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)

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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

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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

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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

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30

40

0 5 10

Load

(N)

Displacement (mm)

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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

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Mode II End Notch Sandwich Test:Numerical Investigations Performed

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• 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

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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

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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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