Session 18 ic2011 ding
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  • 1. Mechanical Characterization ofMicrofibrillated Cellulose (MFC)-poly(lactic acid) nanocomposites Application of the Concept of the Essential Work of Fracture (EWF) Jie Ding, Lech Muszyński , John Simonsen Department of Wood Science and Engineering
  • 2. Background Poly(lactic acid) (PLA) and its products Poly(lactic acid) PLA is a versatile polymer made from renewable agricultural raw materials and is compostable. Applications Known issues • disposable cups, • weak & brittle plates, containers Needs reinforcement • plastic bags • food wraps http://www.ecothefriendlyfrog.co.uk/pla.shtm
  • 3. BackgroundPotential reinforcement: Microfibrillated Cellulose (MFC)MFC are cellulosic fibrilsdisintegrated from plant cell walls(usually aggregates of microfibrils).Typical thickness range: 20-40 nm(aggregates), could be as small as3-10 nm (individual fibrils)(Svagan et al. 2007) Structure and appearance of MFC by SEM (by Jie Ding)
  • 4. BackgroundMFC/Poly(lactic acid)(PLA) CompositesAdvantages:  Both components derived from renewable materials  Both are environmentally friendly  carbon neutral  compostable  Small addition of MFC improves strength and elastic modulus of PLA (Mathew & Oksman 2006)  No satisfactory formulation commercialized to-date
  • 5. Background Many formulations are generated in the search of “the perfect one “ Prototype formulations are generated in small amounts of thin transparent films There is a need for a quick and efficient way of evaluating mechanical properties of new formulations Properties of interest • Strength • Elastic modulus • Toughness
  • 6. ObjectiveDevelop a quick and efficient method forevaluating the strength, elastic modulus and toughnessin thin transparent polymer films.
  • 7. εyyApproach 0.032We have successfully applied non-contact optical methods for full-fieldmeasurement of deformations and 0.024strains in thin transparent films 0.016 0.008 0
  • 8. εyyApproach 0.032Optical methods also allow analysisof failure modes, work to failure andfracture mechanisms. 0.024 0.016 0.008 0
  • 9. ApproachFracture toughness concept OK for brittle materials Stress work to failure  work of fractureNot true for ductile materials: Strainwork to failure  Stress essential work of fracture + work of plastic deformation Wp We Wf = We + Wp Strain
  • 10. ApproachEssential Work of Fracture (EWF)• Represents the energy consumed within the fracture zone where new surface is generated (Kwon and Jar, 2007)• Well correlated to fracture toughness for ductile polymers (Barany et al, 2003)• Therefore it is a material constant, independent of sample geometry (Wu and Mai, 1996)
  • 11. ApproachMeasurement of EWFWf = We + WpLiu & Nairn (1998) used Plasticdouble-edge notched Deformation Zonetension (DENT) specimensWf= welt + wpVpWf/lt=wf= we + βwpl Shape factor
  • 12. Approach Theory of the EWF MethodTypical experimental results for measuring the essentialwork of fracture.wf βwpwe 0 W l Schematic drawing of the relationship between specific total fracture work wf and ligament l Large amount of samples needed
  • 13. OpportunitiesDrawbacks of the traditional EWF experimental Method• Large amount of samples• Assumes knowledge of the shape of the plastic deformation zone (β factor)• Assumes uniform level of plastic deformation within the zoneSolution: optical measurement of strains• The actual distribution of plastic deformation can be readily measured• No need to make assumptions regarding the shape of plastic deformation zone• No need for multiple tests
  • 14. Materials & Methods P After tension failure Strain mapping Permanent strain P
  • 15. Materials & Methods Evaluate EWF using Digital Image Correlation (DIC) Case 1 Case 1 Stress Case 2 Case 3 Strain Stress Case 3 Case 2 Wp + We Stress Strain Wp Strain
  • 16. Materials & MethodsPolyester film• Ductile and transparent (to substitute for MFC/PLA composite)• Identical speckle pattern printed on all specimens• Use double-edge notched specimens and calculate we in both waysTensile tests on thin filmspecimens (Modified ASTM D 882-09) • 1 kN Instron (ElectroPuls specimen E1000) testing frame • Optical measurement of deformations and strains: Digital Image Correlation (DIC), precision ± 0.4 μm
  • 17. Materials & Methods Evaluate EWF using Digital Image Correlation (DIC) Aj Plastic deformation
  • 18. εyyFuture work 0.032No need to notch the specimensbecause we can trace back the strainconcentrations leading to failure 0.024anywhere in the specimen 0.016 0.008 0
  • 19. Preliminary ConclusionIt is possible to measure• Strength• Elastic modulus• ToughnessOn a small set of specimens subjected to a simple tensile test
  • 20. Acknowledgments CSREES/USDA NRI CGP #2008-01522 competitivegrant Lech Muszyński John Nairn John Simonsen All graduate students in my project group
  • 21. jie.ding@oregonstate.edu