1. No category

In-situ Micro-compression
of Unidirectional
Polymer Matrix
Composites
9 April 2015
2d Lt Torin C. Quick1 Dr. Sirina Safiret1,2
Lt Col Chad Ryther1 Dr. David Mollenhauer1
Mr. Robert Wheeler1,3
1Air
Integrity  Service  Excellence
Force Research Laboratory
2University of Dayton Research Institute
3 MicroTesting Solutions LLC
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1
Overview
• Introduction
– Objective
– Previous work
• Experimental
– Approach
– Test fixtures and Setup
• Results
• Conclusion & Future Work
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Objective
• Gain insight into local compressive deformation
behavior of unidirectional(UD) polymer composites
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Previous Work
• Dr. Charles Lu pioneered a novel in-situ compression
test of composites at the micro scale
• Samples of IM7/BMI were prepared using ion
sputtering and FIB milling
• Compressed in a micro-mechanical test fixture in-situ
within an SEM
Lu, Y.C., et. al 2013. “In-Situ Micro-Compression Testing for
Characterizing Failure of Unidirectional Fiber Composites”, Proc.
Amer. Soc. Comp., 28th Technical Conference, State College, PA, 911 September.
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Previous Work
• Difficulty controlling the
indenter displacement after the
onset of failure led to
catastrophic failure of
specimens
• Were able to obtain some
material properties from tests
Lu et al. 2013
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Previous Work Cont.
• Load cell was more compliant
than the test specimen
• The stored energy was
released when the specimen
failed
• Unable to obtain significant
data on the onset or mode of
failure
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Approach
• Create an external method for controlling indenter
displacement after specimen failure
• Test samples at different cross sectional areas in the
micro-scale (i.e. 20 µm, 50 µm, 100 µm)
• Use DIC and X-ray micro-Computed Tomography to
gain insight into damage initiation and propagation
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Sample Fabrication & Geometry
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• Single ply cured and polished
to starting thickness of 50µm
2.5 µm offset
• Shoulders limit indenter
displacement
• FIB milled 100+ hours
• Random & high contrast
pattern applied to surface
• Sputter coated w/ Cr & Pt
• Load fixture capacity restricts
size to 20x20 µm
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Test Method
• Micro test fixture
–
–
–
–
–
100g strain gauge load cell
Piezoelectric actuator
40 µm stroke*
In-situ SEM testing
130 µm sapphire platen
• Testing
– Displacement Control (100nm/sec)
– SEM image every .5µm displacement(~60 sec capture time)
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Micro-testing Results
First micro-pillar test
– Indenter exceeded failure point by ~12 µm
– Arresting shoulders succeeded in preventing total
catastrophic failure – much of top portion destroyed
Indenter
Indenter
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First Micro-pillar Image Series
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DIC Analysis
• 1st Specimen (image size (4096x1510)
• Last two load steps before complete failure
Subset: 39
Step:5
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Second Micro-pillar Test
• Indenter exceeded failure point by only ~1.2 µm
• Far less destruction of the pillar
Indenter
Indenter
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Post Failure
Partial fiber
separated
from matrix
4 Fibers
fractured
~halfway
down the
pillar
Splitting
following
fibers
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Damaged Pillars
10 µm
1st Test
2nd Test
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Micro-testing Results
• 2 micro-pillars were tested
• Displacement control limited damage after failure
Stress (GPa) Modulus (GPa) Strain Force(N) Fiber Volume
FP1
1.18
41.6
0.028
0.313
0.67
FP2
1.72
81.0
0.021
0.497
0.66
Average
1.45
61.3
0.025
0.406
0.67
Manufacture
1.69
150
• Differences may be attributed to fiber distribution
and count – both whole and partial fibers
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Challenges
• SEM image acquisition requires 50-90 sec
– Single scan images are noisy and appear to shift due to
random changes in the rastering path
• Compliant load cell which leads to stored energy in
the fixture
• Loading instability due to piezoelectric actuator
heating
Load vs Displacement
0,0319
Load (g)
0,0317
0,0315
0,0313
0,0311
0,0309
0,0307
0,0305
34
34,2
34,4
34,6
34,8
35
35,2
35,4
35,6
Displacement* (um)
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Conclusions
• Proven that we can limit the extent of damage on
micro-scale compression tests
• Observed several forms of failure within 1 specimen
• Large difference between 2 results (stress &
modulus)
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Future Work
• SEM image averaging for better DIC analysis
• Scan the samples before and after sequential
loadings with X-ray CT
• Acquire test fixture with increased capability
– Enables larger micro-specimens to be tested
– Up to 50x50 µm
• Arrest mechanical testing at damage inception
• Characterize fiber count and distribution on micromechanical properties
• Compliment experiments with a computational model
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Future Work
• Meso-scale testing
– Compare material
properties & failure across
different length scales
– 1.38 mm diameter cylinders
– Acquire CT data sets before
and after failure
– In-situ X-Ray µCT
compression
– Preliminary tests have been
promising
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Thank You
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Back-Up Slides
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FIB Mill
• LYRA User Manual, 2011
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Milling Process
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Cut the Pattern
FIB Milling
Front and Back
Cr followed by Pt
Coating
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Failure During SEM Image Capturing
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Test Comparison
Dimension
Stress (GPa) Strain (%)
Modulus (GPa)
-
-
-
-
FP-1
20 x 13 x 50 µm
1.18
2.8
41.6
FP-2
19 x 15 x 65 µm
1.72
2.1
81.0
-
1.45
2.5
61.3
-
-
-
-
CD-1 1.37mm Ø, 8.19 mm long
1.10
2.9
54.6
CD-2 1.37mm Ø, 8.19 mm long
1.01
2.2
68.2
CD-3 1.40mm Ø, 8.19 mm long
1.17
2.6
74.3
CD-4 1.38mm Ø, 8.20 mm long
1.41
2.6
71.9
Micro-scale
Average
Meso-scale
Average
-
1.17
2.6
67.3
Results from Lu et al.
-
-
-
-
Micro
18 x 15 x 53 µm
1.98
1.04
190
Macro
6.25 x3.17 x 108 mm
0.95
1.33
71
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Specimens from High Speed Machining
Before machining
Ø = 50µm
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Ø = 125µm
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