|TABLE OF CONTENTS||Page|
|Table of Contents||v|
|List of Figures||vii|
|List of Tables||xi|
|1.2||Scope and Organization||2|
|2.0||Characteristics of Thermal Degradation||3|
|2.2||Mechanical and Materials Properties||4|
|25 °C to 177 °C (77 °F to 350 °F)||6|
|177 °C to 371 °C (350 °F to 700 °F)||6|
|460 °C to 2982 °C (560 °F to 5400 °F)||14|
|3.0||Review of NDE Investigations||27|
|3.1||General NDE Methods||27|
|3.2||Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS).||39|
|3.3||Laser Induced Fluorescence (LIF)||45|
Summaries of Principal Work Cited in the Field of Graphite/Epoxy Composite Thermal Degradation
Additional Bibliographic Citations on Graphite/Epoxy Thermal Degradation
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.
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 . 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 . 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.