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Development and Evaluation of Fracture Mechanics Test Methods for Sandwich Composites
2011 Technical ReviewDan Adams, Joe Nelson, Zack BluthUniversity of Utah
FAA Sponsored Project Information
• Principal Investigator: Dr. Dan Adams
• Graduate Student Researchers:Joe Nelson Zack BluthJosh Bluth Brad KuramotoChris Weaver Andy Gill
• FAA Technical Monitor– Curt Davies
• Collaborators:– NASA Langley Research Center (James Ratcliffe)
2
BACKGROUND: Fracture Mechanics Test Methods for Sandwich Composites
• Fracture mechanics test methods for composites have reached a high level of maturity
• Less attention to sandwich composites– Focus on particular sandwich materials– Focus on environmental effects– No consensus on a suitable test configuration or specimen
geometry for Mode I or Mode II fracture toughness testing
3
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RESEARCH OBJECTIVES:Fracture Mechanics Test Methods for Sandwich Composites
• Focus on facesheet-core delamination• Mode I and Mode II
o Identification and initial assessment of candidate test methodologies
o Selection and optimization of best suited Mode I and Mode II test methods
o Development of draft ASTM standards
IDENTIFICATION AND EVALUATION OF CANDIDATE MODE I TEST CONFIGURATIONS
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Piano HingeDelamination
Crack Tip
Applied Load
Plate Support
• Double Cantilever Beam (DCB)
• Modified DCB (MDCB)
• Single Cantilever Beam (SCB) with cantilever beam support
• Three Point Flexure (TPF)
• Plate-Supported Single Cantilever Beam SCB
Elimination of bending of sandwich specimen
Minimal Mode II component (less than 5%)
No significant bending stresses in core
No crack “kinking” observed
Appears to be suitable for a standard test methodPiano Hinge
Delamination
Crack Tip
Applied Load
Plate Support
SELECTED MODE I CONFIGURATION:Single Cantilever Beam (SCB)
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Mode I dominant over range of facesheet thicknesses and crack lengths considered
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90
Perc
ent M
ode
I
Crack Length (mm)
Percent Mode I vs. Crack Length
0.635 mm Facesheet
1.27 mm Facesheet
1.905 mm Facesheet
99.96599.97
99.97599.98
99.98599.99
99.995100
0 10 20 30 40 50 60 70 80 90
Woven carbon/epoxy facesheets, polyurethane foam core
MODE I SENSITIVITY STUDY:
FACESHEET THICKNESS EFFECTS
Single Cantilever Beam (SCB)
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SCB TEST MODE MIXITY:Core Material Effects
Mode I dominant over range of cores considered
Minimal variability among materials and crack lengths
Test appears suitable for a wide range of common core materials
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90
Perc
ent M
ode
I
Crack Length (mm)
Percent Mode I vs. Crack Length
Last-A-Foam FR-6710 Polyurethane Foam
Euro-Composites Nomex Honeycomb ECA 3/16-3.0b
CR III 1/8-5056-0.002 Aluminum Honeycomb
Baltek SuperLite S67 End-Grain Balsa Wood
98
98.5
99
99.5
100
0 10 20 30 40 50 60 70 80 90
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SCB TEST MODE MIXITY:Effects of Crack Placement Within Facesheet/Core Interface Region
Modeled with and without an adhesive layer
Four crack locations Facesheet/core interface
(no adhesive) Within adhesive Above adhesive Below adhesive
No effect of crack location on mode mixity.
TEST METHOD ASSESSMENT:ANALYSIS AND TESTING
• Determination of Acceptable Ranges of Specimen Parameters– Facesheet parameters
Thickness, flexural stiffness, flexural strength– Core parameters
Thickness, density, stiffness, strength– Specimen length and width, initial delamination length
• Use of four different core materials– Nomex honeycomb – Aluminum honeycomb– Polyurethane foam– End-grain balsawood
• Carbon/epoxy facesheets (woven fabric and prepreg)
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Example SCB Test Results:Semi-Stable Crack Growth For Common Core Materials
0
10
20
30
40
0 5 10
Load
(N)
Displacement (mm)
0
20
40
60
80
0 5 10 15 20
Load
(N)
Displacement (mm)
0
20
40
60
80
100
0 5 10 15 20 25
Load
(N)
Displacement (mm)
01020304050
0 5 10 15 20 25Lo
ad (N
)Displacement (mm)
Nomex Honeycomb Aluminum Honeycomb
Polyurethane Foam End-Grain Balsa Wood
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Mechanical testing using three specimen widths1 in. 2 in. 3 in.
Three core materials investigated Aluminum honeycomb Nomex honeycomb Polyurethane foam
SCB Specimen Sizing:
Specimen Width Effects
Three-dimensional finite element modeling GIC variations across specimen width
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Crack front lagging on the free edges due to anticlastic bending of facesheet
Anticlastic bending highly dependent on ν12 of facesheets
SPECIMEN WIDTH EFFECTS:Anticlastic Bending
Interlaminar normal stress at surface of core (Mode I stress)
Symmetry BC
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Increased facesheet stiffness decreases
edge effects
Specimen WidthCenterEdge
GI e
dge/G
I cen
ter
SPECIMEN WIDTH EFFECTS:
Methods For Reducing Variation in GI
Increase facesheet bending stiffness, EI- Thicker facesheet - Addition of doubler (tabbing material)
Reduce ν12 of facesheet Increase specimen width
Specimen Width14
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SCB FACESHEET THICKNESS EFFECTS:Adding Tabbing “Doublers” to Thin Facesheets
Problems with thin facesheets due to large deflections/rotations
Tabbing of upper facesheet predicted to increases accuracy of GIC calculation
Experimental investigation underway
Piano Hinge
Plate Support
Tabbing Doubler
EVALUATION OF MODE II SANDWICH COMPOSITE TEST CONFIGURATIONS
• Three-point End Notch Flexure (3ENF)• Mixed Mode Bending (MMB)• End Load Split (ELS)• Four-point delamination test• Cracked Sandwich Beam (CSB) with hinge• Modified CSB with hinge• Facesheet delamination test• DCB with uneven bending moments• Three-point cantilever • Double sandwich test
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CHALLENGES IN DEVELOPING A SUITABLE MODE II TEST
• Maintaining Mode II dominated crack growth with increasing crack lengths
• Obtaining crack opening during loading
• Obtaining stable crack growth along facesheet/core interface
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Mixed Mode Bend (MMB) Configuration
Delamination Hinge
Modified Cracked Sandwich Beam (CSB) with Hinge
SELECTED MODE II CONFIGURATION:
END NOTCHED SANDWICH (ENS) TEST
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• Modified three-point flexure fixture• High percentage Mode II (>80%) for
all materials investigated• Semi-stable crack growth along
facesheet/core interface• Appears to be suitable for a standard
Mode II test method
Semi-stable delamination propagation
Mode II Test Results:
Foam and Nomex Honeycomb Cores
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Core failure in aluminum honeycomb prior to delamination growth
Mode II Test Results:
Aluminum Honeycomb Core
20
Mode II End Notch Sandwich Test:Numerical Investigations Performed
21
• Mode mixity of crack growth (% GII)• Specimen width effects• Facesheet thickness effects
• Adding doubler to lower facesheet• Crack growth stability
• Specimen length effects• Precrack length effects
Addressing Mode Mixity/Width Variations:Adding Flexural Stiffness to Bottom Facesheet
Frac
tion
of M
ode
II
Distance From Edge, in.0.5 in.
Increasing flexural stiffness (EI) of lower portion of delaminated specimen reduces specimen width effect
Upper/Lower facesheet thickness ratio
Increasing ratio
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Addressing Crack Growth Stability:Specimen Span Length and Precrack Length
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0 0.1 0.2 0.3 0.4 0.5 0.6
Bea
m D
efle
ctio
n(m
)
a/L
Required Displacement for Crack Growth
Region of UnstableCrack Growth
Minimum pre-crack
Pre-crack
Precrack length/Span Length
Appl
ied
Dis
plac
emen
t (m
)
Selection of proper precrack length/span length expected to produce stable crack growth
Experimental investigation underway
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CURRENT FOCUS:
FRACTURE MECHANICS TEST METHODS
Determination of Acceptable Ranges of Sandwich Configurations Facesheet parameters
Thickness, flexural stiffness, flexural strength
Core parameters Thickness, stiffness, strength
Specimen and delamination geometry
Composing draft ASTM standards
24
SUMMARY
• Benefit to Aviation– Standardized fracture mechanics test
methods for sandwich composites Mode I fracture toughness, GIC Mode II fracture toughness, GIIC
– Test results used to predict delamination growth in composite sandwich structures
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Thank you for your attention!
Questions?
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