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NONDESTRUCTIVE EVALUATION METHODS 38 FOR COMPOSITES Thomas S. Jones 38.1 INTRODUCTION of constituents (fiber-resin ratio, resin-starva- The development and selection of nondestruc- tive evaluation (NDE) techniques for application to composite materials and struc- tures presents several challenges and considerations that are quite distinct from the considerations given to the similar processes for metallic materials and structures. A princi- pal consideration is the nature of advanced composites as typically layered, anisotropic materials. The materials of interest include fiber-reinforced plastics such as 'fiberglass' and carbon epoxy, as well as some of the more exotic materials such as metal-matrix or ceramic matrix composites. In some cases, nat- urally occurring composites, such as wood, with its mix of differing density summer and winter growth rings and fibrous structure, may be treated with approaches similar to those used for the man-made composites. The advan- tages offered by composites are focused on the high strength, low weight properties of typical constituent materials. Yet, if the materials are to exhibit high strength, they must be manufac- tured as the designer envisioned and they must maintain their integrity in service. NDE pre- sents a technology to help assure the reliability of the materials. Inhomogeneities that may affect the perfor- mance of a composite include the concentration Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 tion, etc.), orientation and distribution of reinforcement, voids, matrix-reinforcement bonding and similar characteristics. The pur- pose of the NDE is to detect these inhomogeneities and others, including foreign material, fiber breakage, degradation due to moisture, ultraviolet (W) or other reasons, cracks, abrasion, impact damage, fire or exces- sive heat, etc. This chapter describes many methods that can be used to detect anom- alies in composite materials during manufacture and in-service. Although composite materials have been in use on military aircraft for many years, the last few years have seen a rapid escalation in both the number and structural criticality of com- posite applications entering service. The falling price of composite raw materials has vastly increased the number applications in non-aerospace industries, including automo- tive, sports, boating and construction. While the service record for composites has been excellent, they are subject to damage from sources such as overload, hail, lightning, low velocity impact, ballistic rounds and moisture intrusion. Low strain designs, coupled with less critical applications, have made for rela- tively large acceptable flaw sizes and lax inspection criteria. As composite structures representing a more aggressive design criteria and less tolerant materials (such as some of the ceramic matrix materials) begin to accumulate service hours, we will see a new level of
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Visual inspection 839 demands placed on nondestructive inspection
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