Development and Evaluation of Fracture Mechanics Test Methods for Sandwich Composites

Transcription

1 Development and Evaluation of Fracture Mechanics Test Methods for Sandwich Composites 2011 Technical Review Dan Adams, Joe Nelson, Zack Bluth University of Utah

2 FAA Sponsored Project Information Principal Investigator: Dr. Dan Adams Graduate Student Researchers: Joe Nelson Josh Bluth Chris Weaver FAA Technical Monitor Curt Davies Zack Bluth Brad Kuramoto Andy Gill Collaborators: NASA Langley Research Center (James Ratcliffe) 2

3 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 3

4 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 4

5 IDENTIFICATION AND EVALUATION OF CANDIDATE MODE I TEST CONFIGURATIONS 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 Applied Load Delamination Piano Hinge Crack Tip Plate Support 5

6 SELECTED MODE I CONFIGURATION: 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 method Applied Load Delamination Piano Hinge Crack Tip Plate Support 6

7 MODE I SENSITIVITY STUDY: FACESHEET THICKNESS EFFECTS Woven carbon/epoxy facesheets, polyurethane foam core Mode I dominant over range of facesheet thicknesses and crack lengths considered Percent Mode I Percent Mode I vs. Crack Length mm Facesheet 1.27 mm Facesheet mm Facesheet Crack Length (mm) Single Cantilever Beam (SCB) 7

8 SCB TEST MODE MIXITY: Core Material Effects Mode I dominant over range of cores considered Minimal variability among materials and crack lengths Percent Mode I Test appears suitable 30 for a wide range of 20 common core 10 materials Percent Mode I vs. Crack Length Crack Length (mm) Last-A-Foam FR Polyurethane Foam Euro-Composites Nomex Honeycomb ECA 3/16-3.0b CR III 1/ Aluminum Honeycomb Baltek SuperLite S67 End-Grain Balsa Wood 8

9 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. 9

10 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) 10

11 Example SCB Test Results: Semi-Stable Crack Growth For Common Core Materials Load (N) Load (N) Displacement (mm) Displacement (mm) Nomex Honeycomb Aluminum Honeycomb Load (N) Load (N) Displacement (mm) Polyurethane Foam Displacement (mm) 11 End-Grain Balsa Wood 11

12 SCB Specimen Sizing: Specimen Width Effects Mechanical testing using three specimen widths 1 in. 2 in. 3 in. Three core materials investigated Aluminum honeycomb Nomex honeycomb Polyurethane foam Three-dimensional finite element modeling G IC variations across specimen width 12

13 SPECIMEN WIDTH EFFECTS: Anticlastic Bending Crack front lagging on the free edges due to anticlastic bending of facesheet Anticlastic bending highly dependent on ν 12 of facesheets Symmetry BC Interlaminar normal stress at surface of core (Mode I stress) 13

14 SPECIMEN WIDTH EFFECTS: Methods For Reducing Variation in G I Increase facesheet bending stiffness, EI - Thicker facesheet - Addition of doubler (tabbing material) Reduce ν 12 of facesheet Increase specimen width G I edge /G I center Increased facesheet stiffness decreases edge effects Edge Specimen Width Specimen Width Center 14

15 SCB FACESHEET THICKNESS EFFECTS: Adding Tabbing Doublers to Thin Facesheets Problems with thin facesheets due to large deflections/rotations Tabbing Doubler Plate Support Piano Hinge Tabbing of upper facesheet predicted to increases accuracy of G IC calculation Experimental investigation underway 15

16 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 16

17 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 Mixed Mode Bend (MMB) Configuration Delamination Hinge Modified Cracked Sandwich Beam (CSB) with Hinge 17

18 SELECTED MODE II CONFIGURATION: END NOTCHED SANDWICH (ENS) TEST 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 18

19 Mode II Test Results: Foam and Nomex Honeycomb Cores Semi-stable delamination propagation 19

20 Mode II Test Results: Aluminum Honeycomb Core Core failure in aluminum honeycomb prior to delamination growth 20

21 Mode II End Notch Sandwich Test: Numerical Investigations Performed Mode mixity of crack growth (% G II ) Specimen width effects Facesheet thickness effects Adding doubler to lower facesheet Crack growth stability Specimen length effects Precrack length effects 21

22 Addressing Mode Mixity/Width Variations: Adding Flexural Stiffness to Bottom Facesheet Increasing flexural stiffness (EI) of lower portion of delaminated specimen reduces specimen width effect Fraction of Mode II Increasing ratio Upper/Lower facesheet thickness ratio Distance From Edge, in. 0.5 in. 22

23 Addressing Crack Growth Stability: Specimen Span Length and Precrack Length Selection of proper precrack length/span length expected to produce stable crack growth Experimental investigation underway Applied Displacement (m) Beam Deflection(m) Required Displacement for Crack Growth 0 Pre-crack Region of Unstable Crack Growth Minimum pre-crack Precrack length/span a/l Length 23

24 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 24 24

25 SUMMARY Benefit 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 delamination growth in composite sandwich structures 25

26 Thank you for your attention! Questions? 26