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Development and Evaluation of Fracture
Mechanics Test Methods for Sandwich Composites
2013 Technical Review Dan Adams, Zack Bluth, Ryan Braegger
University of Utah
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FAA Sponsored Project Information
• Principal Investigator: Dr. Dan Adams
• Graduate Student Researchers: Ryan Braegger Zach Bluth
• FAA Technical Monitor Curt Davies David Westlund
• Collaborators:
NASA Langley NIAR Boeing Learjet Airbus UTC Aerospace Systems
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RESEARCH OBJECTIVES:
Fracture Mechanics Test Methods for Sandwich Composites
• Focus on facesheet-core debonding • Mode I and Mode II
– Identification and initial assessment of candidate test methodologies
– Selection and optimization of best suited Mode I and Mode II test methods
– Development of draft ASTM standards
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MODE I TEST CONFIGURATION: Candidate Configurations Investigated
Double Cantilever Beam (DCB)
Piano Hinge
Delamination
Crack Tip
Applied Load
Piano Hinge
Delamination
Crack Tip
Applied Load Cantilever
Blocks
Right Support
Rod
Delamination
Crack Tip
Applied Load
Left Support Rod
Center Support
Rod
Piano Hinge
Delamination
Crack Tip
Applied Load
Plate Support
Clamped Double Cantilever Beam (DCB)
Single Cantilever Beam (SCB) Three-Point Flexure
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Piano Hinge
Delamination
Crack Tip
Applied Load
Plate Support
MODE I TEST CONFIGURATION: Single Cantilever Beam (SCB)
Elimination of bending of sandwich specimen
Minimal crack “kinking” observed
Mode I dominant - independent of crack length
Appears to be suitable for standardization
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PARAMETERS INVESTIGATED: Single Cantilever Beam (SCB) Test
• Specimen geometry • Length • Width • Initial crack length
• Facesheet properties • Thickness • Flexural stiffness • Flexural strength
• Core properties • Thickness • Density • Stiffness • Strength
• Mode mixity • Variations across specimen width • Variations with crack length
• Data reduction methods • Thru-thickness crack placement • Anticlastic curvature & curved crack
front • Large rotations of facesheet • Use of facesheet doublers • Facesheet curvature effects
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SCB TEST METHOD DEVELOPMENT: Sandwich Configurations with Thin Facesheets
Concern: Excessive facesheet rotation • Not representative of disbond in actual
sandwich structures • Geometric nonlinearity causes errors
when using conventional data reduction method
Possible Solution: Use of facesheet doublers
Plate Support
Facesheet Doubler • Reduce facesheet rotation
required for disbonding • Allow use of compliance
calibration method of data reduction
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No doubler
0.6 mm doubler
1.6 mm doubler
0
0.5
1
1.5
2
2.5
0 10 20 30 40 50 60 70 80 90
Gc
(N/m
m)
Crack Length (mm)
EFFECTS OF FACESHEET DOUBLER:
Results of SCB Testing With Nomex Honeycomb Core
Adding doubler changes delivered Gc values… …and thru-thickness fracture locations!
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NUMERICAL INVESTIGATION
Facesheet Thickness Effects
• Load applied in each model to produce same GT value – No doubler, “thin” doubler, “thick” doubler
• Considered crack growth at three through-the-thickness locations
• Investigate mode mixity (% GI) • Investigate orientation of max.
principal stress for expected crack growth direction
Near interface 1 mm depth 0.5 mm depth
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• SCB test appears to be Mode I dominant for all cases considered
• Small Mode II component produced by shear stresses in vicinity of crack tip
• Sign of shear stresses change as a function of: – Crack location in core – Thickness of facesheet
• Crack predicted to propagate closer to facesheet/core interface for thinner facesheets
SUMMARY OF FINDINGS: Numerical Investigation
Disbond near interface (in core)
Disbond 1 mm below interface
(in core)
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P
P
EFFECTS OF FACESHEET CURVATURE: Use of Climbing Drum Peel (CDP) Test
• Facesheet curvature during SCB testing is dependent on facesheet thickness
• High curvature produced with thin facesheets not representative of that seen in sandwich structures with disbonds
• Use of Climbing Drum Peel test permits testing with prescribed facesheet curvature
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DETERMINATION OF ENERGY RELEASE RATE, GC: Climbing Drum Peel (CDP) Test
Energy Release Rate, GIC: r2 = flange radius r1 = drum radius + facesheet thickness
w = specimen width
P2
P1 r1
r2
P
P A.T. Nettles, E.D. Gregory and J.R. Jackson, “Using the Climbing Drum Peel (CDP) Test to
Obtain a GIC Value for Core/Face Sheet Bond,” Journal of Composite Materials, Vol 41, 2007.
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CLIMBING DRUM PEEL (CDP) TESTING: Investigating Facesheet Curvature Effects
Standard CDP Fixture ASTM D 1781
r = 2 in.
Large CDP Fixture r = 6 in.
Very Large CDP Fixture r = 12 in.
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CLIMBING DRUM PEEL (CDP) TESTING: Investigating Facesheet Curvature Effects
Standard CDP Fixture ASTM D 1781
r = 2 in.
Large CDP Fixture r = 6 in.
Very Large CDP Fixture r = 12 in.
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PRELIMINARY: Effects of Facesheet Curvature on Apparent Gc
12 in. CDP
6 in. CDP
2 in. CDP
SCB
• [0/90/0]nT IM7/8552 carbon/epoxy facesheets • 3 lb/ft3 Nomex honeycomb core
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Effect of Facesheet Thickness: Single Cantilever Beam (SCB) Specimens
Tested Portion Untested Precrack
3 Ply Facesheet 6 Ply Facesheet 9 Ply Facesheet
Change in fracture location with facesheet thickness
12 in. CDP
6 in. CDP
2 in. CDP
SCB
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Effect of Facesheet Thickness: 6 in. Radius Climbing Drum Peel (CDP) Specimens
3 Ply Facesheet 6 Ply Facesheet 9 Ply Facesheet
Minimal change in fracture location with facesheet thickness
12 in. CDP
6 in. CDP
2 in. CDP
SCB
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Effect of Facesheet Curvature 3 Ply Facesheet Specimens
SCB 2 in. CDP 6 in. CDP 12 in CPD
Minimal change in fracture location with facesheet curvature
12 in. CDP
6 in. CDP
2 in. CDP
SCB Tested Portion Untested Precrack
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RESULTS FROM NUMERICAL INVESTIGATION: Predicted Depth of Crack Growth in Nomex Core
• Increasing depth of crack for increasing facesheet thickness • Crack location independent of test method
Test Method
3 Ply Facesheets
6 Ply Facesheets
9 Ply Facesheets
SCB 0.25 mm (0.010 in.)
0.75 mm (0.030 in.)
1.0 mm (0.039 in.)
2 in. CDP 0.38 mm (0.015 in.)
0.75 mm (0.030 in.)
1.0 mm (0.039 in.)
6 in. CDP 0.38 mm (0.015 in.)
0.75 mm (0.030 in.)
1.0 mm (0.039 in.)
12 in. CDP 0.38 mm (0.015 in.)
0.75 mm (0.030 in.)
1.0 mm (0.039 in.)
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SUMMARY OF PRELIMINARY FINDINGS: Facesheet Thickness Effects
• SCB test results show differences in apparent Gc values and through-thickness locations of crack growth as a function of facesheet thickness
• CDB test results to date do not indicate differences in apparent Gc or through-thickness locations of crack growth as a function of facesheet thickness
• Numerical simulations suggest through-thickness locations of crack growth is a function of facesheet thickness for all test methods investigated
• Additional testing to be performed using specimens from single sandwich panel
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• Maintaining Mode II dominated crack growth with increasing crack lengths
• Obtaining crack opening during loading
• Obtaining stable crack growth along facesheet/core interface
MODE II TEST METHOD DEVELOPMENT: Challenges in Developing a Suitable Test
Delamination Hinge
Mixed Mode Bend
Cracked Sandwich Beam with Hinge
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SELECTED MODE II CONFIGURATION: End Notched Sandwich Test
• 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
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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 Unstable Crack Growth
Minimum pre-crack
Pre-crack
Precrack Length/Span Length
Appl
ied
Dis
plac
emen
t (m
)
• Selection of proper precrack length/span length predicted to produce stable crack growth
• Test results have confirmed this prediction
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END-NOTCHED TEST CONFIGURATIONS: Three-Point Flexure Vs. Cantilever Support
Monolithic Composites: 3 Point End Notch Flexure (3ENF)
(Currently proposed for ASTM standardization)
Sandwich Composites: End Notch Cantilever (ENC)
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END-NOTCHED TEST CONFIGURATIONS: Three-Point Flexure Vs. Cantilever Support
End Notched Cantilever (Symmetric bending)
End Notched Flexure (Unsymmetric bending)
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MODIFIED MODE II CONFIGURATION End Notched Cantilever (ENC) Test
• Cantilever beam configuration • Upward or downward loading • Performance meets or exceeds
3-point flexure configuration for all sandwich configurations considered to date
• Requires specialized fixturing • Allows for reduced specimen
length • Currently under further
examination
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CURRENT STATUS:
Fracture Mechanics Test Methods for Sandwich Composites
• Completion of remaining testing and analysis • Documentation of findings
– FAA Reports
– Journal publications
• Submission of Draft SCB Test Method to ASTM D30 • Summary of findings at European Honeycomb
Sandwich Disbond Growth Workshop (EASA, Cologne, June 2013)
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SUMMARY
Benefits 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 disbond
growth in composite sandwich structures
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