Millimeter-Wave Nondestructive Evaluation of Glass Fiber/Epoxy Composites Subjected to Impact Fatigue


The useful life of a glass fiber/epoxy composite subjected to impact fatigue loading is an important issue in the future design of numerous industrial components. Lifetime predictions have been a problem particularly due to the difficulties encountered in monitoring damage accumulation in composites. It is hypothesized that there is a build up of micro damage, such as matrix micro-cracks and micro-delaminations, even though there is no apparent change in material compliance. A critical level is finally reached at which time the properties of the composite begin to fall and compliance change is evident. In this study the apparent compliance change and the type of damage accumulation is investigated. To measure the compliance change, a test unit was developed that uses a dynamic load measuring system. The load cell measures the load throughout each impact pulse and the compliance and energy absorbed by the specimen is then related to the recorded curve. Initially no change in the impact pulse was apparent; however, after a finite number of cycles the peak load and area under each impact pulse drop, indicating an increase in compliance. Unfortunately, the impact load does not provide information on the form and degree of damage. Thus, millimeter wave nondestructive investigation is used, in conjunction with impact fatigue tests, to examine microstructural aspects of damage initiation and growth. The millimeter wave scanning technique results in detectable damage growth throughout the impact fatigue test. Damage size and growth patterns specific to composites are obvious, and after significant damage can be related to the observable macro damage. Continued development of these investigative techniques promises to enhance the ability of detecting defects and damage growth in fiber reinforced composite materials as well as improving the understanding of impact fatigue initiation in these complex materials.

Meeting Name

Advanced Microwave and Millimeter-Wave Detectors (1994: Jul. 25-26, San Diego, CA)


Electrical and Computer Engineering

Keywords and Phrases

Composite Materials; Defects; Epoxy Resins; Fatigue Of Materials; Glass Fibers; Impact Testing; Monitoring; Nondestructive Examination; Impact Fatigue Loading; Micro-Cracks; Micro-Delamination; Millimeter Waves

International Standard Book Number (ISBN)


International Standard Serial Number (ISSN)


Document Type

Article - Conference proceedings

Document Version


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© 1994 SPIE--The International Society for Optical Engineering, All rights reserved.

Publication Date

01 Jul 1994