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David C. Haeberle Masters Defense Department of Engineering Science and Mechanics June 8, 2001

THE USE OF NANONINDENTATION TO DETERMINE COMPOSITE INTERFACIAL SHEAR STRENGH AND THE EFFECTS OF ENVIRONMENTAL AGING. David C. Haeberle Masters Defense Department of Engineering Science and Mechanics June 8, 2001. Project Objectives.

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David C. Haeberle Masters Defense Department of Engineering Science and Mechanics June 8, 2001

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  1. THE USE OF NANONINDENTATION TO DETERMINE COMPOSITE INTERFACIAL SHEAR STRENGH AND THE EFFECTS OF ENVIRONMENTAL AGING David C. Haeberle Masters Defense Department of Engineering Science and Mechanics June 8, 2001

  2. Project Objectives • Refine and improve microindentation technique for determining interfacial shear strength (IFSS) and characteristics • Compare IFSS measurements for various sizing materials and determine the effect of moisture on IFSS • Relate changes in IFSS to changes in bulk composite properties • Use IFSS results to predict bulk properties

  3. Presentation Outline • Background • Improvements to Microindentation • Analysis of Nanoindentation Results • Comparison of Fiber Sizings • Effect of Hygrothermal Aging on Sizings • Tensile Strength Predictions • Conclusions and Recommendations

  4. Presentation Outline • Background • Improvements to Microindentation • Analysis of Nanoindentation Results • Comparison of Fiber Sizings • Effect of Hygrothermal Aging on Sizings • Tensile Strength Predictions • Conclusions and Recommendations

  5. Designing Fiber/Matrix Interphase Regions • Design interphase materials to have: • good adhesion to both fiber • and matrix • high yield stress Characterize interphase properties to feed into performance model Optimize Performance • Composite modeling to predict lifetime and performance: • Use micro-mechanical models to predict composite mechanical properties • Predict composite fatigue properties • using MRLife™

  6. Why do Sizings Affect Properties? • Processability- • Sizings protect the brittle carbon fiber • Sizings affect the wettability of the carbon fiber • The Interphase (0-1 m) -Causes changes in damage initiation and propagation. • Interdiffusion results in a concentration profile or a mechanical property profile. • Changes in stochiometry of the matrix reactants.

  7. Why do Sizings Affect Properties? • Matrix Plasticization (0-1 m) - An Interphase with no gradients. • Results from a sizing that has diffused to such an extent that no gradients exist. • Fiber/Matrix Adhesion (0-100 nm) - Causes changes in interfacial properties like shear strength. • Results from a sizing that is physically or chemically bonded to the fiber while interacting strongly with the matrix.

  8. Composites Evaluated • Pultruded on a small scale pultruder at Strongwell Corporation • Dow Derakane™ Vinyl Ester Matrix • Hexcel AS4 Carbon Fibers • Three types of sizings • G’, standard industrial sizing (epoxy oligomer) • LSP, carboxl modified poly(hydroxyether) • PVP, poly(vinylpyrrolidone)

  9. Axial Tensile Strength of Pultruded Composites Improved Tensile Strength

  10. 1 Run out 0.9 Phenoxy Sizing 0.8 PVP K90 Sizing G' Sizing 0.7 Normalized Applied Stress 0.6 0.5 0.4 0.3 0.2 100 1000 10000 100000 1000000 10000000 Number of Cycles Unidirectional Fatigue Response (R = 0.1) Improved Fatigue Life

  11. 50 45 40 35 30 Shear Stress (MPa) Low spread Phenoxy sizing 25 PVP K90 sizing G' sizing 20 15 10 5 0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Shear Strain Shear Response of Composite Laminates Increased Ductility Tensile tests on [±45]6s laminates Derakane 441-400 vinyl ester Resin Infusion Molding

  12. Moisture Uptake of Composites at 65 °C

  13. Presentation Outline • Background • Improvements to Microindentation • Analysis of Nanoindentation Results • Comparison of Fiber Sizings • Effect of Hygrothermal Aging on Sizings • Tensile Strength Predictions • Conclusions and Recommendations

  14. Methods for Improving Microindentaion • Increase resolution in load, displacement, and positioning • Eliminate optical determination of debond load • Create a less complex stress-state at the fiber surface • Reduce operation time • Extract true IFSS for micromechanical predictions

  15. ORNL NANO II Indenter

  16. Schematic of Nanoindenter • Operates in load control by an electro-magnetic coil (75nN) • Displacements measured with non-contact capacitance transducers (0.04 nm) • Maximum load of 300 mN • Continuous stiffness measurements • Outfitted with blunt indenter tip (4 mm diameter)

  17. Indenter Depression • 4 mm diameter head • 6-8 mm fiber diameters

  18. G’ Load-Deflection Curve

  19. G’ Stiffness-Load Curve

  20. Progressive Nature of Debonding Fiber Debond Clay Filler Load

  21. 1 3 2 4 5 1 3 2 4 5 Contact Fiber Debond and Debond Propagation Fiber Compression Fiber Buckle Fiber Compression

  22. Presentation Outline • Background • Improvements to Microindentation • Analysis of Nanoindentation Results • Comparison of Fiber Sizings • Effect of Hygrothermal Aging on Sizings • Tensile Strength Predictions • Conclusions and Recommendations

  23. Current Analysis Methods • Shear Lag Analysis • Simple but fails to satisfy the txy = 0 boundary condition at the free surface • Finite Element Analysis • More complex and must be adjusted for each individual fiber • Empirical Solution • Simple but very limited in application

  24. Analysis of Indentation Results • Shear Lag Analysis by Zidi et al. • Debonding occurs a distance h below the surface • Assumes that after debonding a frictional shear force equal to the IFSS is present on the fiber • IFSS calculated from debond load • Does not include plasticity M. Zidi et al., Composites Science and Technology,60, 429 (2000).

  25. Shear Lag Analysis Indenter Fiber bulk composite

  26. Finite Element Analysis 1 Fiber 2 Matrix 3 Bulk Composite

  27. G’ and LSPFEA vs. Shear Lag Results FEA Maximum G’ IFSS within 64%, LSP within 100%of shear lag value Success of FEA is sizing dependent FEA Maximum G’ displacement within 17%, LSP within 30%of experimental value

  28. Effect of Matrix Thickness on FEA Results As Tm IFSS agreement between FEA and Shear Lag improves As Tm displacement agreement between FEA and Shear Lag improves

  29. Effect of Fiber Diameter on FEA Results As df IFSS agreement between FEA and Shear Lag improves As df displacement agreement between FEA and Shear Lag worsens

  30. Presentation Outline • Background • Improvements to Microindentation • Analysis of Nanoindentation Results • Comparison of Fiber Sizings • Effect of Hygrothermal Aging on Sizings • Tensile Strength Predictions • Conclusions and Recommendations

  31. Weibull Cumulative Distributions of IFSS

  32. F F/2 F/2 Grips Network drop Fiber Microbond Test Materials Fiber: AS-4 Matrix: 700 g/mol vinyl ester with 30 wt. % styrene cured with 1.1 wt. % Benzoyl peroxide, 130°C/20 min.,N2 Courtesy: I. C. Kim and Dr. T. H. Yoon, Korea

  33. Sizing Comparison*New Microbond Data Normalized to Old Control Data

  34. LSP and PVPTensile Failure Modes LSP Xt = 2034 MPa PVP Xt = 1964 MPa

  35. Presentation Outline • Background • Improvements to Microindentation • Analysis of Nanoindentation Results • Comparison of Fiber Sizings • Effect of Hygrothermal Aging on Sizings • Tensile Strength Predictions • Conclusions and Recommendations

  36. PVP & LSP Moisture Uptake65 oC Water Bath

  37. LSP Sizing ResultsWeibull Analysis 10% reduction in IFSS after 576 hours of exposure

  38. PVP Sizing ResultsWeibull Analysis 19% increase in IFSS after 576 hours of exposure

  39. G’ Sizing ResultsWeibull Analysis

  40. LSP Wet vs. Dry Weibull Cumulative Distributions 288 hours of exposure wet and dry Wet samples show lower IFSS and a broader distribution 576 hours of exposure wet and dry

  41. PVP Wet vs. Dry Weibull Cumulative Distributions 288 hours of exposure wet and dry Wet samples show lower IFSS and a broader distribution 576 hours of exposure wet and dry

  42. Effect of Matrix Swelling on FEA Results: 0.8 wt% Moisture Effect of matrix thickness on IFSS End result is that matrix swelling causes apparent decrease in IFSS and a broader distribution Matrix swelling affects shear stresses more than fiber translation The effect on matrix swelling is more severe at higher matrix thicknesses Effect of matrix thickness on fiber translation

  43. LSP Moisture ResultsIFSS vs. Tensile Strength Reversible and irreversible component of strength reversible = matrix plasticizationirreversible = interfacial IFSS and tensile strength follow the same trends

  44. Zone of Influence Zone of Influence Interfacial Debonding How does reduction in IFSS cause reduction in TS? Increase in zone of influence increases the probability of local fiber failure and thus bulk composite tensile failure

  45. PVP Moisture ResultsIFSS vs. Tensile Strength Reversible and irreversible component of strength reversible = matrix plasticizationirreversible = interfacial IFSS and tensile strength follow opposite trends

  46. Zone of Influence Zone of Influence Transverse Cracking Interfacial Debonding How can IFSS increase and TS still decrease? Reduction in interphase fracture toughness can lead to rapid debond propagation and brittle fracture, resulting in decreased bulk composite tensile strength

  47. Presentation Outline • Background • Improvements to Microindentation • Analysis of Nanoindentation Results • Comparison of Fiber Sizings • Effect of Hygrothermal Aging on Sizings • Tensile Strength Predictions • Conclusions and Recommendations

  48. Tensile Strength Model • Model developed by Reifsnider and Gao1 • Includes a component of Interfacial Shear Strength • Model computes stress concentrations and the ineffective length based on the ‘Shear Lag’ approach. • It then uses those ineffective lengths to compute the tensile strength of a composite using Batdorf’s2 statistical tensile strength model • Modified to include Interfacial Efficiency by Subramanian, Reifsnider, and Stinchcomb3 (not untilized in this present work) 1 Gao, Z. and Reifsnider, K.L., “Tensile Failure of Composites:Influence of Interface and Matrix Yielding,” Journal of Composites Technology and Research, JCTRER, Vol 14. No. 14, Winter 1992, pp. 201-210. 2 Batdorf, S. B, “Tensile Strength of Unidirectionally Reinforced Composites I and II”, Journal of Reinforced Plastics and Composites, Vol.1, April 1982, 153-164 and 165-176 3 Subramanian S, Reifsnider, K. L and Stinchcomb, W. W.,”Tensile Strength of Unidirectional Composites: The Role of Efficiency and Strength of Fiber-Matrix Interface”, American Society for Testing and Materials, 1995, 289-300

  49. Tensile Strength vs. IFSS

  50. Tensile Strength Predictions Nanonindentation vs. Microbond

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