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Physical Ultrasonics of Composites$
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Dale Chimenti, Stanislav Rokhlin, and Peter Nagy

Print publication date: 2011

Print ISBN-13: 9780195079609

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195079609.001.0001

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Bulk Ultrasonic Techniques for Evaluation of Elastic Properties

Bulk Ultrasonic Techniques for Evaluation of Elastic Properties

3 (p.81) Bulk Ultrasonic Techniques for Evaluation of Elastic Properties
Physical Ultrasonics of Composites

Dale Chimenti

Stanislav Rokhlin

Peter Nagy

Oxford University Press

Currently, the design of most composite components is based on stiffness, and therefore methods for static measurement of stiffness are in wide use. The disadvantages of these methods lie in their destructive nature (the samples must be cut from parts of different orientations), in the difficulty of measuring shear properties, and in the need for extra care when measuring Young’s modulus in off-axis directions. Ultrasonic methods are more accurate and have higher spatial resolution than static measurements. As we showed in Chapter 2, by measuring ultrasonic velocities in several predefined directions, all elastic constants can be determined. The generic method described there is also destructive, however, requiring cutting numerous samples with appropriate fiber orientation. Specialized nondestructive methods for determining the elastic moduli of composite materials are more powerful and they can be applied to composite coupons before, during, and after strength or fatigue testing. It is important to have a fast and inexpensive technique to estimate input parameters for composite design. It is even more important to have a technique to evaluate composites during service to verify that the manufactured elastic stiffnesses match those assumed in the design. Several methods that utilize bulk ultrasonic waves for measurement of composite elastic constants are considered in this chapter. By bulk wave methods, we mean quasilongitudinal and quasitransverse ultrasonic wave velocity measurement methods that are applicable when the sample thickness h is larger than both the ultrasonic pulse space length τV and the wavelength λ (τ is the ultrasonic pulse length in time, and V is the wave speed). Other methods, which are applicable in the range h < τV and which account for wave interference with the boundaries of the specimen, will be considered in the following chapters. The most promising way to evaluate composite elastic properties nondestructively is to measure ultrasonic velocities in different directions in the composite material and reconstruct the elastic constants from these values using some kind of an inversion technique. One possible method has been suggested by Markham in the 1970s, who used ultrasonic waves obliquely incident from water onto a composite plate to measure ultrasonic velocities in various directions and evaluated the results to determine elastic constants.

Keywords:   Christoffel’s equation, Double through-transmission method, Double-transmission time delay, Lamb wave propagation, Self-reference bulk wave method, Snell’s law, Time-of-flight measurements

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