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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. .

HAL Id: jpa-00248878 https://hal.archives-ouvertes.fr/jpa-00248878 Submitted on 1 Jan 1992

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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



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



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



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



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



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



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



III

II

fracture.

~ '~i

~

l'l'

~

D£FOAM£D UND£FOAM£D

j~~~~~

«~~/«~

STRESS

PLASTIC

25

STRAIN

,@.O

20

'@av/8

'

@