PEEK composite materials

Composites Science and Technology 29 (1987) l-15

Rate Sensitivity of Mode II Interlaminar Fracture Toughness in Graphite/Epoxy and Graphite/PEEK Composite Materials A. J. Smiley and R. B. Pipes Center for Composite Materials, University of Delaware, Newark, DE 19716 (USA) (Received 28 August 1986; accepted 13 November 1986)

S UMMA R Y This paper presents results from an experimental study of the effect of rate on the Mode H interlaminar fracture toughness, G , o in graphite~PEEK ( APC2) and graphite/epoxy (AS4/3501-6) laminates. The end notched flexure test geometry was employed for Mode '11 experiments which were performed at room temperature over a range of crosshead speeds from 4"2 x 10-6 to 9"2 x 10- 2 m s - z. The A PC-2 material exhibited ductile crack growth at low rates and brittle crack growth at high rates. The change in fracture mechanism resulted in a decrease in G uc f r o m 1"9 to 0"40 k J m -2 at the upper range o f test rates. The A S 4 / 3 5 0 1 - 6 material exhibited brittle crack growth at all rates. The Gnc values also decreasedat higher test rates from 0"46 to O'06kJm -2"

INTRODUCTION Continuous fiber composites have in the past been utilized mainly for weight savings in secondary structures. Recent advances in composites have led to the development of material systems possessing strength and stiffness properties commensurate with the requirements of primary load-bearing structures. One limitation of many composite systems is their inability to resist defect initiation and propagation when compared to metallic systems. The ability to resist defect propagation is characterized by a materials fracture toughness which is a material property dependent upon several factors. In polymer composite materials these factors include: constituent 1 Composites Science and Technology 0266-3538/87/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

2

A.J. Srniley, R. B. Pipes

and interfacial properties of the fiber and the resin, type of loading (i.e. tensile, shear, tearing), environmental conditions (i.e. temperature, moisture), and loading rate. In order fully to understand the fracture behavior of composite systems, the effect of these factors must be characterized and quantified. Several papers have appeared in the literature on the double cantilever beam test for studying the opening mode (Mode I) of interlaminar crack growth in composite materials. ~-3 The rate effects on G~c are discussed in papers by Daniel and Aliyu, 4 Smiley and Pipes s and Berglund. 6 The use of the end notched flexure (ENF) test to study the shear mode (Mode II) of interlaminar crack growth in composites was first proposed by Russell. 7 Murri and O'Brien and Carlsson et aL 9 have utilized the ENF test geometry to examine the effects of specimen preparation and data reduction methodology on the Mode II interlaminar fracture toughness, G,c. Russell and Street 1°'~1 also employed thg E N F test geometry to investigate the influence of moisture and temperature on G . c . The interlaminar fracture toughness of composite materials is governed by the deformation behavior of the polymer matrix. Hunston et al. ~2 presented results which show a correlation between the Mode I toughness of unreinforced polymers and the Mode I interlaminar toughness of the associated composite laminates. Data published by Kinloch e t aL 13 and Friedrich and Karger-Koesis 14 illustrate that the fracture behavior of expoxy and PEEK polymeric resins exhibit significant dependence on loading rate and temperature. Consequently, one would expect to observe rate effects on the fracture behavior of these polymer-based composite systems. The objective of this work was to determine the rate sensitivity of Mode II interlaminar fracture toughness in continuous fiber composite laminates. The central focus was on a series of ENF tests performed on graphite fiber/ epoxy and graphite fiber/PEEK composite laminates. The rate effects were investigated through careful examination of the crack growth behavior, the fracture toughness values and the microscopic polymer deformation of the E N F specimens over a large range of test rates.

EXPERIMENTAL Materials The graphite fiber reinforced PEEK system utilized was the APC-2 material produced by Imperial Chemical Industries. The fiber volume fraction of the APC-2 material was 0.62. APC-2 is a reinforced thermoplastic polymer and requires a relatively high processing temperature (380°C) and a rapid

Mode H interlaminarfracture toughness

3

cooling rate ( > 10°C min- ~). This was achieved in a compression press fitted with internally cooled platens. An air-water coolant was used to produce an adequate cooling rate of about 40°C min-1. The graphite fiber reinforced epoxy system employed was the AS4/3501-6 system produced by Hercules Co. The prepreg material was processed in an autoclave using the supplierrecommended cure cycle to obtain a fiber volume fraction of 0"66. All material processing was performed in-house at the University of Delaware.

Specimen geometry The E N F test specimen geometry is shown in Fig. 1. It consists of a unidirectional composite beam with a mid-plane crack at one end. The crack length to span ratio, alL, employed was 0-5. During testing a load, P, is introduced at the beam midpoint with the resulting deflection being denoted by 6. By monitoring the beam load-displacement response to fracture, the Mode II interlaminar fracture toughness can be determined. The ENF specimens 'were cut with a diamond-impregnated sawblade from unidirectional panels. The specimen dimensions were 25 x 100mm. The APC-2 specimens averaged 3.3 mm (26 plies) in thickness and the AS4/ 3501-6 specimens averaged 3.0 mm (24 plies) in thickness. The ENF test specimen requires a mid-plane starter crack. This was produced by placing a double layer of 0.025-mm Kapton polyimide film at the panel midplane prior to processing. This leaves a resin-rich region which blunts the crack tip and yields unreasonably high Gnc results. 9 Thus the specimen was precracked before testing. The precracking was performed by pushing a wedged razor blade into a clamped E N F specimen as shown in Fig. 2.

Test conditions The ENF tests were performed at room temperature at crosshead speeds ranging from 4.2 x 10 - 6 to 9"2 x 1 0 - 2 m s - 1 . Each beam was tested at a

E! h Y(X)~----"~C~./~D' -ff~ / ~ ' T P / 2

Fig. 1. End notched flexture test specimen geometry.

4

A.J. Smiley, R. B. Pipes

Fig. 2. Precracking technique utilized for producing starter cracks in end notched flexure test specimens.

constant crosshead speed with one data point being obtained from each specimen. The tests were performed on two different test frames. An Instron 1125 static test frame was utilized for the low and intermediate test rates ranging from 4.2 x 10 - 6 to 4"2 X 1 0 - 3 m s -~. The high rate tests ranged from 3-2 x 10 -3 to 9.2 x 1 0 - 2 m s -~ and were performed on a hydraulic Instron 1322 dynamic test system configured with a high-rate data acquisition system. The data acquisition system employed a quartz crystal load cell and a digital oscilloscope to capture the high-rate response of the ENF specimens.

DATA R E D U C T I O N The interlaminar fracture toughness is characterized by the strain energy release rate, G.15 G--

d

(F-U)

(1)

w da

where F = work done by external force, U = stored elastic strain energy,

Mode II interlaminar.fracture toughness

5

a = crack length, and w = specimen width. G may also be related to the change in compliance with crack extension: ~5

p2 dC

G = -- -2w da

(2)

where P = applied external force and C = beam compliance.

Beam theory method Russell ~ derived an expression for GI[C based on beam theory. That analysis was extended by Carlsson et al. 16 to include shear deformation. The beam compliance of the ENF specimen is given by C-~-

(2L 3 + 3a 3) 2(l'2L+O'9a)h2El"~ ~-~-5 1 + (2La+3a3)G~ 3 ]

(3)

where E 1 =flexural modulus, G13=interlaminar shear modulus, and h = beam half thickness. It follows that, dC 9aZ ( 0.2( El "](h~2~ daa - 8 E ~ h 3 1 + \G13,]\a,/ ,I

(4)

Substitution of eqn (4) into eqn (2) yields the beam theory expression for GIIC

_ GHc

9a2p 2 ( a + 0 2 ( E ~ ( h ~ 2 ~ 16Exw2h3 " \G--~3J\a,/ /

(5)

The critical load and the geometric and material properties of each specimen are utilized to compute the Mode II interlaminar fracture toughness.

Finite element/beam theory method Finite element analysis of the ENF specimen by Gillespie et al.1 ~ indicates that data reduction schemes based upon beam theory underestimate GH by 20-40% depending on the material and specimen geometry. An improved data reduction scheme was suggested 17 to incorporate a finite element correction to the beam theory expression:

9aZP~ ( ( E l ) ( h ) 2) GnF~-16Elw2h 3 ~ + fl ~ a

(6)

The constants ~ and fl are parameters fitted to the numerical results. They are dependent upon the specimen half span, L, and the specimen thickness, 2h. The values of ~ and fl and the values of the material properties utilized in

6

A.J. Smiley, R. B. Pipes

TABLE 1 Parameters and Material Properties Utilized in the Finite Element Data Reduction Scheme for Mode II interlaminar-fracture toughness, Gnc

Material

ct

fl

E1 GPa

G13 GPa

APC-2 AS4/3501-6

0"969 0'969

2"615 2"615

115 128

4-48 6"41

the current study are included in Table 1. All other variables required by eqn (6) are measured from each specimen. Kinetic energy effects

Kinetic energy m a y be an energy-absorbing mechanism in high-rate E N F testing and should be included in the energy balance. The definition of Gnc including kinetic energy is 15 d GHc - w da ( F - U - T)

(7)

where T is the total kinetic energy of the E N F specimen. The kinetic energy of an E N F specimen was analyzed by Smiley 18 who found that the kinetic energy contribution to Gnc for an E N F specimen geometry of alL = 0"5 is dT wd~ --

0"078ph32

(8)

where p is the density of the material and 6 is the crosshead speed of the test machine. Combining eqns (6) and (8) with eqn (7) yields an expression for Gnc which accounts for the kinetic energy contribution: Gi,c

9a2p2 ( fl ( El ) ( h ) 2 ) - 16E1w2h3 o~+ \ G 1 3 J \ a ] / + 0-078ph~$ 2

(9)

Shear displacement rate

The rate sensitivity of the M o d e II interlaminar fracture toughness is characterized by determining GII c a s a function of shear displacement rate. The shear displacement rate in the region of the crack tip, ~ct, is defined as the relative sliding velocity of the crack surfaces at an arbitrarily small distance, e, from the crack tip. Consider the points p and p' on the crack surfaces shown in Fig. 3. Prior to loading the p o i n t s p a n d p ' are coincident at a distance e from the crack tip. During loading the upper crack surface goes

Mode H interlaminar fracture toughness

7

p~

-p

Before Loading

During Loading

Fig. 3. Relative sliding between points on upper and lower crack surfaces during Mode II loading. into tension and p moves away from the crack tip. Concurrently, the lower crack surface goes into compression and p' moves towards the crack tip. The relative sliding distance, u, between p and p' can be determined from Timoshenko's beam theory: 19

246hae u = (2L3 + 3a3)

(10)

Thus the shear displacement rate in the crack tip region is simply 24~ hae tJ.ct -- (2L3 + 3a3)

(11)

F o r the data presented in this paper, e was chosen as two-ply thicknesses, e = 0-25 mm. RESULTS AND DISCUSSION

Load-displacement response The E N F specimen behavior was examined over a large range of crosshead speeds. It was shown 16 that for an E N F beam geometry o f alL = 0"5, the

8

A.J. Smile),, R. B. Pipes

Load, P

Load, P

Displacement, ~

Displacement, ~5

(a)

(b)

Load, P

AS4/3501-6 all speeds Displacement,

(c) Fig. 4.

Mode II load-displacement response: (a) low rate--APC-2 only; (b) intermediate rate--APC-2 only; (c) high rate--APC-2 (all rates--AS4/3501-6).

crack driving force increases with crack length (dG/da > 0). Therefore the crack should grow unstably. This is illustrated in Fig. 4 which shows typical load-displacement responses observed during testing. For the carbon fiber reinforced PEEK material (APC-2), critical crack growth occurred in an unstable fashion. However, at slow crosshead speeds the APC-2 material exhibited subcritical crack growth which produced nonlinear load-displacement response prior to critical crack growth (Fig. 4a). The extent of non-linearity decreases with increasing rate (Fig. 4b). Linear elastic load-displacement response was observed at crosshead speeds > 3.2 × 10- 3 m s - 1 as illustrated in Fig. 4c. Carlsson et al. 9 observed similar subcritical crack growth in APC-2 Mode II specimens. They hypothesized that the non-linear response was due to a combination of

Mode H interlaminar fracture toughness

9

inelastic material behavior (viscoelastic effects and matrix yielding) and stable crack extension. At all rates the critical load was taken at the onset of fast fracture leading to a load drop. The load-displacement response of the carbon fiber reinforced epoxy (AS4/3501-6) exhibited no subcritical crack growth or non-linear behavior at any rate. All crack growth occurred in an unstable fashion as illustrated in Fig. 4c. The critical loads were taken at the onset of crack growth which corresponded to the maximum load on the curve. Fracture

toughness,

G.c

Gnc was computed using the data reduction scheme discussed earlier (eqn (9)). The kinetic energy contribution was found to be negligible over the current range of rates (<0.01%). The data reduction scheme required knowledge of the specimen geometry and material properties as well as the experimental determination of the critical load, Pc- At the low rates this value was taken directly from the load-displacement curve. However, at the high rates (~ > 4.2 x 1 0 - a m s-1) the response of the load instrumentation was too slow to accurately yield the critical load because of the very short duration of the test. Therefore by assuming linear load-displacement response, the critical load was computed from the measured critical displacement, 6 c, and the specimen compliance, C, calculated from eqn (3): (12)

Pc = 6c/C

The toughness was determined for the onset of critical crack growth and therefore is a measure of the initiation fracture energy. The results in Fig. 5 show that APC-2 is tougher than AS4/3501-6 over the range of rates tested. This result was to be expected since APC-2 employs a tough thermoplastic resin (PEEK) for its matrix material while AS4/3501-6 employs a brittle

10.00

'E 2 (.'.'3

APC-2 --*- A S 4 / 3 5 0 1 - 6

100.

.

.

.

- -t- . . . .

H ....

L \

E

b

0.10

o 3 u 0.01 LL

Fig. 5.

1-0'E-8 1'0'E-7 1.C)E-6 10E-5 Shear displacement rate, Oct (ms -1)

Rate sensitivity of Mode II interlaminar fracture toughness, GHc vs uct (log-log).

A. J. Smiley, R. B. Pipes

10 lOOO 'E

"~APC-2 4-AS413501-6

v

lOO. ,.a

=~ olo9 o.o~

1.01z-5

3.0'E-5

5.6E-5

7.6E-5

2-0E-5 40E-5 6-0E-5 Shear displacement rate, Oct (ms -1)

Fig. 6. Rate sensitivityof Mode II interlaminar fracture toughness, GIIC vs Uct(linear-log). epoxy. The results also indicate that both material systems are rate-sensitive in Mode II. Although G.c for APC-2 and AS4/3501-6 remains fairly constant (1.9 and 0.46 kJ m - 2, respectively) for several decades of rate, the toughness begins to decrease with rate at the relatively high rates. The highrate fracture toughness is best illustrated by displaying the data along a linear abscissa as in Fig. 6. Here it can be seen that for APC-2, G~Ic decreases monotonically from 1.9 kJ m - 2 to 0"40 kJ m - 2, while for AS4/3501-6, GHc decreases from an initial value of 0.46kJm -2 to a plateau around 0"06kJm -2. The plateau suggests that a minimum toughness has been reached. Higher rate testing would be required to verify that conclusion.

Fractography Fracture surface examinations were performed with a scanning electron microscope (SEM) to determine the Mode II fracture mechanisms and the nature of material deformation at the initiation sites of the Mode II crack growth. The APC-2 material exhibited two distinct types of fracture behavior. The first was a slow ductile tearing type of crack growth which produced large amounts of plastic deformation as shown in Fig. 7. This type of deformation occurred in specimens tested at low crosshead speeds. The extensive polymer deformation could be responsible for the non-linear loaddisplacement response discussed earlier. The second type of fracture behavior exhibited by APC-2, illustrated in Fig. 8, was a brittle type of crack growth producing much less plastic deformation. This brittle fracture behavior appeared in specimens tested at high crosshead speeds where crack growth occurred in a rapid unstable fashion. The fracture behavior of the AS4/3501-6 material, shown in Fig. 9, did not appear to change appreciably with rate. It is observed that the polymer deformation differs significantly from that observed in the APC-2. That is because the highly cross-linked

Mode II interlaminar fracture toughness

Fig. 7.

Mode II ductile fracture behavior in APC-2.

Fig. 8.

Mode II brittle fracture behavior in APC-2.

11

12

A. J. Smiley, R. B. Pipes

Fig. 9.

Mode II brittle fracture behavior in AS4/3501-6.

epoxy resin does not plastically deform to the same extent as the thermoplastic PEEK resin. As shown, the Mode II deformation behavior in the epoxy is brittle in nature, leaving hackles of resin on the fracture surface.

SUMMARY AND CONCLUSIONS Mode II interlaminar fracture tests were performed on APC-2 and AS4/ 3501-6 composite specimens at room temperature over a large range of crosshead speeds. The rate effects were ascertained through careful examination of each specimen: load-displacement response, fracture toughness, G~xc, and corresponding microscopic deformation behavior as illustrated by the fracture surfaces in the SEM. The APC-2 material exhibited a rate-dependent non-linear loaddisplacement response as a result of subcritical crack growth prior to crack propagation. The extent of non-linearity decreased with increasing crosshead speed and was virtually non-existent at speeds>3.2 x 1 0 - 4 m s -1. The SEM studies showed that at low rates the polymer deformation behavior was ductile in nature with extensive plastic deformation. At the high rates, where no subcritical crack growth occurred

Mode H interlaminar fracture toughness

!3

and linear elastic behavior was observed, the microscopic polymer deformation behavior was brittle in nature. The Mode II interlaminar fracture toughness for APC-2 remained constant at approximately 1.9 k J m -2 up to a shear displacement rate of 2.7 x 10-3 m m s - l . The toughness then decreased monotonically with rate to 0.40kJrn --2 at 7"8 × 10-2 m m s-1. The decrease in Gnc appears to be due to a decrease in the development of plastic deformation during loading. The AS4/3501-6 material exhibited no significant variation in crack growth or material deformation behavior over the range of rates considered. The GHc values showed rate-dependent behavior at shear displacement rates > 2-7 x 10-3 m m s- 1 where the interlaminar fracture toughness decreased from 0 . 4 6 k J m -2 to a plateau of 0.06kJm -2. The plateau suggests that a minimum toughness has been obtained, however, higher rate testing would be required to verify this conclusion. The fractographic results illustrated that the polymer deformation behavior was essentially brittle in nature showing very little effect from rate. A more extensive fractographic analysis is being performed which may identify the subtle changes in the polymer deformation behavior responsible for the decrease in G~ c at the high rates. The results of this study should be useful in the understanding of the fracture mechanisms and damage accumulation of composite materials during impact loading. Future work should focus on the characterization of combined rate and temperature effects. Fracture experiments performed over a similar range of rates and a large range of temperatures should produce a broader spectrum of fracture behavior. A more in-depth fractographic analysis could be utilized to obtain a better understanding of the energy-absorbing mechanisms and how they relate to the interlaminar fracture toughness. ACKNOWLEDGEMENTS The authors would like to thank Imperial Chemical Industries for their personal and financial support of this work. The authors would also like to thank the professional and clerical staff at the Center for Composite Materials, in particular, Jack Gillespie, Leif Carlsson, and Bruce Trethewey. REFERENCES 1. J. M. Whitney, C. E. Browning and W. Hoogsteden, A double cantilever beam test for characterizing Mode I delamination of composite materials, J. Reinfi Plast. Compos., 1 (1982) pp. 297-330.

14

A. J. Smile),, R. B. Pipes

2. D.J. Wilkins, J. R. Eisenmann, R. A. Camin, W. S. Margolis and R. A. Benson, Characterizing delamination growth in graphite/epoxy, in: Damage in Composite Materials, ASTM STP 775, American Society for Testing and Materials, Philadelphia, 1982, p. 168. 3. W. D. Bascom, G. W. Bullman, D. C. Hunston and R. M. Jensen, The width tapered DCB for interlaminar fracture testing, Proc. 29th National SAMPE Symposium, 1982. 4. I. M. Daniel and A. A. Aliyu, Effects of strain rate on delamination fracture toughness of graphite/epoxy, in: Delamination and Debonding of Materials, ASTM STP 876, American Society for Testing and Materials, Philadelphia, 1985, p. 336. 5. A. J. Smiley and R. B. Pipes, Rate effects on Mode I interlaminar fracture toughness in composite materials, J. Compos. Mater., in press. 6. L. Berglund, Rate effects on the interlaminar toughness of PEEK/carbon fiber composites, Licentiate Thesis No. 54, Linkoping University, Sweden (1985). 7. A. J. Russell, On the measurement of Mode II interlaminar fracture energies, Defence Research Establishment Pacific, Victoria, BC, Materials Report 82-0, December 1982. 8. G. B. Murri and T. K. O'Brien, Interlaminar Gl~c evaluation of toughenedresin matrix composites using the end-notched flexure test, presented at the 26th AIAA/ASME/ASCE/AHS Structures, Structural Dynamics, and Materials Conference, held in Orlando, Florida, 15-17 April, 1985. 9. L. A. Carlsson, J. W. Gillespie and B. Trethewey, Mode II interlaminar fracture toughness in graphite/epoxy and graphite/PEEK composites, J. Reins Plast. Compos., 5 (1986) pp. 170-87. 10. A.J. Russell and K. N. Street, Moisture and temperature effects on the mixedmode delamination fracture of unidirectional graphite/epoxy, Delamination and Debonding of Materials, ASTM STP 876, American Society for Testing and Materials, Philadelphia, 1985, p. 349. I 1. A.J. Russell and K. N. Street, Factors affecting the interlaminar fracture energy of graphite/epoxy laminates, in: Progress in Science and Engineering of Composites (eds T. Hayashi, K. Kawata and S. Umekawa), ICCM-IV, Tokyo, 1982, p. 279-86. 12. D. L. Hunston, W. D. Bascom, R. J. Moulton and N. J. Johnston, Matrix resin effects in composite delamination: Mode I fracture aspects, presented at the ASTM Conference Toughened Composites, held in Houston, Texas, 13-15 March 1985. 13. A. J. Kinioch, S. J. Shaw, D. A. Tod and D. L. Hunston, Deformation and fracture behavior of a rubber toughened epoxy. I. Microstructure and fracture studies, Polymer, 24 (1983) pp. 1341-54. 14. K. Friedrich and J. Karger-Kocsis, Temperature and strain-rate effects on the fracture toughness of poly (ether ether ketone) and its short glass glass-fibre composite, Polymer and Composite Group, Technical University, HarburgHamburg, 2100 Hamburg 90, FRG (1986). 15. D. Broek, Elementary Engineering Fracture Mechanics, Martinus Nijhoff Publishers, The Hague, 1982. 16. L.A. Carlsson, J. W. Gillespie and R. B. Pipes, On the analysis and design of the end notched flexure (ENF) specimen for Mode II testing, J. Compos. Mater. 17. J.W. Gillespie, L. A. Carisson and R. B. Pipes, Finite element analysis of the end

Mode H interlaminarfracture toughness

15

notched flexure specimen for measuring Mode II fracture toughness, Compos. Sci. Technol., 27 (1986) p. 177-97. 18. A. J. Smiley, Rate sensitivity of interlaminar fracture toughness in composite materials, MMAE Thesis, University of Delaware, Newark, 1986. 19. S. P. Timoshenko and J. N. Goodier, Theory of Elasticity, McGraw-Hill Inc., New York, 1970.