High-Temperature Tensile Properties of Graphite Fiber-Phthalonitrile Composites

H.N. Jones
Materials Science and Technology Division

T.M. Keller
Chemistry Division

The phthalonitrile resin system developed at the Naval Research Laboratory has the ability to survive elevated temperature exposures up to 371°C (700°F) for extended periods of time. This is in contrast to the epoxy-based systems currently used in Naval aircraft that, depending on the moisture content of the resin, lose their rigidity or shear strength as temperatures approach 150°C (302°F). For the short exposures times typical of missile applications, high-temperature tension tests have recently demonstrated that graphite fiber composites fabricated using the phthalonitrile resin system can retain their tensile strength to temperatures approaching 538°C (1000°F). Epoxy-based graphite fiber-reinforced composites, by comparison, start to lose their tensile strength rapidly at temperatures around 260°C (500°F) for similar short time exposures. The tests demonstrate that the phthalonitrile resin system has great potential for missile structural applications where the time at temperature is on the order of a few minutes.

CONTEXT

Polymeric, graphite fiber-reinforced composites consist of many small-diameter, continuous, graphite fibers embedded in a plastic resin. They have become the dominant material in many of today's highperformance missile and aircraft structures. This is because no other practical or affordable structural material can approach the specific stiffness and strength properties that can be attained with this composite system. The primary factor limiting structural performance in many applications, however, is the temperature capability of the matrix resin. While the absolute strength of the matrix material is low compared to that of the reinforcing fibers, its role in the structural performance of the composite is just as important. The mechanical behavior of a polymer matrix composite material is, in large part, determined by the strength of the interfacial bond between the reinforcing fiber and the polymer. The matrix also provides the load transfer paths between the fibers and around the weak links in them, thus allowing the material to take advantage of the strength and stiffness of the reinforcing fibers. It must be able to provide this function over a range of temperatures and not degrade significantly from exposure to moisture or other fluids.

The most widely used polymeric matrix for composites used in missile and aircraft structures are the epoxy resins that normally cannot be used at temperatures above 200°C (392°F). An additional problem with the epoxy resin systems is the environmental effect of moisture content on the glass transition temperature, i.e., the temperature at which the mechanical properties of the polymer changes from a stiff, glassy solid to a low stiffness, rubbery material. This phenomenon affects the performance of composites using this resin system within the 125-200°C (257-392°F) temperature range. As the moisture content of a graphite-epoxy composite increases, its compressive strength decreases rapidly as temperatures approach 150°C (302°F).

The performance limitations of epoxy resins outlined above have led to intensive research over the years for polymers with better elevated temperature properties and environmental resistance. The polyimide resins, such as PMR-15, have become the most widely used high-temperature matrix material, with an upper temperature limit of 316°C (600°F). Bismaleimide resins (BMI) are another important class, with useful mechanical properties being retained to temperatures approaching 204°C (400°F). While these resin systems offer improved elevated temperature performance, this advantage is often gained at the expense of other properties such as toughness, toxicity of the resin precursors, or resistance to moisture- induced degradation.

At the Naval Research Laboratory (NRL), a new class of high-temperature polymers based on the phthalonitrile system has been developed that has attractive properties for composites.¹ The fully cured resin exhibits good thermal and oxidative stability, and possesses useful long-term mechanical properties up to 371°C (700°F). More significantly, there is no indication of a glass transition or softening up to 500°C (932°F). The uncured resin has a low melt viscosity that allows it to be used in the economical resin transfer molding manufacturing process. Unlike most polymers, it does not emit toxic fumes, nor does it sustain combustion when exposed to flame, which qualifies it for use inside submarines. The maximum temperature limit at which polymer matrix composites are normally considered for use is around 316°C (600°F). However, there are many applications, such as missile structures, where the temperatures encountered are much higher but exist for only a few minutes. An example of this situation is found in some components of medium-range tactical missiles that may experience maximum surface temperatures from aerodynamic heating of from 399-538°C (750-1000°F) for a period of time approaching 70 s.

Our primary objective for this research was to characterize the tensile strength of graphitephthalonitrile unidirectional composites at temperatures well above those explored before, i.e., greater than 316°C (600°F), using short exposure times relevant to missile structural applications.² For a direct comparison, we also performed similar measurements on another composite system, graphite-epoxy, which is widely used in missile and aircraft structures. The failure modes of the test specimens were carefully studied to determine the type of resin degradation involved in the loss in tensile strength with increasing temperature. Additionally, we conducted other experiments, such as the short beam shear test and the sustained load tension test, to further evaluate the high-temperature mechanical behavior of composites using this resin system.

EXPERIMENTAL EQUIPMENT AND METHODS

Measurement of the mechanical properties of polymer matrix composites at elevated temperature presents a number of experimental challenges. Because these materials are often strongly anisotropic in their mechanical properties, the test methods typically used for isotropic metallic materials are not always applicable. Additionally, due to the high-temperature capability of the phthalonitirile resin system, unusual difficulties are encountered in characterizing the elevated temperature tensile strength of composites using it as the matrix.

The primary challenge encountered with high temperature tension testing of polymer matrix composites, beyond that of heating to the test temperature, is that of applying force to the test specimen without damaging the gripped ends. This normally requires a protective layer of material known as a "grip tab," which is applied to the ends to protect the composite from damage by the serrated grip wedges. Problems encountered include crushing inside the grips and grip tab slippage. The first leads to artificially low strength values, and the second prevents sufficient force being applied to the test specimen to measure its strength. If the entire specimen is held at test temperatures approaching 371°C (700°F), such as would normally occur inside standard environmental chambers, most grip tab adhesives will soften and act as a lubricant. This will cause the tabs to slip off before failure of the composite can be induced.

High-Temperature Composite Tension Test System

To overcome these problems, a high-temperature polymer matrix composite tension test system has been developed at NRL (Fig. 1). This system consists of a single-zone furnace with a 152-mm long hot zone capable of achieving temperatures up to 1093°C (2000°F). This furnace is mounted in a 500 kN (55 kip) capacity servohydraulic load frame. In the configuration shown, 100 kN (22 kip) side entry hydraulic grips are used for loading the specimen. Both static and dynamic (fatigue) tests can be performed in this system. The key to successful elevated temperature tension tests of polymer matrix composites is to keep the gripped portion of the test specimen outside of the hot zone of the furnace.

FIGURE 1
High-temperature polymer matrix composite tension test system developed at NRL.The system consists of a single- zone furnace mounted in a 500 kN capacity servohydraulic load frame.For applying force to the test specimen,100 kN capacity side entry hydraulic grips are used.

Single-zone furnaces will develop a significant top to bottom temperature difference unless some means for stirring the furnace atmosphere is provided. To eliminate this temperature stratification, an air jet nozzle is placed at the top of the furnace. A small amount of air is then injected into the furnace through the nozzle to generate turbulence and produce a more even temperature distribution. The furnace also has a cutout on the side for a quartz rod extensometer of the side entry type, as also shown on Fig. 1. This instrument enables measurement of the axial strains in the composite as it is loaded to failure. The furnace can be opened immediately after test specimen failure so that it cools quickly. This feature allows the failure mode of the specimen to be preserved for subsequent examination. Along with the test specimen, a small sample of the composite material, known as a "traveler coupon," is also placed in the furnace. This material, which is not subjected to the destructive forces of the tension test, records the weight loss and other changes in the composite resulting from thermal history alone.

Elevated Temperature Short Beam Shear Test

To gain further insight into the elevated temperature behavior of the phthalonitrile resin when incorporated into a composite, additional tests were developed. One of these is the elevated temperature short beam shear test in which a short sample of the material is loaded in three point bending, as shown in Fig. 2. This experimental method provides a qualitative measure of the variation of the interlaminar shear strength with temperature and has become an important tool for investigating the behavior of a resin system when incorporated into a composite.

FIGURE 2
Elevated temperature short beam shear test for composite materials. This test evaluates the interlaminar shear strength of a composite and has become an important tool for studying the effects of cure cycle variations and environmental exposure on composite mechanical behavior.

Sustained Load Tension Test

One of the standard tests for qualifying a polymer matrix composite material for aircraft engine applications is the sustained load tension test. This test subjects the material to loads between 70 and 90% of the room temperature ultimate strength at temperatures between 288°C (550°F) and 343°C (650°F). These conditions emulate the service environment encountered in some applications where the component must be able to survive long hightemperature exposure times under load without distortion or failure. The material is stressed and held at an elevated temperature until failure occurs. Materials that can survive these conditions for more than 100 h are promising candidates for engine components. A 53 kN capacity lever arm creep testing machine (Fig. 3) was modified for conducting the sustained load tension test on polymer matrix composites. This required the development of special equipment and a furnace (Fig. 4). A sensitive displacement transducer was used to record the deformation of the test specimen as a function of time. This kind of test is one of the most difficult to conduct on composite materials, as it requires near perfect alignment of the load train.

Fig3 Image

FIGURE 3
Lever arm creep testing machine with a 53 kN (12,000 lb) force capacity used to conduct sustained load tension tests on phthalonitrile composites.
Fig4 Image

FIGURE 4
Detailed view of the furnace,gripping arrangement, and displacement measuring system used for the sustained load tension tests.

MATERIALS

The graphite-phthalonitrile composite samples were fabricated by the Naval Air Warfare Center at their Composites Technology Laboratory using a high-temperature autoclave. The starting material was a prepreg tape consisting of IM-7 12K graphite fiber impregnated with a phthalonitrile prepolymer containing 2.4 wt% of bis (4-aminophenoxy-4-phenyl) sulfone (BAPS) curing agent. Eight-ply ([08]) unidirectional panels were laid up by hand using this tape and then cured under pressure at 371°C inside the autoclave. The resulting panels were then cut into 2.54-cm wide strips, 40.6 cm in length along the fiber direction. Aluminum grip tabs were applied using an epoxy adhesive. The graphite-epoxy composite samples used for comparison were fabricated by the Air Force in their composites facility at Wright Aeronautical Laboratories. In this case, a ten-ply ([010]) panel was fabricated from AS4/3501-6 prepreg tape using standard processing techniques and then cut into strips for tension tests. The average room temperature ultimate tensile strength of the graphitephthalonitrile composite was 1.5 GPa, while that of the graphite-epoxy was 2 Gpa. This difference in tensile strength was primarily due to the stronger AS4 graphite fiber used in the epoxy composite.

RESULTS

Figure 5 compares the normalized elevated temperature ultimate strengths of graphite-phthalonitrile and graphite-epoxy. This graph uses the room-temperature ultimate strength as a normalization factor to provide a more direct comparison of the elevated temperature behavior of these two composite systems. The graphite-epoxy composite loses nearly half its strength as the temperature approaches 300°C (572°F). In contrast, the graphite-phthalonitrile composite shows no significant knock down in strength until temperatures are above 500°C (932°F). This retention in strength is due to some very unusual properties of the phthalonitrile resin system. The lack of a glass transition behavior indicates that, unlike most polymers, the resin does not soften upon heating. Additionally, the material exhibits a very low outgassing and a high char yield as it is heated. It is these attributes that enable the resin system to retain significant mechanical strength at temperatures where most polymers would degrade completely.

FIGURE 5
Comparison of normalized ultimate strengths of graphite-epoxy and graphitephthalonitrile unidirectional. The ultimate strength of graphite-epoxy, shown in black, drops nearly 50% at 316°C. The graphite-phthalonitrile, shown in red, does not suffer significant degradation until temperatures are above 482°C.

Composite Failure Modes

Studying the failure mode of the tension test specimens can provide additional information on the behavior of the resin when incorporated into a composite. Figure 6 shows samples of the kinds of failure modes that were observed. The classic room-temperature failure mode of a graphite-epoxy composite is shown as A on Fig. 6, where a localized transverse fracture occurs along with some longitudinal splitting. As the temperature is increased to 260°C (500°F), however, the failure mode changes dramatically. This is shown as B on Fig. 6, where the fracture process has become much more diffuse. This very fine delamination is the result of the extensive softening of the epoxy matrix at elevated temperatures. The graphite-phthalonitrile also exhibits a change in failure mode over the temperature range of 25-482°C (77-900°F), as can be seen by comparing C and D on Fig. 6. This again is the result of changes in the mechanical behavior of the phthalonitrile resin system at high temperature. However, the ability to sustain interfiber shear stresses and remain bonded to the graphite fiber does not appear to be significantly degraded, as indicated by the retention in tensile strength shown in Fig. 5.

FIGURE 6
Comparison of the failure modes exhibited by graphite-epoxy and graphite-phthalonitrile unidirectional composites after a tension test. The graphite-epoxy changes from a predominately transverse fracture at 25°C (A) to a very diffuse delamination at 260°C (B), indicating a softening of the epoxy matrix. The graphite-phthalonitrile also exhibits a change in failure mode as the temperature is increased from 25°C (C) to 482°C (D) where the delamination becomes more widespread. This effect is caused by a change in mechanical properties of the matrix resin at elevated temperature.

Traveler Coupon Weight Loss

Measurements of the weight loss of the traveler coupons indicated that as the test temperature approached 538°C (1000°F) the loss of resin on the surface of the composite became significant (Fig. 7). This loss of matrix material, around 7% of the composite mass, exposed the graphite fibers on the surface and effectively lowered the tensile strength.

FIGURE 7
Surface condition of a graphite-phthalonitrile composite after exposure to 538°C for 400 s where the graphite fibers are exposed from a loss of the phthalonitrile resin due to oxidation

Short Beam Shear Test

High-temperature short beam shear tests were also conducted over a similar temperature range on the graphite-phthalonitrile. The apparent interlaminar shear strength decreased with increasing temperature, which does not, however, correlate with the observed tension test results.

Sustained Load Tension Test

The sustained load tension tests were conducted at 316°C (600°F) where a 40 kN (9000 lb) load was placed on the test specimen. This load produced a stress level of 1.44 GPa, which is around 90% of the average room-temperature strength. Figure 8 shows the displacement records for some of the tests. Failure occurred after exposure times of between 443 and 614 h. These results are very promising for aircraft engine applications. (By way of comparison, Titanium 6Al-4V, a metallic alloy used extensively in aircraft engines has only a tensile strength of 0.9 GPa at 316°C (600°F) and cannot match the performance of graphite-phthalonitrile under these conditions.)

FIGURE 8
Time-displacement records for sustained load tension tests on a graphite-phthalonitrile unidirectional composite at 90% ultimate stress and 316°C. There is very little deformation of the composite until after 400 h of exposure. Failure of the material occurred at exposure times of between 443 (A) and 614 (C) h.

SUMMATION

Graphite-phthalonitrile composites can retain their tensile strength to temperatures approaching 538°C (1000°F) for exposure times on the order 400 s. Above this temperature, the rate of resin loss becomes rapid, with combustion of the composite occurring in an oxidizing environment. Graphite-epoxy, a material currently used in missile and aircraft structures, loses its tensile strength rapidly at 260°C (500°F). Sustained load tension tests indicate that graphite-phthalonitrile has attributes very favorable for some aircraft engine applications in which the material must be able to withstand hundred of hours of exposure without failure or significant distortion. The properties observed in this work indicate that, for high-temperature applications in the 371-538°C (700-1000°F) regime involving exposures on the order of a few minutes, phthalonitrile composites have some unique capabilities not found in other polymer matrix composites. Although many more tests need to be conducted on this composite to qualify it for use in aircraft, missile, and ship structures, the hightemperature capability of the phthalonitrile system offers many opportunities for improvement in the performance of these systems.

ACKNOWLEDGMENTS

The authors thank F.E. Arnold of the Naval Air Warfare Center for supplying the tension test specimens and Dawn D. Dominguez of the NRL Chemistry Division for the short beam shear test specimens.

[Sponsored by ONR]

References
¹ T.M. Keller and T.R. Price, "Amine-Cured Bisphenol-Linked Phthalonitrile Resins," J. Macrmol. Sci.-Chem. A18, 931 (1982).
² H.N. Jones, T.M. Keller, and F.E. Arnold, "High Temperature Tensile Properties of Graphite Fiber-Phthalonitrile Composites," Proceedings of the National Space and Missile Materials Symposium, 27 Feb-2 Mar 2000, San Diego, CA, Interceptors for Missile Defense Section, CD-ROM, Anteon Corp., Dayton, Ohio, 2000.