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EP microcomposites and macrocomposites
Effects of the interface on th e mechan=cal response of CF/EP microcomposites and macrocomposites F. HOECKER and J. KARGER-KOCSIS (Institute for Composite Materials, University of Kaiserslautern, Germany) Received 14 September 1993; revised 2 February 1994
The aim of this study was to investigate the effects of the interfacial bond quality on the mechanical response of composite laminates, for example an epoxy matrix reinforced by continuous carbon fibres of varying surface coating. The fibre/matrix adhesion was characterized by determining the interfacial shear strength q in single-fibre fragmentation and microdroplet pull-off tests. The failure mechanisms were deduced from the stress birefringent patterns (fragmentation test) and from fractographic analysis in a scanning electron microscope (microdroplet pull-off test). Selected interface-relevant properties were evaluated in mechanical tests on laminates. The present paper highlights the problems related to the micromechanical characterization and the interface relevance of data resulting from transverse tensile, transverse flexure and interlaminar shear tests. Furthermore, the effects of the interface on the impact performance of unidirectional and cross-ply laminates were studied. Attempts were made to correlate the macroscopic mechanical response with the interface-related characteristics (q and failure mechanisms). Key w o r d s : interface; mechanical response; carbon/epoxy composites
The mechanical property profile of fibrous composites is mainly determined by the composition and arrangement of their constituents, i.e. matrix and reinforcement, involving parameters such as fibre volume fraction, fibre aspect ratio, fibre orientation and strengths and moduli of the matrix and fibres, respectively. In addition, the nature and characteristics of the interface (or interphase) between fibres and matrix affect the mechanical performance of composites fundamentally. Their property profile can even be 'tailored' for strength, stiffness and toughness requirements by means of interface modification.
transfer and stress redistribution among the fibres, which control the mechanical response in real composites. 'Micromechanical' or 'model' test methods are conducted on single-fibre specimens in order to realize a more well-defined and quantitative determination of the fibre/matrix interfacial shear strength or to study the variation of this term with interface modification. These methods exhibit more clearly defined stress states and thus high interface relevance. However, micromechanical tests lack standardization (i.e. in terms of specimen preparation, testing procedures and data reduction) and 'real composite conditions'.
To characterize the fibre/matrix adhesion or interface modifications qualitatively and comparatively, in principle any test method resulting in interface-relevant data is appropriate. 'Macromechanical' test methods using standardized specimen geometries and testing procedures have the advantage that testing is performed on laminates, i.e. in 'composite-like' conditions. On the other hand, the complicated three-dimensional stress states arising during failure initiation and propagation within the specimens cannot guarantee pure shear failure in the interface. This is, however, a prerequisite for stress
The basic understanding of interfacial effects and the qualification of the mechanical property profile of composites are the driving forces of the widespread use of polymer matrix based composites. Therefore, great efforts are undertaken to correlate the laminate mechanical response with the characteristics of the interface' for particular material systems. In the frame of this project both micromechanical and macromechanical tests were carried out on carbon fibrereinforced epoxy (CF/EP) composite materials of poor and good interfacial bonding, respectively. The present
0010-4361/94/07/0729-10 © 1994 Butterworth-Heinemann Ltd COMPOSITES. VOLUME 25. NUMBER 7 . 1994
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paper highlights the problems related to the micromechanical characterization and the interface relevance of data resulting from transverse tensile, transverse flexure and interlaminar shear tests. Furthermore, the effects of the interface on the impact performance of unidirectional and cross-ply laminates were also investigated, EXPERIMENTAL Materials and fabrication
As a model thermoset composite material CF/EP was chosen, and a hot-curing epoxy system (bisphenol-A based resin Araldit LY 556; anhydride hardener HY 917, 90 phr; heterocylic amine catalyst DY 070, 1 phr; all of them from Ciba-Geigy, Basel, Switzerland) was used as a matrix reference. The fibres selected were polyacrylonitrile-based high tenacity carbon fibres supplied by Akzo (Wuppertal, Germany). The two fibres used in this study varied only in their sizings. Both of them are standard, surface treated and commercially available, and therefore of economic interest. The designations 'Tenax HTA 5131' and 'Tenax HTA 5411' stand for 1.25 wt% EP sizing and 0.15 wt% antistatic agent, respectively. These agents effect 'optimized' (HTA 5131) and 'not optimized' (HTA 5411) adhesion to the epoxy. Unidirectional (UD) and cross-ply laminates for the macroscopic composite tests were produced by wet iliament winding of two rovings (12 k, 800 tex) on a fiat aluminium plate. Their consolidation occurred in an autoclave curing cycle (120°C, 8 h, 0.7 MPa) with subsequent oven cooling. The average fibre volume content ~br for each laminate plate, which varied in the range from 65 to 70 vol%, was established by ashing the matrix. Specimens for the micromechanicai tests were cured in a heating chamber (120°C, 8 h, oven cooling), always using a vacuum-degassed EP resin, Microscopic testing techniques 1. Fragmentation test Although the drawbacks and difficulties of this method, being mostly related to data reduction, are well known 2, tests on single-fibre composites (SFCs) were conducted, The interracial failure mechanisms in SFCs are easy to monitor in situ during the test using transmitted light microscopy and crossed polarizers. The interracial shear strength values, ri, were calculated from 3 3 o'uD - 8 l'
(1)
where D and l' are the fibre diameter and mean fragment length, respectively, and tru is the fibre strength at l'. So, effects like debonding, thermal stresses, etc. are neglected4, and therefore a shear stress controlled interfacial failure is assumed. Nevertheless, the apparent fibre fragment length distributions indicate the shear stress transfer along the fibre/matrix interface. 2. Microdropletpull-off test Microdroplet pull-off tests, as first described by Miller et al. 5, were performed using an adjustable linear microvice, The EP droplet was pulled off at a velocity of 0.2 mm
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COMPOSITES
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min-L The fibre free length was kept constant ( ~ 10 mm) for all tests in order to ascertain similar stored elastic energies for failure initiation and propagation. This test was conducted in addition to the fragmentation mainly for two reasons: (1) to get comparable zqvalues and thus to check the interface sensitivity of these test methods; and (2) to distinguish between adhesive-type failure (poor bonding) and cohesive matrix failure (good bonding) by means of scanning electron microscopy (SEM) examination of the failed sites. The interfacial shear strength value can be directly computed from Fmax r~ - ~rDL (2) where Fmaxis the apparent debonding force (D and L are fibre diameter and embedded fibre length, respectively); alternatively, it can be calculated from the slope of an Fmax/Lplot 6. In the latter case scatter from the linear regression at larger embedded fibre lengths clearly indicates deviation from the assumed shear stress controlled interfacial failure. Macroscopic testing techniques This paper reports only selected topics from a comprehensive research project focused on the effects of the interface on the mechanical response of composites. In particular, transverse tensile and flexural properties as well as interlaminar shear and impact performance are discussed here. Transverse tensile and three-point bending tests were performed according to the ASTM D 3039 and ASTM D 790 standards, respectively; in the latter, a support spanto-depth ratio of 32 was maintained. Moduli were determined from the slope of the stress/strain plot in the range from 0.05 to 0.25% applied strain. The latter was measured by strain gauges and/or incremental transducers. Interlaminar shear properties were determined by the short beam shear test, referring to DIN 29 971 (support span-to-depth ratio of 4), and the transverse Iosipescu shear test 7 utilizing the Wyoming test fixture as proposed by Wilson 8. Instrumented Charpy and drop weight tests were performed in accordance with DIN 53 453 and DIN 53 4432, respectively. Charpy tests were carried out on small, unnotched, standard specimens cut from unidirectional (UD) and cross-ply laminates at ambient temperature. Low energy falling weight impact tests on cross-ply laminates varied in the incident energy (1.25, 1.75 and 4.00 J) to assess the energy absorption and damage accumulation behaviour due to different fibre sizing in the subperforation energy range.
RESULTS A N D DISCUSSION Micromechanicalcomposite characterization 1. Fragmentation test Fig. 1 depicts the failure modes in Tenax HTA 5131/EP and Tenax HTA 5411/EP SFCS as a function of the applied strain. 'Saturation' of the successive fibre fragmentation process was ascertained at 7.6%. It can be
a
b
Fig. 1 Fragmentation mechanisms in CF/EP SFCs for (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP
proved by the apparent stress birefringence patterns that the stress fully accommodates on the fibres after breaking, i.e. ideal stress transfer occurs. This phenomenon indicates good bonding quality between fibre and matrix for Tenax HTA 5131/EP; the mean fibre fragment length derived is 283 pm. For Tenax HTA 5411/EP, successive debonding can be observed, starting from the fractured fibre ends with increasing strain. Therefore, the average fragment length for 'not optimized' bonding is 338 p.m. It must be noted here that the transmitted light microscopy photographs for HTA 5411 are selected to demonstrate the poor bonding and related failure mechanism, although this site is not representative for the whole specimen. It is obvious that the z~values calculated from the HTA 5131 and HTA 5411 mean fibre fragmentation lengths will not be comparable. This is due to the different failure mechanisms, since HTA 5411/EP does not fulfil the prerequisites of Equation (1) (ideal adhesion, yielding interface). Nevertheless, in order to achieve a comparative measure for interfacial shear stress transfer, q is calculated using Equation (1) in both cases, Furthermore, if this equation is considered, there is another shortcoming related to the fibre strength for the fibre free length l' (O'u(/')) 9. This value is, most commonly, calculated from
_~
6000 L I~
, ~, . / W e i b u l l ...... ~ ' - ~. (1")= ~, (Lte~,)( I__~._""~-; L~"~/L= ~1~__~ 1 " ,,L,e,+ -l.. ] ! J
~
5
=.~ ~ • iT.
4000@ .... - ~ ~ ~ 1_ i " ~ ~ - - , Z 3000 ' -
0
lin. Extrapolation 6, (1")= 4407,9- 98,7 * I" ~ - - ~ .......... ' i
~ - ~ - - - - - ~ -
I TI" 2000 i l
..... 1 J
'~ ] ~
5
.......
i~ l [
.... ....... ' ~ ~ ] i L~, [ 1 i L?~, I 10 15 20 Fibre Free Length Ltest[mm]
Fig. 2 Comparison of fibre strength models for Tenax HTA 5131
au(l') __ (~est) I'm O'~(~estest) T (3) This equation is based on Weibull statistical distributions fitted to experimental data determined by tensile test series on single fibres (Cru(Ltest)) using 'measurable' fibre free lengths L~t. In Figs 2 and 3 fibre strength values for 5 and 10 mm fibre free lengths that were calculated from a Weibull fit
COMPOSITES.
NUMBER 7. 1994
731
.~ a. "~
~
6000 ~
5000
~"(I')=°"(L"")/
I [ ~."c'I~
4000
.e
ft.
/Weibull
I
~
"~-~/L---:~..,) l" 4--
, ,,, J I
_
-lin. Extrapolation
--
~u(i') = 3796,7 - 55,5, i --
---.. ~
2, M i c r o d r o p l e t p u l l - o f f test
~
3000
~
~
i
Ltest
|" 200C
0
5
10
t I
' 15
20
Fibre Free Length Ltest[mm] Fig. 3 Comparisonof fibre strength models for Tenax HTA 5411 of strength data gained at Ltes,= 15 mm are compared with the corresponding experimental data for Tenax HTA 5131 and Tenax HTA 5411 filaments. For the single-fibre tensile tests using 15, l0 and 5 mm gauge lengths, the strain velocity was kept constant at 2% min-L This means that the crosshead displacement velocities of the specially designed and instrumented microtension testing machine were 0.3, 0.2 and 0.1 mm min-', respectively. Finally, fibre strengths cru for l' were calculated from Equation (3) and, in addition, by linear extrapolation of the experimental data. It is evident that the Weibull method leads to an 'overestimation' of Cru(l') compared to the linear regression. So, both fibre surface modifications exhibit clear differences in their strength at the critical fibre length. Using linear extrapolation the strength data are rather different (HTA 513 l, 5517 MPa;
Fig. 4
HTA 5411, 3641 MPa), whereas the Weibull method yields similar values (HTA 5131, 5386 MPa; HTA 5411, 5384 MPa). Therefore, ~ values that are calculated by using fibre strengths from linear extrapolation of the mean fragment length are likely to be much more reliableS°"
Based on the SEM photographs taken for matrix droplets on the two fibres (see Fig. 4), one can state that the wettability of both fibres is identical. For both fibre modifications contact angles of approximately 45* were measured for the consolidated samples. In all cases adhesive failure took place at the interface as no matrix residues could be resolved on the fibre surface by SEM (see Fig. 5). Furthermore, adhesive failure can be deduced from the straight lines in the Fm,x/L plots depicted in Fig. 6. This linear proportionality clearly indicates shear stress controlled interracial failure occurring in a sudden and 'catastrophic' manner (sharp and sudden load drop from Fm,x to 0). The r~ values were determined in two ways: (1) from the slope of the FmJL regression line fitted by the leastsquares method; and (2) by direct calculation using Equation (2). As both methods yield similar ri values (Tenax HTA 5131/EP, 93.8 and 95.2 MPa; Tenax HTA 5411/EP, 65.77 and 63.2 MPa), only the average direct z~ data will be given further on.
3. Comparison of rnicromechanical results Fig. 7 compares the zq values determined in the microdroplet pull-off and fragmentation tests on SFCs contain-
Droplet formation in (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP microdroplet pull-off specimens
Fig. 5 Adhesive-type failure in (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP microdroplet pull-off specimens
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COMPOSITES . NUMBER
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30011 II H ............................................................................. IliA 5131 ..................................... I[ F=2,0632L\ _.._... E 200~I ................................................................................................. '\. / ......... u. I[ ~ (9
ing Tenax HTA 5131 and Tenax HTA 5411 fibres. It is very striking that the interfacial shear strengths determined by the microdroplet pull-off test are much higher than those related to the fragmentation test. This phenomenon is already known and fits well in the range of r~ values given for CF/EP in a recent overview by Piggott< For Tenax HTA 5131, r~ seems to reach or even to exceed the matrix tensile strength, determined to be 92.4 MPa. This could not, of course, be proved by matrix residues (SEM), as explained earlier in this paper.
Z'E
O [t F H T ~ .
u.
10
.............
0
90
40
60
80
100
Embedded Fibre Length L [!am] Fig. 6 Maximumdebondingforce vs. embeddedfibre length for the microdropletpull-off test
120'~ . IX 100~ _ _ c ; ~ ~.3 80 ,~N,N,~x-N~
[ ] Tenax HTA 5131 [ ] renax HTA 5 4 1 1
kl............... x'WB~ ,L
60 40-
C
_"
--
-..........
The microdroplet pull-off test exhibits a much higher rating in interface relevance than fragmentation. According to the former technique, the ~ ofTenax HTA 5131/EP (95.2 MPa) is about 50% higher than the HTA 5411 modification (63.2 MPa). An analogously large difference turns out in the SFC fragmentation test (increase of 42%) when the linear extrapolation (LE) technique is used for the determination of the fibre strengths of the mean fragment lengths (HTA 5131, 41.1 MPa; HTA 5411, 28.4 MPa). Although the Weibull data reduction (wB) results in higher ri values (Tenax HTA 5131/EP, 50.2 MPa; Tenax HTA 5411/EP, 42 MPa) than those from the linear extrapolation (leading to a decreasing difference between the ri values from the fragmentation and microdroplet pull-off tests), the interface relevance tends to diminish (increase of 19.5%) owing to different levels of cr,(l') overestimation with respect to the LE method (HTA 5131, 22%; HTA 5411, 48O/o).
acro ec
n;ca, co
os; e
a, ctor;z,
on
A summary of the laminate properties (mean values :1: standard deviations) is given in Table 1,
Microdroplet Fragmentation
1. Transverse tensile and flexural properties
Pulloff Fig. 7 Comparison of the interfacialshearstrengthvaluesobtained from microdroplet and fragmentation tests
In Fig. 8 the experimental strength and modulus data of the laminates studied are depicted for both transverse tensile and transverse three-point bending set-ups. Con-
Table 1. S u m m a r y o f m a c r o m e c h a n i c a l l a m i n a t e p r o p e r t i e s (mean values + s t a n d a r d deviations) Properties Transverse tensile test cr ( M Pa) E (G Pa) G (GPa) Transverse three-point bending test cr (MPa) E(GPa) Short beam shear test r (MPa) Transverse Iosipescu test r (MPa) Charpy test av(UD; mJ mm -2) a v ( C P ; m J m m 2) DI*(UD; %) DI (CP, first peak; %) DI (CP, second peak; %) Drop weight test Relative energy absorption (%) 1.25 J/1.75 J/4.00 J
Tenax HTA 5131/EP
Tenax HTA 5411/EP
37.3 ± 5.7 9.7 J: 0.4 4.7 i 0 . 2
26.2 + 3.4 9.2 :I: 0.6 4.5±0.1
51.7±8.2 8.4:t:0.6
50.4+2,4 8.34-0.5
50.8+3.6
50.74-2.5
37.44-4.4
17.0+3.8
115.5+8.4 105.0t17.2 57.2:1:4.3 83.7 + 1.0 55.0±5.1
92.0:t:10.1 92.1 +12.3 39.84-5.6 81.7 + 9.0 53.2±6.0
88.9/80.0/77.0
89.1/82.6/79.3
*Ductility index
COMPOSITES. NUMBER 7 . 1994
733
Tenax HTA 5131 / EP ~J ,--,
Tenax HTA 5411 / EP
60
i
¢~ 50 a. ~• " 40
lI i
e-
30" O~ C 20"
II
L _
,
8
!
iN
'
i .
004" 10"
ii ---/-
0
~
2~ 0
Tensile Test Fig. 8
13. 10"~
3-P Bending Test
in Tension
in Shear (calc.)
in Flexure
Transverse tensile and flexural properties of Tenax HTA 51 31/EP and Tenax HTA 5411/EP laminates
Fig. 9 Transverse tensile test fracture surfaces of (a) Tenax HTA 5131/EP and (b) Tenax HTA 541 1/EP laminates
sidering the transverse tensile strength data, the influence of the interface is very clear. The value for 5131/EP is 42% higher than for 5411/EP. This enhancement is identical with that of the single-fibre fragmentation test using the LE method. This is rather surprising since the loading conditions of the interface (radial tension vs. shear with respect to the fibre axis in the transverse tensile and single-fibre fragmentation test, respectively)are substantially different. This improvement is attributed to a change in the failure mechanism from adhesive-type interfacial failure (HTA 5411) to a more cohesive matrix failure (HTA 5131). As SEM photographs taken on typical sites of the fracture surfaces (see Fig. 9) show, 'good' interfacial bonding results in a smooth fracture surface where the fibres are still covered and held together by the EP matrix. On the contrary, the 'not optimized' bonding yields a rough fracture surface owing to crack deviation and bifurcation from the main crack plane. The surfaces of the fibres pulled off are bare. The differences between the composites are only slight with respect to the moduli in tension and shear (the latter calculated from Young's moduli and the apparent Poisson's ratio of 0.016 for
be noted that the strength values are higher than those from the transverse tensile tests. This feature can be explained by considering the stress state in the threepoint flexural configuration: critical stresses are confined to a smaller volume that reduces the statistical probability of inherent flaw-induced failure'. Nevertheless, the strengths determined for HTA 5131/EP and HTA 5411/EPare not that much higher than those from the transverse tensile tests and not as high as given elsewhere' for comparable composite systems. The moduli are even lower in transverse flexure than the related values derived from tensile testing. This, in particular, cannot yet be explained by the authors. In contrast to what has been found elsewhere l, the transverse threepoint bending tests do not reveal interfacial changes, even if a large difference between the transverse tensile strengths of Tenax HTA 5131/EP and Tenax HTA 5411/ EP laminates exists. This suggests, contrary to results given elsewhere ~, that the interface relevance is directly correlated with the specimen volume under critical stress, at least for the described transverse testing techniques.
both 5131/EP and 5411/EP),
2. Interlaminar shear properties
as
expected. The results for
transverse tensile properties given in Fig. 8 correlate well with those reported elsewhere ~. Referring to the three-point bending test results, it must
734
COMPOSITES . N U M B E R 7 . 1994
In Fig. 10 the interlaminar shear strengths determined in short beam and transverse Iosipescu shear tests a r e collated. Unexpectedly, the interface relevance seems to
TenaxHTA5131/EP TenaxHTA5411/EP
[] []
60" ""~
,T
¢L ~ 50 ~-, t--'
to those derived from the fragmentation test using the Weibull estimation. Recall that the latter method also levelled off the difference in bonding. The lack in the interface relevance of the short beam shear test is rather surprising, since this method has been established as standard for assessing the quality of fibre/matrix adhesion, especially for thermoset matrix composites. Unfortunately, the reasons for this lack are unknown, since the fractographic analysis is misleading owing to artefacts caused by the separation of the specimens.
,,7
~~.¢, ~
~',,'~ \-,~\,
_ 40
' ,~x,~
30
20
~ ~
~ ~
3. ,mpactproperties (a) Charpy tests. Figs 11 and 12 show the characteristic load/deflection curves from the Charpy tests carried out on the laminates of both unidirectional (UD) and cross-ply (cP) lay-ups. Referring to the UD laminates, improved fibre/matrix adhesion (HTA 5131) results in a 20% increase of the maximum normalized load. Considering the static flexural results where no difference was detected (see Fig. 8) one can conclude that dynamic loading increases the interface relevance. The authors are not yet aware of the reasons for this. For the CP laminates, two sharp load maxima followed by a smooth hump can be seen in the fractograms. It is assumed that
10'
0 Short Beam Shear Test
Fig. 10
Iosipescu Shear Test
Interlaminar shear properties of Tenax HTA 5131/EP and
TenaxHTA5411/EPlaminates
be negligible for the short beam shear test, whereas the transverse Iosipescu test 'works' well in differentiating between 'optimized' and 'not optimized' adhesion. On the other hand, the short beam shear results are similar
Ia
I
300 200 ~
~,
i i' L i
O ....
/b 300 /
,.I
NE
100.
~
0
. . . . 0
O
10
20
200 100 0, r
30
~--:-,
0
10
Deflection [mm]
20
30
Deflection [mrn]
Fig. 11 Charpy test results for unidirectional (a) Tenax HTA 51 31/EP and (b) Tenax HTA 5411/EP laminates (load/deflection curve, load normalized by the specimen depth)
200
11
"
lOO
_
0
o
200
_.
lOO ,,,
~
. . . . . . . . . .
0
--
Z
0
10
20
30
Deflection [mm]
~
0
~
10
~
21
30
Deflection [mm]
Fig. 12 Charpy test results for cross-ply (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP laminates (load/deflection curve, load normalized by the specimen depth)
COMPOSITES.NUMBER7. 1994 735
Tenax HTA 5131 / EP
[~
Tenax HTA 5131 / EP
Tenax HTA 5411 / EP
~
Tenax HTA 5411 / EP
100
X
80 i
80
~
40
._-
0
UD
Cross-Ply
Laminate
Laminate
Fig. 13 Charpy test results for unidirectional and cross-ply Tenax HTA 5131/EP and Tenax HTA 5411/EP laminates (impact resistance)
the first two sharp peaks are related to subsequent delaminations at the tensile side, whereas the following hump is linked to transverse matrix cracking. This will be checked subsequently. Again, the traces are pretty much thesame, butTenaxHTA5411/Eaexhibits, inanalogyto the UD specimens, a lowered peak load level (the Tenax HTA 5131/EP maximum force level increases by 30% with respect to the HTA 5411 modification). Impact resistance and ductility index (ratio of the energy absorbed after the load maximum to the total energy absorption) are given in Figs 13 and 14. While the impact resistance values of the UD and CP laminates are similar, the ductility indices are higher for the cross-ply systems (as far as the first load peak is concerned). This effect becomes even more evident for 'poor' adhesion (HTA 5411) (increases in ductility indices for the cP laminates with respect to the UD laminates: HTA 5131, 46%; HTA 5411,105%). The effect of laminate lay-up on the impact resistance values is marginal. Nevertheless, the interface relevance of the impact resistance and ductility index seems to be most evident for UD laminates (difference in the impact resistance, 26%; difference in the ductility index, 44%--increases for HTA 5131 with regard to
Fig. 15
736
"
~'
~ >
o ~
60
0t UD Laminate
1st peak
2nd peak
Cross-Ply Laminate
Fig. 14 Charpy test results for unidirectional and cross-ply Tenax HTA 51 31/EP and Tenax HTA 5411/EP laminates (ductility index)
HTA 5411). This can be ascribed to a more pronounced matrix deformation in Tenax HTA 5131/EP owing to 'good' fibre/matrix bonding (see Fig. 15).
(b) Subperforation drop weight impact test. Fig. 16 displays the load vs. time traces for the drop weight impact tests performed on cross-ply laminates of the composites with different bonding qualities at various incident energies (1.25, 1.75 and 4.00 J). The main characteristics of these traces and their relation to the failure sequence of laminates are pointed out elsewhere ~. The first load peak on the ascent side of the fractograms (with the subsequent drop) is linked to failure onset by delamination. In general, this load peak 'sharpens' and shifts to earlier times with increasing incident energies. It is obvious from Fig. 16 that this effect is more evident for the 'good' bonding in Tenax HTA 5131/EP (for a 1.25 J incident energy, a load peak can hardly be resolved for the HTA 5411 modification). Furthermore, the load peaks are less sharp for the 'not optimized' bonding modification (HTA 5411), even for high energies. This tendency was confirmed by ultrasonic examinations of the impacted specimens (see Fig. 17). Both the load/time
Charpy test results for unidirectional (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP laminates (fracture surfaces)
C O M P O S I T E S . NUMBER 7 . 1994
8oo
i
600
800-~ b
.....
.......
!.25
J~,.]
:
~ ~ Z
1
2
Q¢]_~~4.~
.__. E
0 0
:
"~g
X ....
I
.2, °
4 0 0r~~1L _ . ~ , . __...~1.75..j % :~ii, 200-jv][
]
3
4
J r, ......
200 { 0 0
Time [ms]
2
3
4
Time [ms]
Fig. 16 Low energy drop weight impact test results for cross-ply (a) Tenax HTA 5131 /EP and (b) Tenax HTA 5411 /EP laminates (normalized load/time curves)
Fig. 17 Low energy drop weight impact test results for cross-ply (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP laminates (C-scan, 5 MHz, defect echo)
curves and the C-scans indicate clearly that Tenax HTA 5131/EP fails in a brittle way, resulting in coarse delaminations even for the lowest impact energy. Tenax HTA 5411/EP, on the other hand, exhibits a higher resistance to damage development by delamination. Since the absorbed energies (see Fig. 18) are the same for both composites, one must suppose that poor bonding encourages transverse matrix cracking rather than delamination. It is obvious, principally, that increasing incident energy shifts the impact response of laminates from 'plastic' to a more 'elastic' type of behaviour in the range of subperforation impact energies. X-ray radiography shows that delamination is the principal failure mechanism for Tenax HTA 5131/EP, whereas for Tenax 541 I/EP an enlarged contribution of matrix cracking to the total energy absorption is observed.
CONCLUSIONS As a result of this study the following concluding remarks may be made. 1) Referring to the micromechanical compositecharacterization, the different adhesion qualities in the investigated carbon fibre/epoxies (CF/EPs)turn out more or less clearly. The microdroplet pull-off test reveals high interfacial shear strength values v~ and superior interface relevance, i.e. the difference between 'optimized' and 'not optimized' bonding, whereas the fragmentation test yields lower r~ values and only marginal interface relevance, as far as Weibull statistics are concerned. More reliable fragment strength values were achieved when instead of the Weibull estimation a linear regression was used; this
COMPOSITES.
NUMBER 7 . 1994
737
[]
Tenax HTA 5131 / EP
[ ] Tenax HTA 5411 / EP 11 . | _~,,'~. 100el . . - i - - .................................. I, • i ~ c ._
sonic C-scans indicate that 'optimized' bonding results in coarse delaminations, whereas 'not optimized' fibre/matrix adhesion encourages transverse matrix cracking rather than delamination. Therefore, p o o r bonding in laminates leads to a higher resistance to d a m a g e development by delamination. Furthermore, increasing the subperforation incident energy decreases the a m o u n t o f absorbed energy (increases the r e b o u n d energy).
~O
,1~
REFERENCES 2 ~ 1.25
~ 1.75
~ 4.00
Incident Impact Energy [d] Fig. 18 Low energy drop weight impact test results for cross-ply Tenax HTA 5131/EP and Tenax HTA 5411/EP laminates (relative
energy absorption)
increased the interface relevance, too. It is worth noting that 'optimized' and ' n o t optimized' bonding are compared in the fragmentation method, the most widespread data reduction o f which is strictly related tO a perfect b o n d i n g and idealized interface (or interphase) behaviour,
2) Increases in the transverse (tensile and flexural) and interlaminar (short beam and transverse Iosipescu shear) properties with enhanced interfacial adhesion (quantified micromechanically) could, in general, be attributed to a change in the failure mechanism from adhesive-type interfacial to a more cohesive matrix failure. In particular, the transverse tensile and transverse Iosipescu shear tests have a high rating in 3)
4)
738
respect o f the interface relevance, While the C h a r p y impact resistances ofunidirectional and cross-ply laminates exhibit similar levels for 'optimized' and ' n o t optimized' b o n d i n g and the influence o f the laminate lay-up is marginal, the ductility indices are higher for the cross-ply laminates, with the interface relevance even more evident for ' n o t optimized' adhesion. The interface relevance o f both impact resistance and ductility index is most conspicuous for UD laminates, F o r the low energy drop weight impact test on crossply specimens, both the load/time curves and ultra-
C O M P O S I T E S . N U M B E R 7 . 1994
1 Drzal, L.T. and Madhukar, M. 'Fibre matrix adhesion and its relationship to composite mechanical properties' J Mater Sci 28 (1993) pp 569-610 and references therein 2 Caldwell, D.L. 'Interfacial analysis' in International Encyclopedia of Composites, Vol 2 edited by S.M. Lee (VCH, New York, 1990) pp 361 377 3 Kelly,A. and Tyson, W.R.V. 'Tensile properties of fibre-reinforced metals: copper/tungsten and copper/molybdenum' J Mech Phys Sofids 13 (1965) pp 32%350 4 Piggott, M.R. 'Interface properties and their influence on fibrereinforced polymers' in Composite Applications--The Role of Matrix, Fibre and Interface edited by T.L. Vigo and B.J. Kinzig (VCH, New York, 1992)pp 221-265 5 Miller, B., Muri, P. and Rebenfeld, L. 'A microbond method for determination of the shear strength of the fibre/resin interface' Compos Sci Techno128 (1987) pp 17 32 6 McAlea, K.P. and Besie, G.J. 'Adhesion between polybutylene terephthalate and E-glass measured with a microdebond technique' Polym Compos 9 (1988) pp 285-290 7 losipeseu, N. 'New accurate procedure for single shear testing of metals' J Mater Sci 2 (1967) pp 537-566 8 Wilson,D.W. 'An overview of test methods used for shear characterization of advanced composite materials Adv Cryo Eng (Mater) 36 (1990) pp 793 810 9 Dai, S.-R. and Piggott, M.R. 'The strengths of carbon and Kevlar fibres as a function of their lengths' Compos Sci Techno149 (1993) pp81-87 10 Favre, J.-P. and Jacques, D. 'Stress transfer by shear in carbon fibre model composites, part l--results of single fibre fragmentation tests with thermosetting resins' J Mater Sci 25 (1990) pp 1373--1380 11 Jang, B.P., Kowbel, W. and Jang, B.Z. 'Impact behaviour and impact fatigue testing of polymer composites' Compos Sci Technol 44 (1992) pp 107-118 and references therein
AUTHORS
The authors are with the Institute for Composite Materials Ltd, University o f Kaiserslautern, PO Box 3049, 67653 Kaiserslautern, G e r m a n y . Correspondence should be addressed to F. Hoecker.
The aim of this study was to investigate the effects of the interfacial bond quality on the mechanical response of composite laminates, for example an epoxy matrix reinforced by continuous carbon fibres of varying surface coating. The fibre/matrix adhesion was characterized by determining the interfacial shear strength q in single-fibre fragmentation and microdroplet pull-off tests. The failure mechanisms were deduced from the stress birefringent patterns (fragmentation test) and from fractographic analysis in a scanning electron microscope (microdroplet pull-off test). Selected interface-relevant properties were evaluated in mechanical tests on laminates. The present paper highlights the problems related to the micromechanical characterization and the interface relevance of data resulting from transverse tensile, transverse flexure and interlaminar shear tests. Furthermore, the effects of the interface on the impact performance of unidirectional and cross-ply laminates were studied. Attempts were made to correlate the macroscopic mechanical response with the interface-related characteristics (q and failure mechanisms). Key w o r d s : interface; mechanical response; carbon/epoxy composites
The mechanical property profile of fibrous composites is mainly determined by the composition and arrangement of their constituents, i.e. matrix and reinforcement, involving parameters such as fibre volume fraction, fibre aspect ratio, fibre orientation and strengths and moduli of the matrix and fibres, respectively. In addition, the nature and characteristics of the interface (or interphase) between fibres and matrix affect the mechanical performance of composites fundamentally. Their property profile can even be 'tailored' for strength, stiffness and toughness requirements by means of interface modification.
transfer and stress redistribution among the fibres, which control the mechanical response in real composites. 'Micromechanical' or 'model' test methods are conducted on single-fibre specimens in order to realize a more well-defined and quantitative determination of the fibre/matrix interfacial shear strength or to study the variation of this term with interface modification. These methods exhibit more clearly defined stress states and thus high interface relevance. However, micromechanical tests lack standardization (i.e. in terms of specimen preparation, testing procedures and data reduction) and 'real composite conditions'.
To characterize the fibre/matrix adhesion or interface modifications qualitatively and comparatively, in principle any test method resulting in interface-relevant data is appropriate. 'Macromechanical' test methods using standardized specimen geometries and testing procedures have the advantage that testing is performed on laminates, i.e. in 'composite-like' conditions. On the other hand, the complicated three-dimensional stress states arising during failure initiation and propagation within the specimens cannot guarantee pure shear failure in the interface. This is, however, a prerequisite for stress
The basic understanding of interfacial effects and the qualification of the mechanical property profile of composites are the driving forces of the widespread use of polymer matrix based composites. Therefore, great efforts are undertaken to correlate the laminate mechanical response with the characteristics of the interface' for particular material systems. In the frame of this project both micromechanical and macromechanical tests were carried out on carbon fibrereinforced epoxy (CF/EP) composite materials of poor and good interfacial bonding, respectively. The present
0010-4361/94/07/0729-10 © 1994 Butterworth-Heinemann Ltd COMPOSITES. VOLUME 25. NUMBER 7 . 1994
729
paper highlights the problems related to the micromechanical characterization and the interface relevance of data resulting from transverse tensile, transverse flexure and interlaminar shear tests. Furthermore, the effects of the interface on the impact performance of unidirectional and cross-ply laminates were also investigated, EXPERIMENTAL Materials and fabrication
As a model thermoset composite material CF/EP was chosen, and a hot-curing epoxy system (bisphenol-A based resin Araldit LY 556; anhydride hardener HY 917, 90 phr; heterocylic amine catalyst DY 070, 1 phr; all of them from Ciba-Geigy, Basel, Switzerland) was used as a matrix reference. The fibres selected were polyacrylonitrile-based high tenacity carbon fibres supplied by Akzo (Wuppertal, Germany). The two fibres used in this study varied only in their sizings. Both of them are standard, surface treated and commercially available, and therefore of economic interest. The designations 'Tenax HTA 5131' and 'Tenax HTA 5411' stand for 1.25 wt% EP sizing and 0.15 wt% antistatic agent, respectively. These agents effect 'optimized' (HTA 5131) and 'not optimized' (HTA 5411) adhesion to the epoxy. Unidirectional (UD) and cross-ply laminates for the macroscopic composite tests were produced by wet iliament winding of two rovings (12 k, 800 tex) on a fiat aluminium plate. Their consolidation occurred in an autoclave curing cycle (120°C, 8 h, 0.7 MPa) with subsequent oven cooling. The average fibre volume content ~br for each laminate plate, which varied in the range from 65 to 70 vol%, was established by ashing the matrix. Specimens for the micromechanicai tests were cured in a heating chamber (120°C, 8 h, oven cooling), always using a vacuum-degassed EP resin, Microscopic testing techniques 1. Fragmentation test Although the drawbacks and difficulties of this method, being mostly related to data reduction, are well known 2, tests on single-fibre composites (SFCs) were conducted, The interracial failure mechanisms in SFCs are easy to monitor in situ during the test using transmitted light microscopy and crossed polarizers. The interracial shear strength values, ri, were calculated from 3 3 o'uD - 8 l'
(1)
where D and l' are the fibre diameter and mean fragment length, respectively, and tru is the fibre strength at l'. So, effects like debonding, thermal stresses, etc. are neglected4, and therefore a shear stress controlled interfacial failure is assumed. Nevertheless, the apparent fibre fragment length distributions indicate the shear stress transfer along the fibre/matrix interface. 2. Microdropletpull-off test Microdroplet pull-off tests, as first described by Miller et al. 5, were performed using an adjustable linear microvice, The EP droplet was pulled off at a velocity of 0.2 mm
730
COMPOSITES
. NUMBER
7 . 1994
min-L The fibre free length was kept constant ( ~ 10 mm) for all tests in order to ascertain similar stored elastic energies for failure initiation and propagation. This test was conducted in addition to the fragmentation mainly for two reasons: (1) to get comparable zqvalues and thus to check the interface sensitivity of these test methods; and (2) to distinguish between adhesive-type failure (poor bonding) and cohesive matrix failure (good bonding) by means of scanning electron microscopy (SEM) examination of the failed sites. The interfacial shear strength value can be directly computed from Fmax r~ - ~rDL (2) where Fmaxis the apparent debonding force (D and L are fibre diameter and embedded fibre length, respectively); alternatively, it can be calculated from the slope of an Fmax/Lplot 6. In the latter case scatter from the linear regression at larger embedded fibre lengths clearly indicates deviation from the assumed shear stress controlled interfacial failure. Macroscopic testing techniques This paper reports only selected topics from a comprehensive research project focused on the effects of the interface on the mechanical response of composites. In particular, transverse tensile and flexural properties as well as interlaminar shear and impact performance are discussed here. Transverse tensile and three-point bending tests were performed according to the ASTM D 3039 and ASTM D 790 standards, respectively; in the latter, a support spanto-depth ratio of 32 was maintained. Moduli were determined from the slope of the stress/strain plot in the range from 0.05 to 0.25% applied strain. The latter was measured by strain gauges and/or incremental transducers. Interlaminar shear properties were determined by the short beam shear test, referring to DIN 29 971 (support span-to-depth ratio of 4), and the transverse Iosipescu shear test 7 utilizing the Wyoming test fixture as proposed by Wilson 8. Instrumented Charpy and drop weight tests were performed in accordance with DIN 53 453 and DIN 53 4432, respectively. Charpy tests were carried out on small, unnotched, standard specimens cut from unidirectional (UD) and cross-ply laminates at ambient temperature. Low energy falling weight impact tests on cross-ply laminates varied in the incident energy (1.25, 1.75 and 4.00 J) to assess the energy absorption and damage accumulation behaviour due to different fibre sizing in the subperforation energy range.
RESULTS A N D DISCUSSION Micromechanicalcomposite characterization 1. Fragmentation test Fig. 1 depicts the failure modes in Tenax HTA 5131/EP and Tenax HTA 5411/EP SFCS as a function of the applied strain. 'Saturation' of the successive fibre fragmentation process was ascertained at 7.6%. It can be
a
b
Fig. 1 Fragmentation mechanisms in CF/EP SFCs for (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP
proved by the apparent stress birefringence patterns that the stress fully accommodates on the fibres after breaking, i.e. ideal stress transfer occurs. This phenomenon indicates good bonding quality between fibre and matrix for Tenax HTA 5131/EP; the mean fibre fragment length derived is 283 pm. For Tenax HTA 5411/EP, successive debonding can be observed, starting from the fractured fibre ends with increasing strain. Therefore, the average fragment length for 'not optimized' bonding is 338 p.m. It must be noted here that the transmitted light microscopy photographs for HTA 5411 are selected to demonstrate the poor bonding and related failure mechanism, although this site is not representative for the whole specimen. It is obvious that the z~values calculated from the HTA 5131 and HTA 5411 mean fibre fragmentation lengths will not be comparable. This is due to the different failure mechanisms, since HTA 5411/EP does not fulfil the prerequisites of Equation (1) (ideal adhesion, yielding interface). Nevertheless, in order to achieve a comparative measure for interfacial shear stress transfer, q is calculated using Equation (1) in both cases, Furthermore, if this equation is considered, there is another shortcoming related to the fibre strength for the fibre free length l' (O'u(/')) 9. This value is, most commonly, calculated from
_~
6000 L I~
, ~, . / W e i b u l l ...... ~ ' - ~. (1")= ~, (Lte~,)( I__~._""~-; L~"~/L= ~1~__~ 1 " ,,L,e,+ -l.. ] ! J
~
5
=.~ ~ • iT.
4000@ .... - ~ ~ ~ 1_ i " ~ ~ - - , Z 3000 ' -
0
lin. Extrapolation 6, (1")= 4407,9- 98,7 * I" ~ - - ~ .......... ' i
~ - ~ - - - - - ~ -
I TI" 2000 i l
..... 1 J
'~ ] ~
5
.......
i~ l [
.... ....... ' ~ ~ ] i L~, [ 1 i L?~, I 10 15 20 Fibre Free Length Ltest[mm]
Fig. 2 Comparison of fibre strength models for Tenax HTA 5131
au(l') __ (~est) I'm O'~(~estest) T (3) This equation is based on Weibull statistical distributions fitted to experimental data determined by tensile test series on single fibres (Cru(Ltest)) using 'measurable' fibre free lengths L~t. In Figs 2 and 3 fibre strength values for 5 and 10 mm fibre free lengths that were calculated from a Weibull fit
COMPOSITES.
NUMBER 7. 1994
731
.~ a. "~
~
6000 ~
5000
~"(I')=°"(L"")/
I [ ~."c'I~
4000
.e
ft.
/Weibull
I
~
"~-~/L---:~..,) l" 4--
, ,,, J I
_
-lin. Extrapolation
--
~u(i') = 3796,7 - 55,5, i --
---.. ~
2, M i c r o d r o p l e t p u l l - o f f test
~
3000
~
~
i
Ltest
|" 200C
0
5
10
t I
' 15
20
Fibre Free Length Ltest[mm] Fig. 3 Comparisonof fibre strength models for Tenax HTA 5411 of strength data gained at Ltes,= 15 mm are compared with the corresponding experimental data for Tenax HTA 5131 and Tenax HTA 5411 filaments. For the single-fibre tensile tests using 15, l0 and 5 mm gauge lengths, the strain velocity was kept constant at 2% min-L This means that the crosshead displacement velocities of the specially designed and instrumented microtension testing machine were 0.3, 0.2 and 0.1 mm min-', respectively. Finally, fibre strengths cru for l' were calculated from Equation (3) and, in addition, by linear extrapolation of the experimental data. It is evident that the Weibull method leads to an 'overestimation' of Cru(l') compared to the linear regression. So, both fibre surface modifications exhibit clear differences in their strength at the critical fibre length. Using linear extrapolation the strength data are rather different (HTA 513 l, 5517 MPa;
Fig. 4
HTA 5411, 3641 MPa), whereas the Weibull method yields similar values (HTA 5131, 5386 MPa; HTA 5411, 5384 MPa). Therefore, ~ values that are calculated by using fibre strengths from linear extrapolation of the mean fragment length are likely to be much more reliableS°"
Based on the SEM photographs taken for matrix droplets on the two fibres (see Fig. 4), one can state that the wettability of both fibres is identical. For both fibre modifications contact angles of approximately 45* were measured for the consolidated samples. In all cases adhesive failure took place at the interface as no matrix residues could be resolved on the fibre surface by SEM (see Fig. 5). Furthermore, adhesive failure can be deduced from the straight lines in the Fm,x/L plots depicted in Fig. 6. This linear proportionality clearly indicates shear stress controlled interracial failure occurring in a sudden and 'catastrophic' manner (sharp and sudden load drop from Fm,x to 0). The r~ values were determined in two ways: (1) from the slope of the FmJL regression line fitted by the leastsquares method; and (2) by direct calculation using Equation (2). As both methods yield similar ri values (Tenax HTA 5131/EP, 93.8 and 95.2 MPa; Tenax HTA 5411/EP, 65.77 and 63.2 MPa), only the average direct z~ data will be given further on.
3. Comparison of rnicromechanical results Fig. 7 compares the zq values determined in the microdroplet pull-off and fragmentation tests on SFCs contain-
Droplet formation in (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP microdroplet pull-off specimens
Fig. 5 Adhesive-type failure in (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP microdroplet pull-off specimens
732
COMPOSITES . NUMBER
7 . 1994
30011 II H ............................................................................. IliA 5131 ..................................... I[ F=2,0632L\ _.._... E 200~I ................................................................................................. '\. / ......... u. I[ ~ (9
ing Tenax HTA 5131 and Tenax HTA 5411 fibres. It is very striking that the interfacial shear strengths determined by the microdroplet pull-off test are much higher than those related to the fragmentation test. This phenomenon is already known and fits well in the range of r~ values given for CF/EP in a recent overview by Piggott< For Tenax HTA 5131, r~ seems to reach or even to exceed the matrix tensile strength, determined to be 92.4 MPa. This could not, of course, be proved by matrix residues (SEM), as explained earlier in this paper.
Z'E
O [t F H T ~ .
u.
10
.............
0
90
40
60
80
100
Embedded Fibre Length L [!am] Fig. 6 Maximumdebondingforce vs. embeddedfibre length for the microdropletpull-off test
120'~ . IX 100~ _ _ c ; ~ ~.3 80 ,~N,N,~x-N~
[ ] Tenax HTA 5131 [ ] renax HTA 5 4 1 1
kl............... x'WB~ ,L
60 40-
C
_"
--
-..........
The microdroplet pull-off test exhibits a much higher rating in interface relevance than fragmentation. According to the former technique, the ~ ofTenax HTA 5131/EP (95.2 MPa) is about 50% higher than the HTA 5411 modification (63.2 MPa). An analogously large difference turns out in the SFC fragmentation test (increase of 42%) when the linear extrapolation (LE) technique is used for the determination of the fibre strengths of the mean fragment lengths (HTA 5131, 41.1 MPa; HTA 5411, 28.4 MPa). Although the Weibull data reduction (wB) results in higher ri values (Tenax HTA 5131/EP, 50.2 MPa; Tenax HTA 5411/EP, 42 MPa) than those from the linear extrapolation (leading to a decreasing difference between the ri values from the fragmentation and microdroplet pull-off tests), the interface relevance tends to diminish (increase of 19.5%) owing to different levels of cr,(l') overestimation with respect to the LE method (HTA 5131, 22%; HTA 5411, 48O/o).
acro ec
n;ca, co
os; e
a, ctor;z,
on
A summary of the laminate properties (mean values :1: standard deviations) is given in Table 1,
Microdroplet Fragmentation
1. Transverse tensile and flexural properties
Pulloff Fig. 7 Comparison of the interfacialshearstrengthvaluesobtained from microdroplet and fragmentation tests
In Fig. 8 the experimental strength and modulus data of the laminates studied are depicted for both transverse tensile and transverse three-point bending set-ups. Con-
Table 1. S u m m a r y o f m a c r o m e c h a n i c a l l a m i n a t e p r o p e r t i e s (mean values + s t a n d a r d deviations) Properties Transverse tensile test cr ( M Pa) E (G Pa) G (GPa) Transverse three-point bending test cr (MPa) E(GPa) Short beam shear test r (MPa) Transverse Iosipescu test r (MPa) Charpy test av(UD; mJ mm -2) a v ( C P ; m J m m 2) DI*(UD; %) DI (CP, first peak; %) DI (CP, second peak; %) Drop weight test Relative energy absorption (%) 1.25 J/1.75 J/4.00 J
Tenax HTA 5131/EP
Tenax HTA 5411/EP
37.3 ± 5.7 9.7 J: 0.4 4.7 i 0 . 2
26.2 + 3.4 9.2 :I: 0.6 4.5±0.1
51.7±8.2 8.4:t:0.6
50.4+2,4 8.34-0.5
50.8+3.6
50.74-2.5
37.44-4.4
17.0+3.8
115.5+8.4 105.0t17.2 57.2:1:4.3 83.7 + 1.0 55.0±5.1
92.0:t:10.1 92.1 +12.3 39.84-5.6 81.7 + 9.0 53.2±6.0
88.9/80.0/77.0
89.1/82.6/79.3
*Ductility index
COMPOSITES. NUMBER 7 . 1994
733
Tenax HTA 5131 / EP ~J ,--,
Tenax HTA 5411 / EP
60
i
¢~ 50 a. ~• " 40
lI i
e-
30" O~ C 20"
II
L _
,
8
!
iN
'
i .
004" 10"
ii ---/-
0
~
2~ 0
Tensile Test Fig. 8
13. 10"~
3-P Bending Test
in Tension
in Shear (calc.)
in Flexure
Transverse tensile and flexural properties of Tenax HTA 51 31/EP and Tenax HTA 5411/EP laminates
Fig. 9 Transverse tensile test fracture surfaces of (a) Tenax HTA 5131/EP and (b) Tenax HTA 541 1/EP laminates
sidering the transverse tensile strength data, the influence of the interface is very clear. The value for 5131/EP is 42% higher than for 5411/EP. This enhancement is identical with that of the single-fibre fragmentation test using the LE method. This is rather surprising since the loading conditions of the interface (radial tension vs. shear with respect to the fibre axis in the transverse tensile and single-fibre fragmentation test, respectively)are substantially different. This improvement is attributed to a change in the failure mechanism from adhesive-type interfacial failure (HTA 5411) to a more cohesive matrix failure (HTA 5131). As SEM photographs taken on typical sites of the fracture surfaces (see Fig. 9) show, 'good' interfacial bonding results in a smooth fracture surface where the fibres are still covered and held together by the EP matrix. On the contrary, the 'not optimized' bonding yields a rough fracture surface owing to crack deviation and bifurcation from the main crack plane. The surfaces of the fibres pulled off are bare. The differences between the composites are only slight with respect to the moduli in tension and shear (the latter calculated from Young's moduli and the apparent Poisson's ratio of 0.016 for
be noted that the strength values are higher than those from the transverse tensile tests. This feature can be explained by considering the stress state in the threepoint flexural configuration: critical stresses are confined to a smaller volume that reduces the statistical probability of inherent flaw-induced failure'. Nevertheless, the strengths determined for HTA 5131/EP and HTA 5411/EPare not that much higher than those from the transverse tensile tests and not as high as given elsewhere' for comparable composite systems. The moduli are even lower in transverse flexure than the related values derived from tensile testing. This, in particular, cannot yet be explained by the authors. In contrast to what has been found elsewhere l, the transverse threepoint bending tests do not reveal interfacial changes, even if a large difference between the transverse tensile strengths of Tenax HTA 5131/EP and Tenax HTA 5411/ EP laminates exists. This suggests, contrary to results given elsewhere ~, that the interface relevance is directly correlated with the specimen volume under critical stress, at least for the described transverse testing techniques.
both 5131/EP and 5411/EP),
2. Interlaminar shear properties
as
expected. The results for
transverse tensile properties given in Fig. 8 correlate well with those reported elsewhere ~. Referring to the three-point bending test results, it must
734
COMPOSITES . N U M B E R 7 . 1994
In Fig. 10 the interlaminar shear strengths determined in short beam and transverse Iosipescu shear tests a r e collated. Unexpectedly, the interface relevance seems to
TenaxHTA5131/EP TenaxHTA5411/EP
[] []
60" ""~
,T
¢L ~ 50 ~-, t--'
to those derived from the fragmentation test using the Weibull estimation. Recall that the latter method also levelled off the difference in bonding. The lack in the interface relevance of the short beam shear test is rather surprising, since this method has been established as standard for assessing the quality of fibre/matrix adhesion, especially for thermoset matrix composites. Unfortunately, the reasons for this lack are unknown, since the fractographic analysis is misleading owing to artefacts caused by the separation of the specimens.
,,7
~~.¢, ~
~',,'~ \-,~\,
_ 40
' ,~x,~
30
20
~ ~
~ ~
3. ,mpactproperties (a) Charpy tests. Figs 11 and 12 show the characteristic load/deflection curves from the Charpy tests carried out on the laminates of both unidirectional (UD) and cross-ply (cP) lay-ups. Referring to the UD laminates, improved fibre/matrix adhesion (HTA 5131) results in a 20% increase of the maximum normalized load. Considering the static flexural results where no difference was detected (see Fig. 8) one can conclude that dynamic loading increases the interface relevance. The authors are not yet aware of the reasons for this. For the CP laminates, two sharp load maxima followed by a smooth hump can be seen in the fractograms. It is assumed that
10'
0 Short Beam Shear Test
Fig. 10
Iosipescu Shear Test
Interlaminar shear properties of Tenax HTA 5131/EP and
TenaxHTA5411/EPlaminates
be negligible for the short beam shear test, whereas the transverse Iosipescu test 'works' well in differentiating between 'optimized' and 'not optimized' adhesion. On the other hand, the short beam shear results are similar
Ia
I
300 200 ~
~,
i i' L i
O ....
/b 300 /
,.I
NE
100.
~
0
. . . . 0
O
10
20
200 100 0, r
30
~--:-,
0
10
Deflection [mm]
20
30
Deflection [mrn]
Fig. 11 Charpy test results for unidirectional (a) Tenax HTA 51 31/EP and (b) Tenax HTA 5411/EP laminates (load/deflection curve, load normalized by the specimen depth)
200
11
"
lOO
_
0
o
200
_.
lOO ,,,
~
. . . . . . . . . .
0
--
Z
0
10
20
30
Deflection [mm]
~
0
~
10
~
21
30
Deflection [mm]
Fig. 12 Charpy test results for cross-ply (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP laminates (load/deflection curve, load normalized by the specimen depth)
COMPOSITES.NUMBER7. 1994 735
Tenax HTA 5131 / EP
[~
Tenax HTA 5131 / EP
Tenax HTA 5411 / EP
~
Tenax HTA 5411 / EP
100
X
80 i
80
~
40
._-
0
UD
Cross-Ply
Laminate
Laminate
Fig. 13 Charpy test results for unidirectional and cross-ply Tenax HTA 5131/EP and Tenax HTA 5411/EP laminates (impact resistance)
the first two sharp peaks are related to subsequent delaminations at the tensile side, whereas the following hump is linked to transverse matrix cracking. This will be checked subsequently. Again, the traces are pretty much thesame, butTenaxHTA5411/Eaexhibits, inanalogyto the UD specimens, a lowered peak load level (the Tenax HTA 5131/EP maximum force level increases by 30% with respect to the HTA 5411 modification). Impact resistance and ductility index (ratio of the energy absorbed after the load maximum to the total energy absorption) are given in Figs 13 and 14. While the impact resistance values of the UD and CP laminates are similar, the ductility indices are higher for the cross-ply systems (as far as the first load peak is concerned). This effect becomes even more evident for 'poor' adhesion (HTA 5411) (increases in ductility indices for the cP laminates with respect to the UD laminates: HTA 5131, 46%; HTA 5411,105%). The effect of laminate lay-up on the impact resistance values is marginal. Nevertheless, the interface relevance of the impact resistance and ductility index seems to be most evident for UD laminates (difference in the impact resistance, 26%; difference in the ductility index, 44%--increases for HTA 5131 with regard to
Fig. 15
736
"
~'
~ >
o ~
60
0t UD Laminate
1st peak
2nd peak
Cross-Ply Laminate
Fig. 14 Charpy test results for unidirectional and cross-ply Tenax HTA 51 31/EP and Tenax HTA 5411/EP laminates (ductility index)
HTA 5411). This can be ascribed to a more pronounced matrix deformation in Tenax HTA 5131/EP owing to 'good' fibre/matrix bonding (see Fig. 15).
(b) Subperforation drop weight impact test. Fig. 16 displays the load vs. time traces for the drop weight impact tests performed on cross-ply laminates of the composites with different bonding qualities at various incident energies (1.25, 1.75 and 4.00 J). The main characteristics of these traces and their relation to the failure sequence of laminates are pointed out elsewhere ~. The first load peak on the ascent side of the fractograms (with the subsequent drop) is linked to failure onset by delamination. In general, this load peak 'sharpens' and shifts to earlier times with increasing incident energies. It is obvious from Fig. 16 that this effect is more evident for the 'good' bonding in Tenax HTA 5131/EP (for a 1.25 J incident energy, a load peak can hardly be resolved for the HTA 5411 modification). Furthermore, the load peaks are less sharp for the 'not optimized' bonding modification (HTA 5411), even for high energies. This tendency was confirmed by ultrasonic examinations of the impacted specimens (see Fig. 17). Both the load/time
Charpy test results for unidirectional (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP laminates (fracture surfaces)
C O M P O S I T E S . NUMBER 7 . 1994
8oo
i
600
800-~ b
.....
.......
!.25
J~,.]
:
~ ~ Z
1
2
Q¢]_~~4.~
.__. E
0 0
:
"~g
X ....
I
.2, °
4 0 0r~~1L _ . ~ , . __...~1.75..j % :~ii, 200-jv][
]
3
4
J r, ......
200 { 0 0
Time [ms]
2
3
4
Time [ms]
Fig. 16 Low energy drop weight impact test results for cross-ply (a) Tenax HTA 5131 /EP and (b) Tenax HTA 5411 /EP laminates (normalized load/time curves)
Fig. 17 Low energy drop weight impact test results for cross-ply (a) Tenax HTA 5131/EP and (b) Tenax HTA 5411/EP laminates (C-scan, 5 MHz, defect echo)
curves and the C-scans indicate clearly that Tenax HTA 5131/EP fails in a brittle way, resulting in coarse delaminations even for the lowest impact energy. Tenax HTA 5411/EP, on the other hand, exhibits a higher resistance to damage development by delamination. Since the absorbed energies (see Fig. 18) are the same for both composites, one must suppose that poor bonding encourages transverse matrix cracking rather than delamination. It is obvious, principally, that increasing incident energy shifts the impact response of laminates from 'plastic' to a more 'elastic' type of behaviour in the range of subperforation impact energies. X-ray radiography shows that delamination is the principal failure mechanism for Tenax HTA 5131/EP, whereas for Tenax 541 I/EP an enlarged contribution of matrix cracking to the total energy absorption is observed.
CONCLUSIONS As a result of this study the following concluding remarks may be made. 1) Referring to the micromechanical compositecharacterization, the different adhesion qualities in the investigated carbon fibre/epoxies (CF/EPs)turn out more or less clearly. The microdroplet pull-off test reveals high interfacial shear strength values v~ and superior interface relevance, i.e. the difference between 'optimized' and 'not optimized' bonding, whereas the fragmentation test yields lower r~ values and only marginal interface relevance, as far as Weibull statistics are concerned. More reliable fragment strength values were achieved when instead of the Weibull estimation a linear regression was used; this
COMPOSITES.
NUMBER 7 . 1994
737
[]
Tenax HTA 5131 / EP
[ ] Tenax HTA 5411 / EP 11 . | _~,,'~. 100el . . - i - - .................................. I, • i ~ c ._
sonic C-scans indicate that 'optimized' bonding results in coarse delaminations, whereas 'not optimized' fibre/matrix adhesion encourages transverse matrix cracking rather than delamination. Therefore, p o o r bonding in laminates leads to a higher resistance to d a m a g e development by delamination. Furthermore, increasing the subperforation incident energy decreases the a m o u n t o f absorbed energy (increases the r e b o u n d energy).
~O
,1~
REFERENCES 2 ~ 1.25
~ 1.75
~ 4.00
Incident Impact Energy [d] Fig. 18 Low energy drop weight impact test results for cross-ply Tenax HTA 5131/EP and Tenax HTA 5411/EP laminates (relative
energy absorption)
increased the interface relevance, too. It is worth noting that 'optimized' and ' n o t optimized' bonding are compared in the fragmentation method, the most widespread data reduction o f which is strictly related tO a perfect b o n d i n g and idealized interface (or interphase) behaviour,
2) Increases in the transverse (tensile and flexural) and interlaminar (short beam and transverse Iosipescu shear) properties with enhanced interfacial adhesion (quantified micromechanically) could, in general, be attributed to a change in the failure mechanism from adhesive-type interfacial to a more cohesive matrix failure. In particular, the transverse tensile and transverse Iosipescu shear tests have a high rating in 3)
4)
738
respect o f the interface relevance, While the C h a r p y impact resistances ofunidirectional and cross-ply laminates exhibit similar levels for 'optimized' and ' n o t optimized' b o n d i n g and the influence o f the laminate lay-up is marginal, the ductility indices are higher for the cross-ply laminates, with the interface relevance even more evident for ' n o t optimized' adhesion. The interface relevance o f both impact resistance and ductility index is most conspicuous for UD laminates, F o r the low energy drop weight impact test on crossply specimens, both the load/time curves and ultra-
C O M P O S I T E S . N U M B E R 7 . 1994
1 Drzal, L.T. and Madhukar, M. 'Fibre matrix adhesion and its relationship to composite mechanical properties' J Mater Sci 28 (1993) pp 569-610 and references therein 2 Caldwell, D.L. 'Interfacial analysis' in International Encyclopedia of Composites, Vol 2 edited by S.M. Lee (VCH, New York, 1990) pp 361 377 3 Kelly,A. and Tyson, W.R.V. 'Tensile properties of fibre-reinforced metals: copper/tungsten and copper/molybdenum' J Mech Phys Sofids 13 (1965) pp 32%350 4 Piggott, M.R. 'Interface properties and their influence on fibrereinforced polymers' in Composite Applications--The Role of Matrix, Fibre and Interface edited by T.L. Vigo and B.J. Kinzig (VCH, New York, 1992)pp 221-265 5 Miller, B., Muri, P. and Rebenfeld, L. 'A microbond method for determination of the shear strength of the fibre/resin interface' Compos Sci Techno128 (1987) pp 17 32 6 McAlea, K.P. and Besie, G.J. 'Adhesion between polybutylene terephthalate and E-glass measured with a microdebond technique' Polym Compos 9 (1988) pp 285-290 7 losipeseu, N. 'New accurate procedure for single shear testing of metals' J Mater Sci 2 (1967) pp 537-566 8 Wilson,D.W. 'An overview of test methods used for shear characterization of advanced composite materials Adv Cryo Eng (Mater) 36 (1990) pp 793 810 9 Dai, S.-R. and Piggott, M.R. 'The strengths of carbon and Kevlar fibres as a function of their lengths' Compos Sci Techno149 (1993) pp81-87 10 Favre, J.-P. and Jacques, D. 'Stress transfer by shear in carbon fibre model composites, part l--results of single fibre fragmentation tests with thermosetting resins' J Mater Sci 25 (1990) pp 1373--1380 11 Jang, B.P., Kowbel, W. and Jang, B.Z. 'Impact behaviour and impact fatigue testing of polymer composites' Compos Sci Technol 44 (1992) pp 107-118 and references therein
AUTHORS
The authors are with the Institute for Composite Materials Ltd, University o f Kaiserslautern, PO Box 3049, 67653 Kaiserslautern, G e r m a n y . Correspondence should be addressed to F. Hoecker.