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Dynamic Mechanical Response of
Brain Tissues
2nd Symposium on TBI, University of Maryland, 19 May 2011
Weinong Wayne Chen
Schools of Aeronautics/Astronautics and Materials Engr.
Purdue University, West Lafayette, IN
(765) 494-1788, [email protected]
Tusit Weerasooriya
Steve Son
Eric Nauman
Riyi Shi
Collaborators:Farhana Pervin
Xu Nie
Brett Sanborn
Yun Ge
Kolsky Bar
Lindholm, 1964
Some of the Kolsky Bars at Purdue
Non-Uniform Loading on Soft Specimens
After Gray, 2000
Note: Lead is harder than most
soft tissues.
Non-homogeneous Deformation
– Uniform deformation along specimen thickness
– Related to dynamic stress equilibrium in most cases
Two-dimensional Effects in the Specimen
– Friction effect
– Radial inertia in specimen
Dynamic Characterization of Soft Materials
Modified Kolsky Bar for Soft Materials
Some Tissues Dynamically Characterized
Livers
Lungs
SkinKidneys
Tendon
Reticular membrane
Specimen
cross-sections
Tendons under tension
Muscles under
compression and tension
White Matter
Gray matter
MTS
SHPB
Dynamic Properties of Gray and White Matters
Scatter in Response of Bovine Gray Matter
0
200
400
600
800
1000
1200
0 5 10 15 20 25 30 35 40 45
Strain (%)
Str
ess
(KP
a)
Strain Rate: 400/s
Strain Rate: 1200/s
Strain Rate: 1800/s
Error Bar
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60
Strain (%)
Str
ess
(KP
a)
Strain Rate : 0.01/s
Strain Rate: 0.1/s
Error Bar
High-rate Results
Low rates
Brain Tissues from Different Animals
0 10 20 30 40 50 60 70
0
100
200
300
400
En
gin
ee
rin
g S
tre
ss (
KP
a)
Engineering Strain (%)
Male Hog Gray Matter
Female Hog Gray Matter
Strain Rate :1000/s
0 10 20 30 40 50 60 70
0
100
200
300
400
En
gin
ee
ring
Str
ess (
KP
a)
Engineering Strain (%)
Hog
Steer
Lamb
Gray Matter @ 1000/s
0 10 20 30 40 50 60 70
0
400
800
1200
1600
En
gin
ee
rin
g S
ters
s (
KP
a)
Engineering Strain (%)
Yorkshire Gilt
Cross-bred Hog
Gray Matter @ 3000/s
Male/Female
Pig/Cow/Lamb
Purebred/Crossbred
Specimen dimension:
ϕ10 mm outer diameter
ϕ5 mm inner diameter
1.7 mm thickness
Camera frame rate: 50,000 fps
STRIKER
PULSE SHAPER
FLANGE
MOMENTUM TRAP
STRAIN GAGE
SPECIMEN
PLEXIGLAS
HIGH SPEED CAMERA
PRESET GAP
WHEATSTONE
BRIDGEPRE AMPLIFIER
OSCILLOSCOPE
Brain Tissue Lateral Deformation
A Washer-shaped Gel Specimen under Compression
66% Peak Axial Strain
Strain Rate ~2,000/s
G ~ 200 kPa
Strain Rate ~2,000/s
G ~ 5 MPa
Disturbances in Measured Axial Stresses
x
p
p
x
x
o
x
x
o aap
2
22
3
2
18116
3
pE
E
x
zyxx
1
1
pzy
5.0
pE xx
xE p
Aluminum 3.5 GPa ~7 MPa 0.002
Plexiglass 0.2 GPa ~3 MPa 0.015
Soft Tissue 1.1 kPa ~2 MPa 1800
xEp
Measured axial stress
dominated by pressure from
inertia, friction, etc.
Inertia pressure (Forrestal and Warren, 2010)
Ideally, p ~ 0
05.0%5 x
When
Impact rod (TW, 0.375” Dia.)
Impact pin
Impact sleeveIncident tube
Specimen sleeve
SpecimenStrain gage
Specimen adapter
Impact rod
Impact sleeve
Impact pin
Cushion (fixed to rigid wall)
Kolsky Torsion Bar for Dynamic Shear Response
Kolsky Torsion Bar for Dynamic Shear Response
Dynamic shear response under torsional loading
No radial-inertia effect.
No stress concentrations at the edges.
Pure shear properties of the material at high rates.
• “Desk-top” Kolsky torsion bar setup
Camera model: Cordin 550
Frame rate=50,000 fps
Outer adapter, fixed to torque
sensor.
Inner adapter, connected to
torsion bar.
Gel sample
No slippage or debonding at the
specimen-adapter interface.
High-speed Imaging of Deformation
Shear stress-strain curves
(Note the small stress amplitude)
Shear strain rate history
0 50 100 150 200 250 3000
400
800
1200
1600
2000
0
400
800
1200
1600
2000
Sh
ea
r S
tre
ss (
Pa
)
Sh
ea
r S
tra
in R
ate
(1
/s)
Time (microsecond)
Strain rate history
Stress history
0.00 0.04 0.08 0.12 0.16 0.200
600
1200
1800
2400
3000
Sh
ea
r S
tre
ss (
Pa
)
Shear Strain
Dynamic Shear Stress-Strain Responses
Ring-shaped Specimen
O.D.=19 mm
I.D.=14.3 mm
Thickness=2 mm
Dynamic Shear Strain Rate and Strain
0 100 200 300 400 500 6000
100
200
300
400
500
600
700
800
900
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
Sh
ea
r str
ain
Sh
ea
r str
ain
ra
te (
1/s
)
Time (microseconds)
Shear strain rate history
Shear strain history
Dynamic Stress-Strain Curves
0.0 0.1 0.2 0.3 0.4 0.5 0.60
1
2
3
4
5
E
qu
iva
len
t str
ess (
KP
a)
Equivalent strain
Sample #1
Sample #2
Sample #3
Sample #4
Sample #5
A Comparison of Axial/Shear Responses
0.0 0.1 0.2 0.3 0.4 0.5 0.60
1
2
3
4
5
Equiv
ale
nt str
ess (
KP
a)
Equivalent strain
Sample #1
Sample #2
Sample #3
Sample #4
Sample #5
Shear strain rate=700/s
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70
300
600
900
1200
1500
1800
Equiv
ale
nt S
tress (
KP
a)
Equivalent Strain
Strain rate=2000/s
Summary
• Uniaxial brain tissue compression experiments
too sensitive to disturbances.
• Necessary to separate volumetric and shear
responses.
• Novel dynamic shear experimental methods
developed, calibrated, and used for brain
tissue characterization.