Technical justification of repairs to composite laminates

Composite repair in the commercial aviation environment is largely evaluated and applied using rules of thumb and excessively conservative repair configurations. This is primarily due to the perception that the material is inherently too complex for reliable analysis and the data needed for such an analysis would be too difficult for a commercial operator to acquire or would not be accurate due to variables in the application process. Most composite design engineers have been exposed to the classical laminate theories, with their labor-intensive matrix analysis techniques, which require computerized calculation to make them feasible. Although computer advancement has made this approach more acceptable, the deep levels of understanding and large numbers of mechanical properties needed for accuracy make it impracticable for a commercial operator to design repairs. Some analysis methods, originally created to optimize design of graphite laminates in manufacturing prior to classical analysis, can be utilized with a fraction of the effort and still provide accurate results. These techniques reduce the complexity of composites to a level of difficulty that is manageable in a repair design environment. The equations to accomplish this have been found to be valid for other common substrate materials and their hybrid combinations. The majority of mechanical properties necessary for the accomplishment of such an analysis can be determined by directly measuring the capability of the material to be used in the repair process. This not only lessens the number of mechanical property variables needed to perform an accurate calculation but also takes into account any variability that exists between the processes used in repair. In addition, when comparing the results of an original structure with that of a repair analyzed using the same method, any bias inherent in the analysis technique will be nullified in their comparison. This type of analysis makes it possible for a commercial operator to provide accurate and feasible technical justification for repairs to composite laminates. © 1999 Elsevier Science Ltd. All rights reserved.