FRACTURE MECHANICS TEST METHODS

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Augustus Wilkerson
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1 DEVELOPMENT AND EVALUATION OF FRACTURE MECHANICS TEST METHODS FOR SANDWICH COMPOSITES Dan Adams Department of Mechanical Engineering University it of Utah Salt Lake City, UT AMTAS A t 2012 M ti AMTAS Autumn 2012 Meeting Seattle, WA October 31, 2012

2 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 Boeing Airbus NIAR Learjet UTC Aerospace Systems

RESEARCH OBJECTIVES: Fracture Mechanics Test Methods for Sandwich Composites Focus on facesheet-core core debonding Mode I and Mode II Identification and

3 RESEARCH OBJECTIVES: Fracture Mechanics Test Methods for Sandwich Composites Focus on facesheet-core 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 d

dominant - independent of crack length Appears to be suitable for

4 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 Applied Load Delamination Crack Tip Plate Support Piano Hinge

PARAMETERS INVESTIGATED: Single Cantilever Beam (SCB) Test Specimen geometry

Flexural stiffness Flexural strength Core properties Thickness Density Stiffness

reduction methods Thru-thickness crack placement Anticlastic curvature & curved

5 PARAMETERS INVESTIGATED: Single Cantilever Beam (SCB) Test Specimen geometry Mode mixity Length Width Initial crack length Facesheet properties Thickness Flexural stiffness Flexural strength Core properties Thickness Density Stiffness Strength 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

SCB TEST METHOD DEVELOPMENT: Sandwich Configurations with Thin Facesheets

actual sandwich structures Geometric nonlinearity causes errors when

facesheet doublers Reduce facesheet rotation required for disbonding

6 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 Reduce facesheet rotation required for disbonding Allow use of compliance calibration method of data reduction Facesheet Doubler Plate Support

$thickness fracture locations! 2.$ 5 Gc (N/ /mm) 1 0.6 mm doubler 0.

7 EFFECTS OF FACESHEET DOUBLER: Different Doubler Thicknesses Produce Different G c Values and different thru-thickness thickness fracture locations! mm doubler 1.5 Gc (N/ /mm) mm doubler No doubler Crack Length (mm)

NUMERICAL INVESTIGATION Effects of Thin Facesheets & Facesheet

doubler, thin doubler, thick doubler Consider crack growth at

(% G I ) Investigate orientation of max.

8 NUMERICAL INVESTIGATION Effects of Thin Facesheets & Facesheet Doublers Load applied in each model to produce same GT value No doubler, thin doubler, thick doubler Consider crack growth at three through-the-thickness locations Investigate mode mixity (% G I ) Investigate orientation of max. principal stress for expected crack growth direction At interface 0.5 mm depth 1 mm depth

9 FACESHEET DOUBLER EFFECTS: No Doubler Facesheet Crack Core Crack at interface Shear Stress Gradient 99.3% G I

10 FACESHEET DOUBLER EFFECTS: Thin Doubler Glass Doubler Facesheet Crack Core At interface 97.2% G I Glass Doubler Facesheet Core Above Crack Crack Core 0.5 mm depth 99.9% G I

11 FACESHEET DOUBLER EFFECTS: Thick Doubler At interface 93.8% G I 98.3% G I 0.5 mm depth 1 mm depth 99.8% G I

stresses change as a function of: Thickness of facesheet Crack location in core Crack predicted to

12 SUMMARY OF FINDINGS: Numerical Investigation SCB test appears to be Mode I dominant for all cases considered Mode II component produced by shear stresses in vicinity i it of crack tip Sign of shear stresses change as a function of: Thickness of facesheet Crack location in core Crack predicted to propagate closer to facesheet/core interface for thinner facesheets Use of doublers to reduce facesheet rotation is not recommended

produced with thin facesheets not representative of that seen in sandwich structures

13 EFFECTS OF FACESHEET CURVATURE: Use of Climbing Drum Peel (CDP) Test Facesheet curvature during SCB testing is P 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 P

) ( r 2 r 1 ) P P 2 r 2 r 1 P 1 P A.T. Nettles, E.D. Gregory and J.R.

14 DETERMINATION OF ENERGY RELEASE RATE, G C : Climbing Drum Peel (CDP) Test Energy Release Rate, G IC : P 2 P 1 G IC = P 2 P 1 r 2 = flange radius wr 1 r 1 = drum radius + facesheet thickness w = specimen width ( ) ( r 2 r 1 ) P P 2 r 2 r 1 P 1 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.

15 Single Cantilever Beam (SCB) Versus Climbing Drum Peel (CDP) 9Ply( Thick ) Facesheet SCB Test w/ CC SCB Test w/ MBT CDP Test

16 Single Cantilever Beam (SCB) Versus Climbing Drum Peel (CDP) 6Ply( Medium ) Facesheet SCB Test w/ CC SCB Test w/ MBT CDP Test

17 Single Cantilever Beam (SCB) Versus Climbing Drum Peel (CDP) 3Ply( Thin ) Facesheet SCB Test w/ CC SCB Test w/ MBT CDP Test

18 Effect of Facesheet Thickness: Single Cantilever Beam (SCB) Specimens 3 Ply Facesheet 6 Ply Facesheet 9 Ply Facesheet Untested Tested Portion Precrack

19 Effect of Facesheet Thickness: Climbing Drum Peel (CDP) Specimens 3 Ply Facesheet 6 Ply Facesheet 9 Ply Facesheet Untested Tested Portion Precrack

20 SUMMARY OF PRELIMINARY FINDINGS: Climbing Drum Peel Testing G C measurements from Climbing Drum Peel and Single Cantilever Beam tests in agreement for thicker facesheets G C measurements from Single Cantilever Beam tests t are reduced d for thin facesheets Slight through-thickness difference in fracture location with facesheet thickness for both test methods

21 CURRENT FOCUS: Effects of Facesheet Curvature on Apparent G c f Preliminary design of a large radius Climbing Drum Peel fixture

22 MODE II TEST CONFIGURATION: Edge-Notched Sandwich Configurations Monolithic Composites: 3 Point End Notch Flexure (3ENF) (Currently proposed for ASTM standardization) Sandwich Composites: End Notch Cantilever (ENC)

23 MODE II END NOTCHED CANTILEVER TEST: Symmetrical Bending Version of 3-ENF End Notched Flexure (Unsymmetric bending) End Notched Cantilever (Symmetric bending)

PROPOSED MODE II CONFIGURATION End Notched Cantilever (ENC)

(tension) or downward wa d (compression) o Predicted

all sandwich configurations considered to date Improved crack

24 PROPOSED MODE II CONFIGURATION End Notched Cantilever (ENC) Test Cantilever beam configuration i Can be loaded upward (tension) or downward wa d (compression) o Predicted performance meets or exceeds that of 3-ENF configuration for all sandwich configurations considered to date Improved crack growth stability Appears to be suitable for a standard Mode II test method

$SUMMARY Benefits to Aviation Standardized fracture mechanics test methods for sandwich composites Mode I fracture$

25 SUMMARY Benefits to Aviation Standardized fracture mechanics test methods for sandwich composites Mode I fracture toughness, G IC Mode II fracture toughness, G IIC Test results used to predict disbond growth in composite sandwich structures

Durability of bonded aircraft structure. AMTAS Fall 2016 meeting October 27 th 2016 Seattle, WA

Durability of bonded aircraft structure AMTAS Fall 216 meeting October 27 th 216 Seattle, WA Durability of Bonded Aircraft Structure Motivation and Key Issues: Adhesive bonding is a key path towards reduced