Nondestructive Evaluation of Composites
Authors: Hagemaier Donald 
Published: October 2008

Composite structures have been used in the fabrication of aircraft structures for the past three decades. Most primary aircraft structures are made from carbon epoxy composites while fairing and antenna .ire made from fiberglass and Kevlar composites. This paper describes the application of nondestructive testing (NDT) for both fabrication and in service inspections. In fabrication, all laminate structures receive ultrasonic inspection while honeycomb parts receive additional radiographic inspection. Ultrasonic and bond testing is used to detect voids, porosity, and disbonds. Additionally, these inspections require personnel qualification, approved procedures, built-in reference standards for instrument calibration, and accept reject criteria. To reduce inspection costs, automated inspections are performed.@ Generally, NDT is not required for in-service inspection except for critical areas of primary structure. However, impact damage, disbonds, water intrusion into honeycomb, does occur necessitating the use of NDT to determine the extent of damage prior to repair. Manual NDT methods are used to evaluate small areas of damage while automated ultrasonic methods, electronic shearography, and thermography (infrared) methods are used for large area scanning. Acoustic emission inspection is sometimes used for proof testing or monitoring of pressure vessels.

Introduction

The inspection of composite structures is a complex task which requires considerable skill. To ensure inspections are conducted with maximum effectiveness, consistency and reliability, it is necessary that the inspector be equipped with a certain level of knowledge and have access to proper inspection equipment. It is imperative that inspectors have a good understanding of the basic fabrication and repair techniques as well as specific knowledge of the part under inspection. Additionally, inspectors must be familiar with the capabilities and limitations of the various NDT methods.

Preproduction Tests

This phase includes first-article manufacturing and testing, specification preparation, design and reliability review and supplier selection. During this phase, good communication is needed among the design, stress, materials and process, and NDT engineers.

As the R&D hardware and test specimens are fabricated, they are inspected by a variety of applicable NDT methods. As a result of these inspections, quality acceptance standards are established for the production parts. The qualitative and quantitative influence of flaws and property variability are determined. By screening the incoming materials or parts, it is possible to classify them into various quality groupings. Correlation of the destructive and NDT test results enable engineering to make sound judgments for fabrication.

Fabrication Inspection

The fabrication phase includes material and process control, supplier surveillance, product improvement, increased production output, and cost reduction. Materials and parts are inspected for conformity and systems or components are tested for reliability and quality assurance. The inspection standard requires that the parts be inspected to a detailed inspection plan and that the inspections be carried out by qualified and approved inspection personnel and laboratories. Manufacturing Flaws.