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epoxy composite
Composites 26 (1995) 207 214
~ U T T N E E R WORTHM A N N
Printed in Great Britain 0010-4361/95/$10.00
The effect of stacking sequence on impact damage in a carbon fibre/epoxy composite S. A. Hitchen* and R. M. J. Kemp Materials and Structures Department, DRA Farnborough, Farnborough, Hants, GU14 6TD, UK (Received March 1994; revised June 1994) The effect of stacking sequence on impact damage in a carbon fibre/toughened epoxy composite was studied. The major form of damage was delamination, which initiated at almost every interface through the panel. During the impact event the force-time response was monitored and the energy absorbed analysed in terms of an initiation and a propagation energy. The energy absorbed in delamination initiation was influenced by the stacking sequence, being increased by placing 45~ fibres in the surface plies and by increasing the number of dissimilar interfaces. The residual energy absorbed in delamination propagation was found to increase linearly with increasing total delamination area. The compression-after-impact strength was related to the maximum delamination area. (Keywords: carbon/epoxylaminates; impact damage; stacking sequence)
INTRODUCTION Carbon fibre composites offer many benefits over conventional structural materials as strength and stiffness, for example, can be tailored to meet specific design requirements by careful selection of the laminate stacking sequence 1. These materials also show high energy absorption under gross failure conditions (i.e. crashworthiness), but are prone to localized subsurface damage under relatively low energy impact loads. Such impacts may arise on aircraft structures due to dropped loads and runway debris. The localized damage is termed 'barely visible impact damage' (BVID) and is potentially a source of mechanical weakness, particularly under subsequent compression loading. Considerable research has, therefore, been devoted to analysing the impact properties and post-impact compression behaviour 2 with a view to improving impact resistance. At present, only a limited amount of work has concentrated on the effect of stacking sequence on impact resistance. Dorey 3 showed that laminates containing _+45~ surface plies offered superior impact resistance and improved residual strength compared with those having 0 ~ surface layers. This was attributed to the increased flexibility of the composite increasing its ability to absorb energy elastically. Hong and Liu 4 and Liu s studied the impact resistance of a glass/epoxy composite having the stacking sequence [0J05/05], where 0 = 0, 15, 30, 45, 60 and 90 ~ They found
* To whom correspondence should be addressed 9 Crown Copyright, 1995. Published with the permission of the Controller of HMSO, London
a dramatic increase in delamination area as 0 increased, with the energy required for delamination initiation decreasing as 0 increased. The energy required for delamination initiation was also influenced by the number of dissimilar interfaces, increasing as the number of interfaces increased 4. Clark 6 used an analytical approach to predict the position and size of delaminations through the laminate thickness under impact loading. Larger delaminations were predicted and observed to occur as the angle between adjacent plies increased, which supports the observations of Hong and Liu 4'5. Strait e t al. 7 investigated the effect of stacking sequence on the energy absorbed during impact penetration tests of a carbon/epoxy composite having three main stacking sequences, cross-ply, quasi-isotropic and [0/+ 45], with minor variations in ply order in each basic lay-up geometry. The results showed no clear-cut effects of stacking sequence in terms of the energy absorbed in delamination initiation for the three main laminate types although the absorbed energy was influenced by the minor changes in lay-up for each basic geometry. Other studies 8'9, however, concluded that the stacking sequence had little or no effect on the energy absorption or the extent of damage, particularly when the variations in stacking sequence were relatively minor. The effect of stacking sequence on the impact resistance of composite laminates is, therefore, not yet fully understood. This study investigated the effect of stacking sequence on impact damage and compression after impact (CAI) in a carbon fibre/toughened epoxy composite. The energy absorbed during the impact event was monitored and has been related to the development of damage.
COMPOSITES Volume 26 Number 3 1995 207
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp EXPERIMENTAL
Material The effect of stacking sequence on impact damage and CAI was evaluated on a carbon fibre/toughened epoxy T800H/924C. Six different 16-ply panels, 600 mm by 300 mm, each containing equal numbers of 0 ~ and 45 ~ plies, were autoclaved at 180~ under 700 kPa pressure in accordance with the recommended cure schedule. The following layer stacking sequences were investigated: Panel 1 - [(+ 45,02)2]s Panel 2 - [(+ 45)2,04]s Panel 3 - [(+ 45,0, - 45,0)z]~ Panel 4 - [(02, + 45)2]~ Panel 5 [04,(_ + 45)2]~ Panel 6 - [(0, + 45,0, - 45)2]s After post-curing the panels were ultrasonically Cscanned to ensure panel integrity.
Impact testing The composite panels were impacted using a commercial instrumented falling weight test machine to a level of 7 J using an impact mass of 2 kg, an impact velocity
of 2.5 m s-1 and a 10 mm diameter hemispherical tup. During the impact event the panel was clamped between two steel rings of internal diameter 100 mm and the impact head was captured after impact to prevent secondary strikes. The energy absorbed during the impact event was monitored by the instrumented impactor, with the primary output being force versus time data which were subsequently analysed into absorbed energy values. The impacts were repeated at 100 mm intervals across the panel as recommended in the C R A G ~~standard, with five impacts being performed on each panel. After impact the panels were again ultrasonically C-scanned.
Post-impact analysis Compression properties. After impact and C-scan inspection, coupons of dimensions 250 mm by 20 mm for plain compression tests and 250 m m by 50 mm for CAI tests were cut using a diamond slitting wheel. Each CAI specimen contained an impact site. Aluminium end-
4~
a
b
3
1
1L.6
312
4.8
6.4
8.0
1'.8
312
Time, ms
4.8
84
8.0
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C
d 3
z
z ~2
,E
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1
2
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6
2
0
Time. ms
4
10
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4
3.0
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2.3
z z
e52
c~I-5 o
o
0.8
0
, 1.6
i i 3.2 4.8 Time, ms
Figure 1 Typical force
288
COMPOSITES
Volume
versus
, 6.4
8.0
1.6
3.2 4.8 Time, ms
6.4
8.0
time curves: (a) panel 1; (b) panel 2; (c) panel 3; (d) panel 4; (e) panel 5; (f) panel 6
26 Number
3 1995
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp tabs were bonded onto the ends of the plain compression test specimens, but not on the CAI specimens. All tests were performed to the CRAG 1~ standard with an antibuckling guide to support the coupon during compression loading to prevent macrobuckling. Specimens were tested to failure using a screw-driven test machine and the failure loads recorded.
Damage analysis. The damage sustained during the impact event was analysed using a range of techniques. The maximum delamination area for every impact was determined from the C-scans performed after impact. Additional tests were then carried out on one impact site from each panel. Initially the specimens were X-rayed using conventional shadow X-radiography with a zinc iodide penetrant used to reveal the extent of delamination, splitting and fibre fracture. The through-thickness damage distribution was determined using a de-ply technique. The specimens were impregnated with a solution of gold chloride, placed in a furnace to partially 'burn-off' the matrix and separated into the individual plies. The delaminations were indicated by a gold residue from which the area of each delamination was measured. RESULTS
Force-time data Typical force versus time plots for the six panels recorded during the impact event are shown in Figure 1. Typical plots of energy versus time for panel 5 together with the corresponding force versus time plot are shown in Figure 2. The absorbed energy increased to a maximum and then fell to zero, but the maximum energy occurred some time after the peak force had been reached. Similar trends were obtained for the other five 3.0
2.3
5~1.5 ,2
\
0.8
\, \\.
\ 0
1',6
'
Emax 7"0
3!2 i Time, ms
L 48
1 64
80
t
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3
\
~ 3.5 c tla
\
Compression behaviour The plain compression strength and the CAI strength for the six panels are listed in Table 2. The plain compression failure stresses were influenced by the stacking sequence, with values ranging from 881 MPa for panel 2 to 659 MPa for panel 5. Comparison of the different lay-ups revealed similarities between the plain compression failure stresses in panels 1 and 2, panels 3 and 6, and between panels 4 and 5. The residual strength after impact was lower than the plain compression failure strength, with values ranging from 344 MPa for panel 1 to 273 MPa for panel 5. Impact damage analysis Damage morphology. The stacking sequence influenced both the shape and size of the maximum delamination area as shown by the X-radiographs of Figure 3. An approximately circular damage zone was observed in panel 2 whereas the other five panels contained elongated maximum delaminations parallel to the fibre direction in the surface plies. All six panels contained a crush zone at the impact site which sustained considerable fibre damage. In addition to delamination, splitting was observed which tended to be most extensive in the plies furthest from the impact site. The position, size, orientation and shape of the delaminations through the panel thickness are shown by the de-ply tracings of Figures 4a-f. Delaminations initiated at almost every dissimilar ply interface through the panel thickness and tended to be oriented parallel to the fibres in the lower ply (i.e. furthest from the impact site). Delamination area. The distribution of damage through the thickness of the six panels as determined by the de-ply technique is shown in Figures 5a and b. Delamination area tended to increase towards the back face furthest from the impact site. The maximum and total delamination areas, determined from the C-scans and the de-ply tracings respectively, are listed in Table 2. It is evident from these results that the maximum delamination area is influenced by the stacking sequence, with panel 5 showing the largest maximum delamination area (1620 mm 2) and panel 6 the smallest (360 mm2). Table 1 Mean energy values recorded during the impact event (average of four specimens) for the six panels determined from the force v e r s u s time curves
\\\ \\
panels. The peak energy (energy at peak force) and the maximum energy were obtained from the force-time plots and the mean value of the four curves analysed are listed in Table 1. The panels containing 45 ~ surface plies gave higher peak energies (6.23 to 6.41 J) than panels containing 0 ~ surface plies. The maximum energy was approximately constant for all six panels and appeared to be independent of the stacking sequence.
\-,
Panel 1.8
0
1.6
3.2
48
6.4
80
Time, ms
Figure 2 A typical force energy
versus
v e r s u s time curve and the corresponding time curve for panel 5
l 2 3 4 5 6
Peak energy. E~ (J)
M a x i m u m energy, E .... (J)
6.31 6.23 6.41 6.06 5.98 6. l 1
6.65 6.6 6.55 6.63 6.7 6.66
Residual energy, Er (J) 0.34 0.37 0.14 0.57 0.72 0.56
COMPOSITES Volume 26 N u m b e r 3 1995
209
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp Table 2
Delamination areas together with the pre- and post-impact compression data for the six panels
Panel
Maximum delamination area (mm2)a
Total delamination area (mm2)b
Plain compression strength (MPa) a
Compression after impact strength (MPa) ~
1 2 3 4 5 6
510 870 650 765 1620 360
1350 1360 870 1510 2070 2t70
880 881 718 660 659 724
344 318 298 312 273 339
_+58 +_50 -+ 21 + 12 + 26 +44
_+ 118 -+ 99 -+ 35 _+ 11 + 92 "+43
+_35 _+64 -+ 5 _+ 12 +4 _+ 11
~ Average of four values b Single specimen assessed using a de-ply technique
Figure 3
210
X-ray penetrant radiographs: (a) panel 1; (b) panel 2; (c) panel 3; (d) panel 4; (e) panel 5; (f) panel 6
COMPOSITES Volume 26 Number 3 1995
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp
9 O0
-
+
9
0 0 0 0
a
',
/
%
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Figure 4 De-ply tracings showing the distribution of delaminations through the thickness: (a) panel 1; (b) panel 2; (c) panel 3; (d) panel 4; (e) panel 5; (f) panel 6
COMPOSITES Volume 26 Number 3 1995
211
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp 1360 800
800
9 Panel 1
04
"
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I
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9 Panel 3
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't
o Panel 4
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Distribution of delaminations through the panel thickness (determined using a de-ply technique): (a) panels 1, 2 and 3; (b) panels 4, 5
The total delamination area was always greater than the maximum delamination area, with values ranging from 870 mm 2 for panel 3 to 2170 mm z for panel 6. The panels containing 45 ~ surface plies gave smaller total delamination areas (870 to 1360 mm 2) than the panels containing 0 ~ plies (1510 to 2170 mm2). DISCUSSION The results have shown that altering the layer stacking sequence influenced a range of properties including the peak force during impact, delamination area, and the pre- and post-impact compression strength. In this section a novel analysis of the impact energy data is used to relate the energy values recorded during the impact event to delamination development. The CAI properties are related to delamination damage and the effect of stacking sequence on the damage morphology is discussed.
these factors to the initiation energy, E~, and the maximum absorbed energy, Emax, is shown in Figure 6. Ei was significantly increased by placing 45 ~ fibres in the surface plies and slightly increased by increasing the number of dissimilar interfaces. Ema• was approximately constant for all six panels and appeared to be independent of the number of dissimilar interfaces and the orientation of the fibres in the surface layers. Another feature of the impact data was the further increase in absorbed energy after the peak force had been reached (i.e. after El). It can reasonably be assumed that all damage has been produced when the maximum
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.
.
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Delamination area and absorbed energy Previous work by Srinivason et al." subjected a range
of polymer composites to increasing impact energies. A 'step' in the load versus displacement curve was observed which occurred at constant load for the different impact energies and corresponded to the formation of extensive delamination. This load 'step' was, therefore, the threshold load for delaminations to occur. In the current work on the T800H/924C carbon fibre/toughened epoxy system, the force-during the impact event increased with time to a maximum value and showed a sudden drop after the peak load for all six panels (Figure 1). This load drop, therefore, corresponded to the 'step' observed by Srinivason et al.1] and hence is assumed to correspond to delamination formation or initiation. The corresponding absorbed energy at the 'step' is, therefore, taken to be the energy absorbed in delamination initiation and is termed Ei. The maximum absorbed energy during the impact event is termed Em~x. The laminates used in this investigation consisted of equal numbers of 0 ~ and + 45 ~ plies, but differed in terms of the number of dissimilar interfaces and the orientation of the fibres in the surface layers. The relevance of
212
COMPOSITES Volume 26 N u m b e r 3 1995
6,4
t--
LU 6.2
5.8 6
8 Number
10
14
of dissimilar interfaces
x - Ei 4 5 ~ s u r f a c e p l i e s ~-EmaxO
12
~ surface plies
- - Ei 0 ~ s u r f a c e p l i e s ~ Emax45 ~ surface plies
Figure 6 Effect of the number of dissimilar interfaces on the energy at peak force (El) and the maximum energy (Emax)
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp absorbed energy (Emax) is reached since it corresponds to the maximum deflection value. The energy absorbed between the initiation (Ei) and maximum (Emax) value is, therefore, assumed to be absorbed by the process of delamination extension and will be termed the residual energy (Er), defined as follows: Er = Emax
E~
(1)
The residual energies for the six panels, which are listed in Table 1, were clearly influenced by the stacking sequence, being higher for lay-ups containing 0 ~ surface plies. Panels comprising surface 0 ~ plies also showed a larger total delamination area (Table 2) and a plot of the total delamination area versus"residual energy (Figure 7) shows a correlation between these parameters. The residual energy increased with increasing total delamination area in an approximately linear relationship and this supports the assumption that the residual energy corresponds to the energy absorbed during delamination extension. The slope of the line shown in Figure 7 corresponds to an energy release rate for delamination extension and was found to be 380 J m 2. The values of the strain energy release rate measured for unidirectional T800H/924C material were Gk = 300 J m -2 and Gilt -580 J m -2. The estimated strain energy release rate for delamination growth, therefore, lies between these two measured values. The novel analysis proposed in this work has analysed the energy absorbed during impact in terms of an initiation and propagation energy. The energy absorbed in delamination initiation was influenced by the stacking
Compression after impact The stacking sequence was found to influence both the pre- and post-impact compression strength as shown in Table 2. The CAI strength, however, showed no trend with either the number of dissimilar interfaces or the orientation of the fibres in the surface plies. A similar effect of stacking sequence on the maximum delamination area was also found and, as expected from these observations, the CAI strength was related to the maximum delamination area as shown in Figure 8. The residual strength decreased with increasing maximum delamination area and was, therefore, independent of the maximum delamination shape or through-thickness position. Since a simple relationship between the maximum delamination area and an energy parameter was not evident, the maximum delamination area and hence the residual strength, may have a complex dependence on both the initiation and propagation energies. Future work on other materials may clarify this relationship.
Delamination shape The interply delamination shape varied considerably with stacking sequence and through-thickness position as shown in Figures 4a-f The delamination shapes can be generalized as circular, diamond and elongated as illustrated in Figures 9a-c. The elongated delamination was oriented parallel to the fibre direction in the ply below the delamination as
0.8 84
/ I /
0.6
sequence, being increased by placing 45 ~ fibres in the surface plies and by increasing the number of dissimilar interfaces. The residual energy absorbed in delamination propagation was found to increase linearly with increasing total delamination area. An application of this analysis is as a non-destructive method for determining the total delamination area from the force time data without the need for destructive de-ply inspection.
/ /
0
/ /
400
/ / / / /
~- 0.4
[.1.1
360
/ / /
t A
/ / /
i1J 13_
/
320
2~
/ /
0.2
O
/ /
280
/ / / /
240
/
400
0
800
1,200
1,600
2,000
Total Delamination Area sqmm
200 200
600
1,000 MDA
z, P1
+ P2
~ P3
~ P4 x P5
o P6 - - c a l . l i n e ]
Figure 7 Relationship between the total delamination area (measured using de-ply technique) and the residual energy (Er). The points are the experimental data and the dashed line was calculated using linear regression
l z, P1
+ P2
~ P3
~ P4
1,400
1,800
sqmm x P5
o P6
--cal.
line
Figure 8 Relationship between the compression after impact (CA1) strength and the maximum delamination area (MDA) (measured from C-scans). The points are experimental data and the dashed line was calculated using linear regression
COMPOSITES Volume 26 Number 3 1995
213
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp
a
b
impact has been analysed by relating the damage distribution to the energy absorption characteristics during impact. The compression after impact strength was determined by the maximum delamination area, being decreased as the delamination area increased. The total delamination area was influenced by the stacking sequence and was related to the energy absorbed during impact by a novel analysis which distinguished the delamination initiation energy E~ from the delamination extension energy Er and the maximum absorbed energy Em~x. The delamination initiation energy was increased by placing 45 ~ plies in the surface layers and by increasing the number of dissimilar interfaces which therefore reduced the energy available for delamination extension. This reduced extension energy Er resulted in a reduced total delamination area. The delamination shape was influenced by splitting and fibre fracture in the ply below the delamination. ACKNOWLEDGEMENTS
c
d
Figure 9 Schematic diagram showing the four characteristic delamination shapes: (a) circular; (b) diamond; (c) elongated; (d) waisted
predicted by the stiffness mismatch model of Hong and Liu 4 and the peel separation model of Clarke 6. The elongated delamination often featured splitting in the lower ply which is thought to promote delamination extension. Another feature was 'waisting' which promoted a lemniscate or 'peanut' shaped delamination as illustrated in Figure 9d. This delamination shape appeared to be generally associated with a combination of delamination and fibre fracture in the lower ply and the energy absorbed by this medium may reduce lateral delamination growth. CONCLUSIONS The current work has shown that the stacking sequence influences both the pre- and post-impact compression strengths and affects the impact damage in 16-ply carbon fibre/toughened epoxy laminate. The effect of a 7 J
214
COMPOSITES Volume 26 Number 3 1995
This work was supported by the MOD. The authors would also like to thank Mr J. Coleman for fabricating the panels tested in this work and Mr C. Molyneux for the compression testing. REFERENCES 1 Cantwell, W.J. and Morton, J. Composites I991, 22, 347 2 Chen, W.S., Rogers, C., Cronkhite, J.D. and Martin, J. J. Am. Helicopter Soc, 1986, 60 3 Dorey, G. in 'Failure Mode of Composite Materials with Organic Matrices and Their Consequences in Design', AGARD CP 163, 1975, paper 8 4 Hong, S. and Liu, D. Exp. Mech. 1989, 115 5 Liu, D. J. Compos. Mater. 1988, 22, 674 6 Clark, G. Composites, 1989, 20, 209 7 Strait, L.H., Karasek, M.L. and Amateau, M . F . J . Compos. Mater. 1989, 26, 1 8 Morton, J. and Goodwin, E.W. Compos. Struct. 1989, 13, 1 9 Davies, C.K.L., Turner, S. and Williamson, K H . Composites 1985, 16, 270 10 Curtis, P.T. (Ed.) RAE Tech. Rep. TR 88012, Royal Aerospace Establishment, 1988 II Srinivason, K., Jackson, W.C. and Hinkley, J.A. '36th [nt. SAMPE Symp.' 15 18 April 1991, Covina, CA, pp. 850-862
~ U T T N E E R WORTHM A N N
Printed in Great Britain 0010-4361/95/$10.00
The effect of stacking sequence on impact damage in a carbon fibre/epoxy composite S. A. Hitchen* and R. M. J. Kemp Materials and Structures Department, DRA Farnborough, Farnborough, Hants, GU14 6TD, UK (Received March 1994; revised June 1994) The effect of stacking sequence on impact damage in a carbon fibre/toughened epoxy composite was studied. The major form of damage was delamination, which initiated at almost every interface through the panel. During the impact event the force-time response was monitored and the energy absorbed analysed in terms of an initiation and a propagation energy. The energy absorbed in delamination initiation was influenced by the stacking sequence, being increased by placing 45~ fibres in the surface plies and by increasing the number of dissimilar interfaces. The residual energy absorbed in delamination propagation was found to increase linearly with increasing total delamination area. The compression-after-impact strength was related to the maximum delamination area. (Keywords: carbon/epoxylaminates; impact damage; stacking sequence)
INTRODUCTION Carbon fibre composites offer many benefits over conventional structural materials as strength and stiffness, for example, can be tailored to meet specific design requirements by careful selection of the laminate stacking sequence 1. These materials also show high energy absorption under gross failure conditions (i.e. crashworthiness), but are prone to localized subsurface damage under relatively low energy impact loads. Such impacts may arise on aircraft structures due to dropped loads and runway debris. The localized damage is termed 'barely visible impact damage' (BVID) and is potentially a source of mechanical weakness, particularly under subsequent compression loading. Considerable research has, therefore, been devoted to analysing the impact properties and post-impact compression behaviour 2 with a view to improving impact resistance. At present, only a limited amount of work has concentrated on the effect of stacking sequence on impact resistance. Dorey 3 showed that laminates containing _+45~ surface plies offered superior impact resistance and improved residual strength compared with those having 0 ~ surface layers. This was attributed to the increased flexibility of the composite increasing its ability to absorb energy elastically. Hong and Liu 4 and Liu s studied the impact resistance of a glass/epoxy composite having the stacking sequence [0J05/05], where 0 = 0, 15, 30, 45, 60 and 90 ~ They found
* To whom correspondence should be addressed 9 Crown Copyright, 1995. Published with the permission of the Controller of HMSO, London
a dramatic increase in delamination area as 0 increased, with the energy required for delamination initiation decreasing as 0 increased. The energy required for delamination initiation was also influenced by the number of dissimilar interfaces, increasing as the number of interfaces increased 4. Clark 6 used an analytical approach to predict the position and size of delaminations through the laminate thickness under impact loading. Larger delaminations were predicted and observed to occur as the angle between adjacent plies increased, which supports the observations of Hong and Liu 4'5. Strait e t al. 7 investigated the effect of stacking sequence on the energy absorbed during impact penetration tests of a carbon/epoxy composite having three main stacking sequences, cross-ply, quasi-isotropic and [0/+ 45], with minor variations in ply order in each basic lay-up geometry. The results showed no clear-cut effects of stacking sequence in terms of the energy absorbed in delamination initiation for the three main laminate types although the absorbed energy was influenced by the minor changes in lay-up for each basic geometry. Other studies 8'9, however, concluded that the stacking sequence had little or no effect on the energy absorption or the extent of damage, particularly when the variations in stacking sequence were relatively minor. The effect of stacking sequence on the impact resistance of composite laminates is, therefore, not yet fully understood. This study investigated the effect of stacking sequence on impact damage and compression after impact (CAI) in a carbon fibre/toughened epoxy composite. The energy absorbed during the impact event was monitored and has been related to the development of damage.
COMPOSITES Volume 26 Number 3 1995 207
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp EXPERIMENTAL
Material The effect of stacking sequence on impact damage and CAI was evaluated on a carbon fibre/toughened epoxy T800H/924C. Six different 16-ply panels, 600 mm by 300 mm, each containing equal numbers of 0 ~ and 45 ~ plies, were autoclaved at 180~ under 700 kPa pressure in accordance with the recommended cure schedule. The following layer stacking sequences were investigated: Panel 1 - [(+ 45,02)2]s Panel 2 - [(+ 45)2,04]s Panel 3 - [(+ 45,0, - 45,0)z]~ Panel 4 - [(02, + 45)2]~ Panel 5 [04,(_ + 45)2]~ Panel 6 - [(0, + 45,0, - 45)2]s After post-curing the panels were ultrasonically Cscanned to ensure panel integrity.
Impact testing The composite panels were impacted using a commercial instrumented falling weight test machine to a level of 7 J using an impact mass of 2 kg, an impact velocity
of 2.5 m s-1 and a 10 mm diameter hemispherical tup. During the impact event the panel was clamped between two steel rings of internal diameter 100 mm and the impact head was captured after impact to prevent secondary strikes. The energy absorbed during the impact event was monitored by the instrumented impactor, with the primary output being force versus time data which were subsequently analysed into absorbed energy values. The impacts were repeated at 100 mm intervals across the panel as recommended in the C R A G ~~standard, with five impacts being performed on each panel. After impact the panels were again ultrasonically C-scanned.
Post-impact analysis Compression properties. After impact and C-scan inspection, coupons of dimensions 250 mm by 20 mm for plain compression tests and 250 m m by 50 mm for CAI tests were cut using a diamond slitting wheel. Each CAI specimen contained an impact site. Aluminium end-
4~
a
b
3
1
1L.6
312
4.8
6.4
8.0
1'.8
312
Time, ms
4.8
84
8.0
Time, ms
C
d 3
z
z ~2
,E
,,o
1
2
4
10
6
2
0
Time. ms
4
10
6 Time, ms
4
3.0
e 3
2.3
z z
e52
c~I-5 o
o
0.8
0
, 1.6
i i 3.2 4.8 Time, ms
Figure 1 Typical force
288
COMPOSITES
Volume
versus
, 6.4
8.0
1.6
3.2 4.8 Time, ms
6.4
8.0
time curves: (a) panel 1; (b) panel 2; (c) panel 3; (d) panel 4; (e) panel 5; (f) panel 6
26 Number
3 1995
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp tabs were bonded onto the ends of the plain compression test specimens, but not on the CAI specimens. All tests were performed to the CRAG 1~ standard with an antibuckling guide to support the coupon during compression loading to prevent macrobuckling. Specimens were tested to failure using a screw-driven test machine and the failure loads recorded.
Damage analysis. The damage sustained during the impact event was analysed using a range of techniques. The maximum delamination area for every impact was determined from the C-scans performed after impact. Additional tests were then carried out on one impact site from each panel. Initially the specimens were X-rayed using conventional shadow X-radiography with a zinc iodide penetrant used to reveal the extent of delamination, splitting and fibre fracture. The through-thickness damage distribution was determined using a de-ply technique. The specimens were impregnated with a solution of gold chloride, placed in a furnace to partially 'burn-off' the matrix and separated into the individual plies. The delaminations were indicated by a gold residue from which the area of each delamination was measured. RESULTS
Force-time data Typical force versus time plots for the six panels recorded during the impact event are shown in Figure 1. Typical plots of energy versus time for panel 5 together with the corresponding force versus time plot are shown in Figure 2. The absorbed energy increased to a maximum and then fell to zero, but the maximum energy occurred some time after the peak force had been reached. Similar trends were obtained for the other five 3.0
2.3
5~1.5 ,2
\
0.8
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1',6
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Emax 7"0
3!2 i Time, ms
L 48
1 64
80
t
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3
\
~ 3.5 c tla
\
Compression behaviour The plain compression strength and the CAI strength for the six panels are listed in Table 2. The plain compression failure stresses were influenced by the stacking sequence, with values ranging from 881 MPa for panel 2 to 659 MPa for panel 5. Comparison of the different lay-ups revealed similarities between the plain compression failure stresses in panels 1 and 2, panels 3 and 6, and between panels 4 and 5. The residual strength after impact was lower than the plain compression failure strength, with values ranging from 344 MPa for panel 1 to 273 MPa for panel 5. Impact damage analysis Damage morphology. The stacking sequence influenced both the shape and size of the maximum delamination area as shown by the X-radiographs of Figure 3. An approximately circular damage zone was observed in panel 2 whereas the other five panels contained elongated maximum delaminations parallel to the fibre direction in the surface plies. All six panels contained a crush zone at the impact site which sustained considerable fibre damage. In addition to delamination, splitting was observed which tended to be most extensive in the plies furthest from the impact site. The position, size, orientation and shape of the delaminations through the panel thickness are shown by the de-ply tracings of Figures 4a-f. Delaminations initiated at almost every dissimilar ply interface through the panel thickness and tended to be oriented parallel to the fibres in the lower ply (i.e. furthest from the impact site). Delamination area. The distribution of damage through the thickness of the six panels as determined by the de-ply technique is shown in Figures 5a and b. Delamination area tended to increase towards the back face furthest from the impact site. The maximum and total delamination areas, determined from the C-scans and the de-ply tracings respectively, are listed in Table 2. It is evident from these results that the maximum delamination area is influenced by the stacking sequence, with panel 5 showing the largest maximum delamination area (1620 mm 2) and panel 6 the smallest (360 mm2). Table 1 Mean energy values recorded during the impact event (average of four specimens) for the six panels determined from the force v e r s u s time curves
\\\ \\
panels. The peak energy (energy at peak force) and the maximum energy were obtained from the force-time plots and the mean value of the four curves analysed are listed in Table 1. The panels containing 45 ~ surface plies gave higher peak energies (6.23 to 6.41 J) than panels containing 0 ~ surface plies. The maximum energy was approximately constant for all six panels and appeared to be independent of the stacking sequence.
\-,
Panel 1.8
0
1.6
3.2
48
6.4
80
Time, ms
Figure 2 A typical force energy
versus
v e r s u s time curve and the corresponding time curve for panel 5
l 2 3 4 5 6
Peak energy. E~ (J)
M a x i m u m energy, E .... (J)
6.31 6.23 6.41 6.06 5.98 6. l 1
6.65 6.6 6.55 6.63 6.7 6.66
Residual energy, Er (J) 0.34 0.37 0.14 0.57 0.72 0.56
COMPOSITES Volume 26 N u m b e r 3 1995
209
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp Table 2
Delamination areas together with the pre- and post-impact compression data for the six panels
Panel
Maximum delamination area (mm2)a
Total delamination area (mm2)b
Plain compression strength (MPa) a
Compression after impact strength (MPa) ~
1 2 3 4 5 6
510 870 650 765 1620 360
1350 1360 870 1510 2070 2t70
880 881 718 660 659 724
344 318 298 312 273 339
_+58 +_50 -+ 21 + 12 + 26 +44
_+ 118 -+ 99 -+ 35 _+ 11 + 92 "+43
+_35 _+64 -+ 5 _+ 12 +4 _+ 11
~ Average of four values b Single specimen assessed using a de-ply technique
Figure 3
210
X-ray penetrant radiographs: (a) panel 1; (b) panel 2; (c) panel 3; (d) panel 4; (e) panel 5; (f) panel 6
COMPOSITES Volume 26 Number 3 1995
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp
9 O0
-
+
9
0 0 0 0
a
',
/
%
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9169
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d
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+
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9
f i
0
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i
I
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50 m m
Figure 4 De-ply tracings showing the distribution of delaminations through the thickness: (a) panel 1; (b) panel 2; (c) panel 3; (d) panel 4; (e) panel 5; (f) panel 6
COMPOSITES Volume 26 Number 3 1995
211
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp 1360 800
800
9 Panel 1
04
"
E E 600
I
c o
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zx Panel 6
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9 Panel 3
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,,., . . . . . . . . . . . . .
2
/,
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6
8 10
12 lZ, 16
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site
a Figure 5 and 6
't
o Panel 4
04
9 Panel 2
b
Distribution of delaminations through the panel thickness (determined using a de-ply technique): (a) panels 1, 2 and 3; (b) panels 4, 5
The total delamination area was always greater than the maximum delamination area, with values ranging from 870 mm 2 for panel 3 to 2170 mm z for panel 6. The panels containing 45 ~ surface plies gave smaller total delamination areas (870 to 1360 mm 2) than the panels containing 0 ~ plies (1510 to 2170 mm2). DISCUSSION The results have shown that altering the layer stacking sequence influenced a range of properties including the peak force during impact, delamination area, and the pre- and post-impact compression strength. In this section a novel analysis of the impact energy data is used to relate the energy values recorded during the impact event to delamination development. The CAI properties are related to delamination damage and the effect of stacking sequence on the damage morphology is discussed.
these factors to the initiation energy, E~, and the maximum absorbed energy, Emax, is shown in Figure 6. Ei was significantly increased by placing 45 ~ fibres in the surface plies and slightly increased by increasing the number of dissimilar interfaces. Ema• was approximately constant for all six panels and appeared to be independent of the number of dissimilar interfaces and the orientation of the fibres in the surface layers. Another feature of the impact data was the further increase in absorbed energy after the peak force had been reached (i.e. after El). It can reasonably be assumed that all damage has been produced when the maximum
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n
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.
.
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Delamination area and absorbed energy Previous work by Srinivason et al." subjected a range
of polymer composites to increasing impact energies. A 'step' in the load versus displacement curve was observed which occurred at constant load for the different impact energies and corresponded to the formation of extensive delamination. This load 'step' was, therefore, the threshold load for delaminations to occur. In the current work on the T800H/924C carbon fibre/toughened epoxy system, the force-during the impact event increased with time to a maximum value and showed a sudden drop after the peak load for all six panels (Figure 1). This load drop, therefore, corresponded to the 'step' observed by Srinivason et al.1] and hence is assumed to correspond to delamination formation or initiation. The corresponding absorbed energy at the 'step' is, therefore, taken to be the energy absorbed in delamination initiation and is termed Ei. The maximum absorbed energy during the impact event is termed Em~x. The laminates used in this investigation consisted of equal numbers of 0 ~ and + 45 ~ plies, but differed in terms of the number of dissimilar interfaces and the orientation of the fibres in the surface layers. The relevance of
212
COMPOSITES Volume 26 N u m b e r 3 1995
6,4
t--
LU 6.2
5.8 6
8 Number
10
14
of dissimilar interfaces
x - Ei 4 5 ~ s u r f a c e p l i e s ~-EmaxO
12
~ surface plies
- - Ei 0 ~ s u r f a c e p l i e s ~ Emax45 ~ surface plies
Figure 6 Effect of the number of dissimilar interfaces on the energy at peak force (El) and the maximum energy (Emax)
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp absorbed energy (Emax) is reached since it corresponds to the maximum deflection value. The energy absorbed between the initiation (Ei) and maximum (Emax) value is, therefore, assumed to be absorbed by the process of delamination extension and will be termed the residual energy (Er), defined as follows: Er = Emax
E~
(1)
The residual energies for the six panels, which are listed in Table 1, were clearly influenced by the stacking sequence, being higher for lay-ups containing 0 ~ surface plies. Panels comprising surface 0 ~ plies also showed a larger total delamination area (Table 2) and a plot of the total delamination area versus"residual energy (Figure 7) shows a correlation between these parameters. The residual energy increased with increasing total delamination area in an approximately linear relationship and this supports the assumption that the residual energy corresponds to the energy absorbed during delamination extension. The slope of the line shown in Figure 7 corresponds to an energy release rate for delamination extension and was found to be 380 J m 2. The values of the strain energy release rate measured for unidirectional T800H/924C material were Gk = 300 J m -2 and Gilt -580 J m -2. The estimated strain energy release rate for delamination growth, therefore, lies between these two measured values. The novel analysis proposed in this work has analysed the energy absorbed during impact in terms of an initiation and propagation energy. The energy absorbed in delamination initiation was influenced by the stacking
Compression after impact The stacking sequence was found to influence both the pre- and post-impact compression strength as shown in Table 2. The CAI strength, however, showed no trend with either the number of dissimilar interfaces or the orientation of the fibres in the surface plies. A similar effect of stacking sequence on the maximum delamination area was also found and, as expected from these observations, the CAI strength was related to the maximum delamination area as shown in Figure 8. The residual strength decreased with increasing maximum delamination area and was, therefore, independent of the maximum delamination shape or through-thickness position. Since a simple relationship between the maximum delamination area and an energy parameter was not evident, the maximum delamination area and hence the residual strength, may have a complex dependence on both the initiation and propagation energies. Future work on other materials may clarify this relationship.
Delamination shape The interply delamination shape varied considerably with stacking sequence and through-thickness position as shown in Figures 4a-f The delamination shapes can be generalized as circular, diamond and elongated as illustrated in Figures 9a-c. The elongated delamination was oriented parallel to the fibre direction in the ply below the delamination as
0.8 84
/ I /
0.6
sequence, being increased by placing 45 ~ fibres in the surface plies and by increasing the number of dissimilar interfaces. The residual energy absorbed in delamination propagation was found to increase linearly with increasing total delamination area. An application of this analysis is as a non-destructive method for determining the total delamination area from the force time data without the need for destructive de-ply inspection.
/ /
0
/ /
400
/ / / / /
~- 0.4
[.1.1
360
/ / /
t A
/ / /
i1J 13_
/
320
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/ /
0.2
O
/ /
280
/ / / /
240
/
400
0
800
1,200
1,600
2,000
Total Delamination Area sqmm
200 200
600
1,000 MDA
z, P1
+ P2
~ P3
~ P4 x P5
o P6 - - c a l . l i n e ]
Figure 7 Relationship between the total delamination area (measured using de-ply technique) and the residual energy (Er). The points are the experimental data and the dashed line was calculated using linear regression
l z, P1
+ P2
~ P3
~ P4
1,400
1,800
sqmm x P5
o P6
--cal.
line
Figure 8 Relationship between the compression after impact (CA1) strength and the maximum delamination area (MDA) (measured from C-scans). The points are experimental data and the dashed line was calculated using linear regression
COMPOSITES Volume 26 Number 3 1995
213
Effect of stacking sequence on impact damage: S. A. Hitchen and R. M. J. Kemp
a
b
impact has been analysed by relating the damage distribution to the energy absorption characteristics during impact. The compression after impact strength was determined by the maximum delamination area, being decreased as the delamination area increased. The total delamination area was influenced by the stacking sequence and was related to the energy absorbed during impact by a novel analysis which distinguished the delamination initiation energy E~ from the delamination extension energy Er and the maximum absorbed energy Em~x. The delamination initiation energy was increased by placing 45 ~ plies in the surface layers and by increasing the number of dissimilar interfaces which therefore reduced the energy available for delamination extension. This reduced extension energy Er resulted in a reduced total delamination area. The delamination shape was influenced by splitting and fibre fracture in the ply below the delamination. ACKNOWLEDGEMENTS
c
d
Figure 9 Schematic diagram showing the four characteristic delamination shapes: (a) circular; (b) diamond; (c) elongated; (d) waisted
predicted by the stiffness mismatch model of Hong and Liu 4 and the peel separation model of Clarke 6. The elongated delamination often featured splitting in the lower ply which is thought to promote delamination extension. Another feature was 'waisting' which promoted a lemniscate or 'peanut' shaped delamination as illustrated in Figure 9d. This delamination shape appeared to be generally associated with a combination of delamination and fibre fracture in the lower ply and the energy absorbed by this medium may reduce lateral delamination growth. CONCLUSIONS The current work has shown that the stacking sequence influences both the pre- and post-impact compression strengths and affects the impact damage in 16-ply carbon fibre/toughened epoxy laminate. The effect of a 7 J
214
COMPOSITES Volume 26 Number 3 1995
This work was supported by the MOD. The authors would also like to thank Mr J. Coleman for fabricating the panels tested in this work and Mr C. Molyneux for the compression testing. REFERENCES 1 Cantwell, W.J. and Morton, J. Composites I991, 22, 347 2 Chen, W.S., Rogers, C., Cronkhite, J.D. and Martin, J. J. Am. Helicopter Soc, 1986, 60 3 Dorey, G. in 'Failure Mode of Composite Materials with Organic Matrices and Their Consequences in Design', AGARD CP 163, 1975, paper 8 4 Hong, S. and Liu, D. Exp. Mech. 1989, 115 5 Liu, D. J. Compos. Mater. 1988, 22, 674 6 Clark, G. Composites, 1989, 20, 209 7 Strait, L.H., Karasek, M.L. and Amateau, M . F . J . Compos. Mater. 1989, 26, 1 8 Morton, J. and Goodwin, E.W. Compos. Struct. 1989, 13, 1 9 Davies, C.K.L., Turner, S. and Williamson, K H . Composites 1985, 16, 270 10 Curtis, P.T. (Ed.) RAE Tech. Rep. TR 88012, Royal Aerospace Establishment, 1988 II Srinivason, K., Jackson, W.C. and Hinkley, J.A. '36th [nt. SAMPE Symp.' 15 18 April 1991, Covina, CA, pp. 850-862