Impact resistance and energy absorption mechanisms in hybrid composites

Composites Science and Technology 34 (1989) 305-335

Impact Resistance and Energy Absorption Mechanisms in Hybrid Composites

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin & R. H. Zee Composites Research Laboratories, Materials Engineering Program, 342 Ross Hall, Auburn University, Alabama 36849, USA (Received 2 February 1988; accepted 5 July 1988)

ABSTRACT The response of hybrid composites to low-velocity impact loading has been investigated. The energy absorbing mechanisms of laminates containing various fibers were studied primarily by means of the instrumentedfalling dart impact testing technique. Static indentation tests and scanning eleetron microscopy ( S E M ) were also employed to assist in the identification of failure mechanisms. The composites containing polyethylene ( PE ) fibers, which were of high strength and high ductility, were found to be effective in both dissipating impact energy and resisting through penetration. Polyester (PET)fiber reinforced epoxy also exhibited superior impact characteristics even though the PET fabric layers without epoxy did not have good modulus or ductility. Good energy absorbing capability was also observed in epoxy reinforced with woven fabrics made of high-performance Nylon fibers. Nylon, PE and PET fibers were found to enhance the impact resistance of graphite fiber composites. Upon impact loading, the composites containing either PE or P E T fibers in general exhibited a great degree of flexural plastic deformation and some level of delamination, thereby dissipating a significant amount of strain energy. Hybrids containing Nylon fabric showed analogous behavior, but to a lesser degree. The stacking sequence in hybrid laminates was found to play a critical role in controlling plastic deformation and delamination. This implies that the stacking sequenee is a major factor governing the overall energy absorbing capability of the hybrid structure. The 305

Composites Science and Technology 0266-3538/89/$03-50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

306

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

penetration res&tance of hybrid composites appeared to be dictated by the toughness (strength plus ductility) of their constituent fibers. The fiber toughness must be measured under high strain rate conditions.

1 INTRODUCTION Low-velocity impact loading often creates invisible internal damage to graphite fiber composites. Composites made from low- or medium-ductility fibers are also known to suffer from inferior resistance to high-velocity projectile penetration. The study of impact response of composites has become an area of great academic and practical interest in the composite communities.1 - 2 5 , 3 2 - 3 6 Several approaches have been taken to improve the impact resistance of composite materials. These include the control of fiber/matrix interfacial adhesion,37,38 matrix modifications (in particular, rubber toughening), 5'1°'35 lamination design (e.g. selection of laminate stacking sequence), 24"39 introduction of through-the-thickness reinforcements (braiding, 3-D weaving, and stitching)ff °'41 insertion of interlaminar 'interleaf' layersff 2 fiber hybridization 13'16"17'25-29"33-81 and utilization of high-strain fibers. 18,21,22 The last approach appears to be very promising since a major cause of the impact related problems in high performance composites is believed to be related to the low strain to failure of the reinforcement fibers. This problem has been addressed in one of our previous publications. 81 In addition to improving understanding of the subject of fiber toughness, this report also addresses the effects of interlaminar hybridization. The purpose of the present investigation was to achieve an understanding of the roles of various fiber properties, 2s- 31 such as strength and ductility, on impact and penetration resistance of fibrous composites. It is anticipated that such understanding can be used to improve the composite response to impact loading, especially in a hybrid geometry. Although the fiber ductility has been recognized as a key factor in composite impact response, in many previous studies 18'21'22 the tensile fracture strain of monofilaments was utilized as an index for ductility. However, this property was normally obtained by low strain-rate tension tests. In the present study the impact properties of various single-fiber woven fabric samples, with or without resin impregnation, were utilized when interpreting the impact response of hybrids. The deformation and fracture mechanisms in hybrid composites containing these fabrics under various loading conditions were determined. In addition the major material parameters dictating such mechanisms were identified. Efforts were focused on the effects of fiber hybridization on the mechanical behavior of composites.

Impact resistance and energy absorption in hybrid composites

307

2 EXPERIMENTAL 2.1 Materials and sample preparation The raw materials used and their suppliers are listed in Table 1. A wide range of high-performance fibers were used to identify the effects of the constituent fiber strength and ductility on the energy absorption mechanisms of hybrid composites. Each laminate sample was prepared by stacking preimpregnated fabric layers in an open mould, followed by press curing at 60°C for 1 hour. Each sample was then machined into four 10cm x 10cm specimens. These specimens were used for falling-weight impact and indentation tests. TABLE 1

Materials and Suppliers

Material

Reinforcement style

Relevant characteristics (relative values)

Polyethylene fibers Graphite fibers Aramid fibers

Plain weave

Nylon fibers

Plain weave

Polyester fibers Glass fibers

Plain weave

High strength and high strain High strength and low strain High strength and medium stram Low strength and very high stram Low strength and very high stram High strength and low strain Low curing temperature

Epoxy resin Curing agent

Plain weave Plain weave

Plain weave

Supplier

Spectra 900 from Allied Signal Hercules AW193P Kevlar-49 fabrics from Burlington Compet-Nylon from Allied Signal Compet-PET from Allied Signal Unknown Ciba-Geigy 507 Ciba-Geigy 956

Three different groups of laminates were fabricated. The first group includes bare fabric samples without epoxy, plus the corresponding singlelayer laminate samples, where the fabric was fully impregnated with epoxy resin. This group was tested to determine the roles of fibers, matrix and interface in affecting the impact behavior of a laminate. The second group consists of several two-layer hybrids to provide better understanding of the functions of each fabric type in the overall impact response of a hybrid. In the third group, specimens composed of five fabric/epoxy layers with different fabric type and stacking were used to study these parameters on impact properties.

308

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

2.2 Instrumented falling-weight impact test Impact behavior was determined with a Dynatup model 730 drop weight facility manufactured by the General Research Corporation. The laminate was clamped horizontally over an annular support with an inner diameter of 7.5cm. Weights collected in a box with an impacting nose tip of 1"27cm diameter were dropped through 1-5 m to hit the laminate at the center of the span. The weights and the drop height were varied to glovea range of incident energies and velocities. An impact velocity of 4"5 m/s and incident energy of 110 J were utilized throughout this study. The impactor was equipped with an instrumented tip (a load transducer) whose output was fed to a data acquisition board installed in a dedicated IBM PC-AT computer. Load as a function of time during impact was obtained as well as an integral curve for the energy absorbed by the specimen. These two types of data representation were analyzed with the IBM computer. The load and energy at a maximum load point are symbolized by Pm and E m. With E t representing the total energy absorbed by the specimen in complete penetration, the energy absorbed in the crack propagation phase will be Ep = E t - E m. The ductility index (D.I.) is then calculated from D.I. = E J E m.

2.3 Compression/indentation tests In order to observe periodically the deformation and failure processes in a composite under 'quasi-impact' conditions, static identation tests were conducted with the same annular support as that used in the impact case described above. The same impactor was used as with the indentation tip. The crosshead of the Instron universal testing machine was controlled at a speed of 0-1 in/min (2.54mm/min) to facilitate simultaneous detection of acoustic emission (AE) signals. This latter technique was carried out to assist in the identification of various microfailure processes operating during compression/indentation loading.

3 RESULTS

3.1 Graphite-Nylon/epoxy hybrids (Gr-Ny/EPO) The properties of four model laminates subjected to a low-velocity impact are shown in Fig. 1. The laminates tested include four specimens each of the following samples: Nylon/epoxy single-ply (curve Ny), graphite/epoxy single-ply (curve Gr), and two-ply Nylon-graphite/epoxy with Nylon layer (NyGr) and graphite layer (GrNy) facing the impact direction, respectively.

Impact resistance and energy absorption in hybrid composites

309

o

GrNy

.

t.2

.6

1.8 TIME( msec )

~J~lCltlml ]d S of OYrlayed Curvea~ NY-~q-03 MY-'6G-04

2.4

I--~--Ot

to

3.0

I-lO--Oi

(a)

Jangl ,.J

i

'.0

.9

I

t.fi

i

2.4 TIME{ msec)

SpI C| ~ ]d a of ~ l r l i y l d CurytE NV,-mt.-03 ~ 4

Ny-,h'y,-W'~

I

3,2

4.0

Grin'-"02

(b) Fig. 1. (a) Impact characteristics of two-layer graphite-Nylon hybrid laminates in comparison with those of single-layer samples. (b) Impact behavior of two-layer composites•

310

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

Both the load versus time and energy versus time curves are shown in Fig. 1. From these curves information such as the material stiffness (the initial slope of a load versus time curve), yield point, maximum load (Pmax), energy at maximum load (Em), energy after maximum load (Ep), ductility index (D.I. -- Ep/Em), and total energy absorbed (Et) can be obtained. Sample Gr and sample GrNy exhibited very similar initial slope of the load versus time curve. This slope is higher than those found in samples Ny and NyGr. This suggests that the early part of the impact response is controlled primarily by the property of the layer facing the impactor. Graphite/epoxy has a higher modulus than the Nylon counterpart, giving samples Gr and GrNy higher initial slope values than samples Ny and NyGr in the load versus time curve. Although the tensile strength ( O ' m a x = 2070-2340MPa) and modulus (E = 345-690 GPa) of a graphite fiber are respectively 2-3 times and 70-130 times higher than amax (965 MPa) and E (5.17 GPa) of a Nylon fiber, the maximum load and impact energy absorbed in the graphite/epoxy composites were considerably lower than in the Nylon/epoxy composites. This is probably caused by the inferior impact toughness of graphite fibers, which possess high tensile strength and modulus but very low fracture strain values. It is of interest that the 'impact ductility', represented by the time-tofailure in the impact load verus time curve of graphite/epoxy laminate (curve Gr), is only half that of Nylon/epoxy laminate (curve Ny). However, the tensile fracture strain (~f = 0-6-0.8%) of graphite filaments is 25-33 times smaller than that (ef = 20%) of Nylon filaments used. This implies that the tensile fracture strain, usually obtained from a slower-rate tensile test, cannot be used directly as an index of impact ductility for fibers embedded in composites subjected to high-rate loading. One might expect the impact properties of a two-ply laminate to be simply additive, i.e. equal to the sum of the properties of the constituent layers. This is obviously not the case in the present example (Fig. 1) and other hybrids studied in this report. It turns out that the single Nylon/epoxy layer (sample Ny) possesses the highest maximum load and the greatest total energy absorption capability, even greater than those of the two-layer laminates (samples GrNy and NyGr). A great degree of plastic deformation was observed in sample Ny. The specimen was compressed and deflected by the tip of the impactor in such a way that the specimen protruded to form a dome along the rim of the circular support. In addition, an extensive whitening phenomenon, obviously induced by fiber/matrix debonding, was observed. These deformation mechanisms (indentation, deflection, debonding) dissipate a significant amount of impact energy prior to perforation and penetration of the specimen by the impactor. The impact behavior of sample NyGr is quite similar to that of sample Ny,

Impact resistance and energy absorption in hybrid composites

311

although Ny is slightly tougher and more ductile. Both exhibited a high maximum load and large energy at maximum load, followed by a rapid drop in the load level. The energy dissipation after maximum load, Ep, is significantly lower than E m, resulting in a very small ductility index value. The impact response of NyGr appeared to be dominated by the front layer (i.e. the Nylon/epoxy layer) and a major portion of the impact energy was dissipated before this layer experienced the maximum load. The wavy portion beyond the initial linear portion of the NyGr curve, but prior to the maximum load, is probably caused by a combination of the interlaminar separation (delamination) and the back layer tensile crack propagation. This was confirmed by intermittently stopping the indentation tests to permit careful examination of the deformed samples. Incorporating a graphite layer appeared to have constrained the ability of a Nylon layer to undergo plastic deformation, thereby significantly reducing its load-bearing and energy-absorbing capability. Final failure occurred when the Nylon layer was fully penetrated. Sample GrNy is identical to NyGr in composition and configuration but opposite in loading direction. Sample GrNy was loaded with the graphite layer being the front surface facing the impactor. Plastic deformation seemed to occur by indentation of the front layer in conjunction with delamination, the latter mechanism operating prior to and during the perforation phase of the back surface (Nylon layer). Again the presence of a graphite layer restricted the ability of the Nylon ply to deflect and deform plastically. After maximum load, where perforation ocurred in the graphite layer but not in the Nylon layer, sample GrNy was still capable of dispersing a considerable amount of impact energy, leading to a relatively high ductility index. Figure l(b) shows the impact behavior of the two-ply laminates. Both the Pmaxand the E, values of the two-ply laminates (NyNy and GrGr) are higher than those of the corresponding single-ply samples (Ny and Gr in Fig. l(a)), but not exactly doubled. The impact resistance of a graphite composite can be improved considerably by incorporating a selected amount of ductile Nylon fabric. The stiffness of a ductile laminate can be enhanced ifa layer of graphite fabric is added to receive the impactor.

3.2 Graphite-polyester/epoxy hybrids (Gr-Es/EPO) The impact response of Gr-Es/EPO hybrids (Fig. 2) is rather similar to that of Gr-Ny/EPO hybrids (Fig. 1). Both samples are composed of a highmodulus but low-ductility layer (Gr) and a high-ductility but low-modulus layer (Ny or Es). Similarity in the initial slope of the load versus time curve between samples EsGr and Es, and yet different from Gr and GrEs (Gr and

312

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee o

uQ.

ES o

EsGr X

o

c:,k~

•

i

2 Q

.0

t.3

2.6

3.9 TIME{ msec )

5.2

6.5

Spec~lien Id s of Ovnrlayed Curves: Es-Gp-O3

Es-GP-O4

I-6r-0$

I-Es-O!

(a) o

Jan01'

'

eta

/,/,//It o

/

./~.E,

~/,/ i

Lu

co

GrGr

. . . . .

t

t

.0

1.6

~ecliien

3.2

i

4,B TIME[ msec )

L

6.4

8.0

Id s of Overlayed Curves: Gr6r-O2 EsEs--03

(b) Fig, 2. (a) Impact response of single-layer lamina and two-layer hybrids based on graphite/ and polyester/epoxy, (b) Impact response of two-layer composites.

Impact resistance and energy absorption in hybrid composites

313

GrEs also have similar initial slope values), again suggests that the indentation of the front surface was responsible for the first deformation of the specimen. In contrast to the relatively straight and kink-free load versus time curves observed in the single-fiber laminates Es, Gr (Fig. 2(a)), EsEs and GrGr (Fig. 2(b)), the curves for both EsGr and GrEs hybrids are rather wavy except at the initial elastic range. In both EsGr and GrEs, the first break from the linear behavior always occurred near or above 0.23 kN, which was the maximum load experienced by the single Gr/EPO layer. This suggests that the first crack in these hybrids, irrespective of the type of front surface, always takes place in the more brittle graphite layer; a similar phenomenon was also observed in the Nylon-graphite hybrids. Propagation of this initial crack was probably followed by interlaminar delamination. After this first break, the slope of curve GrEs gradually decreased to a value comparable to that of curve Es; the load was then carried practically by the polyester layer alone. In contrast, the load versus time curve of EsGr, where polyester (Es) layer faced the impact direction, did not exhibit a decrease in slope after the first or subsequent breaks. In both EsGr and GrEs, final failure occurred as a result of perforation of the tougher polyester layer. Both hybrids showed lower values of Praax, Et and maximum deflection values as compared with the single-layered Es sample. It is also clear that these impact properties are not mathematically additive. Compared to sample GrEs, EsGr was more heavily deformed and exhibited a more extensive whitening phenomenon, most probably caused by the fiber/epoxy interfacial debonding. Both showed similar interlaminar delamination and were fully penetrated, Sample EsGr was capable of bearing a higher load, dissipating a greater amount of energy, and deflecting to a greater extent. Such differences in impact response with a different impact direction were also noted in many other two-ply hybrid laminates.

3.3 Graphite-polyethylene/epoxyhybrids (Gr-Pe[EPO) The load-time traces in Fig. 3, in combination with the photographs of the impact loaded specimens shown in Fig. 4, again illustrate several important points. First, a tough fabric like Pe can impart a great level of impact resistance to a brittle composite like graphite/epoxy. Both the maximum load and impact energies of the graphite/epoxy composite were increased dramatically. Second, the impact response of a hybrid depends critically upon the stacking order with respect to the impact direction. By analogy with the case of graphite/polyester hybrids, with the tough layer coming in contact with the impactor first, the hybrid was able to bear a higher load and to absorb a greater magnitude of impact energy. Compressive indentation

314

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

c:J

Jangl

o~

~.0

.8

i.6

2.4

3.2

4.0

TIME{ msec ) Specimrl Id a of Ovenlayed Curves: Pe-Gr-Oi Pe-Br-02

Gr'Gr-02

(a) o

un

.

c3

dangl un r~

u-J zcu

cza~

aJ

Gr

/

w

o

cd

o

'.0

2.4

4.B 7.2 TIME{ msec)

~e(~).lieri Id 3 9f 0vePlay6d CAJPve9: Pe-Gr-0t Pe-Gr,-02

l-#e-0t

9.6

12.0

I-Gr-~t

(b) Fig. 3. (a) i m p a c t load-displacement curves obtained from graphite-PE/epoxy hybrids and graphite-graphite composites. (b) Comparison of impact behavior between two-layer hybrids and single-layer composites•

Impact resistance and energy absorption in hybrid composites

315

i I i iil : ~

(a)

(c)

ii¸

(b)

(d)

Fig. 4. Photographs of PE-graphite hybrids: (a) and (b) are taken from the same specimen, (a) showing the graphite (front) face and (b) the PE (back) face; (c) and (d) show the PE and Gr side, respectively,of a specimen loaded from the PE side. again represented the first stage of deformation and was responsible for the elastic stiffness. At a slightly greater deflection both constituents would share the load, and the first 'yield drop' in the load versus time curve of PeGr (Fig. 3) was believed to be due to the initiation of a tension crack on the back surface (the graphite layer; Fig. 4(d)). Propagation of this tension crack proceeded simultaneously with delamination (Fig. 4(c)). These processes were followed by the final perforation of the Pe layer. It should be noted that the pictures shown in Fig. 4(a) and (b) were taken from the same specimen

316

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

with the former showing the front surface (graphite layer facing the impactor) and the latter the back surface. Figures 4(c) and 4(d) represent the same specimen loaded from the Pe side (picture (c)). A dome was formed when the specimen was impacted from the Pe side (Fig. 4(d)), but not when loaded from the graphite side. Also, a greater degree of delamination was found in the former. Third, the high-modulus graphite fabric, if placed in a proper position, can improve the stiffness of Pe composites. The final point is concerned with the synergism of hybrids (Fig. 3(b)). Under impact testing conditions, the graphite and Pe fabrics appeared to show a positive hybrid effect in maximum load, i.e. Pmax measured was greater than the rule-of-mixture prediction if the Pe layer faced the impact direction. However, with either impact direction, the impact energies of PeGr and GrPe showed the existence of a negative hybrid effect. 3.4 Graphite-Kevlar/epoxy hybrids (Gr-Kv/EPO) Efforts were also made to study the impact behavior of Kevlar-graphite hybrids. Figure 5(a) shows a comparison of the impact response between KvKv and GrGr. The Kevlar/epoxy laminates had slightly higher values for Pmaxand E t than the graphite/epoxy laminates. All the specimens from both groups were fully penetrated, and little permanent deflection was observed. Hybridization of Gr with Kv therefore was not expected to improve the impact properties of graphite/epoxy composites significantly. Figure 5(b) shows that the two-ply hybrids are slightly better when loaded from the Kevlar surface, but both KvGr and GrKv are not much superior to the pure graphite/epoxy (GrGr) laminates. Again, all the impact loaded specimens were penetrated through without undergoing any significant deflection (no dome).

3.5 Kevlar-polyethylene/epoxyhybrids (Kv-Pe/EPO) The impact properties of Kv-Pe/EPO are very similar to those of GrPe/EPO. This is not surprising considering the fact that KvKv and GrGr are not much different in their impact response. The test results of a Kv-Pe/EPO series of samples are shown in Fig. 6. The photographs in Fig. 7 indicate that hybridization with a Pe layer significantly increases the degree of delamination and enhances plastic deformation in Kevlar. Figures 7(a) and 7(b) show the back and front surfaces, respectively, of an impact loaded KvKv laminate while (c) and (d) show those ofa PeKv laminate loaded from the Pe side.

Impact resistance and energy absorption in hybrid composites

t

J.:

317

oa~

.A

/ '.0

,6

t.2

2.4

t.9 TIME[ msec )

3.0

Id e of 0viirlayedCurves: Kv-KV-0L 6r~-0t

Spllclmin

(a)

~ IKvGr~ ~t

/

....

KvGr : ./'~" ~vG~ " 7 ~

~

oa

.

0=

'7

,/ o

'.0

.6

t.2

t.9 TIME[msec ]

2.4

3.0

Speclaien[d a of Over,layedCoP,tea: i(y-Ge-O3 Kv-Gr-04

(b) Fig. 5.

Impact response of(a) Kevlar-Kevlar and graphite-graphite/epoxy laminates and (b) KvGr hybrid composites loaded from opposite directions.

318

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

i o

dangi

--'

-

<::,__ ~o

o{

~dm

i

.~//

:0

i.0

KvKv

2.0

3.0 TIME( rnsec )

4.0

5.0

Speclllefl Id a of Overlzfed Curves:

KV-#E-Ot

KV-~E-04

Kv-I(v-Ot

(a) o

Jangl

f -

p;//P°

tr~

o

~L

Z~

/

o

KvPe

~X

2.4

.0

4.8

7.2

9.6

t2.0

TIME(msec)

]d s of OveMayea C~ves: KV-PE-Ot I(V-~E-O4

Spectllen

I-Kv-Ol

I-Pe-Ot

(b) Fig. 6.

(a) Impact properties of Kevlar-PE/epoxy hybrid and KvKv composites. (b) Impact response of hybrids in comparison with that of single-ply laminates.

Impact resistance and energy absorption in hybrid composites

(a)

(b)

(c)

(d)

319

Fig. 7. Impact loaded KvKv (a,b) and PeKv (c, d) specimens; (c) shows the back (Kv) face and (d) the front (PE) face.

3.6 Five-layer hybrid laminates Summarized in Figs 8(a) and 8(b) are the instrumented impact test data of various graphite-polyester/epoxy hybrid laminates. Figure 8(a) shows the total energy (Et), the energy at maximum load (Era) and the energy after maximum load (Ep). Each of these parameters is expressed as a function of the relative volume proportion of PET fibers (i.e. Es/(Es + Gr)). The data points that are connected to form a curve represent the alternate hybrid laminates (GrGrEsGrGr, GrEsGrEsGr, EsGrEsGrEs) or single-fiber

320

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

120

I

I

I

1

I

1

I

I

I

110

O A ~ PET I00

•

•

•

[] A ~

90

Gr.

+`ace

Impact Impact

+`ace

Symmetric

laminates

/ /-

80

q

iI

70

>-

I

60

nl W Z W

50 40 30 20 10 0

10

0

20

30

AI

I

50

60

40

VOLUME

FRACTION

i

OF

i

I

I

70

80

98

PET

00

(Z)

(a) 10

1

A •

9

1

PET i m p a c t "~r. i m p a c t

I

I

+`ace +`ace

I

0 ~

\

?

9. g

I

l

PET i m p a c t Gr. i m p a c t

<

8

Q (E O /

I

1

+'ace +'ace

>

\

X 2

W Z >-

5

U

(E

Q

3 2 1

0 0

o

1

to

I

2o

I

3o

VOLUHF

I 4o

I 50

FRFICTION

I

so

~x r~l

I

70

80

OF P E T

(%)

l

90

t ~ o8

(b) Fig. 8. Summary of impact properties of five-layered graphite-PET/epoxy hybrid laminates. (a) Total energy, energy at maximum load and energy absorbed after maximum load. (b) Maximum load and ductility index.

Impact resistance and energy absorption in hybrid composites

321

laminates (GrGrGrGrGr, EsEsEsEsEs). The isolated data points off the connected curves represent those unsymmetric laminates (EsEsGrGrGr, EsEsEsGrGr) that were loaded from the Es side (open symbol) and Gr side (solid symbol), respectively. The impact load versus time curves of two identical EsEsEsGrGr specimens loaded from different directions are different. Nevertheless, the specimens in both cases were fully penetrated. A slightly greater amount of energy was dissipated in the laminate when the graphite side faced the impact direction. This is in direct contrast to the behavior of the corresponding two-layer versions, where the laminate was more impact resistant when the polyester layer faced the impactor. Irrespective of the loading direction, these unsymmetric laminates appear to absorb more energy and withstand a higher load than their alternatesequence counterparts. The above data indicate that PET fibers can be used effectively to improve the impact resistance of the graphite/epoxy composites. Both E t and E mwere significantly increased with an increasing PET content. However, Ep was only slightly higher with a corresponding increase in the PET volume. Therefore, a major portion of the energy was absorbed prior to maximum load. Similar trends were found in the case of PeGr five-ply hybrids, al When either four or five layers of Pe or PET were included in the laminate, no through penetration was observed. In these materials, a great amount of energy was consumed in deforming the laminate and creating delamination. The center of the specimen was expected to experience the highest load, leading to some fiber breakage in the back surface. For the alternate-sequence hybrid laminates containing less than four layers of either PE or PET fabric, a lesser degree of plastic deformation occurred during impact loading. Again specimens were fully penetrated by the impactor. Dagger-shaped delamination zones can be observed visually from both surfaces of a loaded specimen because of the color contrast between the graphite (black) and PET or Pe (white) layers. The maximum load seems to be indicative of the initiation of the penetration crack. The ductility index of the PET-graphite hybrids, like that of the PE based hybrids, s~ decreases with increasing energy absorption capability. A third group of five-layer hybrids studied were Kevlar-graphite reinforced epoxy laminates. Adding one or two layers of Aramid fabric actually resulted in a reduction in Et, E m and, to a lesser degree, Ep. sl A significantly higher level of energy was dissipated after the maximum load was reached (Ep>> Era), suggesting the presence of failure mechanisms different from those in other materials investigated. The ductility indices increased as more and more Kevlar fabric layers were added to the graphite/epoxy systems. This trend is unique to this group of hybrids. Again the stacking sequence was found to play a significant role in determining the

322

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

impact response, with the unsymmetric samples generally exhibiting a greater energy absorption capability, in agreement with earlier observation.

4 DISCUSSION

4.1 Impact failure mechanisms The impact resistance of a composite is measured by the total energy dissipated in the material prior to final failure. This energy is equal to the sum of the dynamic wave energy dispersed and the energy absorbed during plastic deformation, plus the energy needed for creating new surfaces. The latter includes delamination and through-the-thickness cracks. These latter cracks usually result from a combined action of indentation (or compression) on the front surface and flexural tension stresses on the back surface. The microfailure mechanisms operating during this phase of loading include matrix cracking, fiber/matrix debonding, fiber breakage, and fiber pull-out, as evident from SEM examination of the fracture surfaces. A close scrutiny of the load-time traces obtained in the present impact study provided direct evidence to support the hypothesis that indentation represents the very first stage of impactor-material interaction. Depending on the geometry, composition and stacking sequence of the hybrid laminates, this stage was followed by one or more of the processes to be described below. For a stiff single-fiber laminate such as graphite/epoxy loaded by a small projectile traveling at a sufficiently high speed, the laminate could not respond quickly enough in flexure. The high stresses generated close to the point of impact result in perforation and through penetration provided the incident energy is sufficiently high. The laminate would experience no permanent deflection and little delamination, leading to a relatively clean circular hole as a result of through penetration. This behaviour was observed in graphite/epoxy laminates as shown in Figs 7(a), 7(b) and 9(a). Because of their brittle nature, the total energy dissipation capability by these structures was also very low. If the laminate was flexible but not tough, such as Nylon fiber reinforced epoxy, the initial indentation would be followed by a combination of slight flexure and continued indentation. In this case, the degree of flexure or deflection would be small before the laminate was perforated (Fig. 9(b)). This degree of flexure depends critically on the impact toughness (not tensile toughness) of the reinforcement fibers. Although the Nylon fibers possess a tensile strain as high as 20%, they were not able to respond quickly enough as the Nylon fabric was impact loaded. 8~ It was found that very few Nylon

Impact resistance and energy absorption in hybrid composites

323

Fig. 9. Photographs of various two-ply laminates after impact testing. fibers, if any, were permitted to extend fully during impact, resulting in a poor energy dissipation, albeit still better than that of graphite fibers. The Pe fiber/epoxy laminates, being flexible and tough, can respond quickly enough in flexure after the initial impact contact. The initial indentation could not perforate the laminate because the fibers were of high strength and ductility. During impact, the total energy absorbed by the bare Pe fabric without epoxy was found to be 15 times higher than that of graphite fabric and 4 times higher than that of Nylon fabric. 81 Coated with epoxy matrix, the single-layer PE laminate was 100 times and 15 times better than the corresponding graphite and Nylon materials, respectively (see Figs 1-3). The high impact toughness of PE fibers was found to enhance the perforation resistance of the laminate. This allows a great extent of flexure to occur, leading to a considerable degree of plastic deformation. Several

324

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lm, R. H. Zee

plastic deformation mechanisms were found to operate during this deflection stage. These included fiber/matrix debonding, fiber pulling, matrix yielding and cracking. Occasionally, a small number of fibers were found broken on the back surface. Some delamination was also evident in the heavily deformed samples (Fig. 9(c)). No single-fiber laminates containing more than one layer of Pe fabric were fully penetrated, in contrast to other fabric systems. The PET fiber/epoxy systems were also found to be very flexible and relatively tough. These laminates also showed a great tendency to undergo flexural plastic deformation, thereby dissipating a great amount of strain energy. Compared to PE/epoxy laminates, the PET-based materials showed somewhat inferior impact response. However, they were still far superior to both Nylon- and Kevlar-based laminates. This was rather surprising considering that the PET fibers and Nylon fibers employed in the present study were rather similar in filament tensile behavior and the Nylon fabric exhibited a better impact response. The bare fabric impact properties of both fibers were considerably poorer than those of the Kevlar fabric. The Pmaxand E t were 0.62kN and 4.69J for PET, 1.06kN and 7.89J for Nylon, and 1.58 kN and 15.5 J for Kevlar woven fabric, respectively. Coated with epoxy resin, the fabric became a single-layer laminate, and these laminates behaved in a totally opposite way. The Pmax and E t for these laminates were 2.29 kN and 30"33 J for PET, 1.0 kN and 6.29 J for Nylon, and 0.39 kN and 1.39 J for Kevlar, respectively (Table 2). The PET fabric and epoxy resin, when combined together, were synergistic in impact response, showing a 3.7 times and 6-5 times improvement in Pmaxand E t, respectively, over the fabric alone. The bare fabric samples of PET were penetrated and did not show any sign of flexing or plastic deformation. By contrast, the corresponding single-layer laminates were severely flexed prior to penetration. The plastic deformation behavior was characterized by an extensive stress whitening phenomenon, primarily caused by the fiber/matrix interfacial debonding. However, adding epoxy resin actually slightly degraded the impact properties of Nylon fabrics, although both the fabric and the laminate samples were penetrated and deformed very little. Under comparable impact conditions, the Kevlar woven fabric was heavily deformed and was capable of absorbing a significant amount of impact energy. This was anticipated based on the relatively good tensile toughness of Kevlar fibers. When impregnated with epoxy resin, however, the maximum load and total energy dropped by factors of 4 and 11, respectively. The laminate was incapable of responding in flexure or undergoing any plastic deformation. The exact causes for this poor impact response are still under investigation. It is possible that the tendency for the Kevlar fibers to defibrillate, which makes them particularly durable in

Impact resistance and energy absorption in hybrid composites

325

TABLE 2 Average Impact Properties of Single-Layer Fabric/Epoxy and Double-Layer Hybrid Laminates

Sample

Pmax (kN)

E m (J)

Ep (J)

E t (J)

D.L

Gr/EPO Ny/EPO Es/EPO Kv/EPO Pe/EPO G/EPO

0.25 1"06 2-10 0.38 3.99 0.45

0.25 5.89 21.74 0.42 70.86 0.99

0.83 0"73 1-94 0.85 20.53 0.66

1.08 6.62 23"68 1-27 91.39 1.65

3.22 0.124 0"089 2.024 0.290 0.666

GrGr/EPO GrNy/EPO NyGr/EPO NyNy/EPO

0"39 0'59 0.82 1-49

0'62 1-88 3.34 6"99

2"09 2"57 2.54 2' 15

2-71 4.45 5'88 9' 14

3.371 1.367 0"760 0"308

GrGr/EPO GrEs/EPO EsGr/EPO EsEs/EPO

0"39 1.28 1.87 3"84 3-74

0-62 9'85 14.3 63-71 55-24

2"09 0"89 1"80 28'39 3.04

2"71 10.74 16-10 92-10 58.28

3.371 0"090 0.126 0-446 0"055

GrGr/EPO GrPe/EPO PeGr/EPO PePe/EPO

0'39 1"90 3'15 5.79

0-62 7"50 19.40 98.83

2'09 5.78 3-50 7'07

2'71 13-28 22-90 105"90

3.371 0"771 0"180 0.072

KvKv/EPO KvPe/EPO PeKv/EPO PePe/EPO

0"42 3-25 3.77 5'79

0"77 22'77 31.81 98.83

2'88 3"03 2-52 7"07

3"65 25"80 34.33 105.90

3"780 0" 133 0'079 0"072

Observations

Clean crack Slightly flexed Dome, flexed Dome, flexed

No penetration Penetration

Dome, no penetration

Gr = graphite, Ny = Nylon, Es = polyester, Kv = Kevlar, G = glass, Pe = polyethylene (all woven fabric form); EPO = epoxy resin.

bending, was hindered in the presence of epoxy resin. Existence of epoxy resin is also expected to increase the probability to put the Kevlar fibers under compressive stress fields. The relatively poor compression strength of Kevlar fibers is another possible cause of this problem. Somewhat positive synergism was observed in the composites of glass fibers and epoxy resin. At the moment of impactor-material contact during impact, a certain amount of dynamic energy is dissipated in the material and the specimen fixture system. The damping capacity of the material is expected to play a role in determining the amount of the dynamically damped energy. Polymerbased fibers such as PE, polyamide (both Aramid and Nylon) and polyester

326

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

possess great damping coefficients and therefore, should be more effective in absorbing the dynamic energy. The remaining energy will have to be consumed in generating plastic deformation and cracks. The composites containing polyethylene and polyester fibers were observed to undergo extensive plastic deformation. These fibers seem to enhance the tendency for a composite to undergo more uniform deformation, thus allowing a greater portion of the material to share the load. This would assist in avoiding or delaying the more detrimental localized deformation, where a small area of the material carries all the load. This latter inhomogeneous deformation phenomenon was observed in the graphite- and glass-fiber composites. 4.2 Effects of constituent fiber toughness As summarized in Table 2, under comparable conditions the PE fiber-based composites possess the greatest energy absorbing capability. With an impact speed of 4.53 m/s and an incident energy of 104 J, practically 90% and 100% of the incident energy was absorbed by the single- and double-ply polyethylene composites, respectively. The two-ply samples were not perforated or penetrated. These specimens have obviously undergone extensive plastic deformation, as evidenced by the deformation of a dome. The PET/epoxy laminates exhibit the second largest total energy (Et) among all the two-ply laminates tested. Again severe formation and a certain level of delamination were also observed. Several two-layer specimens were not fully penetrated. The impact toughness of the EsEs laminate is followed, in descending order, by NyNy, KvKv and GrGr composites. No significant permanent deformation (except near the perforation) was visible in the last three materials. All specimens have been perforated and penetrated. The data presented in Table 2 and Fig. 10 clearly show that a great maximum load generally leads to a high magnitude of total absorbed energy, suggesting that the impact toughness in these composites is controlled by the overall composite strength. Composite strength is believed to be dictated by the fiber properties. The ductility index (D.I.) of composites was found to decrease with increasing total impact energy. This means that, for tougher laminates, more energy is dissipated prior to maximum load than afterwards. This observation again confirms that tough fibers are essential to achieving composite impact toughness. The graphite fibers used in the present study were only capable of extending to a maximum strain of 0-75% (Table 3). This would only provide a minimal fracture energy (< 80 k j/m2). The fracture strains of these fibers will be exceeded at the local pin-loaded spot; therefore through penetration of such composites is anticipated. Although ductile fibers like Nylon-6 could

Impact resistance and energy absorption in hybrid composites

327

lie

10{} Ow,

90 80 v

>-

70

nl W Z W

60

.J

58 48 $

0

38 28 1~1

I

.s *:

Single-layer

i

I

,

,;s

~

2Js

~s

4

41s

MRX. LORD ( k N ) ~: l]oublm-layer #abric/epoxy;

~ s~s hybrid

laminates

Fig. 1O. Relationship between the total energy absorbed and the maximum load experienced by the laminates.

be extended, under tension test conditions, to a final strain of 20% (Table 3) in the absence of epoxy resin, the overall strain of a composite is limited practically by the epoxy ductility. Using 4% composite strain as an example, Nylon fibers cannot be stretched much more than 4% and the strain energy density in a Nylon filament with a modulus (E) of 5.17 GPa is only 4.1 MPa. The corresponding toughness o f a PE fiber with E = 117 GPa is estimated to TABLE 3 Estimation of Fiber Toughness Fiber

High-modulus carbon High-strength high-strain carbon Kevlar-49 Kevlar-29

Glass S-glass PE Nylon High-strength polyester

Tensile strength (MPa)

Young's modulus (GPa)

Fracture strain (%)

2 070 2 344

380-690 345

0-75 0"6

2 410-4 830 2 800 2 758 3 450 4 585 2 590 965.3 1 034

230-390 124 65 72"4 89-6 117 5"17 9'65

1"8 2'3 4'0 1"3 5"4 4 20 22

Toughness at ~ = 4% (MPa) 9

7 40 32 56 15 66 65 4-1 7"7

328

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee 48

,

,

~

,

~

,

S~NGLE FBBRIC LSYER

i

~

~

i

,

E)

38

>o~ w z w

28

Q w m 0~ o

E)

m

10

C9

E) 8 8

QI Q I 18 28

I 38 FIBER

I 48

I 58

I 68

I 78

TENSILE TOUGHNESS U P T O MSX. S T R S I N

I 88

I 98

I 188

I

118

20

(MP~)

Fig. 11. Correlation between the total impact energy absorbed by a single fabric layer (without epoxy) and the fiber tensile toughness for various types of fibers. be 65 MPa (Table 3). For a high-modulus carbon fiber with a tensile strength of 2070 MPa and tensile strain of 0.75%, the tensile strain energy density is only 9"0 MPa. This concept of fiber toughness suggests that a great amount of strain energy can be absorbed in a composite provided that the majority of the PE fibers are permitted to extend to their full deformation capacity. Regular graphite fibers will not be as effective in absorbing strain energy because of their own limited failure strains. Nylon fibers will not be effective either, simply because of their low modulus and strength and the constraints imposed by the limited maximum composite strain. These fibers were not given the opportunity to extend to their full capacity during impact loading. A large tensile strain of fibers will not necessarily translate to a high fiber toughness in composite applications, unless the strength (and modulus) of the fiber is high and the composite strain does not become a limiting factor. Further, the fiber strength and strain values commonly used to estimate the fiber toughness were measured by tensile tests, which were usually conducted at relatively low speeds compared to the impact tests. As illustrated in Fig. 11, these values cannot be used directly in interpreting the composite impact behavior. Figure 11 shows the results of our attempts to correlate the tensile toughness of the individual fibers with the impact toughness of the corresponding fabric samples for various fiber types.

Impact resistance and energy absorption in hybrid composites

329

Apparently, a high tensile toughness does not necessarily lead to a high fabric impact toughness. Nylon and polyester fibers are represented by the two data points with the highest fiber tensile toughness. These high tensile toughness values are a result of the high fracture strains. However, their fabric products do not exhibit high impact energy. Instrumented impact testing of single-layer fabric specimens, both with and without matrix resin, provided relatively more reliable data. The techniques of high rate impact testing of composites were also discussed in two recent papers, s2's3 The fiber toughness, matrix ductility and interfacial adhesion must all work together to ensure a high composite impact resistance.

4.3 Effects of hybridization Higher residual thermal stresses are expected to be present in interlaminar hybrids than in the corresponding single-fiber laminates because of the differences in fiber thermal expansion coefficients and elastic constants. These residual stresses are also expected to be maximal near the interface between two different layers and, therefore, will affect delamination. The effect has been experimentally observed in hybrids. Hybridization was also found to change the ability of a lamina to flex or to deform plastically during impact. A vivid example came from the observation that a two-ply PePe/epoxy laminate was severely flexed to form a sharp dome whereas a GrPeGrPeGr, also containing two layers of Pe/epoxy, was fully penetrated without undergoing any significant plastic deformation in flexure. The latter exhibited a lower maximum load and lower energy absorption level than the former. Both PeGr and GrPe samples showed inferior impact properties to the single-layered Pe/epoxy. Adding a layer of graphite to a Pe lamina significantly reduced the degree of plastic deformation. The same phenomenon was also found in the case of PETbased materials. Stiffplies had the tendency to inhibit their neighboring plies from flexing. In all the two-ply hybrids studied, the laminate exhibited better impact properties when the tougher layer faced the impact direction. However, the behavior of the five-ply hybrids was much more complex. From our data it appears that, as far as impact resistance is concerned, the stacking fabric approach (unsymmetric stacking sequence) is a better configuration than the alternating sequence. This is in agreement with earlier findings. 26 When the glass fabric faced the direction of impact, the laminate was found 26 to absorb a significantly larger amount of impact energy. The authors 26 suggested that this resistance to penetration was achieved by utilizing the high compressive properties of the glass and/or graphite fiber, resulting in more effective initial energy dissipation. The remaining energy was then

330

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

absorbed by the PE fabric by deflection (plastic deformation) without exhibiting through penetration. In our present study, however, two conflicting groups of data were obtained. In one group of five-layered hybrids containing glass fabric (from one source) and PE fabric, generally more energy was dissipated with the glass layers facing the impactor. However, the other hybrids containing glass fabric from a second supplier exhibited the opposite trend. In the case of graphite-PE hybrids, better energy absorbing characteristics were observed when the PE fabric faced the impact direction. In the latter two groups of hybrids (containing 3 or 4 layers of PE), the material at the back face (either glass or graphite) was severely damaged, thereby dissipating a moderate amount of loading energy. Delamination was observed to occur between the PE portion and the glass (or graphite) portion, dissipating additional energy. This delamination might have taken place before or during the deflection phase of the PE layers. Additional delamination, although not visually observed, was expected to occur at other locations of the laminate. The PE fibers were capable of being stretched to a large extent to allow the laminate to deform rather than fracture. In the case of five-layered PET-graphite hybrids, the impact properties were slightly better when the graphite layer faced the impactor. The causes for the existence of such a wide variety of behavior have yet to be determined.

5 CONCLUSIONS The energy absorbing mechanisms of hybrid laminates in response to impact loading were investigated. The following conclusions have been reached: 1. The polyethylene, PET and Nylon fibers, when combined with epoxy resin, have been shown to absorb large amounts of energy prior to failure. These fibers can be used in hybrids to improve the impact resistance of various composite materials. A large impact toughness of the constituent fibers is essential for improving the impact resistance of hybrid laminates. 2. The impact energies of the interlaminated hybrids generally showed a negative hybrid effect, i.e. slightly lower energy dissipation than that predicted by the rule of mixtures. However, the maximum loads often showed a positive synergism. 3. The technique of interlaminate hybridization was found to enhance delamination under impact loading. Delamination was found to be an effective energy absorbing process in hybrids. 4. The impact load versus displacement traces confirm the speculation that indentation of the front surface represents the very first stage of loading. The front surface controls the initial laminate stiffness during impact.

Impact resistance and energy absorption in hybrid composites

331

Perforation induced by indentation was found to be the most important failure mechanism in the brittle laminates studied. The laminates containing tough fibers were capable of resisting perforation by flexing during impact. This process of plastic deformation to form a dome helped to dissipate a major portion of the strain energy. 5. Many different macroscopic failure mechanisms in hybrid laminates can occur during impact loading. Laminates with an alternating stacking sequence usually exhibited a combination of through penetration and delamination, the latter being in the dagger shape and visible from both sides. For unsymmetric hybrid laminates containing two or three layers of PE or PET fabric, the impact energy absorption capability depends on which side faces the impact direction. In general, the unsymmetric hybrids have better impact properties than their alternating sequence counterparts. In most cases, failure in these materials involved perforation, delamination and some tearing of the more brittle layers in conjunction with deflection of the tougher layers, provided the tougher side faced the impact direction. When the more rigid side was struck first, these stiff layers were perforated with a lesser degree of plastic deformation. With some exceptions, this process was followed by through penetration of the tougher layers, leading to an inferior energy absorption capability. 6. The ductility index was generally found to decrease with increasing energy absorbing capability of a laminate, implying that, for tougher composites, more energy will be dissipated to reach the maximum load than afterwards. This observation again suggests the significance of fiber properties in controlling the strength and, therefore, the impact resistance of composites. ACKNOWLEDGEMENTS The financial support for this project was provided by the Army Research Office (grant DAAL03-86-G-0211). PE, Nylon and polyester fibers were supplied by Allied Signal Inc. We are very grateful for this support.

REFERENCES 1. ASTM STP 568, Foreign Object Impact Damage to Composites. American Society for Testing and Materials, Philadelphia, Pennsylvania, 1975. 2. ASTM STP 497, Composite Materials: Testing and Design (2nd Conf.). ASTM, Philadelphia, 1975. 3. ASTM STP 936, Instrumental Impact Testing of Plastics and Composite Materials. ASTM, Philadelphia, 1987.

332

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

4. Wardle, M. W. & Zahr, G. E., Instrumented impact testing of Aramidreinforced composite materials. In Ref. 3, pp. 219 35. 5. Beaumont, P. W. R., Riewald, P. G. & Zweben, C., Methods for improving the impact resistance of composite materials. In Ref. 1, pp. 134-58. 6. Cordell, T. M. & Sjoblom, P. O., Low velocity impact testing of composites. In Proc. Am. Soc.for Composites, First Technical Conf., 7-9 Oct. 1986, Dayton, Ohio, pp. 297-312. 7. Goering, J. C., Impact response of composite laminates. Proc. Am. Soc. for Composites, 1st Tech. Conf., Oct. 1986, Dayton, Ohio, pp. 326-45. 8. Wu, H. T. & SPringer, G. S., Impact damage of composites. Proc. Am. Soc.for Composites, 1st Tech. Conf., Oct. 1986, Dayton, Ohio, pp. 346-51. 9. McQuillen, E. J. & Gause, L. W., Low velocity transverse normal impact of graphite epoxy composite laminates. J. Comp. Mater., 10 (1976) 79-81. 10. Takeda, N., Sierakowski, R. L. & Malvern, L. E., Wave propagation experiments on ballistically impacted composite laminates. J. Comp. Mater., 15 (1981) 157-74. 11. Wardle, M. W., Impact damage tolerance of composites reinforced with Kevlar Aramid fibers. Proc. ICCM-IV on Progress in Science and Engineering of Composites, Tokyo, 1982. 12. Dorey, G., Relationships between the impact resistance and fracture toughness in advanced composite materials. AGARD Conference Proceedings, No. 288, August 1980. 13. Dorey, G., Sidey, G. R. & Hutchings, J., Impact properties of carbon fibre/Kevlar 49 fibre hybrid composites. Composites, 9 (1978) 25-32. 14. Dorey, G., Impact and crashworthiness of composite structures. In Structural Impact and Crashworthiness, ed. G. A. O. Davies, Elsevier Applied Science, 1984, Vol. 1, p. 155. 15. Wardle, M. W. & Tokarsky, E. W., Drop weight impact testing of laminates reinforced with Kevlar Aramid fibers, E-glass, and graphite. Composite Technol. Rev., 5(1) (1983) 4 10. 16. Adams, D. F. & Miller, A. K., An analysis of the impact behavior of hybrid composites. Mater. Sci. and Eng., 19 (1975) 245-60. 17. Adams, D. F., A scanning electron microscopic study of hybrid composite impact response. J. Mater. Sci., 10 (1975) 1591-602. 18. Cantwell, W. J., Curtis, P. T. & Morton, J., An assessment of the impact performance of CFRP reinforced with high-strain carbon fibers. Comp. Sci. and Technol., 25 (1986) 133~48. 19. Bless, S. J., Hartman, D. R. & Hanchak, S. J., Ballistic performance of thick S-2 glass composites. Symp. on Comp. Mater. in Armament AppL, 20-22 August 1985, UDR-TR-85-88A, 1985. 20. Bless, S. J., Hartman, D. R., Okajima, K. & Hanchak, S. J., Ballistic Penetration of S-2 Glass Laminates. Owens-Corning Fiberglass Corp., Tech. Publ., 4-ASP14493, 1987. 21. Cantwell, W. J. & Morton, J., Ballistic perforation of CFRP. In Proc. Con/'. on Impact of Polymeric Materials, Guildford, Surrey, Sept. 1985. 22. Curtis, P. T., An Initial Evaluation of a High-Strain Carbon Fiber Reinforced Epo.~v. Royal Aircraft Establishment, TR 84004, 1984. 23. Bradshaw, J., Dorey, G. & Sidey, R., hnpact Response of Carbon Fiber Reinforced Plastics. Royal Aircraft Establishment, Tech. Report 12240, 1973.

Impact res&tance and energy absorption in hybrid composites

333

24. Walter, R. W., Johnson, R. W., June, R. R. & McCarthy, J. E., Designing for integrity in long-life composite aircraft structures. A S T M STP 636 (1977) 228-47. 25. Zimmerman, R. S. & Adams, D. F., Impact Performance of Various Fiber Reinforced Composites as a Function of Temperature. Technical Publ. of Allied Fibers Technical Center, Petersburg, Virginia, 1987. 26. Cordova, D. S. & Bhatnagar, A., High Performance Hybrid Reinforced Fiber Composites: Optimizing Properties with PE Fibers. Tech. Publ. of Allied Fibers Tech. Center, Petersburg, Virginia, 1987. Also at 32nd SAMPE Conf. and Exhibits, Anaheim, California, 4-9 April 1987. 27. Adams, D. F., Zimmerman, R. S. & Chang, H. W., Properties of a polymermatrix composite incorporating Allied Spectra 900 PE fibers. Proc. Int. S A M P E Symp., Anaheim, California, 19 March 1985. 28. Cordova, D. S., Coffin, D. R., Young, J. A. & Rowan, H. H., Effects of polyester fiber characteristics on the properties of hybrid reinforced thermosetting composites. Proc. Ann. Conf. of Reinforced Plastics~Composites Institute, 30 (1984). 29. Cordova, D. S., Rowan, H. H. & Lin, L. C., Computer modeling of hybrid reinforced thermoset composites. Proc. Ann. Conf. of Reinforced Plastics~Composites Institute, 41 (1986). 30. Cordova, C. W., Rowan, H. H. & Young, J. A., Improved thermoset polyurethanes utilizing Nylon fiber reinforcement. Proc. Ann. Conf. of Reinforced Plastics/Composites Institute, 41 (1986). 31. Broutman, L. J. & Mazor, A., Mechanical properties of PET and Nylon fiber reinforced epoxies. Proc. Ann. Conf. of Reinforced Plastics~Composites Institute, 41 (1986). 32. Cantwell, W. J., An investigation into the impact resistance of CFRP. MSc thesis, Imperial College, London, 1982. 33. Cantwell, W. J., Curtis, P. T. & Morton, J., Impact and subsequent fatigue damage growth in carbon fiber laminates. Int. J. Fatigue, 6 (1984) 113. 34. Cantwell, W. J., Curtis, P. & Morton, J., Post-impact fatigue performance of carbon fiber laminates with non-woven and mixed-woven layers. Composites, 14 (1983) 301. 35. Williams, J. G. & Rhodes, M. D., Effects of resin on impact damage tolerance of graphite/epoxy laminates. A S T M STP 787 (1982) 450. 36. Greszczuk, L. B., Damage in composite materials due to low velocity impact. In Impact Dynamics, ed. J. A. Zukas et al., John Wiley & Sons, New York, 1982, pp. 55-94. 37. Drzal, L. T., Rich, M. J. & Lloyd, P. F., Adhesion of graphite fibers to epoxy matrices. I. The role of fiber surface treatment. J. Adhesion, 16 (1982) 1-30. 38. Theocaris, P. S., The mesophase and its influence on the mechanical behavior of composites. In Charac. of Polymers in the Solid State (I), Adv. in Polymer Sci. Series, No. 66, ed. H. H. Kausch & H. G. Zachmann. Springer-Verlag, New York, 1985, 150-87. 39. Hong, C. S., Suppression of interlaminar stresses of thick composite laminates using sublaminate approach. Adv. Mater. Technol. 87 (SAMPE Symp.), 32 (1987) 558-65. 40. Jang, B. Z. & Chung, W. C., Structure-property relationships in threedimensionally reinforced fibrous composites. In Advanced Composites: Latest Developments. ASM International, 1986, 183-92.

334

B. Z. Jang, L. C. Chen, C. Z. Wang, H. T. Lin, R. H. Zee

41. Ko, F. K., Chu, H. & Ying, E., Damage tolerance of 3-D braided intermingled carbon/PEEK composites. In Ref. 40, pp. 75-88. 42. Krieger, R. B., Jr, An adhesive interleaf to reduce stress concentrations between plys of structural composites. Adv. Mater. Technol. 87 (SAMPE Symp.), 32 (1987) 279-86. 43. Poursartip, A., Riahi, G., Teghtsoonian, E., Chinatambi, N. & Mulford, N., Mechanical properties of PE fiber/carbon fiber hybrid laminates. ICCM- VI and ECCM-II, Vol. 1. Elsevier Applied Science, 1987, pp. 209-20. 44. Aveston, J. & Sillwood, J. M., Synergistic fiber strengthening in hybrid composites. J. Mater. Sci., 11 (1976) 1877-83. 45. Bader, M. G. & Manders, P. W., The strength of hybrid glass/carbon fiber composites. Part I. Failure strain enhancement and failure mode. J. Mater. Sci., 16 (1981) 223345. 46. Bader, M. G. & Manders, P. W., Part II. A statistical model. J. Mater. Sci., 16 (1981) 2246-56. 47. Bunsell, A. R. & Harris, B., Composites, 5 (1974) 157-64. 48. Aveston, J. & Kelly, A., Phil. Trans. Roy. Soc., A294 (1980) 519-34. 49. Xing, J., Hsiao, G. C. & Chou, T. W., A dynamic explanation of hybrid effects. J. Comp. Mater., 15 (1981) 443-61. 50. Harlow, D. G., Statistical properties of hybrid composites. Proc. Roy. Soc., A389 (1983) 67-100. 51. Ishikawa, T. & Chou, T. W., Elastic behavior of woven hybrid composites. J. Comp. Mater., 16 (1982) 2-19. 52. Pitkethly, M. J. & Bader, M. G., Failure modes of hybrid composites consisting of carbon fiber bundles dispersed in a glass fiber epoxy resin matrix. J. Phys. D: Appl. Phys., 20 (1987) 315-22. 53. Kim, R. Y. & Soni, S. R., Suppression of free-edge delamination by hybridization. ICCM- V (1985) 1557-72. 54. Mai, Y. W., Andonian, R. & Cotterell, B., On polypropylene-celluiose fibercement hybrid composites. S A M P E Symp. (1981) 1687-99. 55. Harris, B. & Bunsell, A. R., Impact properties of glass fiber/carbon fiber hybrid composites. Composites, 6 (1975) 197-201. 56. Arrington, M. & Harris, B., Composites, 9 (1978) 149-52. 57. Marom, G., Fischer, S., Tuler, F. R. & Wagner, H., Hybrid effects in composites. J. Mater. Sci., 13 (1978) 1419-26. 58. Parratt, N. J. & Potter, K. D., Mechanical behavior of intimately-mixed hybrid composites. S A M P E Symp. (1981) 313 26. 59. Wagner, H. D. & Marom, G., On composition parameters for hybrid composite materials. Composites, 13 (1982) 18. 60. Wagner, H. D., Elastic properties and fracture of hybrid composite materials. PhD thesis, Hebrew Univ. of Jerusalem, 1982. 61. Wagner, H. D. & Marom, G., Delamination failure in hybrid composites: effect of stacking sequence. 38th Ann. Conf. RP/Comp. Inst., Soc. of Plastic Industry, 7-11 Feb. 1983, Session 12-E. 62. Adams, D. F. & Zimmerman, R. S., Static and impact performance of PE fiber/graphite fiber hybrid composites. S A M P E Journal, Nov./Dec. (1986) 10-16. 63. Zweben, C., Tensile strength of hybrid composites. J. Mater. Sci., 12 (1977) 1325-37.

Impact resistance and energy absorption in hybrid composites

335

64. Chou, T. W. & Fukuda, H., Stiffness and strength of hybrid composites. Proc. 1st Japan-US Conf. on Composite Materials, Tokyo, 1981. 65. Chou, T. W. & Kelly, A., Mechanical properties of composites. Ann. Rev. Mater. Sci., 10 (1980) 229-59. 66. Bhatia, N. M., Strength and fracture characteristics of graphite-glass intraply hybrid composites. In Composite Materials: Testing and Design (6th Conf.), ASTM STP 787, ed. I. M. Daniel. Am. Soc. for Testing and Materials, 1982, pp. 183-99.

67. Sivakumaran, K. S., On the vibration characteristics of interlaminated hybrid composite plates. ICCM- V (1985) 1703-10. 68. Chamis, C. C. & Lark, R. F., Hybrid Composites--State-of-the-Art Review: Analysis, Design, AppL and Fabrication, NASA TMX-73545, 1977. 69. Chamis, C. C. & Sinclair, J. H., Prediction of Properties of Intraply Hybrid Composites, NASA TM 79087, 1979. 70. Fukuda, H. & Chou, T. W., A statistical approach to the strength of hybrid composites. Proc. ICCM IV (Tokyo) (1982) 821. 71. Fukuda, H. & Chou, T. W., Stress concentrations around a discontinuous fiber in a hybrid composite sheet. Trans. Japan Soc. Comp. Mater., 7 (1981) 37-42. 72. Fukunaga, H., Chou, T. W. & Fukuda, H., Strength of intermingled hybrid composites. J. Reinforced Plastics and Composites, 3 (1984) 145. 73. Ji, X., On the hybrid effect and fracture mode of interlaminated hybrid composites. Progress in Science and Engineering of Composites (Proc. ICCM IV) (1982) 1137-44. 74. Chamis, C. C., Hanson, M. P. & Serafini, T. T. In Composite Materials: Testing and Design (2nd Conf.), ASTM STP 497. Am. Soc. for Testing and Materials, 1972, pp. 324-49. 75. Beaumont, P. W. R., Riewald, P. G. & Zweben, C., Methods for improving the impact resistance of composite materials. In Foreign Object Impact Damage to Composites, ASTM STP 568. Am. Soc. for Testing and Mater., 1974, pp. 134-58. 76. Dorey, G., Sidey, G. R. & Hutching, J., Impact properties of carbon fiber/Kevlar 49 fiber hybrid composites. Composites, 9 (1978) 25-32. 77. Dorey, G., The use of hybrid to improve the composite reliability. Proc. Technical Symposium III on Design and Use of KEVLA R A ramid in Aircraft. Du Pont, Oct. 1981. 78. McColl, I. R. & Morley, J. G., Crack growth in hybrid fibrous composites. J. Mater. Sci., 12 (1977) 1165-75. 79. Chou, T. W., Hybrid and textile structural composites: an overview. Proc. Int. Symp. on Comp. Mater. andStruct., June 1986, Beijing, China, ed. T. T. Loo & C. T. Sun, pp. 181-6. 80. Li, J., Mai, K. & Zeng, H., Studies on the mechanical properties and fracture morphology of hybrid carbon/glass reinforced hybrid matrix (PSF/ESF) composites. In Ref. 79, pp. 936-42. 81. Jang, B. Z., Chen, L. C., Hwang, L. R. & Zee, R. H., The response of fibrous composites to impact loading. Polymer Composites (in press). 82. Harding, J. & Welsh, L. M., Impact testing of fiber reinforced composite materials. In Progresses in Science and Engineering of Composites (Proc. ICCM IV, Tokyo), ed. T. Hayashi, K. Kawata & S. Umekawa, 1982, pp. 845-52. 83. Kawata, K., Hashimoto, S. & Takeda, N., Mechanical behavior in high velocity tension of composites. In Ref. 82, pp. 829-36.