Session 18 ic2011 ding
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Transcript of Session 18 ic2011 ding
Mechanical Characterization of Microfibrillated Cellulose (MFC)-poly(lactic acid)
nanocomposites
Jie Ding, Lech Muszyński , John Simonsen
Department of Wood Science and Engineering
Application of the Concept of the Essential Work of Fracture (EWF)
Poly(lactic acid) PLA is a versatile polymer made from
renewable agricultural raw materials and is compostable.
Background
Poly(lactic acid) (PLA) and its products
Applications
• disposable cups,
plates, containers
• plastic bags
• food wraps
http://www.ecothefriendlyfrog.co.uk/pla.shtm
Known issues
• weak & brittle
Needs reinforcement
Potential reinforcement: Microfibrillated Cellulose (MFC)
Background
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)
Background
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
MFC/Poly(lactic acid)(PLA) Composites
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
Background
Develop a quick and efficient method for
evaluating the
strength,
elastic modulus and
toughness
in thin transparent polymer films.
Objective
0.016
0.008
0
0.024
0.032
εyy
Approach
We have successfully applied non-contact optical methods for full-field measurement of deformations and strains in thin transparent films
0.016
0.008
0
0.024
0.032
εyy
Approach
Optical methods also allow analysis of failure modes, work to failure and fracture mechanisms.
Approach
Fracture toughness concept OK for brittle materials
work to failure work of fracture
Not true for ductile materials:
work to failure
essential work of fracture
+ work of plastic deformation
Wf = We + Wp Strain
Str
ess
Str
ess
Strain
We
Wp
• 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)
Essential Work of Fracture (EWF)
Approach
Measurement of EWF
Wf = We + Wp
Liu & Nairn (1998) used
double-edge notched
tension (DENT) specimens
Wf= welt + wpVp
Wf/lt=wf= we + βwpl
Approach
Plastic
Deformation
Zone
Shape factor
Typical experimental results for measuring the essential
work of fracture.
Theory of the EWF Method
βwp
we
wf
Schematic drawing of the relationship between specific total fracture work wf and ligament l
l
Large amount of samples needed
Approach
0 W
• 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
Opportunities
Drawbacks of the traditional EWF experimental Method
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
Materials & Methods
P
P
tension
Strain
mapping
After
failure
Permanent
strain
Evaluate EWF using Digital Image Correlation (DIC)
Case 1
Materials & Methods
Case 2
Case 3S
tre
ss
Strain
Strain
Str
ess
Strain
Str
ess
Wp
Wp + We
Case 1
Case 2
Case 3
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
E1000) testing frame
• Optical measurement of
deformations and strains:
Digital Image Correlation
(DIC), precision ± 0.4 μm
specimen
Evaluate EWF using Digital Image Correlation (DIC)
Aj
Plastic
deformation
Materials & Methods
0.016
0.008
0
0.024
0.032
εyy
Future work
No need to notch the specimens because we can trace back the strain concentrations leading to failure anywhere in the specimen
Preliminary Conclusion
It is possible to measure
• Strength
• Elastic modulus
• Toughness
On a small set of specimens subjected to a simple tensile test
Acknowledgments
CSREES/USDA NRI CGP #2008-01522 competitive
grant
Lech Muszyński
John Nairn
John Simonsen
All graduate students in my project group