Jan 1, 1992 - H. Couque, J. Lankford, A. Bose. To cite this ... Bose. Southwest. Research. Institute,. 6220. Culebra. Road,. San. Antonio, ...... Corporation,.
Tensile fracture and shear localization under high loading rate in tungsten alloys H. Couque, J. Lankford, A. Bose
To cite this version: H. Couque, J. Lankford, A. Bose. Tensile fracture and shear localization under high loading rate in tungsten alloys. Journal de Physique III, EDP Sciences, 1992, 2 (11), pp.2225-2238. .
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Phys.
J.
III
France
2
(1992)
2225-2238
NOVEMBER
1992,
PAGE
2225
Classification
Physics
Abstracts
81.40E
46.30N
Tensile fracture and in tungsten alloys Couque,
H.
October1991,
7
A.
and
Institute,
Research
Southwest
(Received
Lankford
J.
localization
shear
high loading
rate
Bose
6220
Road,
Culebra
revised
under
9
July 1992,
San
Antonio,
accepted15
78228-0510,
Texas
U-S-A-
July 1992)
tensile compressive loading rate and microstructure the and on microstructurally dissimilar allays has been investigated. tungsten characterized tllrough fracture toughness tests performed Dynamic tensile fracture properties were intensity loading rate of 10~ MPa $ s~ ~, and by tensile testing at a strain rate of at a stress 103 s-I. Shear banding phenomena investigated by means of compression tests performed at were nickel-cobalt-tungsten alloys were of 5 x103 s-I. Under rapid loading conditions, strain rates interface found tougher than nickel-iron-tungsten alloys; the tungsten/tungsten be to was Quantitative micromodeling using simple identified goveming microstructural factor. the as fracture found to provide a models of correlating toughness with microstructures. was mean Compression-induced shear within localization found to be facilitated characterized systems was by either elongated tungsten particles or an adiabatic shear-prone matrix. The shear band width observed proportional to tungsten particle size. to be was Abstract.
failure
influence
The
properties
of
of
three
Introduction. of
Because used
are
WHA
as
has
penetrators
generally high density,
their kinetic
involved
[1-5].
penetrator
energy
high While
strain few
properties are, in fact, distinguish dynamic tensile
strength,
ductility,
tungsten heavy alloys (WHA) efficient development of more rapid loading rates that in occur mechanical these high strain rate
the
testing to simulate the questioned whether
rate
studies
have
microstructure and
and
However,
materials.
sensitive
compression
[6, 7],
mechanical
the
generic
response
have
micromechanisms
recently
that
been
clarified
tensile
(stress-
[8]. The
objective
of this
paper
is to
report
the
influence
of
microstructure
on
the
behavior toughness) and compressive failure of three tungsten alloys covering a wide rang in ductility and strength. Although tensile properties are known to provide important information with regard to integrity, knowledge of compressive failure provides penetrator insight conceming actual penetration performance. In particular, the latter is thought to be localization, which lead to self sharpening of the related to shear penetrator during impact, can facilitate penetration. and thereby
strain/fracture
JOURNAL
2226
DE
PHYSIQUE
III
N°
II
Materials.
newly developed, liquid-phase, 90 weight percent tungsten alloys compositions, and mean Designations, investigation. for this tungsten grain size chosen were table given in I. Since each alloy contains (mean intercept length of the tungsten grain) are constituent, I-e-, Fe, Co, remaining elemental reflects the tungsten and nickel, its designation respectively. and Mn, characterized by a it is evident that all microstructures Figure I shows the three are fcc matrix. embedded within ductile grains of bcc nearly pure network contiguous tungsten a characterized by elongated tungsten directional swaged Co alloy The texture a possesses grains, and in addition tungsten precipitates are present in the matrix [7]. For the Co material, performed with loading and swaging directions parallel. The newly mechanical the tests were commercial,
Two
Fig. is
I.
Typical
horizontal.
and
one
microstructures
:
a) Fe, b) Co, cl Mn
the
swaging
direction
of the Co
microstructure
N°
II
Table
Material
I.
RATE
90
W-8
Ni-2
Fe
91
W-6
Ni-3
Co
Mn
90
W-4
Ni-6
Mn
in
Grain
Size
(~m)
Co
Reduction
2227
Tungsten
Treatment/Condition
(wt.9b)
Fe
(*)
LOADING
conditions.
Composition
Alloy Designation
HIGH
UNDER
LOCALIZATION
SHEAR
23.5
As-sintered
Swaged
22.9
25 9b
7.0
As-sintered
area~
developed Mn alloy [9] has the (intercept) size of 7 ~m, and based propensity especially high intrinsic and in alloy is somewhat porous, Experimental
approach.
Compression
and
tensile
tests
microstructure,
finest
with
thermomechanical
on
for
adiabatic
shear.
consequence
only
was
performed
were
As
at
can
be
[9],
should
figure lc, compression.
in
varying
rates
have
from
an
this
from
seen
characterized
strain
grain
tungsten
average
an
consideration
10-4
to
Quasi-static hydraulic test data obtained by using a servo-controlled were machine while under displacement conditions, dynamic in a split control tests run were Hopkinson for [10] of bar adapted both compressive and tensile [I I] loading. modes pressure The compression specimens 6.35 in diameter in cylinders and 12.70 length were mm mm tensile specimens 7.62 mm in gage length and 3.18 mm in diameter used. were Quasi-static and dynamic fracture initiation conducted intensity loading tests at stress were x103
5
s-I
of I and 10~ MPa
Kj, size
30.5
W
of
mm,
=
in
tested
were
using precracked
s~
thickness
15.2
B
machine
test
monitored,
were
while
compact
specimens.
Static
specimens
of
planar
22.7 of prefatigued crack length ao mm displacement control. Load crack opening ans deduced from growth was compliance measure-
mm,
=
conventional
a
displacement
Q~
and
=
under
crack
ments.
A the
special coupled background on
referred
design [12,
references
to
bars
pressure the
13].
of primary consist components specimen to release the stored specimens. These experiments
specimen
to
a
load
of
(CPB) technique was used for dynamic fracture testing. For is the reader development of the experimental apparatus of the CPB experiment. The schematic Figure 2 shows a
and
two
were
bars
pressure
rapidly, energy by conducted
corresponding
445kN
to
to
energy,
store
and
two
preloading applied
an
a
notched,
prefatigued the stress
round
compact
starter
fracture
and bars starter pressure of 390MPa. The test
and secured with wedges, as shown in bars initiated by introducing a sharp cut subsequently high-speed air wheel and circumferential notch of the specimen using a cutter into the starter unloading (compressive) pulse in the specimen drill. Failure of the initiates stress starter an specimen This the axial displacement rate to transmits stress arms. pressure bars, which a rapid failure duration of and has 100 the the pulse has a rise time of about corresponding to starter ~s amplitude part associated with the unloading of the two separated constant stress pressure a 79.0 and a thickness 30.5 Each specimen have a planar size W bars. a height H mm, mm, 15.2 mm. Since displacement rate is applied at the load line of the two about the B same specimens tested simultaneously [13], each specimen was precracked to a different prefatigued obtain dynamic loading 22.7 crack length ao=13.5 and two rates. to as mm mm ao
specimens figure 2.
were
Fracture
then
of the
inserted
starter
into
slots
specimen
in
=
=
=
the
was
=
JOURNAL
2228
DE
PHYSIQUE
N°
III
II
# PRELOADING
-PRECRACKEII COMPACT
CONNECTION-
SIIIE-GR1l0vEll TYPE
lCTl
SPECIMEN
PRESSURE NOTCH
BAR
#I
ROUND
STARTER
38,1
MM
SPECIMEN ~~~
~~
STRAIN ,
CRACK GAGES j27
MM
(
i
1
'
952
MM
PRESSURE BAR
WEDGES
~,
EDDY
Fig.
2.
Schematic
Specifically,
#2
of
CURRENT
coupled
the
$
TRANSDUCER
pressure
bars
experiment
with
specimens
of
different
crack
size.
specimens. Linear elastic toughness based on the was formula 2.6 [14]. For the specimen of longer standard for a compact specimen of ratio H/W crack length, the toughness was calculated using crack opening displacement at measured 12.5 mm from the load line at the onset of crack growth. This crack opening displacement was measured using an eddy current transducer while the of crack growth was from deduced onset strain performed at a location lo mm beyond the prefatigued crack tip, see measurements figure 2. For the specimen of shorter crack length, only an estimate of the toughness is provided since no crack opening displacement was recorded. The toughness calculated was using an estimate of the crack opening displacement at load line at the onset of crack growth. approximated from the bar strain history at The crack opening displacement at load line was analysis [13], while the from the specimen using one-dimensional 127 mm starter stress wave fracture
a
loading
mechanics
rate
used
ratio
to
of 4
was
evaluate
reached
the
plane
with
these
strain =
two
fracture
N°
LOCALIZATION
SHEAR
11
of
onset
beyond
growth prefatigued
crack the
tip,
crack
toughness
Fracture
from
deduced
was
strain
figure using
see
validated
was
UNDER
HIGH
LOADING
performed
measurements
2229
RATE
at
location
a
lo
mm
2. elastic
the
fracture
criterion,
mechanic
I-e-,
yield stress at a 0.2 percent 103 s-I for quasi-static offset strain dynamic loading strain of 10-4 s-I and and rates at conditions, respectively. For specimen sizes not satisfying 2.5(Kj~/«~)2, I-e-, ductile type fracture, the toughness was calculated using the fracture parameter Jj~. This procedure was required only for the Fe microstructure quasi-static conditions [13]. under specimen
greater
or
equal
to
2.5(Kj~/«~)~,
«~ is the
where
fracture.
Tensile
Tensile
(e),
size
data
for
the
Fe
and
microstructures,
Co
figure
in
summarized
are
3 and
II.
table
in
of
terms
alloys
These
true
(«~)
stress
and
natural
strain
in representative of the extremes under tungsten alloy systems
are
strength and ductility that can be obtained with conventional quasi-static increasing strain rate, the strength conditions. With differential between the two alloys remains approximately On the other hand, the ductility differential is constant. considerably reduced as a consequence of the proportionately larger ductility loss suffered by as-sintered the Fe alloy. The Fe alloy has significant hardening capability, which decreases somewhat with strain On the that the swaged Co alloy soitens rate. contrary, to a degree softening increases with strain This type of hardening and behavior previously rate. was quantified using a simple power law relationship [7], and is reported in table II.
l =
=
t
10'~i~
10~
s'~
0 g[%]
Table
Alloy
Strain
Rate
(s-I)
Yield
Stress
10-4 103
Fe
Fe
10-4 103
Co
Co
(*)
properties.
Tensile
II.
«
K(e~f, =
2
with
«
the
flow
(MPa)
Hardening
N
(*)
Maximum
665
0.145
33.0
140
0.014
15.9
676
0.008
9, I
250
J012
5.5
stress
and
e~ the
plastic
strain
(eY~~'~
~
Strain
e~~ 0,1).
JOURNAL
2230
PHYSIQUE
DE
N°
III
11
80 60 Fe
K
~° ~~
. jmPa~mj
20 0
o-~
j
I
~
s
MPa$
-
ig.
]
l s
.
Table
III.
Measured
Rate
and
toughnesses.
calculated
Yieldstress
Fracture
Time
at
Griffith
Critical
Calculated
Stress
Distance
Toughness
rfJ
xj~
t~~
(~~)~
«~*
x
[WI
[MPai
[MPa]
[~m]
~MPa
Measured
K(
li]
Fe
Ductile
108
665
12.5
65
71
Fe
Brittle
35
076
3
034
8.3
20
28
Fe
Brittle
6
133
3
034
8.3
22
19
Co
Brittle
iY
676
4
022
23
47
46
Co
Brittle
19.5
235
4
022
23
40
35
(a)
:
yield
stress
initiation tj~ is the microstructure A
Figure toughness
at
limit
the
time,
and
B
=
2
plastic
the
Young
E the
36.4
of
2
moduli
and
zone
for
(«~)~ the
Fe
=A
Ln
e
with
+B
microstructure
e
s~
106 3.2
and
B
936.5,
x
106
106
=1.22(«~)~/(Etj~)
29.5
A
Kt/ij~
Kj
~MPa
and
[18] for
where
the
Co
011.5.
data. the toughness Under quasi-static conditions, the ductility, trend which did higher prevail the not at rate, was a Co alloy is tougher. the less ductile While the toughness of the Fe alloy where decreases dramatically with rising load rate, that for Co material only slightly. The toughness decreases fracture interpreted simple models, details of which provided in results using the were are reference [13]. A summary follows to indicate different modeling approaches used for the the principally modes. observed fracture two For
the
4
and
table
found
Fe
summarize
III
to
increase
microstructure,
Figure
with
initiation
of
fracture
was
region next (during a
observed
to
be
ductile
under
static
crack. prefatigued Damage evolution tip plastic zone fracture increment), schematically within the crack represented in figure 6, appeared to proceed as follows. deformation Initial fairly uniform at critical strain both interrupted local by multiple cracking of the tungsten and matrix at was a inclined at an angle, 20-50°, with regard to the loading direction cracks tungsten grains. These cracks resulting from interaction of non-coplanar parallel twins With shear cleavage [15]. are transferred the crack tip stress field is the tungsten grains enabled to carry local stresses, to the conditions.
sat
shows
the
overload
to
the
II
N°
SHEAR
UNDER
LOCALIZATION
HIGH
2231
RATE
LOADING
Profile ~~~
Fafigued
AA
3
3
Zone _
,
1
A ,
)
a2)
Fafigued Zone /
#
A
A
bl)
,-,, ,
t t
,
Zone
',
i
A
b2) It
Fatigued
.>
Zone
~
"
,,
',
,
'
,
, ,
I
j
'
j
i 1
.,
4
5.-Scanning
Fig. fracture the
specimens
failure
followed
electron :
tungsten
adjacent
al) Fe, bl) Co and of the dynamic
process :
fractographs
~-f
was
I, =
deduced
from
matrix
2,
stereographic tungsten/matrix
to
fracture
prefatigued specimens
view.
The
the
interface
failure
3,
crack
tips
of
the
a2) Fe, b2) Co. mechanisms
tungsten/tungsten
are
quasi-static Schematic
of
indicated
interface
as
4. =
JOURNAL
2232
Fig.
6.
of
Schematic
ductile
the
PHYSIQUE
DE
initiation
N°
III
II
fracture.
~ '~i
~
l'l'
~
D£FOAM£D UND£FOAM£D
j~~~~~
«~~/«~
STRESS
PLASTIC
25
STRAIN
,@.O
20
'@av/8
'
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