 215 psi) Gross weight = 634,500 lbs Each gear carries 47.5% loading
= 301,387.5 lb
Aircraft Gear Configuration
12FE Model Assumptions
225 in
Longitudinal Joint
Overlay=5”; AC=1.3888910-5 1/F
Concrete slabs=18” PCC=5.510-6 1/F
CTB=8”; CTB=7.510-6 1/F35F
35F
13F
Subgrade
5F
TPCC=-1.25F/in
TAC=-1.5F/in
Thermal Loading
The 2-D FE model is a reasonable approximation of 3-D geometry for the purpose of fracture study.
Material properties: Elastic (Due to the limitation of J-integral approach)
Subgrade: Winkler foundation (Linear spring)
Thermal loading: Simplified thermal loading rates and thermal coefficients were used
Load transfer efficiency (LTE): 2 node spring element (Stiffness in the vertical direction) (Hammons 1998)
LTE
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Description of FE Fracture Model
PCC-1 PCC-2
Both Gears Loading
Undeformed
Deformed
PCC-3 PCC-5 PCC-6
Joint-1 Joint-4Joint-2 (with a crack) Joint-5
PCC-4
Joint-3
Crack Tip
AC Overlay
PCC
Subbase
x
y Center Position = 236 in
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Stress Contour around Crack Tip
Crack Tip
Joint
Tension Zone
Stress Concentration
PCC
AC
Existing Crack
Existing Crack
Crack Tip
The extent of the zone of material tension, along with the stress concentration at the crack tip can be observed.
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Mode-I Stress Intensity Factor (KI)
-1200
-1000
-800
-600
-400
-200
0
200
400
600
0 100 200 300 400 500
Center Position of Gear Loading from a Crack Tip (inch)
SIF
(K
I)
0.1 in 0.3 in0.5 in 1.0 in1.5 in 2.5 in3.5 in 4.5 in
(a) Gear Loading Responses without Temperature Loading
(b) Gear Loading Responses with Temperature Loading
0
500
1000
1500
2000
2500
3000
3500
0 100 200 300 400 500
Center Position of Gear Loading from a Crack Tip (inch)
SIF
(KI)
0.1 in 0.3 in0.5 in 1.0 in1.5 in 2.5 in3.5 in 4.5 in
Tension
Compression
The most critical tensile conditions were when the gear was located 67”
and 337” away from the crack, not directly over the crack. The cases with longer existing crack lengths tended to be more
sensitive to fracture responses. When temperature loading is applied in combination with certain gear
loading, the critical tensile responses were significantly increased.
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(a) Responses without Temperature Loading
(b) Responses with Temperature Loading
Mode-II Stress Intensity Factor (KII)
-200
-100
0
100
200
0 100 200 300 400 500
Center Position of Gear Loading from a Crack Tip (inch)
SIF
(K
II)
0.1 in 0.3 in0.5 in 1.0 in1.5 in 2.5 in3.5 in 4.5 in
-200
-100
0
100
200
0 100 200 300 400 500
Center Position of Gear Loading from a Crack Tip (inch)
SIF
(KII)
0.1 in 0.5 in1.0 in 1.5 in2.5 in 3.5 in4.5 in
In the case of Mode-II responses, the critical shear conditions are found
to be under the edge loading condition, e.g., under the edge of one tire
loading among the four wheels represented in the 2D model.
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One Gear vs. Both Gears
0
400
800
1200
0 100 200 300 400 500
Center Position of Gear Loading from a Crack Tip (inch)
SIF
(K
I)
One Gear - KI
Both Gear - KI
(a) Mode-I Stress Intensity Factor (KI) (b) Mode-II Stress Intensity Factor (KII)
-200
-100
0
100
0 100 200 300 400 500
Center Position of Gear Loading from a Crack Tip (inch)
SIF
(K
II)
One Gear - KII
Both Gear - KII
While the fracture responses have similar trends, the differences between single and dual gear loading becomes larger at the critical loading positions.
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Summary J-integral approach was verified and implemented
into FE fracture pavement model to consider the critical responses under various aircraft gear loadings combined with simplified thermal loading.
The critical tensile condition for the pavement studied occurred when the gear load was positioned away from the existing joint (counter flexure)
Thick PCC over CTB
Large Aircraft Tire Radius and Multiple Wheels
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Summary (cont.) J-integral approach has limitations
(Elastic, Fixed-length cracks)
Cohesive Zone Modeling Approach Overcomes these Limitations
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#2 - CZM Approach
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sep
d
0
)(Gf
Cohesive Zone Model
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Crack Mouth Opening Displacement (CMOD) Gage
Schematic of DC(T) Fracture Test
Used to Obtain Fracture Energy, and can be used to Estimate Material Tensile
Strength
FE Model Input
• Elastic properties– Young’s modulus (E)– Poisson’s ratio (ν)
• Viscoelastic properties– Creep compliance
• Fracture properties– Fracture energy (Gf)
– Tensile strength (St)
• The others– Layer thickness– LTE– Subgrade support– Thermal coefficient– Friction between PCC
and granular subbase– Gear loading time (e.g.,
0.1 sec = 50 mph)– Pavement temperature
profiles (EICM)
Thermal Loading Only
- 1:00pm – 7:00am, 18 Hours -
(Fracture Energy = 200 J/m2, Tensile Strength = 3.56 MPa)
Joint 1 Joint 2(CZM)
Joint 3 Joint 4 Joint 5
Critical Pavement Cooling Cycle
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-16
-14
-12
-10
-8
-6
-4
-2
0
-10 0 10 20 30 40
Temperature (Fahrenheit)
Dep
th (
inch
)
1:00 PM 2:00 PM3:00 PM 4:00 PM5:00 PM 6:00 PM7:00 PM 8:00 PM9:00 PM 10:00 PM11:00 PM 12:00 AM1:00 AM 2:00 AM3:00 AM 4:00 AM5:00 AM 6:00 AM7:00 AM
1:00pm(Jan. 4th) – 7:00am (Jan. 5th, 1999) : 18 hours
AC Overlay
PCC
4”
9”
Existing AC4”
Pavement temperature profiles were determined by the enhanced integrated
climate model (EICM, 2003).
O’Hare Airport (ORD) 4L-22R
Example of Thermal Analysis
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Thermal + Gear Loading
- 3:00am – 4:00am, 1 Hour + Gear Loading -
(Fracture Energy = 200 J/m2, Tensile Strength = 3.56 MPa)
Joint 1 Joint 2(CZM)
Joint 3 Joint 4 Joint 5
Gear Loading Position
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Example of Crack Propagation
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Summary Two different fracture models were applied to
investigate the mechanism of reflective cracking in the airfield overlay system.
Both J-integral and CZM approaches can be used to obtain a non-arbitrary assessment of critical responses in airfield overly systems (HMA/PCC).
Both fracture approaches provide useful tools to study the complex mechanism behind reflective cracking in airfield overlay systems.
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RC Research DirectionCurrent and Proposed
Complete CZM Simulations
Compile Major Comprehensive Report (with Recommendations) by April
Make Recommendations Surrogate Model Feasibility and Promising Directions
Document Needs for Additional Field Data (and Supporting Lab Data)
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Etc.
Post-Doc – Su Kai
Qinwu Xu (China Penn State)
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Thank You !!