1.0INTRODUCTION
1.1Background

Polymer matrix composites are being used increasingly in a variety of important applications because of their excellent mechanical properties and low weight, combined with advancements in manufacturing technology. In particular, graphite fiber reinforced epoxy composites are playing an important role in the production of high performance vehicles as well as critical aerospace structures and primary structures of commercial and military aircraft. For these applications, the safety and reliability of these materials must be assured. Of particular concern in the aerospace industry is the tendency of some graphite/epoxy composites to become irreversibly damaged when exposed to elevated temperatures. Organic resin systems used in the manufacture of graphite/epoxy composites are susceptible to significant and irreversible thermal oxidative degradation following subjection to temperatures above or within a narrow window of their upper service temperatures. When exposed to temperatures sufficiently high enough to cause resin degradation, these materials experience a drop in the glass transition temperature that effectively lowers the upper service temperature and significantly reduces the room-temperature mechanical strength properties of the composite [1,2]. Below a certain exposure threshold, these composites can appear visually and microscopically to be undamaged but, in fact, have lost a significant percentage ((60%) of their original strength [3]. In addition, embrittlement and cracking of the surface causes a loss in the impact strength of the material. Therefore graphite/epoxy composite structures exposed to overheat conditions can suffer irreversible and catastrophic damage in a very short time.

In aircraft and aerospace applications, graphite/epoxy composites can be exposed to damaging levels of heat as a result of fire or operational service. Typical heat damage can result from fires, lightning strikes, supersonic dashes, jet engine exhaust, heating blankets, or curing ovens/autoclaves. Heat damage can also result from exposure to hot gases from missile efflux or the ground reflected engine efflux from vertical or short-takeoff and landing aircraft. Depending on the application, composite components may be nonuniformly heated to temperatures in excess of recommended maximum values (either short term or prolonged exposure); these components, although not visibly blistered or delaminated, may have been seriously degraded and may be no longer flight worthy. Characterization of this damage is complicated by numerous variables inherent to heat damage, such as the method of heat application, exposure environment, cool-down environment and the thermal history of the material. Degradation mechanisms are also dependent on the chemical structure of the polymer matrix, the composite processing conditions, and any coatings which may be present.

In the case where graphite/epoxy laminates are exposed to excessive levels of thermal energy such that they exhibit heat induced surface blisters and/or internal delaminations, this type of damage may be repaired by removal of the visibly damaged material and the application of a repair patch. However, repair personnel cannot be sure that the material surrounding the damaged site, to which the repair patch will be attached, has not also been severely degraded by exposure to the thermal energy.

The effect of heat damage on graphite/epoxy composites presents a unique challenge for nondestructive evaluation (NDE) in that the damage often results in loss of strength in these materials without producing physical flaws. Also, the damage may result as a combination of thermally cycling the composite above the glass transition of the polymer, and oxidative degradation of the polymer or the polymer-fiber interface [1,2]. Most traditional NDE techniques such as, ultrasonics, radiography, and thermography, are capable of detecting physical anomalies such as cracks and delaminations. However, to be effective for thermal degradation they must be capable of detecting initial heat damage which occurs on a molecular scale.

1.2Scope and Organization

This Report provides a review of the current understanding of heat damage in graphite/epoxy composites and methods for measuring and detecting heat damage. Information was identified and acquired by searching a number of computer databases as well as performing manual library searches and making personal inquiries. Information was sought to determine the principal thermal degradation mechanisms in graphite/epoxy composites; to identify laboratory analytical methods for measuring physical properties affected by heat damage in graphite/epoxy composites; and to evaluate the state-of-the-art for nondestructively characterizing thermal degradation in advanced composites. Considerable information, over the span of a number of years, was found on heat degradation and affected properties; therefore Section 2 provides a comprehensive overview of the characteristics of heat damage including degradation mechanisms, mechanical and material properties affected by heat, and progress in theoretically modeling heat damage. A table is provided in Appendix A which summarizes principal literature citations dealing with graphite/epoxy thermal degradation. In addition to the references cited in the text, additional bibliographic citations are included in Appendix B. Considerably less information was found on the nondestructive characterization and detection of heat damage in composites. A review of the current state-of-the-art of the nondestructive evaluation of heat damage in graphite/epoxy composites is provided in Section 3. Included is reported work documented in the literature as well as recent research and development currently in progress. Conclusions are presented in Section 4, and references are included in Section 5.

4.0CONCLUSIONS

Although considerable information exists on mechanical and material property changes caused by heat exposure, the basic failure mechanisms are still not well understood. Work currently in progress at Wright Laboratories is addressing this issue by developing an extensive test matrix for characterizing failure mechanisms for a variety of heat exposure conditions [35]. There is existing evidence to suggest that the failure mechanisms and property changes associated with moderate temperatures, long time exposure are much different than those associated with high temperature, short time exposure such as would be encountered in a fire. Existing evidence indicates that the damage associated with moderate temperature, long time conditions is much more incipient and measurably degrades the matrix before there is any visual evidence. Thus, NDE techniques such as ultrasonics that might work for detecting high heat damage, such as delaminations, will not necessarily work to detect the more subtle property changes associated with modest temperatures.

Several promising NDE techniques are currently under development, primarily based on thermal, infrared, and radioscopy methods; however, from the standpoint of practical application, the large variety of aircraft configurations, materials, and heat exposure conditions complicates the issue. A more complete understanding of the failure mechanisms associated with different conditions and configurations will help determine the most viable NDE approaches for practical application. The selected method(s) must be easy to perform under field conditions. Also, the equipment must be small, lightweight, inexpensive; easy to use; reliable; and provide repeatable results.

As discussed in Section 3.0, promising results for characterizing mechanical property degradation associated with incipient heat damage at moderate levels of heat exposure have been reported using the chemical NDE methods of Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and Laser Induced Fluorescence (LIF). As has been reported, the combined use of LIF (to identify the existence of damage) and the phenyl DRIFTS component (to provide damage verification, location, and size) appears to be an effective NDE procedure for measurement of residual strength of overheated composite structure. These techniques essentially detect molecular changes of the polymer matrix that result from elevated temperatures. Since these techniques are based on chemical analysis of composite laminate properties, they offer potential for use in cure monitoring, quality control, and material characterization. A drawback to accepted application of these methods is that the concept of employing NDE methods based on chemical surface analysis to predict bulk mechanical properties may be questionable. Reports from the recent Overheated Composites Workshop indicated that for practical application to thick composites, overheated thick laminates require depth of damage information to assess damage and to effect repair [35]. NDI must be able to evaluate the surface area of damage and the depth of damage. To address this issue, the DRIFTS and LIF chemical NDE methods should be demonstrated using thicker composite laminates.

In a report by Navy workers at the Conference on NDE and Characterization of Graphite Epoxy Composites, results were presented of NDE evaluations on graphite/epoxy samples that had been sealed, primed and painted [49]. Heat damage was induced in graphite/epoxy composite coupons by exposing the painted side to a radiant heat source at temperatures ranging from 177 °C to 649 °C (350 °F to 1200 °F) for one minute. Results of the NDE measurements were correlated with the strength properties of the composites. At higher temperatures, ultrasonics was effective in detecting delaminations in samples that were exposed to temperatures greater than 593 °C (1100 °F). On the other hand, DRIFTS measurements were found to indicate the onset of heat damage before reduction in composite strength; however, high scatter levels were observed for measurements between damaged and undamaged specimens which tended to negate the effectiveness of DRIFTS.

While the LIF method appears promising for detecting and quantifying heat damage in graphite/epoxy composites, there are a number of remaining technical issues that must be addressed in order to fully specify the strengths and limitations of the method. Details of the thermal damage mechanisms and of the correlation between fluorescence signatures and specific cure reaction and degradation products are not well understood. Direct correlation between spectral properties and other bulk physical testing methods, such as glass transition temperatures, Tg, must also be established. Factors such as environmental conditions and components age, along with the effect of surface coatings (such as paint) and surface conditioning (such as aerobic vs. anaerobic exposure) need to be evaluated. Finally, while applicability to other resin systems is anticipated, these need to be studied to determine the breadth of possible use for this technique.