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DEVELOPMENT AND EVALUATION OF FRACTURE MECHANICS TEST METHODS
FOR SANDWICH COMPOSITES
Andy Gill presenting for Dr. Dan Adams
Department of Mechanical Engineering University of Utah Salt Lake City, UT
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Principal Investigator: Dr. Dan Adams Graduate Student Researchers:
Chris Weaver Andy Gill Brad Kuramoto Josh Bluth
FAA Technical Monitor Curt Davies
FAA Sponsored Project Information
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BACKGROUND: FRACTURE MECHANICS TEST METHODS
FOR SANDICH 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
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RESEARCH OBJECTIVE
Develop fracture mechanics test methods for sandwich composites Focus on facesheet core
delamination Both Mode I and Mode II Suitable for ASTM standardization
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RESEARCH APPROACH: THREE PHASE PROGRAM
PHASE I: Identification and initial assessment of candidate test methodologies
PHASE II: Selection and optimization of best suited Mode I and Mode II test methods
PHASE III: Development of draft ASTM standards
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PHASE I (REVIEW): Identification and initial assessment of
candidate test methodologies Identify candidate Mode I and Mode II test
methodologies Literature review- Lead to five Mode I and eight Mode II
configurations Modifications from adhesive and composite laminate tests Original concepts were also created
Identification of materials and geometries currently in use for structural sandwich composites
Assessment of candidate test configurations using finite element analysis
Select promising configurations for mechanical testing
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PHASE I CONTINUED Identification and initial assessment of
candidate test methodologies Three core materials (12-14 mm thickness)
Polyurethane foam core with density of 160 kg/m3 (10 lb/ft3) Nomex honeycomb core Aluminum honeycomb core
Two facesheet materials (1.3-1.5 mm thickness each) Woven carbon/epoxy, VARTM processed Unidirectional carbon/epoxy, secondary bonding
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PHASE I CONTINUED:
Finite element analysis of initial test configurations Evaluate fracture mode mixity (i.e. Mode I vs. Mode II) Analyze stress state within specimen Monitor crack opening after load application (Mode II) Determine suitable loading geometries Select promising Mode I and Mode II test configurations for
mechanical testing
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PHASE I CONTINUED:
Finite element modeling ANSYS 8.0 software Two-dimensional, plane strain, geometrically nonlinear
analyses Crack path created with a row of overlapping nodes,
coupled beyond crack tip Crack closure method used to calculate energy release
rates, GI and GII
Core
Facesheet
Coupled Nodes
Crack
Crack Tip Y
X
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Identification of Mode I test configurations Double Cantilever Beam (DCB)
Significant Mode II component Significant bending stresses in core Crack “kinking” for Nomex
honeycomb core Specimen rotation due to off axis loading Determined to be unsuitable for a
standard test method
Modified DCB (MDCB) Significant Mode II component Crack “kinking” for Nomex
honeycomb core Determined to be unsuitable for a
standard test method
PHASE I MECHANICAL TEST RESULTS:
Mode I Investigation
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Single Cantilever Beam (SCB) with cantilever beam support
Significant Mode II component Crack “kinking” for Nomex
honeycomb core Determined to be unsuitable for a standard test method
Three Point Flexure (TPF) Significant bending stresses in
core Extra machining operations
required for specimen Determined to be unsuitable for
a standard test method
PHASE I MECHANICAL TEST RESULTS:
Mode I Investigation
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Plate-Supported SCB (MSCB) 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 method
Piano Hinge
Delamination
Crack Tip
Applied Load
Plate Support
PHASE I MECHANICAL TEST RESULTS:
Mode I Investigation
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Identification of Mode II test configuration 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 Facesheet delamination test DCB with uneven bending moments Three-point cantilever Double sandwich test
PHASE I RESULTS: Mode II Investigation
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PHASE I RESULTS: Mode II Investigation
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
Only two of the ten investigated test configurations produced any form of interlaminar stable crack growth
Modified CSB (MCSB) Mixed Mode Bending (MMB)
Seven test configurations experienced crack “kinking”, the other unstable
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Mixed Mode Bending (MMB) Crack opening as delamination
propagates for foam core Possible to achieve high
percentage Mode II (>90%) using short lever arm lengths
Semi-stable crack growth for foam core
Crack “kinking” for Nomex honeycomb core
Core crushing for aluminum honeycomb core
Not well suited for a standard Mode II test method
PHASE I MECHANICAL TEST RESULTS:
Mode II Investigation
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Modified Cracked Sandwich Beam with Hinge Creates crack opening as
delamination propagates 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
PHASE I MECHANICAL TEST RESULTS:
Mode I Investigation
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PHASE II ACTIVITIES: Further Development of
Mode I and Mode II Test Methods
Sensitivity study – determination of acceptable range of specimen parameters
Development of suitable test fixturing Development of suitable test procedures Development of suitable data analysis methods
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SENSITIVITY STUDIES: Determination of Acceptable Ranges
of Specimen Parameters
Facesheet parameters Thickness, flexural stiffness, flexural strength
Core parameters Thickness, density, stiffness, strength
Specimen and delamination geometry
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Piano Hinge
Delamination
Crack Tip
Applied Load
Plate Support
Use of plate-supported Single Cantilever Beam (SCB) test
Focus on two parameters of concern Sandwich core material
Facesheet thickness
Investigate mode mixity for range of delamination lengths
CURRENT FOCUS: Mode I Sensitivity Study
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MODE I SENSITIVITY STUDY: Effect of Core Material on %Mode I
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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MODE I SENSITIVITY STUDY: Effect of Facesheet Thickness
Mode I dominant over range of facesheet thicknesses 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
99.965 99.97
99.975 99.98
99.985 99.99
99.995 100
0 10 20 30 40 50 60 70 80 90
Woven carbon/epoxy facesheets, polyurethane foam core
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CURRENT FOCUS: Mode I Test Fixture Development
22
Ability to test 1 in. to 3 in. wide sandwich specimens
Edge clamp restraints to lower panel support
Translating fixture base
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Use of Modified Cracked Sandwich Beam Determination of acceptable range of specimen
parameters Core thickness, stiffness Facesheet flexural stiffness
Investigate mode mixity and crack opening for range of delamination lengths
CURRENT FOCUS: Mode II Sensitivity Study
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MODE II SENSITIVITY STUDY: Effect of Core Material
0.00E+00
5.00E+06
1.00E+07
1.50E+07
2.00E+07
2.50E+07
3.00E+07
3.50E+07
4.00E+07
4.50E+07
0 10 20 30 40 50 60 70
Long
itudi
nal M
odul
us (P
a)
Critical Crack Size (mm)
Longitudinal Direction Modulus of Core vs. Critical Crack Size Varying the cores in plane
modulus has little affect on % Mode II Foam, Nomex, and aluminum
honeycomb all remained above 90%
Failure of test decided when there is core/face- sheet interaction
In plane modulus of core affects crack length at which interaction begins
Use trend line to develop MCSB core material test limits
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CURRENT FOCUS: Mode II Test Fixture Development
25
Modified three-point flexure configuration
Emphasis on minimizing specialized specimen preparation-core removal
Proposed design would support top face sheet without need of core removal
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UPCOMING ACTIVITIES: Further Development of
Mode I and Mode II Test Methods
Sensitivity study – determination of acceptable range of specimen parameters Computational simulations to determine limits Experimental validation of limits
Fabrication and evaluation of test fixturing Development of suitable test procedures Development of suitable data analysis methods
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Thank You For Your Time Any Questions