Composite materials are desirable for aeronautical and aerospace applications for many reasons including their high strength-to-weight ratios, fatigue and corrosion resistance, design adaptability, and performance capabilities in harsh environments. Because of these qualities, composites are useful in many applications such as in armor, helmets, and helicopters, and as structural components.

Example of a layup of a laminated [453/04/902/60] composite of unidirectional plies, with schematic (left) and stacking order (right)

However, when in-service, composite materials experience very different damage mechanics than metals. Performance and quality of composite materials can suffer from fatigue, environmental conditions, and external damage just as metals can, but due to their inherent complexity and the difficulty of detecting damage in composites with traditional inspection techniques, maintaining and guaranteeing the safety of composite structures is a challenging problem.

Strategies such as structural health monitoring (SHM), a system with which to monitor a structure in real time, are particularly useful for composite materials, because they combat these difficulties by giving warning of any changes to the system. Implementing reliable SHM strategies into composite structures will allow the Army to fully take advantage of the performance capability of composite materials while increasing efficiency by saving time and expense on repair, decreasing the need to take structures out-of- service for inspection, decreasing the occurrence of sudden failures, and extending the lifetime of composite structures by enabling efficient maintenance.

This project studies damage progression in composite materials as monitored by multiple techniques used in nondestructive inspection to further understand the material itself and the capabilities of various nondestructive evaluation (NDE) techniques. Correlating and validating NDE strategies for sensing damage and operational effects on composite materials will add to the knowledge of composites and efficient strategies to monitor these materials and use them to their full capabilities.

There are many types of composites used in industry based on materials such as glass fiber, Kevlar, and carbon fiber combined with various types of epoxy. Carbon fiber-reinforced polymers are particularly useful in necessarily low-weight but high-strength applications. Unidirectional pre-preg composites, which have fibers oriented in the same direction held together by an epoxy matrix, are one of the most commonly used of these materials. Individual plies can be oriented and stacked together to create a laminated composite structure with specific material properties. The accompanying figure illustrates an example of a laminated composite of unidirectional plies. In this way, a designer can create a composite laminate with tailored material properties necessary for a specific application.

This unique design capability and adaptability coupled with potential high strength-to-weight ratios make composite materials extremely desirable for high-performance applications. However, composites have relatively complicated and situation-dependent damage mechanics. Composite materials experience multiple damage types that can occur together or in sequence to contribute to a weakened structural state or final failure. The main modes of failure are matrix cracking, delamination, fiber breakage, and fiber buckling, which all can contribute individually or together to the progressive degradation, weakening and final failure of a composite structure. In fatigue loading, the types of failure that predominantly occur and are a signal of potential areas for final failure are matrix cracking and delamination.

This work was done by Colleen Rosania, Mulugeta A. Haile, Natasha C. Bradly, Asha Hall, Michael Coatney, and Fu-Kuo Chang for the Army Research Laboratory. ARL-0224

This Brief includes a Technical Support Package (TSP).
Sensing Applied Load and Damage Effects in Composites with Nondestructive Techniques

(reference ARL-0224) is currently available for download from the TSP library.

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