Contact:
Sarah Mouring
Department of Naval Architecture & Ocean Engineering
United States Naval Academy
590 Holloway Rd., Mail Stop 1 1 D
Annapolis, MD  21402
Tel: (410) 293-6442
mouring@usna.edu
Advanced Composites Research Center

Fundamental understanding of the mechanisms by which damage accumulates over a period of time in laminates is essential to a sound design process of composite materials and structures made from advanced composite laminate. Fiber breakage and matrix micro-cracking constitute such mechanisms that lead progressively to ply fracture, followed by laminate cracking and ultimately to component failure. Engineers often view fracture mechanics as a macroscopic phenomenon and perform tedious tests to learn about materials strength. The classical continuum theories have provided most of our theoretical tools, such as elasticity. However, technology is demanding that we understand materials failure where the continuum picture is no longer valid and an atomistic understanding is required. An important example is the dynamics of brittle fracture. Hence, application of physical modeling requires the identification and understanding of failure mechanisms in the material at the micron level and of the component at larger scales. This would provide a basis for understanding the differences in failure behavior of the material and structure.

Non-destructive evaluation (NDE) procedures provide a means for identifying the fatigue mechanism required to perform the physical modeling. The NDE Laboratory of the US Naval Academy has recently conducted a series of experiments using focused ultrasonics (UT) for this purpose. Feasibility experiments were performed on three grp panels, two made at the Academy and one made at NSWC, Carderock, using the high-resolution UT imaging - acoustic microscope - a capability of the Academy's NDE Laboratory. The ultrasonic images clearly showed zones within the image that were very distinct. The panels were cut into small sections, using the UT images as a guide, and a series of loading tests were done that established a correlation between the images and the structural integrity of these panels. This demonstrated the feasibility of the UT acoustic microscope to infer physical properties of these composite panels.

The proposed work tasks consist of refining the high-resolution UT imagery to be even more predictive of physical properties of the composites. Parameters from the frequency spectrum of the UT waveforms, recorded during the imaging scan, will be used to create "frequency domain" images. It is well documented that frequency domain parameters can be used to both detect and discriminate between different defect categories in composites, such as micro-cracking, delaminations, and porosity. Consequently, images can be constructed such that colored regions within the image are indicative of a particular defect class, such as micro-cracking, at those locations.

The acoustic microscope, in conjunction with subsequent destructive tests, will be incorporated into physical models that will identify failure mechanisms. If successful, the UT acoustic microscope can be configured as a portable PC-based field instrument. So, the NDE, combined with the models, can both be implemented in a portable field-rugged system.

Specifically, the proposed objectives of this work are to: (1) extend the UT feasibility experiments to a larger population of composite components, (2) examine the frequency spectrum of the UT waveforms, recorded during image creation data acquisition, to identify parameters (e.g., peak frequency) that correlate with physical properties and different defect types, such as micro-cracking, debonds, and porosity, (3) perform a series of destructive tests, based on zones identified in the UT images, to further refine the correlation between these zones and subsequent destructive analysis results, and (4) make a first pass at creating models that integrate the UT non-destructive images with the underlying actual structural integrity, from the destructive tests, that can be used as a predictive means for assessing the structural integrity of these composites.