ENGINEERED MATERIALS
Multi-phase Materials Produced by
Tailoring (Design) the Microstructure and Interfaces
TOUGHENING MECHANISMS
" Crack Deflection/Kinking
" Microcracking
Phase Transformation
Small and Large Scale Crack Bridging
METHODS
Fiber/Whisker Reinforcement
" Ductile Phases or Laminates
TOUGHENING OF STRUCTURAL CERAMICS
CRITICAL ENERGY RELEASE RATE
REOUIRED FOR CRACK DEBONDING
FIBER FAILURE SUPPRESSED
AT MATRIX CRACK FRONT
0
V/1
71
Ir////,
o
b
tube. land over 6
ctm
weal: intit0.. t, help
and Stain (more energy dissipation
j*°0d'!"g
match moduli to- favor kinki
KNOWLEDGE BASE - INTERFACES IN CMCs
INTERFACE PROPERTIES CONTROL BEHAVIOR OF CMCs
' RATIO OF INTERFACE (DEBOND) TO FIBER TOUGHNESS :
-
a/Cam i~- 1l4
FIBERS DEBOND RATHER THAN BREAK
s
RESIDUAL STRAIN MUST BE SMALL (&ate& 3 x 10 IC) .AND NEGATIVE
- PREVENTS THERMAL CRACKING ,OF FIBERS AND MATRIX
- INTERFACE IN MODERATE TENSION
bl, V
" INTERFACIAL COEF OF FRICTION SMALL ( k 4, 0.1)
- INTERFACE SLIDING RESISTANCE 2 ; 't 4 40 MPa
- FIBERS BREAK FAR FROM MATRIX CRACK PLANE ( = > LONGER PULLOUT LENGTHS)
BEHAVIOR OF CERAMIC MATRIX COMPOSITES
*
MECHANICS MODEL WHICH GUIDES THE TAILORING OF
THE CONSTITUENTS AND THE INTERFACE FOR SPECIFIC
COMPOSITE BEHAVIOR
* IDENTIFY MECHANISMS CONTRIBUTING TO TOUGHENING
AND QUANTIFY THE DISSIPATED ENERGY
* ESTABLISH EXPERIMENTALLY AND MODEL ANALYTICALLY
THE SEQUENCE, EXTENT AND INTERACTIONS OF THE
DAMAGE MECHANISMS
* ANALYTICALLY SIMULATE PROGRESSIVE DAMAGE AND
INTERACTIONS, AND PREDICT FAILURE MODES
KNOWLEDGE BASE - FAILURE MODES IN CMCs
BENDING FAILURE
FAILURE IN TENSION
1 ° 1
11 """" 11 G"s""
Orwan ""
IW~"" H" wl
4a
Y
`
O
TENSILE STRESS-STRAIN CURVE
Fibw e1N%AU
F." . . 0
0
" 6' ~6~m
'
wao"
cr*ctblp, 0 0
w zc
ma
II.IIIIdiIIaU1I
il .1luellmun
IIilitillivid
WEAK INTERFACE
* HIGH-STRENGTH FIBERS
* TENSILE RESIDUAL STRESSES
NORMAL TO INTERFACE
Ceramic Toughening by Ductile Metal Laminates
25gm
14 0 M,
U
.r,
N
01.0
".j
pj-
~rGtG,l~.r t.
R-curve
U
c~S
i.,
w
Damage Zone Size, L
(b) Cavitation-Bridging
Reimanis, Dalgleish and Evans, Acta Meal, 1991
Varias, Suo and Shih, J . Mech . Phys . Solids, 1991, 1992
MICROMECHANICAL MODEL FOR
CLEAVAGE DEBONDING
Elastic-brittle Griffith model for crack growth:
CJcleave
= 2'Yint-
Energy release rate for a penny-shaped crack on an interface :
9=
F, and F,. depend on
(For
/12/1111
2 -1- F,,-r2)1
- y1 2c.
7rjzii
vl and v2 (Willis, 1972) .
Equate the right hand sides of (*) and (**) and rearrange to get the
fracture stress,
711
Tint
~
f
vi FQ Sc
F,. Now 2yint N 1 Jm-2 ,
Here S = (1 + 'T 2 /Q2 ) and we took FT
c = 250 nm, and for alumina, ~.c = 158 GPa and v = 0.22. For tension
dominated load states S .^: 1,
= 6-
af ;ze 770 MPa.
The above value agrees with the hoop stress determined by finite element analysis of the interface crack geometry for the measured failure
loads corresponding to several remote phase angles . The hoop stress,
calculated at distances of 3 to 5 hum ahead of the crack tip, range from
500 to 750 MPa.
t";s tY
t 0:~9 Z
r/h
substrates, subjected to a
Inset : a metal foil bonded between two ceramic
distribution ahead of the crack
remote Mode I stress intensity factor. The mean stress
and Shih, 1991 .)
tip is plotted for several loading levels. (After Varias, Suo
Growth and
Competition Between Near-Tip Void
Cavitation
Remote High Triaxiality Debonding/
Defects
(a) Near-tip void growth,
~~
2.06oX
(b) Remote Cavitation, &c ~ 0.4 6o h
Debonding/Cavitation is favoured if h < 5X
High Triaxiality Debonding and Cavitation
2.0
2.0
2.5
1. .: ..- 2 .7 -..~.__
2.5
2 .0 2.?
2.7
/ 2.5
0.0
-1.0
2.5
v=/ao=2 .5
IKI/(vow) = 14.58
-1.0
0.0
1.0
Xl/h
2.0
3.0
4.0
Varias, Suo and Shih
(JMPS, 1991, 1992)
* Cavitation/Debonding at a distance of several foil thicknesses ahead
of the crack tip.
9 This failure mode is triggered when the mean stress exceeds about
five times the tensile yield stress of the metal.
The large spacing between the debonded zones and the crack tip allows
the intact metal patches to bridge the crack, leading to rising crack
growth resistance curves .
The maximum foil thickness which would allow the above bridging
mechanism to develop has been identified .
CERAMIC-METAL/POLYMER LAMINATES
Laminates with alternate ceramic and metal/polymer sheets are tough
under quasi-static and dynamic loading conditions and offer advantages in
weight and strength for biaxially loaded structures .
* The rapidly rising fracture resistance curve is brought about by the arrest
of the main damage front at the ceramic-metal/polymer interface and the
diffusion of the load from the main crack front by a bridging mechanism.
a
9'J
u
U
"tQ
25 gm
R-curve
U
cd
V
w
Damage Zone Size, L
* As the applied load is raised, damage by cavity growth and decohesion
spreads along opposite edges of the interface (see figures above) . The the
intact ligament bridges these interfacial flaws . The increase in the size of
the bridging zone results in rapidly rising crack growth resistance.
* The next stage of damage propagation involves the fracture of the next
ceramic layer. Once again, the above bridging mechanism is activated (see
figures below) .
1
°
7F
w
Ductile Material
~,,~,,~,
:: Ceramic
i :...
awnw
: