1. Mechanical Characterization of
Microfibrillated 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. Background
Potential reinforcement: Microfibrillated Cellulose (MFC)
MFC are cellulosic fibrils
disintegrated from plant cell walls
(usually aggregates of microfibrils).
Typical thickness range: 20-40 nm
(aggregates), could be as small as
3-10 nm (individual fibrils)
(Svagan et al. 2007)
Structure and appearance of MFC
by SEM (by Jie Ding)

4. Background
MFC/Poly(lactic acid)(PLA) Composites
Advantages:
 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. Objective
Develop a quick and efficient method for
evaluating the
 strength,
 elastic modulus and
 toughness
in thin transparent polymer films.

7. εyy
Approach 0.032
We have successfully applied non-
contact optical methods for full-field
measurement of deformations and 0.024
strains in thin transparent films
0.016
0.008
0

8. εyy
Approach 0.032
Optical methods also allow analysis
of failure modes, work to failure and
fracture mechanisms. 0.024
0.016
0.008
0

9. Approach
Fracture toughness concept OK for brittle
materials
Stress
work to failure  work of fracture
Not true for ductile materials: Strain
work to failure 
Stress
essential work of fracture
+ work of plastic deformation
Wp
We
Wf = We + Wp Strain

10. Approach
Essential 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. Approach
Measurement of EWF
Wf = We + Wp
Liu & Nairn (1998) used
Plastic
double-edge notched Deformation
Zone
tension (DENT) specimens
Wf= welt + wpVp
Wf/lt=wf= we + βwpl
Shape factor

12. Approach
Theory of the EWF Method
Typical experimental results for measuring the essential
work of fracture.
wf
βwp
we
0 W
l
Schematic drawing of the relationship between specific
total fracture work wf and ligament l
Large amount of samples needed

13. Opportunities
Drawbacks 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 zone
Solution: 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 & Methods
Polyester 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 ways
Tensile tests on thin film
specimens
(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. εyy
Future work 0.032
No need to notch the specimens
because we can trace back the strain
concentrations leading to failure 0.024
anywhere in the specimen
0.016
0.008
0

19. Preliminary Conclusion
It is possible to measure
• Strength
• Elastic modulus
• Toughness
On a small set of specimens subjected to a simple tensile test

20. Acknowledgments
 CSREES/USDA NRI CGP #2008-01522 competitive
grant
 Lech Muszyński
 John Nairn
 John Simonsen
 All graduate students in my project group

21. jie.ding@oregonstate.edu