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The tensioned push-out test for fibre-matrix interface characterisation under mixed mode loading
Materials Science and Engineering, A160 (1993) 1-5
1
The tensioned push-out test for fibre-matrix interface characterisation under mixed mode loading M. C. Watson and T. W. Clyne Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ (UK) (Received June 22, 1992)
Abstract A novel test procedure is suggested for the exploration of interfacial mechanical properties in fibre-reinforced composites. This is based on the established single fibre push-out test, which has the advantage of being applicable to specimens which can be routinely produced from normal unidirectional long fibre composite material. The suggested modification involves the application of equal biaxial in-plane tension while the push-out testing is carded out. This allows the interface to be subjected to various combinations of mode I and mode II loading, ranging from pure opening to pure shear. Hence it should be possible to obtain data characterising the resistance to interracial debonding and disengagement
over all of this range. Illustrative data are presented here for titanium-basedcomposites,in the form of measured critical shear stresses for various applied normal stresses at the interface.
1. Introduction
There have over recent years been significant advances concerning stresses and fracture mechanics at bimaterial interfaces [1-5]. For interfaces which are significantly less tough than the neighbouring bulk materials, a crack (i.e. a debonding event) can continue to follow an interface, even though the stress intensity at the crack tip is not purely mode I (crack opening) and may have a substantial mode II (shearing) component [6, 7]. The mixity of crack tip loading mode can vary between pure mode I and pure mode II, depending on the loading geometry and elastic properties of the two materials. This homogeneity is commonly characterised by the so-called phase angle, % which is the angle with a tangent given by the ratio of the mode II to mode I stress intensity factors at the crack tip. This mixity is important because the toughness, i.e. the critical strain energy release rate, Gic, can vary quite markedly with q~ -- see Fig. 1. A large shear component tends to result in the crack tip being shielded (particularly if the interface is geometrically rough), with more frictional work being done in the wake of the crack -- leading to a larger Gic value. Various tests have been devised [8-12] to measure Gic, as shown in Fig. 1. However, many of these are applicable only to planar interfaces; those suitable for fibre-matrix interfaces tend to be limited to large values of ~0 (i.e. predominantly shear loading). Tests 0921-5093/93/$6~00
developed for fibre composites [13], such as pull-out [14-16] and full fragmentation [17] tests, are in general aimed at identification of critical stress levels for debonding or sliding under pure shear loading. Moreover, for many tests the specimen must be produced in a special operation which differs from the normal composite manufacturing route and may thus create different interracial microstructures and residual stresses. One of the few tests which can be applied to normal fibrous composite material is the single fibre pushout (or push-down) test. There is therefore considerable interest in this procedure, in spite of the apparent limitation to pure shear loading (~p -- 90°). Fibre push-out and push-down testing has received considerable attention recently, in terms of both experimental and theoretical work [18-25]. This has included the derivation of various shear lag-type analytical solutions to the stress field. More recently, finite element study [26] and photoelastic examinations [27] have shown that the variations in shear stress along the length of the fibre are less pronounced than predicted by shear lag models. Debonding is stimulated near the free surface under essentially pure shear loading. (In fact, the applied load generates a normal stress on the interface by differential Poisson expansion, and residual thermal stresses are also present, but since both effects usually generate radial compression, there is no mode I loading.) Once debonding has started, it will propagate along the~ length of the fibre. After the © 1993 - Elsevier Sequoia. All rights reserved
2
M.C. Watson, T. W. Clyne
/
Tensionedpush-out testforfibre-matrix interface characteristics Load
Double
[ I
Cantilever I~ Beam
¥~0"-5" ~ from frictional
4-point bend ~
"41ff-'m~mm ~
Composite specimen (fibres parallel
/
t° load) lnduction
'
work, plastic
deformation, ete ~ite ter n-out)
.......
#~ Brazil nut
~
~ 70" - 90"
0
i ~ 0 Tif(6d~, Platen
Fig. 2. Schematic illustration of the diffusion bonding set-up used to produce specimens for the test.
Fig. 1. Schematic illustration of the dependence of G~,.on % plus
depictions of various tests. Different tests for the measurement of Gi~.involve a wide range of ~0values. initiation of debonding, motion will be at first partly and then purely by frictional sliding at the interface. While debonding is taking place, the load may rise or fall, depending on the debonding and frictional sliding stresses. The push-out test can also be used to obtain Gic data. Analyses have been presented [19, 28, 29] of the energy balance during push-out. These depend on monitoring of the load/displacement behaviour during progressive debonding. In the present paper, a variant of the push-out test is proposed in which the specimen is subjected to equal biaxial in-plane tension while an axial compressive load is applied to a fibre. This should in principle allow any value of ~p from 0 ° to 90 ° to be generated, depending on the proportion of in-plane and axial stresses (and the presence of residual stresses) -- see Fig. 1. The information given here is limited to a brief outline of the principle and some experimental information; a preliminary set of test results is presented, obtained with a set-up giving only the peak load needed for push-out. Interfacial work of fracture data cannot be obtained without continuous load/displacement monitoring, but the results serve to demonstrate that the test procedure is viable and that an effect of the type expected was observed on changing the mode mixity.
2.. Material and specimen preparation The composite used in this investigation was supplied by BP plc in the form of a 30-ply panel, prepared by hot isostatic pressing of T i - 6 A I - 4 V foils and SiC monofilaments (approximately 35% by volume). The monofilaments were W-cored, stoichiometric SiC, 100/zm in diameter, with a duplex coating of graphitic carbon and TiB2 (approximately 1 /zm of each). A
small specimen, 5 mm square and 2 mm thick, was cut from this panel, the fibres lying normal to the square surface. In order to facilitate gripping, the composite specimen was diffusion bonded into a larger panel of unreinforced alloy. This was done by stacking six alloy foils, each 20 mm square and 500/zm thick and placing the composite specimen on top, after chamfering the edges to an angle of 45 °. This assembly was then diffusion bonded together -- see Fig. 2. This was done at 850 °C for 30 min under a vacuum of about 0.6 Pa, with a maintained stress of 150 MPa. The composite square became immersed in the unreinforced material during this treatment, with the fibres remaining vertical. After bonding, the specimen was cut and ground, then thinned by careful metallographic polishing to produce a 'foil' of thickness around 100/zm.
3. Testing procedure The test set-up allowed biaxial loads to be applied in the plane of the specimen, whilst supported from beneath by a plinth with a hole of diameter about 200 /zm, into which the fibres could be pushed -- see Fig. 3. Before the specimen was placed in the grips, small strain gauges were attached on the unreinforced material, parallel to the two in-plane loading directions. Readings from these were monitored during adjustment of the loads to ensure that an equal biaxial stress state was generated. Since the specimen was relatively thin, there was a danger of significant curvature arising during handling and loading -- both in-plane and when indenting individual fibres. In order to compensate for the effect of this curvature on the strain gauge readings, gauges were also attached on the underside of the specimen. The applied in-plane loads could then be adjusted until the mean readings of each pair of strain gauges were the
M. C. Watson, T. W. Clyne
Matrix
/
Tensioned push-out test for fibre-matrix interface characteristics
Load appliedwith pyramidal indenter ~ Composite
. . . . . .
&3 ]
~I
1.2
i
3
i
\
I
Fibre (SIC) ~
Ra~., s,~ss
I---..... Hoop Stress
I.I
Matrix (~)
o
~I.0 /..
-'"
~0.9
....................
Composite(Ti-35%SiC)
/
0.8
(a)
/
/
0.7
i
Strain gauges (parallel to 2 //direction)
Applied
i
Strain gauges (parallel to 1 direction) ~\
I00
,,i
101 Fibre radii from centre
10 0
102
',,,
(b)
0 ~//////~/////////~
",
Hole
Composite(Ti-35%SiC)
-I00
Fig. 3. Schematic illustration of the loading set-up.
Matrix (Ti) Fibre (SIC)
Radial Stress Hoop Stress
~-200
same, since this corresponded to the volume-averaged in-plane stresses being equal in the two directions. The specimen was placed in the grip assembly, which was then moved around on the base until a selected fibre was centred in the optical imaging system of the indenter. The in-plane stress was then generated via simple screw arrangements on one half of each pair of grips. Compressive loads were then applied axially to the fibre with a conventional microhardness pyramidal indenter, the fibre being viewed after each loading operation. The applied axial load was progressively increased until pushout was seen to have occurred.
4. Results and discussion Interpretation of the results from the test as described above is quite straightforward. Firstly, for any given in-plane load, the strain gauge readings must be converted to a radial stress across the fibre/matrix interface. (The actual value will then be the sum of this and any residual stress present in the unloaded composite.) This can, for the case of an equal biaxial stress, be carried out using an analytical stress model, at least to a good approximation. For example, the coaxial cylinder model [30, 31] can be used; this gives the radial distribution of the three principal stresses within a set of infinite coaxial cylinders subjected to specified axial and radial loads (and temperature change). The model can be applied with the known in-plane stress input as an applied radial stress and the applied axial stress taken as zero. In fact, this leads to an error since,
. . . . . .
i
100
. . . . .
i
i
101
102
Fig. 4. Predicted [31] variations with radial location of the radial and hoop stresses within a system composed of a central SiC fibre, a surrounding cylinder of composite material (Ti-35%SIC) and an outer cylinder of unreinforced (Ti) matrix. Stress distributions are shown for: (a) an applied radial stress (stresses given as a ratio to this); (b) a temperature decrease of 500 K (absolute values of stresses given).
even with no applied axial stress, the model predicts a non-zero axial stress (from Poisson effects), whereas in practice the axial stress should be negligible in a thin slice of this type. Also, since the system is taken as being of finite extent in the plane, the radial and hoop stresses will not necessarily be equal, even at the periphery, whereas the assumed boundary conditions require an isotropic in-plane stress at the far field. However, in practice these are rather small errors for the systems of interest. This is illustrated in Fig. 4(a), which shows the predicted [31] variation in radial and hoop stress as a function of radial position for a system composed of a central fibre, a surrounding circular region of composite (treated as a continuum) and an outer region of unreinforced matrix. Property data shown in Table 1 were used in generating this figure. The radii of these regions have been selected so as to correspond approximately to the specimen of interest here. Errors introduced from the fibre not being in the centre of the composite, and the composite surrounding the fibre not being a homogeneous con-
4
M.C. Watson, T. W. Clyne
/
Tensionedpush-outtestforfibre-matrix interface characteristics
TABLE 1. Thermophysical data Property
100
Fibre (SIC) Composite Matrix (Ti-35%SiC) (Ti-6AI-4V)
@ 7o
Axial E (GPa) Trans. E (GPa) Axial v
Trans. v Axial a ( 1 0 -6 K-l) Trans. a (10-6 K -' )
450 450 0.2
0.2 4.0 4.0
232 155 0.3
0.25 5.0 7.0
115 115 0.35
,~
6o
4o
0.35 8.0 8.0
lO
tinuum, could be significant but are unlikely to be very large. It can be seen from Fig. 4(a) that, for the specimen of interest, the normal stress across the fibre-matrix interface will be about 25% higher than the far field in-plane stress (rl: Or -- 1.25ol
__Opo 4S
Eme
I
i
10
20
,
i
i
30
40
,
I
l
50
60
,
/
I
70
80
,
I 90
,
i 100
Interracialradial tensilestrbss (MPa)
Fig. 5. Experimental pushout data from the tensioned pushout test, applied to as-fabricated Ti-6AI-4V/35%SiC, expressed in
the form of inteffacial shear strengths as a function of applied interracial radial tensile stress.
stress required for frictional sliding. It is possible that the apparent levelling off in critical shear stress with increasing normal tension corresponds to a transition from frictional sliding to debonding being the process dictating the peak push-out load. It will certainly be of interest to study debonding behaviour in this test, with continuous monitoring of load-displacement behaviour, in order to explore this possibility and to establish the interracial fracture toughness. This can then be evaluated as a function of the phase angle % although it will be necessary to measure the thermal residual stress in order to establish % It is clear from the data in Fig. 4(b) that thermal residual stresses are expected to be significant, even when allowance is made for some stress relaxation during cooling from the fabrication temperature.
5. Conclusions (2)
where s is the aspect ratio (depth/diameter) of the fibre, which is close to unity in the present case. The value of the in-plane biaxial stress al is related to the strain gauge reading e s by the equation
al
0
(1)
In practice, it will often be important to take account of residual thermal stresses, at least for metal matrix composites. For example, Fig. 4(b) shows the stress distribution in the specimen according to the coaxial cylinder model for a temperature decrease of 500 K (a lower value than the actual decrease being taken to allow for some stress relaxation during cooling). It can be seen that a compressive stress of over 200 MPa is predicted at the interface. Preliminary experimental data are shown in Fig. 5, giving the experimental pushout stress, Opo (converted to an interfacial shear stress for frictional sliding, rfr -see below), as a function of the interracial radial tensile stress, Or, being imposed via the in-plane tension. The push-out stress is thought in this system [32] to represent the onset of frictional sliding, with an effectively uniform shear stress along the length of the fibre given by Tfr
0
(3)
( 1 - vm)
where E m and Vm are the Young's modulus and Poisson's ratio of the matrix. (Typical strain gauge readings were a few hundred microstraln.) The interfacial radial stress is then found from o 1 using eqn (1). These are preliminary data, but they do appear to indicate that a systematic trend is present in terms of applied radial tension causing a reduction, in the shear
A novel testing procedure has been described for the investigation of interracial mechanical properties in fibre-reinforced composites. This involves the superposition of normal (radial) tensile stress at the interface and the shear stress imposed by conventional push-out testing. This will allow the mechanical response of the interface to be characterised over a range of opening/ sheafing mode combinations. A method of applying the test has been described and illustrative data have been presented for a Ti-6AI-4V/35%SiC monofilament composite, obtained using, a conventional microhardness indenter. In addition, an analytical approach is described allowing the experimental data to be convetted to shear and normal stresses at the interface, for the case of both being constant along the length of the fibre. It is shown that the application of normal tension
M. C. Watson, T. W. Clyne
/
Tensioned push-out test for fibre-matrix interface characteristics
leads to a significant d e c r e a s e in the p e a k shear stress n e e d e d to p u s h out the fibre. In considering these results, the residual (compressive radial) t h e r m a l stresses s h o u l d be t a k e n into account, since they are likely to have a significant influence o n the effective b a l a n c e of o p e n i n g and shearing m o d e s , at least for metal (and p o l y m e r ) matrix composites.
17
18 19
References 1 J.R. Rice, Elastic fracture mechanics concepts for interracial cracks, J. Appl. Mech. Trans. ASME, 55(1988) 98-103. 2 M.S. Hu, M. D. Thouless and A. G. Evans, The decohesion of thin films from brittle substrates, Acta Metall., 36 (1988) 1301-1307. 3 M. Hu, The cracking and decohesion of thin brittle films, Mater. Res. Soc. Symp., 130(1989) 213-218. 4 A. G. Evans, M. Riihle, B. J. Dalgleish and P. G. Charalambides, The fracture energy of bimaterial interfaces, Mater Sci. Eng., A, 126(1990) 53-64. 5 Z. Suo and J. W. Hutchinson, Interface crack between two elastic layers, Int. J. Fract., 43(1990) 1-18. 6 A. G. Evans and J. W. Hutchinson, Effects of non-planarity on the mixed mode fracture resistance of bimaterial interfaces, Acta. Metall., 37(1989) 909-916. 7 M.-Y. He and J. W. Hutchinson, Kinking of a crack out of an interface, J. Appl. Mech., 56 (1989) 270-278. 8 C. Atkinson, R. E. Smelser and J. Sanchez, Combined mode fracture via the cracked Brazilian disk test, Int. J. Fracture, 18(1982) 279-291. 9 P. G. Charalambides, J. Lund, A. G. Evans and R. M. McMeeking, A test specimen for determining the fracture resistance of bimaterial interfaces, J. Appl. Mech. Trans. ASME, 56 (1989) 77-82. 10 H. C. Coa and A. G. Evans, An experimental study of the fracture resistance of bimaterial interfaces, Mech. Mater., 7 (1989) 295-304. 11 P. G. Charalambides, H. C. Coa, J. Lund and A. G. Evans, Development of a test for measuring the mixed mode fracture resistance of bimaterial interfaces, Mech. Mater., 8 (1990) 269-283. 12 J.-S. Wang and Z. Suo, Experimental determination of interfacial fracture toughness curves using brazil-nut sandwiches, Acta Metall. Mater., 38(1990) 1279-1290. 13 I. Verpoest, M. Desaeger and R. Keunings, in H. Ishida (ed.), Critical review of direct micromechanical test methods for interfacial strength measurements in composites, in Controlled Interphases in Composite Materials, Elsevier, 1990, pp. 653-666. 14 P. Lawrence, Some theoretical considerations of fibre pullout from an elastic matrix. J. Mater Sci., 7(1972) 1-6. 15 P. S. Chua and M. R. Piggott, The glass fibre-polymer interface: I. Theoretical considerations for single fibre pullout tests, Comp. Sci. Technol., 22 (1985) 33-42. 16 R.R. Kieschke and T. W. Clyne, in E R. Jones (ed.), Pull-out testing of SiC monofilaments in a spray-deposited Ti matrix,
20
21 22
23
24
25 26 27
28 29 30 31
32
5
in Interracial Phenomena in Composite Materials, Butterworths, 1989, pp. 282-293. Y. Le Petitcorps, R. Pailler and R. Naslain, The fibre/matrix interfacial shear strength in titanium alloy matrix composites reinforced by SiC or B CVD filaments, Comp. Sci. Technol., 35 (1989) 207-214. D. B. Marshall, An indentation method for measuring matrix-fibre frictional stresses in ceramic composites, J. Am. Ceram. Soc., 67(1984) C259-260. D, B. Marshall and W. C. Oliver, Measurement of interfacial mechanical properties in fibre-reinforced ceramic composites, J. Am. Ceram Soc., 70 (1987) 542-548. T. P. Weihs and W. D. Nix, in S. I. Anderson, H. Lilholt and O. B. Pederson (eds.), In situ measurements of the mechanical properties of fibres, matrices and interfaces in metal matrix and ceramic matrix composites, in Metallic and Ceramic Composites, Riso National Laboratory, 1988, pp. 497-502. C. H. Hsueh, Elastic load transfer from partially embedded axially loaded fibre to matrix, J. Mater Sci. Lett., 7 (1988) 497-500. D. K. Shetty, Shear lag analysis of fibre push-out (indentation) tests for estimating interfacial friction stress in ceramic-matrix composites, J. Am. Ceram. Soc., 71 (1988) 107-109. C. H. Hsueh, Evaluation of interfacial shear strength, residual clamping stress & coefficient of friction for fibrereinforced ceramic composites. Acta. Metall. Mater., 38 (1990) 403-409. C. H. Hsueh, J. D. Bright and D. K. Shetty, Interfacial properties of SiC-borosilicate glass composites evaluated from pushout and pullout tests, J. Mater. Sci. Lett., 10 (1991) 135-138. R. N. Singh and M. Sutcu, Determination of fibre-matrix interracial properties in ceramic matrix composites by a fibre push-out technique, J. Mater. Sci., 26 ( 1991 ) 2547-2556. M. N. Kallas, D. A. Koss, H. T. Hahn and J. R. Hellman, Interfacial stress state present in a "thin slice" fiber push-out test, J. Mater. Sci., 27 (1992) 3821-3826. M. C. Watson and T. W. Clyne, The use of single fibre pushout testing to explore interfacial mechanics in SiC monofilament-reinforced Ti. Part I. A photoelastic study of the test, Acta Metall. Mater., 40(1992) 135-140. R.J. Kerans, R. S. Hay and N. J. Pagano, The role of the fiber matrix interface in ceramic composites, Ceram. Bull., 68 (1989)429-442. R. J. Kerans and T. A. Parathasarathy, Theoretical analysis of the fiber pullout and pushout tests, J. Am. Ceram. Soc., 74 (1991) 1585-1596. Y. Mikata and M. Taya, Stress field in a coated continuous fibre composite subjected to thermo-mechanical Ioadings, J. Comp. Mater., 19(1985)554-579. C. M. Warwick and T. W. Clyne, Development of a composite coaxial cylinder stress analysis model and its application to SiC monofilament systems, J. Mater. Sci., 26 (1991) 3817-3827. M. C. Watson and T. W. Clyne, The use of single fibre pushout testing to explore interfacial mechanics in SiC monofilament-reinforced Ti. Part II. Application of the test to composite material, Acta Metall. Mater., 40 (1992) 141-148.
1
The tensioned push-out test for fibre-matrix interface characterisation under mixed mode loading M. C. Watson and T. W. Clyne Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ (UK) (Received June 22, 1992)
Abstract A novel test procedure is suggested for the exploration of interfacial mechanical properties in fibre-reinforced composites. This is based on the established single fibre push-out test, which has the advantage of being applicable to specimens which can be routinely produced from normal unidirectional long fibre composite material. The suggested modification involves the application of equal biaxial in-plane tension while the push-out testing is carded out. This allows the interface to be subjected to various combinations of mode I and mode II loading, ranging from pure opening to pure shear. Hence it should be possible to obtain data characterising the resistance to interracial debonding and disengagement
over all of this range. Illustrative data are presented here for titanium-basedcomposites,in the form of measured critical shear stresses for various applied normal stresses at the interface.
1. Introduction
There have over recent years been significant advances concerning stresses and fracture mechanics at bimaterial interfaces [1-5]. For interfaces which are significantly less tough than the neighbouring bulk materials, a crack (i.e. a debonding event) can continue to follow an interface, even though the stress intensity at the crack tip is not purely mode I (crack opening) and may have a substantial mode II (shearing) component [6, 7]. The mixity of crack tip loading mode can vary between pure mode I and pure mode II, depending on the loading geometry and elastic properties of the two materials. This homogeneity is commonly characterised by the so-called phase angle, % which is the angle with a tangent given by the ratio of the mode II to mode I stress intensity factors at the crack tip. This mixity is important because the toughness, i.e. the critical strain energy release rate, Gic, can vary quite markedly with q~ -- see Fig. 1. A large shear component tends to result in the crack tip being shielded (particularly if the interface is geometrically rough), with more frictional work being done in the wake of the crack -- leading to a larger Gic value. Various tests have been devised [8-12] to measure Gic, as shown in Fig. 1. However, many of these are applicable only to planar interfaces; those suitable for fibre-matrix interfaces tend to be limited to large values of ~0 (i.e. predominantly shear loading). Tests 0921-5093/93/$6~00
developed for fibre composites [13], such as pull-out [14-16] and full fragmentation [17] tests, are in general aimed at identification of critical stress levels for debonding or sliding under pure shear loading. Moreover, for many tests the specimen must be produced in a special operation which differs from the normal composite manufacturing route and may thus create different interracial microstructures and residual stresses. One of the few tests which can be applied to normal fibrous composite material is the single fibre pushout (or push-down) test. There is therefore considerable interest in this procedure, in spite of the apparent limitation to pure shear loading (~p -- 90°). Fibre push-out and push-down testing has received considerable attention recently, in terms of both experimental and theoretical work [18-25]. This has included the derivation of various shear lag-type analytical solutions to the stress field. More recently, finite element study [26] and photoelastic examinations [27] have shown that the variations in shear stress along the length of the fibre are less pronounced than predicted by shear lag models. Debonding is stimulated near the free surface under essentially pure shear loading. (In fact, the applied load generates a normal stress on the interface by differential Poisson expansion, and residual thermal stresses are also present, but since both effects usually generate radial compression, there is no mode I loading.) Once debonding has started, it will propagate along the~ length of the fibre. After the © 1993 - Elsevier Sequoia. All rights reserved
2
M.C. Watson, T. W. Clyne
/
Tensionedpush-out testforfibre-matrix interface characteristics Load
Double
[ I
Cantilever I~ Beam
¥~0"-5" ~ from frictional
4-point bend ~
"41ff-'m~mm ~
Composite specimen (fibres parallel
/
t° load) lnduction
'
work, plastic
deformation, ete ~ite ter n-out)
.......
#~ Brazil nut
~
~ 70" - 90"
0
i ~ 0 Tif(6d~, Platen
Fig. 2. Schematic illustration of the diffusion bonding set-up used to produce specimens for the test.
Fig. 1. Schematic illustration of the dependence of G~,.on % plus
depictions of various tests. Different tests for the measurement of Gi~.involve a wide range of ~0values. initiation of debonding, motion will be at first partly and then purely by frictional sliding at the interface. While debonding is taking place, the load may rise or fall, depending on the debonding and frictional sliding stresses. The push-out test can also be used to obtain Gic data. Analyses have been presented [19, 28, 29] of the energy balance during push-out. These depend on monitoring of the load/displacement behaviour during progressive debonding. In the present paper, a variant of the push-out test is proposed in which the specimen is subjected to equal biaxial in-plane tension while an axial compressive load is applied to a fibre. This should in principle allow any value of ~p from 0 ° to 90 ° to be generated, depending on the proportion of in-plane and axial stresses (and the presence of residual stresses) -- see Fig. 1. The information given here is limited to a brief outline of the principle and some experimental information; a preliminary set of test results is presented, obtained with a set-up giving only the peak load needed for push-out. Interfacial work of fracture data cannot be obtained without continuous load/displacement monitoring, but the results serve to demonstrate that the test procedure is viable and that an effect of the type expected was observed on changing the mode mixity.
2.. Material and specimen preparation The composite used in this investigation was supplied by BP plc in the form of a 30-ply panel, prepared by hot isostatic pressing of T i - 6 A I - 4 V foils and SiC monofilaments (approximately 35% by volume). The monofilaments were W-cored, stoichiometric SiC, 100/zm in diameter, with a duplex coating of graphitic carbon and TiB2 (approximately 1 /zm of each). A
small specimen, 5 mm square and 2 mm thick, was cut from this panel, the fibres lying normal to the square surface. In order to facilitate gripping, the composite specimen was diffusion bonded into a larger panel of unreinforced alloy. This was done by stacking six alloy foils, each 20 mm square and 500/zm thick and placing the composite specimen on top, after chamfering the edges to an angle of 45 °. This assembly was then diffusion bonded together -- see Fig. 2. This was done at 850 °C for 30 min under a vacuum of about 0.6 Pa, with a maintained stress of 150 MPa. The composite square became immersed in the unreinforced material during this treatment, with the fibres remaining vertical. After bonding, the specimen was cut and ground, then thinned by careful metallographic polishing to produce a 'foil' of thickness around 100/zm.
3. Testing procedure The test set-up allowed biaxial loads to be applied in the plane of the specimen, whilst supported from beneath by a plinth with a hole of diameter about 200 /zm, into which the fibres could be pushed -- see Fig. 3. Before the specimen was placed in the grips, small strain gauges were attached on the unreinforced material, parallel to the two in-plane loading directions. Readings from these were monitored during adjustment of the loads to ensure that an equal biaxial stress state was generated. Since the specimen was relatively thin, there was a danger of significant curvature arising during handling and loading -- both in-plane and when indenting individual fibres. In order to compensate for the effect of this curvature on the strain gauge readings, gauges were also attached on the underside of the specimen. The applied in-plane loads could then be adjusted until the mean readings of each pair of strain gauges were the
M. C. Watson, T. W. Clyne
Matrix
/
Tensioned push-out test for fibre-matrix interface characteristics
Load appliedwith pyramidal indenter ~ Composite
. . . . . .
&3 ]
~I
1.2
i
3
i
\
I
Fibre (SIC) ~
Ra~., s,~ss
I---..... Hoop Stress
I.I
Matrix (~)
o
~I.0 /..
-'"
~0.9
....................
Composite(Ti-35%SiC)
/
0.8
(a)
/
/
0.7
i
Strain gauges (parallel to 2 //direction)
Applied
i
Strain gauges (parallel to 1 direction) ~\
I00
,,i
101 Fibre radii from centre
10 0
102
',,,
(b)
0 ~//////~/////////~
",
Hole
Composite(Ti-35%SiC)
-I00
Fig. 3. Schematic illustration of the loading set-up.
Matrix (Ti) Fibre (SIC)
Radial Stress Hoop Stress
~-200
same, since this corresponded to the volume-averaged in-plane stresses being equal in the two directions. The specimen was placed in the grip assembly, which was then moved around on the base until a selected fibre was centred in the optical imaging system of the indenter. The in-plane stress was then generated via simple screw arrangements on one half of each pair of grips. Compressive loads were then applied axially to the fibre with a conventional microhardness pyramidal indenter, the fibre being viewed after each loading operation. The applied axial load was progressively increased until pushout was seen to have occurred.
4. Results and discussion Interpretation of the results from the test as described above is quite straightforward. Firstly, for any given in-plane load, the strain gauge readings must be converted to a radial stress across the fibre/matrix interface. (The actual value will then be the sum of this and any residual stress present in the unloaded composite.) This can, for the case of an equal biaxial stress, be carried out using an analytical stress model, at least to a good approximation. For example, the coaxial cylinder model [30, 31] can be used; this gives the radial distribution of the three principal stresses within a set of infinite coaxial cylinders subjected to specified axial and radial loads (and temperature change). The model can be applied with the known in-plane stress input as an applied radial stress and the applied axial stress taken as zero. In fact, this leads to an error since,
. . . . . .
i
100
. . . . .
i
i
101
102
Fig. 4. Predicted [31] variations with radial location of the radial and hoop stresses within a system composed of a central SiC fibre, a surrounding cylinder of composite material (Ti-35%SIC) and an outer cylinder of unreinforced (Ti) matrix. Stress distributions are shown for: (a) an applied radial stress (stresses given as a ratio to this); (b) a temperature decrease of 500 K (absolute values of stresses given).
even with no applied axial stress, the model predicts a non-zero axial stress (from Poisson effects), whereas in practice the axial stress should be negligible in a thin slice of this type. Also, since the system is taken as being of finite extent in the plane, the radial and hoop stresses will not necessarily be equal, even at the periphery, whereas the assumed boundary conditions require an isotropic in-plane stress at the far field. However, in practice these are rather small errors for the systems of interest. This is illustrated in Fig. 4(a), which shows the predicted [31] variation in radial and hoop stress as a function of radial position for a system composed of a central fibre, a surrounding circular region of composite (treated as a continuum) and an outer region of unreinforced matrix. Property data shown in Table 1 were used in generating this figure. The radii of these regions have been selected so as to correspond approximately to the specimen of interest here. Errors introduced from the fibre not being in the centre of the composite, and the composite surrounding the fibre not being a homogeneous con-
4
M.C. Watson, T. W. Clyne
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Tensionedpush-outtestforfibre-matrix interface characteristics
TABLE 1. Thermophysical data Property
100
Fibre (SIC) Composite Matrix (Ti-35%SiC) (Ti-6AI-4V)
@ 7o
Axial E (GPa) Trans. E (GPa) Axial v
Trans. v Axial a ( 1 0 -6 K-l) Trans. a (10-6 K -' )
450 450 0.2
0.2 4.0 4.0
232 155 0.3
0.25 5.0 7.0
115 115 0.35
,~
6o
4o
0.35 8.0 8.0
lO
tinuum, could be significant but are unlikely to be very large. It can be seen from Fig. 4(a) that, for the specimen of interest, the normal stress across the fibre-matrix interface will be about 25% higher than the far field in-plane stress (rl: Or -- 1.25ol
__Opo 4S
Eme
I
i
10
20
,
i
i
30
40
,
I
l
50
60
,
/
I
70
80
,
I 90
,
i 100
Interracialradial tensilestrbss (MPa)
Fig. 5. Experimental pushout data from the tensioned pushout test, applied to as-fabricated Ti-6AI-4V/35%SiC, expressed in
the form of inteffacial shear strengths as a function of applied interracial radial tensile stress.
stress required for frictional sliding. It is possible that the apparent levelling off in critical shear stress with increasing normal tension corresponds to a transition from frictional sliding to debonding being the process dictating the peak push-out load. It will certainly be of interest to study debonding behaviour in this test, with continuous monitoring of load-displacement behaviour, in order to explore this possibility and to establish the interracial fracture toughness. This can then be evaluated as a function of the phase angle % although it will be necessary to measure the thermal residual stress in order to establish % It is clear from the data in Fig. 4(b) that thermal residual stresses are expected to be significant, even when allowance is made for some stress relaxation during cooling from the fabrication temperature.
5. Conclusions (2)
where s is the aspect ratio (depth/diameter) of the fibre, which is close to unity in the present case. The value of the in-plane biaxial stress al is related to the strain gauge reading e s by the equation
al
0
(1)
In practice, it will often be important to take account of residual thermal stresses, at least for metal matrix composites. For example, Fig. 4(b) shows the stress distribution in the specimen according to the coaxial cylinder model for a temperature decrease of 500 K (a lower value than the actual decrease being taken to allow for some stress relaxation during cooling). It can be seen that a compressive stress of over 200 MPa is predicted at the interface. Preliminary experimental data are shown in Fig. 5, giving the experimental pushout stress, Opo (converted to an interfacial shear stress for frictional sliding, rfr -see below), as a function of the interracial radial tensile stress, Or, being imposed via the in-plane tension. The push-out stress is thought in this system [32] to represent the onset of frictional sliding, with an effectively uniform shear stress along the length of the fibre given by Tfr
0
(3)
( 1 - vm)
where E m and Vm are the Young's modulus and Poisson's ratio of the matrix. (Typical strain gauge readings were a few hundred microstraln.) The interfacial radial stress is then found from o 1 using eqn (1). These are preliminary data, but they do appear to indicate that a systematic trend is present in terms of applied radial tension causing a reduction, in the shear
A novel testing procedure has been described for the investigation of interracial mechanical properties in fibre-reinforced composites. This involves the superposition of normal (radial) tensile stress at the interface and the shear stress imposed by conventional push-out testing. This will allow the mechanical response of the interface to be characterised over a range of opening/ sheafing mode combinations. A method of applying the test has been described and illustrative data have been presented for a Ti-6AI-4V/35%SiC monofilament composite, obtained using, a conventional microhardness indenter. In addition, an analytical approach is described allowing the experimental data to be convetted to shear and normal stresses at the interface, for the case of both being constant along the length of the fibre. It is shown that the application of normal tension
M. C. Watson, T. W. Clyne
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Tensioned push-out test for fibre-matrix interface characteristics
leads to a significant d e c r e a s e in the p e a k shear stress n e e d e d to p u s h out the fibre. In considering these results, the residual (compressive radial) t h e r m a l stresses s h o u l d be t a k e n into account, since they are likely to have a significant influence o n the effective b a l a n c e of o p e n i n g and shearing m o d e s , at least for metal (and p o l y m e r ) matrix composites.
17
18 19
References 1 J.R. Rice, Elastic fracture mechanics concepts for interracial cracks, J. Appl. Mech. Trans. ASME, 55(1988) 98-103. 2 M.S. Hu, M. D. Thouless and A. G. Evans, The decohesion of thin films from brittle substrates, Acta Metall., 36 (1988) 1301-1307. 3 M. Hu, The cracking and decohesion of thin brittle films, Mater. Res. Soc. Symp., 130(1989) 213-218. 4 A. G. Evans, M. Riihle, B. J. Dalgleish and P. G. Charalambides, The fracture energy of bimaterial interfaces, Mater Sci. Eng., A, 126(1990) 53-64. 5 Z. Suo and J. W. Hutchinson, Interface crack between two elastic layers, Int. J. Fract., 43(1990) 1-18. 6 A. G. Evans and J. W. Hutchinson, Effects of non-planarity on the mixed mode fracture resistance of bimaterial interfaces, Acta. Metall., 37(1989) 909-916. 7 M.-Y. He and J. W. Hutchinson, Kinking of a crack out of an interface, J. Appl. Mech., 56 (1989) 270-278. 8 C. Atkinson, R. E. Smelser and J. Sanchez, Combined mode fracture via the cracked Brazilian disk test, Int. J. Fracture, 18(1982) 279-291. 9 P. G. Charalambides, J. Lund, A. G. Evans and R. M. McMeeking, A test specimen for determining the fracture resistance of bimaterial interfaces, J. Appl. Mech. Trans. ASME, 56 (1989) 77-82. 10 H. C. Coa and A. G. Evans, An experimental study of the fracture resistance of bimaterial interfaces, Mech. Mater., 7 (1989) 295-304. 11 P. G. Charalambides, H. C. Coa, J. Lund and A. G. Evans, Development of a test for measuring the mixed mode fracture resistance of bimaterial interfaces, Mech. Mater., 8 (1990) 269-283. 12 J.-S. Wang and Z. Suo, Experimental determination of interfacial fracture toughness curves using brazil-nut sandwiches, Acta Metall. Mater., 38(1990) 1279-1290. 13 I. Verpoest, M. Desaeger and R. Keunings, in H. Ishida (ed.), Critical review of direct micromechanical test methods for interfacial strength measurements in composites, in Controlled Interphases in Composite Materials, Elsevier, 1990, pp. 653-666. 14 P. Lawrence, Some theoretical considerations of fibre pullout from an elastic matrix. J. Mater Sci., 7(1972) 1-6. 15 P. S. Chua and M. R. Piggott, The glass fibre-polymer interface: I. Theoretical considerations for single fibre pullout tests, Comp. Sci. Technol., 22 (1985) 33-42. 16 R.R. Kieschke and T. W. Clyne, in E R. Jones (ed.), Pull-out testing of SiC monofilaments in a spray-deposited Ti matrix,
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in Interracial Phenomena in Composite Materials, Butterworths, 1989, pp. 282-293. Y. Le Petitcorps, R. Pailler and R. Naslain, The fibre/matrix interfacial shear strength in titanium alloy matrix composites reinforced by SiC or B CVD filaments, Comp. Sci. Technol., 35 (1989) 207-214. D. B. Marshall, An indentation method for measuring matrix-fibre frictional stresses in ceramic composites, J. Am. Ceram. Soc., 67(1984) C259-260. D, B. Marshall and W. C. Oliver, Measurement of interfacial mechanical properties in fibre-reinforced ceramic composites, J. Am. Ceram Soc., 70 (1987) 542-548. T. P. Weihs and W. D. Nix, in S. I. Anderson, H. Lilholt and O. B. Pederson (eds.), In situ measurements of the mechanical properties of fibres, matrices and interfaces in metal matrix and ceramic matrix composites, in Metallic and Ceramic Composites, Riso National Laboratory, 1988, pp. 497-502. C. H. Hsueh, Elastic load transfer from partially embedded axially loaded fibre to matrix, J. Mater Sci. Lett., 7 (1988) 497-500. D. K. Shetty, Shear lag analysis of fibre push-out (indentation) tests for estimating interfacial friction stress in ceramic-matrix composites, J. Am. Ceram. Soc., 71 (1988) 107-109. C. H. Hsueh, Evaluation of interfacial shear strength, residual clamping stress & coefficient of friction for fibrereinforced ceramic composites. Acta. Metall. Mater., 38 (1990) 403-409. C. H. Hsueh, J. D. Bright and D. K. Shetty, Interfacial properties of SiC-borosilicate glass composites evaluated from pushout and pullout tests, J. Mater. Sci. Lett., 10 (1991) 135-138. R. N. Singh and M. Sutcu, Determination of fibre-matrix interracial properties in ceramic matrix composites by a fibre push-out technique, J. Mater. Sci., 26 ( 1991 ) 2547-2556. M. N. Kallas, D. A. Koss, H. T. Hahn and J. R. Hellman, Interfacial stress state present in a "thin slice" fiber push-out test, J. Mater. Sci., 27 (1992) 3821-3826. M. C. Watson and T. W. Clyne, The use of single fibre pushout testing to explore interfacial mechanics in SiC monofilament-reinforced Ti. Part I. A photoelastic study of the test, Acta Metall. Mater., 40(1992) 135-140. R.J. Kerans, R. S. Hay and N. J. Pagano, The role of the fiber matrix interface in ceramic composites, Ceram. Bull., 68 (1989)429-442. R. J. Kerans and T. A. Parathasarathy, Theoretical analysis of the fiber pullout and pushout tests, J. Am. Ceram. Soc., 74 (1991) 1585-1596. Y. Mikata and M. Taya, Stress field in a coated continuous fibre composite subjected to thermo-mechanical Ioadings, J. Comp. Mater., 19(1985)554-579. C. M. Warwick and T. W. Clyne, Development of a composite coaxial cylinder stress analysis model and its application to SiC monofilament systems, J. Mater. Sci., 26 (1991) 3817-3827. M. C. Watson and T. W. Clyne, The use of single fibre pushout testing to explore interfacial mechanics in SiC monofilament-reinforced Ti. Part II. Application of the test to composite material, Acta Metall. Mater., 40 (1992) 141-148.