| Book Review: Nondestructive Characterization of Composite Media | |
The properties of a composite are determined not only by the raw
materials used, but also by how those materials are put together
during the manufacturing process. Variations in ambient conditions or
layup methods can mean that a structure made one day will have
different properties than one made the next. Witness coupons can be
layed up with the part, but that is still not the same as measuring
the part properties directly.
In Nondestructive Characterization of Composite Media,
Ronald Kline shows how ultrasonic test methods can be used to directly
measure material properties. Such methods can measure elastic moduli,
fiber volume fraction and porosity. Because the methods are
nondestructive and don't require loading of the part, they can be used
on finished products, not just test coupons.
|
|
|
The first half of NDC covers the theory of elastic wave propagation in anisotropic media, with a focus on orthotropic composites. This portion of the book requires a good knowledge of anisotropic elasticity theory, especially as it is applied to composites, and acoustic wave theory.
Because of its highly theoretical nature, the first half of the book would be of interest mainly to graduate students or researchers active in this field. The second half covers experimental techniques, and should have a broader appeal. Although it is detailed enough for the researcher, it still gives a good overview of the capabilities and limitations of the characterization methods.
Ultrasonics allow the direct measurement of the Cij stiffness matrix components. The data show that the assumptions of transverse isotropy can be in error by 15-25% for boron-aluminum and boron-epoxy composites. In other words, the stiffness matrix of real structures is not symmetric. Some of this effect is explained by the preferential pressure direction during cure (normal to the laminate but not transverse).
The methods can also show the effects of non-uniform cure. In the case of a unidirectional flat plate, for example, fiber volume fraction can vary by up to 20%. The effects are especially pronounced at the panel edges. Because the stiffness matrix is directly measured, it is possible to plot the variation in moduli as a function of position. This data could then be used to perform a finite element analysis using actual material properties, not just uniform predicted properties.
Measuring material properties, as opposed to simply identifying defects, is a relatively new application of ultrasonics. Dr. Kline points out those areas where more research is needed, and those areas where the method is not yet refined enough to give accurate results. NDC was published in 1992, so it is likely significant advances have been made in the field since that time. Still, NDC provides a good theoretical background to the topic, and serves as a starting point for research into experimental methods.
Details: Nondestructive Characterization of Composite Media, by
Ronald A. Kline, published by
Technomic Publishing Company, 1992, ISBN 0-87762-925-0.
Introduction; List of Symbols; 1. Basic Governing Equations; 2. Wave
Surfaces; 3. Energy Propagation; 4. Bulk Wave Propagation Anisotropic
Media; 5. Guided Waves; 6. Experimental Consideration for Ultrasonic
Measurements; 7. Methods for Elastic Modulus Reconstruction from
Ultrasonic Data; 8. Experimental Characterization for Dynamic Modulus
Measurement in Anisotropic Media: Phase Velocity vs. Group Velocity;
9. Ultrasonic Modulus Measurements in Composite Media; 10. Composite
Microstructure Characterization; References; Index