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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

5

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?