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Advances in Composites Testing
Dan AdamsUniversity of UtahSalt Lake City, UT
March 21, 2018
Overview:New ASTM Standards Under Development at the U of U
• Fracture Mechanics Test Methods for Sandwich Composites
• Durability of Composite Adhesive Bonds
• Energy Absorption/Crashworthiness
• Damage Tolerance Testing of Sandwich Composites
Fracture Mechanics Test Methods for Sandwich Composites:
Background• Fracture mechanics test methods
for composites have reached a high level of maturity
• Less attention to sandwich composites
• Recent concerns regarding facesheet/core disbond in aviation industry
End Notched Flexure TestASTM D 7905
Double Cantilever Beam TestASTM D 5528
Objectives:Fracture Mechanics Test Methods for Sandwich Composites
• Focus on facesheet/core disbonding• Mode I and Mode II test methods
• 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
Identification and Evaluation of Candidate Mode I Test Configurations
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Piano Hinge
Delamination
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)
Sandwich specimen precracked at one end
Specimen secured to fixture base plate
Upward loading of upper facesheet using bonded hinge
Measurement of load, displacement, crack length
Calculation of critical energy release rate, Gc
Currently being balloted for ASTM standardization
Selected Mode I Test Configuration:Single Cantilever Beam (SCB) Test
Mode II Sandwich Disbond Testing:Challenges in Test Method Development
• Obtaining stable crack growth along facesheet/core interface
• Maintaining Mode II dominated crack growth with increasing crack lengths
• Obtaining crack opening during loading
SELECTED MODE II TEST CONFIGURATION:Separated End Notched Flexure (S-ENF) Test
• Modified three-point flexure test configuration
• High % Mode II (>80%) for all sandwich configurations studied
• Use of tensioned wire to achieve facesheet/core separation
• No core removal required• Adjustable wire height and span
Overview:New ASTM Standards Under Development at the U of U
• Fracture Mechanics Test Methods for Sandwich Composites
• Durability of Composite Adhesive Bonds
• Energy Absorption/Crashworthiness
• Damage Tolerance Testing of Sandwich Composites
Background:The Metal Wedge Crack Durability Test
• Bonded aluminum double cantilever beam loaded by forcing a wedge between adherends• Assembly placed into test
environment (ex: 50⁰ C, 95% RH)• Crack growth ∆a due to environmental
exposure measured following prescribed time period• Able to asses quality of bond quickly by
causing rapid hydration of oxide layers
ASTM D 3762: “Standard Test Method for Adhesive-Bonded Surface Durability of Aluminum (Wedge Test)”
Update to ASTM D 3762 standard underway following University of Utah/FAA research investigation
• Variable flexural stiffness of composite adherends• Environmental crack growth dependent on
adherend flexural stiffness• Flexural stiffness must be
within an acceptable range or…
• Must tailor wedge thickness for composite adherends
or…• Must use another quantity to assess durability
• Restrictions in fiber orientation adjacent to bonded interface• Failure in the composite laminate prior to failure in the
adhesive or at the bondline
Development of a Composite Wedge Test:Additional Complexities
Use of Fracture Toughness, GcTo Assess Environmental Durability
Consider composite adherends as cantilever beams• Measured values of crack length, a• Known value of beam deflection, δ
δ = t/2 (half of wedge thickness)
Tip deflection of a cantilever beam:
Strain energy due to bending:
Strain energy release rate:
.
a = crack lengtht = wedge thicknessh = adherend thicknessb = specimen widthT = load to deflect tip of beamEf = flexural modulus
= fracture toughness
Correction factor for crack tip rotation
• Unidirectional IM7/8552 carbon/epoxy adherends
• AF163-2K film adhesive• “Ideal Bond”: Grit-blast &
acetone wipe bond surfaces• Multiple adherend thicknesses
to produce different Ef
• 13, 15, 17, 19, 21, 23 ply thicknesses
• 122°F (50°C) and 95% humidity environment
Experimental Investigation:
Composite Wedge Test Development
Effects of Composite Adherend Thickness:Crack Length Measurements
Crack length measurements change with varying adherend flexural stiffness…
Crack Length, a Crack Growth, ∆a
Effects of Composite Adherend Thickness:Fracture Toughness Values
Apparent facture toughness values remain relatively constant
• Use of fracture toughness Gc for durability assessment of bonded composites appears promising
• Positive feedback from FAA and aerospace industry• Currently comparing measured fracture toughness values with results from other test methods
• Preparing ASTM standard for balloting
Composite Wedge Test Development:Status
Overview:New ASTM Standards Under Development at the U of U
• Fracture Mechanics Test Methods for Sandwich Composites
• Durability of Composite Adhesive Bonds
• Energy Absorption/Crashworthiness
• Damage Tolerance Testing of Sandwich Composites
• Metal structures: plastic deformation• Composite structures: brittle‐type fracture
Images courtesy of Oak Ridge National Laboratory
Background:Differences in Energy Absorption Mechanisms
What’s Happening at the Crush Front?Crush Failure Modes Observed
Fiber Splaying• Long axial cracks• Frond formation• Delamination
dominated
Fragmentation• Short axial cracks• Shear failure from compressive stresses• Extensive fiber fracture
Brittle Fracture• Intermediate length cracks• Combines characteristics from other failure modes
What’s Happening at the Crush Front?Crush Failure Modes Observed
Fiber Splaying• Long axial cracks• Frond formation• Delamination
dominated
Fragmentation• Short axial cracks• Shear failure from compressive stresses• Extensive fiber fracture
Brittle Fracture• Intermediate length cracks• Combines characteristics from other failure modes
Fiber Splaying• Long axial cracks• Frond formation• Delamination
dominated
Fragmentation• Short axial cracks• Shear failure from compressive stresses• Extensive fiber fracture
Brittle Fracture• Intermediate length cracks• Combines characteristics from other failure modes
Energy Absorption
What’s Happening at the Crush Front?Crush Failure Modes Observed
Flat Coupon Testing: Current University of Utah Crush Test Fixture
Flat Coupon Crush Testing:Use of Unsupported and Pin‐Supported Base
Unsupported TestingFor Flat Sections
Pin‐Supported TestingFor Curved Sections & Corners
• Measure energy absorption for both support conditions
• For use in crush predictions of structural members
• Emphasis on FAA Crashworthiness Certification
• Testing to support analysis development and evaluation
• Focus on composite cargo floor stanchion in transport aircraft
Design‐value development
Material propertyevaluation
Componenttests
Sub‐component tests
Structural elements tests
Allowable development
Material specification development
Material screening and selection
Full‐scaletests Analysis validation
CMH-17 Crashworthiness Working Group:Crashworthiness of Composite Cargo Floor Stanchion
Floor Beam
Strut #3
Frame & Skin
Strut #3
Overview:New ASTM Standards Under Development at the U of U
• Fracture Mechanics Test Methods for Sandwich Composites
• Durability of Composite Adhesive Bonds
• Energy Absorption/Crashworthiness
• Damage Tolerance Testing of Sandwich Composites
Damage Tolerance Test MethodsFor Sandwich Composites
Currently under development at Univ. of Utah)
• Preferred damage tolerance test method for monolithic laminates
• High interest level for sandwich composites
• Constant bending moment and zero shear in damaged section of panel
• Damaged facesheet can be placed under compression or tension stress
Edgewise Compression Four‐Point Flexure
Thank you for your attention!
Questions?