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Center for Mechanics of Solids, Structures and Materials
Department of Aerospace Engineering and Engineering Mechanics
“Measure what is measurable, and make measurable what is not so.” - Galileo GALILEI
Dynamic fracture
Krishnaswamy Ravi-Chandar
Lecture 4 presented at the
University of Pierre and Marie Curie
May 21, 2014
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 2/46
Outline
• Crack surface roughness development
• Crack front waves
• Crack branching
• Tape peeling
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 3/46
Dynamic crack evolution
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 4/46
Surface roughening
Mirror Mist Hackle
Branch
Ravi-Chandar and Knauss, Int J Fract, 1984
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 5/46
Dynamic crack front evolution
Mirror Mist Hackle
Branch
Ravi-Chandar and Knauss, Int J Fract, 1984
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 6/46
Microcracking model for dynamic fracture
Ravi-Chandar and Knauss, Int J Fract, 1984 – Homalite - 100
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 7/46
Surface roughening and limiting speed
• Microcracking, instability
– R-C and Knauss, IJF, 1984, R-C and Yang, JMPS, 1996
• Dynamic instability and microbranching
– Sharon and Fineberg, PRL, 1996
• Crack twisting – mode III perturbations
– Hull, J Mat Sci, 1997
• Microcracking, roughening
– Bonamy and coworkers, 2010,…
Lp
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 8/46
Surface roughening - PMMA
Ravi-Chandar and Yang, J Mech Phys Solids, 1997 – PMMA
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 9/46
Formation of conic marks
a
100 mm
b
5 mm
Smekal, 1953 Ravi-Chandar and Yang, J Mech Phys Solids, 1997 ; Ravi-Chandar, Int J Fract, 1998 – PMMA
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 10/46
‘Plane’ of the microcracks - microbranches
Ravi-Chandar and Yang, J Mech Phys Solids, 1997 – PMMA
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 11/46
Nucleus of the conic
Ravi-Chandar, Int J Fract, 1998 – PMMA
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 12/46
Statistics of conic marks - 1
0
500
1000
1500
2000
2500
3000
3500
0 100 200 300 400 500 600 700 800
Region aRegion bRegion c
Den
sity
of
Co
nic
Mar
kin
gs
- m
m-2
Position - mm
Ravi-Chandar and Yang, J Mech Phys Solids, 1997 – PMMA
Focal Length - microns
Fre
qu
en
cy
0
1
2
3
0 1 2 3 4 5 6 7 8
Focal Length - microns
Fre
qu
en
cy
0
1
2
3
0 1 2 3 4 5 6 7 8
Focal Length - microns
Fre
qu
en
cy
0
1
2
3
0 1 2 3 4 5 6 7 8
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 13/46
Statistics of conic marks - 2
Scheibert, Guerra, Celarie, Dalmas and Bonamy, PRL, 2010
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 14/46
Speed of microcracks?
• Assume that microcracks grow with the same speed
– If microcrack speeds are of equal, then the shape of the conic is independent of the speed (R-C and Yang, 1997)
– Dalmas et al (IJF, 2013) show this by examining the detailed shapes of conics
• How fast do the microcracks grow?
– It is not really possible to examine this experimentally. To be determined by other means!
– This is still an open issue
• Can we simulate this? – just the kinematics!
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 15/46
Nucleation and growth model: Flaw Nuclei
• Nucleation is by cavity growth
• Spacing and density are governed by the stress level
• Measured densities are in the range of 500 to 2500 nuclei per mm2.
Ravi-Chandar and Yang, J Mech Phys Solids, 1997
0 100 200 300 400 500 600 700 800 900 10000.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 16/46
nucleus
active crack front
dn, nucleation distance
s, spacing between nuclei
Nucleation and growth model: Nucleation
• When the stress at a flaw nucleus reaches a critical value, the nucleus becomes an active microcrack; calculation of this requires detailed knowledge of the flaw dimensions and the detailed stress field in its vicinity.
• Here we assume that when an active crack front approaches a nucleus to within a critical nucleation distance, dn the flaw is nucleated into a microcrack.
Ravi-Chandar and Yang, J Mech Phys Solids, 1997
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 17/46
Nucleation and growth model: Growth
• Constant microcrack speed
– all active microcracks grow with the same speed regardless of the density of microcracks
• Constant energy flux
– microcrack speed depends on the number of active microcracks and the available energy
Ravi-Chandar and Yang, J Mech Phys Solids, 1997
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 18/46
Constant microcrack speed
Ravi-Chandar and Yang, J Mech Phys Solids, 1997
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 19/46
Constant power input
Position
No
rma
lize
d e
ns
em
ble
cra
ck
sp
ee
d
0
1
2
3
4
0 5 10 15
Mac
rocr
ack
sp
eed
Position
Position
Mic
roc
rac
k s
pe
ed
0
0.5
1
1.5
0 5 10 15
Mic
rocr
ack
sp
eed
Position
Ravi-Chandar and Yang, J Mech Phys Solids, 1997
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 20/46
Comments…
• Differences in density of conic marks between different types of PMMA need to be reconciled
– Bonamy et al have one to two orders of magnitude lower conic marks per unit area
• Quantitative modeling of the deformation within the fracture process zone – cavitation, crack initiation, growth, coalescence,…
• Continuum modeling based on heterogeneity?
– Line models and quantitative calibration/comparison
– Crack front waves
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 21/46
Crack front waves – a quick summary
Morrissey and Rice, JMPS, 2000
0
ˆ ( , ) ˆˆ( , ) ( , )G k
P k A kG
0( , ) ( , )A z t a z t v t
… obtained from perturbation solution of Willis and Movchan; has a simple zero corresponding to a propagating mode with speed Cf ; this is the crack front wave.
ˆ( , )P k
f 0.96 0.97 RC C
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 22/46
Source
Wallner lines
Field, J Contemporary Physics, 1971
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 23/46
Wallner lines
Photographs: Jill Glass, Sandia National Laboratories
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 24/46
Mode III perturbations
Bonamy and Ravi-Chandar, 2003
x z
y
12.7 mm
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
6.00E-06 6.50E-06 7.00E-06 7.50E-06 8.00E-06 8.50E-06
f
MHz
l
mm
t
ns
0.5 6880 2000
5 688 200
20 172 50
Wavelength and duration for glass
-3
-2
-1
0
1
2
3
0.00 0.05 0.10 0.15 0.20 0.25
Distance traveled - m
Pu
lse
am
pli
tud
e -
V
Glass
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 25/46
25
Specimen P
v= 890 m/s
f = 5 MHz
10 mm
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 26/46
26
Specimen AC
v= 440 m/s
f = 5 MHz
12.7 mm
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 27/46
27
Specimen AI
v= 440 m/s
f = 20 MHz
10 mm
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 28/46
Crack-ultrasonic pulse interaction
l
vl/Cs
Shear wave
Crack front
Cs
v
q
x z
y
x
z
v
m/s
f
MHz
lx l v/Cs
mm
480 5 96
867 5 174
895 20 45 Attenuation is similar to that
of bulk glass
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 29/46
Results of Fineberg et al
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 30/46
Summary on crack fronts
• Theory predicts that they exist – Important in roughness generation; bulk waves can also
deliver energy to the cracks and cause roughness, but these CF waves are more effective.
– Difficult to distinguish from Wallner lines
• Small amplitude plane wave perturbation – Perfectly linear response to mode III perturbations
– Responds to wavelength of input, speed of crack – no inherent characteristic length!
– No persistent interaction between different pulses
– No persistence when perturbation is removed
• Need larger amplitude perturbations?
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 31/46
Dynamic crack branching
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 32/46
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 33/46
Crack branching
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0 50 100 150 200 250
Time - ms
Dy
na
mic
Str
es
s In
ten
sit
y F
ac
tor,
KI -
MP
a m
1/2
0
40
80
120
160
200
Cra
ck
po
sit
ion
, a
- m
m
a
KI
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 34/46
Microcracking model for dynamic fracture
Ravi-Chandar and Knauss, Int J Fract, 1984 – Homalite - 100
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 35/46
Control of surface roughening and branching
Ravi-Chandar and Knauss, IJF, 1984
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 36/46
Fracture mechanics
Lp
Inner Problem: Are the details of the failure mechanisms and fracture processes important only in determining the fracture energy, G?
Outer Problem: Knowledge of constitutive laws and balance equations is adequate to calculate the energy release rate, G
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 37/46
Crack branching analysis
• Stress field based
– Yoffe (1951): v~0.65 CR
• Energy-based
– Eshelby (1970)
– Freund (1972): v~0.53 CR
– Adda-Bedia et al. (2005, 2007) -0.25
0
0.25
0.5
0.75
1
1.25
1.5
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Angle (Radians)
Ho
op
Str
ess
- f
v/Cd=0.0
0.1
0.2
0.3
0.35
0.4
221
( ) dynamic energy release rateI IG A v KE
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 38/46
Energy balance calculations of Adda-Bedia et al
0( , ) ( ) ( ,0), , ,p p pK t v k v K t p I II III
0( , ) ( ) , ', ( ,0)p l pl l
l
K t v k v H v v K tl
Adda-Bedia et al. 2005, 2007,…
Not all the Hpl are available, but H33
was calculated by Adda-Bedia et al, corresponding to the mode III problem.
0
33( , ) ( ) , ', ( ,0)III III IIIK t v k v H v v K tl
2
0
33
1( ) , ( ,0)
2IIIG g v H v K tl
m
;G v G v G G
0.39 ; 0.22(39.6 ); 0sv C vl
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 39/46
In-plane modes
Adda-Bedia et al. 2005, 2007,…
2 2
0 0
11 21
1( ) , ( ,0) ( ) , ( ,0)
2I I II IIG g v H v K t g v H v K tl l
m
;G v G v G G
H11 and H21 are not available, but Adda-Bedia argues that in the limit of zero crack speed, these should behave similarly to H33, corresponding to the mode III problem.
0.518 ; 0.13(23.4 ); 0Rv C vl
influences the critical speed, but not the branching angle
vG
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 40/46
Dynamic lifting of a tape
• Inextensible, flexible tape with tension T, mass per unit length r
• … transverse deflection
• … transverse velocity
• … slope (assumed small)
,w w
w wt x
( , )w x t
T
l ct c 2 21
2L w Twr
2 22
2 20 speed of transverse waves
w wc c T
x tr
Expect piecewise linear solutions:
In the absence of adhesion in the string, you can show that
If is prescribed, then
0w w
s w wt x
s c
ds
sdt
f f s f s
( , )w x t
( , )w x t
( , )w x t
/w w c w
Burridge and Keller, 1978, Freund, 1990, Duomochel et al, 2008,…
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 41/46
Dynamic peeling of a tape with adhesion
Now let us consider adhesion of the tape to the substrate with an adhesive energy G
,w w
w wt x
( , )w x t
T
l st sExpect piecewise linear solutions:
But another equation is needed to determine
This is derived from energy balance: dynamic energy release rate
0w w
s w wt x
Consider pulling one end of a semi-infinite tape at a constant speed, . As a result, a peel front moves with a speed s
s
2 2 2/2
TG L s L s w w c
w
22
21
2
T sG w
c
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 42/46
Dynamic peeling of a tape with adhesion
,w w
w wt x
( , )w x t
T
l st s
Impose Griffith condition: G = G
122
21 qs
sw w
c
2 /qsw T G
0 0.2 0.4 0.6 0.8 10
1
2
3
4
5
Normalized Velocity
No
rma
lize
d S
lop
e
22
21
2
T sG w
c
G
2 2 2
2 2 22 /
s w c
c T w c
G
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 43/46
Let us peel quasi-statically:
At this point, the peel front encounters a weak patch; let us fix d
Quasi-statically, crack will extend until
Peeling front encountering a weak patch
2 1
1 2 1
0 x l
l x
G G
G G
2 2 2
1 2
2 / ; 0
2 /
qsw T w
d l T
G
G
2 1 12 /qs qsw w T G
2
1 1
equilibrium crack length
qs
al
l
G
G
1 12 /qsw T G
1l
d 2qsw
qs
al
1qsw
2G 1G
However, the crack will propagate dynamically
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 44/46
But, the crack will advance dynamically, at a speed
A kink wave will propagate towards the fixed end in the peeled portion of the tape
Dynamic peeling front encountering a weak patch
2 1
1 2 1
0 x l
l x
G G
G G
ct
1l
h
c
s
1 1,w w
2qsw
1 1crack tip: 0sw w
1 2 1 2kink: 0w w c w w
2 2 1 2 1 1
dynamic energy release rate
2 / 2 /cw w w w w s T G
1 2 1
1 2 1
1
2qs
w
w
G G
G G
s
1l
c
s c
t
x
c
2 1
2 1
/ 1
/ 1
s
c
G G
G G
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 45/46
Next, we consider reflection from the fixed end of the kink wave
This will arrive at the location at
Dynamic peeling front encountering a weak patch
1l
c
s c
t
x
c
al
t
ax l 1 /at l l s
1l
d 2qsw1 1,w w
s
0,r rw w
c
1 1kink: 0r rw w c w w
11 1 1
2
1r qs qs
sw w w w
c
G
G
The peel front arrests and everything becomes quiescent!
21
1
qs
a al l lG
G
Ravi-Chandar, CENTER FOR MECHANICS OF SOLIDS, STRUCTURES AND MATERIALS 46/46
Extensions to this one-d problem
• Continue loading at x = 0; must consider multiple reflections
• Introduce speed dependence to the energy G(v)
• Consider flexibility – straightforward extension to the bending problem
• Consider extensible strings – more complex due to the presence of longitudinal and transverse waves