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Showing papers by "William H. Prosser published in 1992"


Journal ArticleDOI
TL;DR: In this paper, a high fidelity transducer was used to determine the direction of motion of a source in the case of aluminum and graphite/epoxy composite materials using the Reissner-Mindlin theory combined with lamination theory.
Abstract: Acoustic emission was interpreted as modes of vibration in finite aluminum and graphite/epoxy plates. The `thin plate'' case of classical plate theory was used to predict dispersion curves for the two fundamental modes described by the theory and to calculate the shapes of flexural waveforms produced by a vertical step function loading. There was good agreement between the theoretical and experimental results for the aluminum. Composite materials required the use of a higher order plate theory (Reissner-Mindlin) combined with lamination theory in order to get good agreement with the measured velocities. Plate modes were shown to be useful for determining the direction of motion of a source. Thus, with a knowledge of the material, it may be possible to ascertain the type of the source. For example, particle impact on a plate could be distinguished from a crack growing in the plate. A high fidelity transducer was needed to distinguish the plate modes. After evaluating several types of transducers, a broadband ultrasonic transducer was found which satisfied the fidelity requirement and had adequate sensitivity over the 0.1 to 1 MHz range. The waveforms were digitized with a 5 MHz transient recorder. The dispersion curves were determined from the phase spectra of the time dependent waveforms. The aluminum plates were loaded by breaking a 0.5 mm. pencil lead against the surface of the plate. By machining slots at various angles to the plane of a plate, the direction in which the force acted was varied. Changing the direction of the source motion produced regular variations in the measured waveforms. Four composite plates with different laminate stacking sequences were studied. To demonstrate applicability beyond simple plates, waveforms produced by lead breaks on a thin-walled composite tube were also shown to be interpretable as plate modes. The tube design was based on the type of struts proposed for Space Station Freedom''s trussed structures.

79 citations


Journal ArticleDOI
TL;DR: In this paper, simulated acoustic emission signals were induced in a thin-walled graphite/epoxy tube by means of lead breaks (Hsu-Neilsen source).
Abstract: Simulated acoustic emission signals were induced in a thin-walled graphite/epoxy tube by means of lead breaks (Hsu-Neilsen source). The tube is of similar material and layup to be used by NASA in fabricating the struts of Space Station Freedom. The resulting waveforms were detected by broad band ultrasonic transducers and digitized. Measurements of the velocities of the extensional and flexural modes were made for propagation directions along the tube axis (0 degrees), around the tube circumference (90 degrees) and at an angle of 45 degrees. These velocities were found to be in agreement with classical plate theory.

41 citations


01 Jan 1992
TL;DR: In this article, the phase velocity of antisymmetric plate waves is determined through spectral analysis of signals recorded from a lead break source on the surface of graphite epoxy laminates.
Abstract: A recently developed ultrasonic techique which has been successful in monitoring the integrity of advanced structures is described. The technique is based on a two-transducer contact type arrangement that can be used to determine the dispersion curves of guided waves generated within the specimen. The phase velocity of antisymmetric plate waves is determined through spectral analysis of signals recorded from a lead break source on the surface of graphite epoxy laminates. The influence of all five stiffness constants on the dispersion curves of both symmetric and antisymmetric plate waves is theoretically investigated for propagation at 0, 45, and 90 deg to the fibers. All but c12 were found to have a strong influence on the dispersion curves in this frequency range. The fit with the theoretically predicted dispersion curves was excellent for propagation parallel and perpendicular to the fibers in the unidirectional specimen and good for the other cases considered.

14 citations


Patent
17 Jun 1992
TL;DR: In this article, a method for determining the angle of impact of an object on a thin-walled structure was proposed, based on the analysis of the acoustic waves which result when an object impacts a structure, where transducers are placed on and in the surface of the structure which sense the wave caused in the structure by impact.
Abstract: A method for determining the angle of impact of an object on a thin-walled structure which determines the angle of impact through analysis of the acoustic waves which result when an object impacts a structure. Transducers are placed on and in the surface of the structure which sense the wave caused in the structure by impact. The waves are recorded and saved for analysis. For source motion normal to the surface, the antisymmetric mode has a large amplitude while that of the symmetric mode is very small. As the source angle 22 increases with respect to the surface normal, the symmetric mode amplitude increases while the antisymmetric mode amplitude decreases. Thus, the angle of impact is determined by measuring the relative amplitudes of these two lowest order modes.

4 citations


01 Jan 1992
TL;DR: In this paper, the effects of nonlinear elasticity on energy flux deviation in undirectional gr/ep composites are examined using acoustoelasticity theory and the second-and third-order elastic stiffness coefficients for T300/5208 GR/ep.
Abstract: The effects of nonlinear elasticity on energy flux deviation in undirectional gr/ep composites are examined. The shift in the flux deviation is modeled using acoustoelasticity theory and the second- and third-order elastic stiffness coefficients for T300/5208 gr/ep. Two conditions of applied uniaxial stress are considered. In the first case, the direction of applied uniaxial stress was along the fiber axis (x3), while in the second case it was perpendicular to the fiber axis along the laminate stacking direction (x1). For both conditions, the change in the energy flux deviation angle from the condition of zero applied stress is computed over the range of propagation directions of 0 to 60 deg from the fiber axis at two-degree intervals. A positive flux deviation angle implies the energy deviates away from the fiber direction toward the x1 axis, while a negative deviation means that the energy deviates toward the fibers. Over this range of fiber orientation angles, the energy of the quasi-longitudinal and pure mode transverse waves deviates toward the fibers, while that of the quasi-transverse mode deviates away from the fibers.

3 citations


01 Jan 1992
TL;DR: In this paper, the effects of nonlinear elasticity on energy flux deviation in unidirectional graphite/epoxy (gr/ep) composites were studied. And the authors showed that changes in composite materials which alter the elastic properties such as moisture absorption by the matrix or fiber degradation, can be detected nondestructively by measurements of the energy flux shift.
Abstract: In isotropic materials, the direction of the energy flux (energy per unit time per unit area) of an ultrasonic plane wave is always along the same direction as the normal to the wave front. In anisotropic materials, however, this is true only along symmetry directions. Along other directions, the energy flux of the wave deviates from the intended direction of propagation. This phenomenon is known as energy flux deviation and is illustrated. The direction of the energy flux is dependent on the elastic coefficients of the material. This effect has been demonstrated in many anisotropic crystalline materials. In transparent quartz crystals, Schlieren photographs have been obtained which allow visualization of the ultrasonic waves and the energy flux deviation. The energy flux deviation in graphite/epoxy (gr/ep) composite materials can be quite large because of their high anisotropy. The flux deviation angle has been calculated for unidirectional gr/ep composites as a function of both fiber orientation and fiber volume content. Experimental measurements have also been made in unidirectional composites. It has been further demonstrated that changes in composite materials which alter the elastic properties such as moisture absorption by the matrix or fiber degradation, can be detected nondestructively by measurements of the energy flux shift. In this research, the effects of nonlinear elasticity on energy flux deviation in unidirectional gr/ep composites were studied. Because of elastic nonlinearity, the angle of the energy flux deviation was shown to be a function of applied stress. This shift in flux deviation was modeled using acoustoelastic theory and the previously measured second and third order elastic stiffness coefficients for T300/5208 gr/ep. Two conditions of applied uniaxial stress were considered. In the first case, the direction of applied uniaxial stress was along the fiber axis (x3) while in the second case it was perpendicular to the fiber axis along the laminate stacking direction (x1).