The effect of fibre orientation on the tensile strength of fibre-reinforced metals

.I. Mech.

Phys.

Solids,

THE

1966,

Vol.

14, pp. 49 to 64.

EFFECT

TENSILE

Pergamon

Press Ltd.

OF FIBRE

STRENGTH

Printed

in Great

ORIXNTATION

Britain.

ON THE

OF E’IHRE-R&JIiVFORCED

METALS

7’1rl~ ukinlate tensile strwgtlr o!’ film-reiuforwcl nrctal conqwsitcs has been investigated as a funrtion of librc orientation, using composites of *luu~iuium rcinfoorccd with stainless steel wires and with silica fibrcs. For ullidircctlonall~-:lliRnc(l fihcs the strcngtll and the mode of composite In specimens containing fracture can be correl;ltctl cluan I itatircly \5itI1 t Iw librv orientation. fibrcs at IIIOTTtlwl one oriclltation, intcraclion between fibrcs inhibits an exact quantitative intcrprctation, altlrougll qunlitati\xdy the cor111:osilc strwgt11 mid fracture behaviour can be cslkaincd in terllls of tlkc smlw gc~~er:il lxinciplw wl~ic:hgovern tlw beha\-iour of unirlirectionallyaligned librc cor:qrositcs. I.

~NI’~
CossmEli.inm interest has been shown in recent years in the reinforcement of botll metals and plastics with strong fibres. The bulk of the experimental work has been concerned with in\-estigations of tensile properties, especially ultimate

tensile

st,rcngth.

Metals

reinforced

with

uniaxially-aligned

strong

fibres

can

possess high tensile strengths when t&cd in a direction parallel to the fibres, for example, M~~DAsE:~,s.JECR and W~XOS (l!KO), SI.TTO~\; and CIIORNE(1063) and C‘~c.~rc’lr I.EY and I~AIGR (1%X). Hone\-er. in practical applications of fibre-reinforced materials, it will not always be possible to guarantee that stresses will only be apl~lietl in a direction parallel to the fibrcs. nor will it be possible to guarantee pcrfcct

fibre alignment

orientation

is ilnportant

in all cases. in tlctrrmining

IIvnc~

a knowlcdgc

of the effects of fibre

tlie range of usefulness of these composite

materials. The effect of fibrc orientation on tensile strength has been treated theoretically by SIWWEJ.I. and I,rl- (1961) and by KISLLY and ~).LVIICS (1965). Three possible thilurc mechanisms in a nlatcrial reinforced with long, urlitlirectionally-aligned fibrcxs arc post~ilatetl, A+. fibrc failure, matrix failure in shear. or matrix failure in plant strain. The opcrati\-e mechanism is dictated by the angle Q between the fibrc direction

and the direction

of the applied stress.

A4ssumirg that the libres have a higher breaking strength but a lower breaking strain than the matrix, then. provided that a minimum volume fraction of fibre is exceeded. the strength of a composite with fibres unidirectionally-aligned direction of the applied stress is gi\.cn 1,). 4.9

in the

Thus when the fibrcs control failure, the breaking

shoulrl

incrcasc as tllc angle between

the applied

strygth

of tlw composite

stwss iiit(l tlw filw

tlirwtiorl

inwcascs. (ii)

Shear failure

At intermediate

in

tlze nutria

valurs of B failure can take place by shrar of tlrc rrratris OII il ‘1’1~ fractItrc stress tlrcn is gi\sc‘trI+

plane parallel to the Abrcs.

E’ailurc call also owur 1)). 7rfi is the ultimate shrar stwss of the matrix. slwar at the fibrc/matris intcrfaw OI at au\\- other intcrfacc within tlrcbwmposit( parallel to the fibres (for csamplc, the :~l~tIrlil~i,,~rlj,lIlrrlit~i~~~~ eoatiug bountlariw wl1cre

in the prcsent~ aliunliniunl-silira s}~e~ii~~t~~~s). 7m then rqtrescnts the failure stress in shear at the weakest interface. Tltc maximum stccngth of the cwtupositc will bc obtained at the high& \aluc of 0 at which composite failure is cwut relic-tl 1)). equation (2).

(iii)

This critical aiiglc. O,,,:,,, is givcii

Fuiktre qf the nu.7tvi,c ilr plntx

1,)

strairr

At high values of 0 the matrix can fail by llow transverse to the fibrcs. t.lic necessary applied stress being gi\-err 1~)

whwe q,, is the ultimate tensile stress of the matrix, or alternativ~ely tlic wcakwt intcrfaw, in plant strain. Viglwc 1 illustrates sc~lrematieally Irow the strength

of the composit-cx rrr~tl~~~

each of the three modes of failure wilt \-ary with fibre orientation. I
The effect

of fibre orientation

and Wt7IzF (lQ64)

on the tensile

strength

ou the shear deformntion

tfle flow stress in shear in a coastrsinctl

of thin brazed

matris Similar

least l.3

of Abre-reinforced

metals

joints

indicate

51 that

can be increased bp a factor of at stndics by I\~Y_YTT and WULFF

for a smalI joint thickness. the tcnsilc strength of thin brazed joints indicate that the nftimate tcnsilc stress of the constrninrtd matrix can be incrcnsetl by* a factor of at least 3 in thin joints. These rcsl&s suggest that increased values for matrix: sheer and (1957)

on

tensile

strength

sho~~ld br

whcrc the itltcr-iibrc

calctk&r~ strengths of composites, This cffcct may, Itowcver, be ofket because

assuit~31

~hcn

sl)aci~k,17is small.

of the ncu+uniform thickness of matrix swtions, lending to concentration of stress anti I~c~cc l~wm:ttur~ i’ailurc iti t‘cgions of’ close librc I~ro~iJn~t~. The f~~r~~~oir~~ discussion Icncts to the cotkcIusioik that it. is tlcsirablc mcasurcmrr~ts of tlic cTEcctile matrix or intcrlhcc strain ill the systems

wtlcr

sttldy.

wltctle~w

i, (MATRIX OR INTERFACE PLANE

0

I_*-

..I_..._**

this is possible.

FRACTUREi

* 45

ORIENTATION

FIG. 1. Varidion

STRX:N

to obt,ain direct experimental strength in shear and plane

YO OF FIBRES TO

of U.‘l’.S. of a tibre-rcinforccd

AXIS

!degreer)

met.:d with

orientation

(schematic),

To date, no systematic csperimcntal work on the variation of composite strength wit,11fibre orientation appears to have been performed on fibre-reinforced metals. However, isolated observations on the cflect of Bbre orientation on fracture Yor cxamplc, HERTZBERG (1QBS) bchaGour do tend to confirm the predictions. has sl~own that for a system consisting of fihfes of cllro~~i~~~t in a copper matrix, fa.iIure occurs by shear parallel to the fibrcs for fibres aIigned at ~$5” to the tensile axis and, for fibrcs at 90” to the tensile axis, flow OCCLWS transwrse to the fibres. Other data for metals arc sparsr, but some information does exist on the effects of orientation on the tensile strength of sheets of fibre-reinforced plastics, for example, D.wrs and %_wi
(i) (ii)

(iii)

(b).

Typical longiturlin:tl taper-section of :Ilu~lliniutll-SO~~silica lart~irratc. Mqnification, X 20.

.-=

WARP

CORNERS

(b) 0268

024% 0 250s

contained a very high proportion of failures within the grips. Later tests were performed using restrained aligned grips? similar to those used previously by CIUI~X~~~,ICI* and BarrErr (196-l), whicf~ pnventcd grip nlis~li~nrnen~ and hence atso reduced specimen distortion, All srtbscquent tests were thus performed with these grips. Test pieces of all types were tested at room temperature and test pieces of system (iii) were also tested at 400°C. Fracture surfaces were examined with ;1,bench stereo-microscope and metalfographic e~anljl~a~~onwas also performed On n lo~g~tudil3al taper-scctiun adja.ccnt to the fracture. By these means the mode of fracture could be clearly es~~bIishe~~ for each specimen. In order to obtain a realistic \-due for the matrix inter-coating horrndary shear strcagth. 71n. in the aluminimn-silica systems under study, measurements of the shear streq$h in a direction parallel to the fibres were made using a method recommended by BR.AIX~IW (1%~). Specimens were prepared as previously as a hot-pressed block of fibrc aligned along the specimen axis. From this block, a cylindricaf specimen was Lusncd to the shape shown in Fig. 3b. By using standard ~~o~~ns~eldNo. 12 cylindrical grips, the end could be sheared off a specimen and t.lle shear strength calculated from the ~~ax~lnl,l~~l applied load. Tests were performed both at room temperature and at 400°C on specimens containing both 40 and 50 volume % sifica.

The effect of fibre orientation on the strength of t,hese composites is shown in Figs. -Eand 5 for aluminirrm-stainless steel and aluminium-tiO”/O silica rcspcctivcly. The asymmetrical tensile properties result in rotation of the fibres towards the test axis and the resultant. reduction in B is accommodat.ed by shear deformation of t.he matrix. Some allowance for t.his change in Ebre orientation is necessary when assessing the result,s and t~husthe test results, which are plotted at the original orien~ati~)n~are arrowed in the direction of decreasing 8,

j .i

Pm.

4,

Variation

of

I_T.T,S. of

unitlirectlonillly-aii~rl~(~ orientation.

LllrllliiniunI-stairll~ss

stcd

wil II

The effect of Abre orientation

Q until the whole aluminium-50 consistently

specimen

‘A silica obtained

Laminated

on the tensile strengt,h of fibre-reinforced

55

metals

fractured in this manner at 30’ and above. In the fracture along the coating boundaries was

specimens,

for specimens

e

sheets of o~icntatio~~ &

The initial experiments

containing (Type

fibres at 5’ and above.

II)

on this type of specimen

were undertaken

in an attempt

to overcome the specimen distortion and tendency to grip misalignment experienced earlier, by using a ‘ balanced ’ specimen which had no built-in tendency to distort asymmetrically. However, from the initial results it was soon apparent that these specimens were capable of improved tensile properties and merited a more detailed study. (i)

_ll’uttlitliultl-stait1Zc~s.ssteel

The tensile strength values obtained are shown in Fig. 6. As predicted from simple theory, a slight increase in tensile strength with 8 was observed. A particularly noteworthy feature is the relatively large angle to which a high tensile strength can bc maintained. The value at * 20” differs little from the O” value and even at

0

‘0 \

‘lo-

t

2

\

\

5

\\

\

\

20 -

0

IO

20

30 ORIENTATION

FIG. 6. Variation

& 30”, only a 20%

\

\

\ ‘\

40

\ ‘.\

‘\

‘_

-VA._

50

OF FIBRES

&I TC

AXIS

------___~

was observed.

_L%I

1

fdegrec:)

of U.T.S. of laminated aluminium-stainless

fall in strength

* 80

70

steel with orientation.

Satisfactory

fractures

close to

the centre of the gauge length were obtained from all this series of specimens. The nature of the fracture changed as $ B was increased. Up to & 20” fracture occurred as a clean break across the specimen, sometimes in a ‘ cup and cone ’ shape. At * 30’ fracture occurred along the wire/matrix interface in one set of laminations, breaking the wires in the alternating layers. The 60’ specimen showed

.

I

The effect, of film2 orientation on the temilc strength of fi~rc-reit~force~l ntetais

occurred were

primarily

always

jagged

as a result of fibre breakage, since

some

of

the

frnctxre

although

the fracture

propagated

along

17

surfaces

aluminium/

aluminium

coating boundaries; this was in cork& to the ‘ clean breaks ’ formed in the alunlirlium-stainless steel specimens at low angles. As B was increased beyond * HOO, an increasing proportion of the fracture occurred along the aluminium/ aluminium coating boundaries and some separation of the laminations became noticoablr,

(iii)

until at j, 60” fracture

occurred

completely

in this manner.

‘fl!l,trirziurrt-.lo(~~, silicn The tensile strengths

of these composites

incticate that the extra alLl~~i~~i~~t1~

added to cushion the ftbres and prevetlt breakage has produced the desired effect. The plot of strength against orielltation, Fig. 0. indicates higher strength at all ~alucs of B than Fig. 7, and a fall from the fully aligned strength w 20% occurs ‘at the & 30” orientation. Dissolution experiments graphic examination considerably

reduced

of longitudinal

taper-sections

by the additional

‘l&e type of fracture surfaces observed

indicated

x-aluc of OJ~? and metalio-

that fibre damngr was

ahnninium.

varied with B in a similar manner to the

nluminium-30 “/: silica specimens previously tested, although slightly less jag& surfaces were obser\-cd at angles below :t_ !N’, where fracture of the fibres was t,lrc controlling- fracture mechanism for the composite. In Gw

of the interesting

results obtained

at room temperature,

further icsts

were also performed at WOT, where the shear strength of the aluminium matris is considerably reduced. The tensile strength results obtained from these specimens arc shown in Fig. 10. The nature of the fractures obserred again \-aried with B. lip to & lo* composite fracture followed fibre fracture, althoq$ the fracture surface was generally (although not invariably) in the form of a ‘ 17,’ following the alternate Iaycrs of fibre. Bryond f IO’ failure occurred completely by shear of the matrix with complete separation of all the layers, as typified by t,hc specimen

58 shosvr~

1’.

w.

in Fig. 11. Considerable

specimens axis during

.IncxsoN

and I).

elongation

(‘II.A?.CIIL1<\

and plastic ~~efor~~l~~tiol~ occlrrreti ilt the

at high values of & 8, where the fibrcs swung round towards the spcc*irncn testing.

6oti

FIBRE

-

FRACTURE

ORIENTATION

OF

ilBRES

SHEAR

FRACTIJRE

TCJ AXIS

idegrees!

fractures showed that the type of fracture obtained in a given lamination dcpendcd on the orientation and the test temperature in exactly the same manner as obscr\~etl previously

in laminated

specimens.

Satisfactory shear fr;lctnres along the alltrr~irli~~rn/al~~rl~it~i~Lrr~ coating hourttfarks (and also through the alurn~t~iu~~foil where ~~l)plicablc) were obtained from all the test specimens. The results are presented in Table 1. The effect of the additional la.yers of aluminium

foil is small, being confined to a slight increase in shear strength

at room temperature.

-_

___.-._ .

The effect of fibre orientation on the tensile strength of fibre-reinforced metals

ISOTROPIC

CROSSPLIED

50 t 40

I”IG. 12.

Variation

of U.T.S. of

f

’ crossplied ’ and ‘ isotropic ’ lay-ups of alun~iniun~-40~0 silica with orientation.

CROSSPLIED

4OO’C

IsoPRoPIC

4QoY

SC

"Z .

9 040 t

2 5

30

-a ‘p_._ ---

0

1

IO

20

30

4 ORIENTATION

I’-IG.

1%

Variation

50 TO

IO AXIS

20

t

30

40

(degrees)

of U.T.S. of ‘ crossplied’ and ‘isotropic’ silica with orientation at 400°C.

lay-ups of aluminiunl-40~0

The effect of fibre orientation on the tensile strength of fibre-reinforced metals

63

Complete randomization of fibre orientation is not a desirable method of retaining strength in a composite as this limits packing to TO--30% fibre by volume. Furthermore

if compaction

occurs

in the

excessive fibre damage from mec~lanieal occur under pressure. For two-dimensionaf method is to prepare nnidirectiollally-aligned

composite

manufacturing

sequence,

interaction between tangled fibres may forms, such as sheets or plates, a better

composites from a series of laminations, each containing fibrcs. Uy arranging the laminations at various orienta-

tions, the degree of composite isotropy can be varied in a controlled addition. because each lamination contains lnlictire~tionally-alijirlect problem of fibre packing does not occur.

SCOTCHPLY

fashion. fibres,

In thcb

1002

120.000

0

10

20

30 ORIENTATIC’N

FIG. 14.

43

50 TO

AXIS

60

70

80

90

:degreesi

Variation of LJ.T.S. of ’ crossplied ’ and ’ isotropic ’ lny-ups of ’ Scotcllply 1002 ’ with 0rieJlt~~ion.

Strength data are available for a. considerable number of lay-ups of fibrereinforced plastics, such as ‘ Scot&ply 1002 ’ (see DAVIS and Zr~~~~owsrc~). The ‘ crossplied ’ and ’ isotropic ’ lay-ups represent the simplest approaches towards more isotropic sheet materials, and their strength variation with tibre orientation is shown in Fig. 14 in comparison with unitlirectionally-aligned composites. Similar curves result for the reinforced metal matrix except that due to the increased and tensile strength of the metal matrix the alignment is not so critical.

shear It is

therefore feasible that similar types of application for fibre-reinforced metals in sheet form will be possible as are at present successfully employing fibre reinforced plastics.

(i) The simple and DAVIES essentially

theoretical equations of STOWELL and LIE (1961) and KELLY (1965) relating tensile strength to fibre orientation have been confirmed for ~~ni~~irectiona~l~-aligne~l fibre composites of

64

(ii)

(iii)

(if

)

(\-I (1-i)