Showing papers by "Ajit Mal published in 1992"

Wave Attenuation in Fiber-Reinforced Composites

Abstract: Attenuation of waves in graphite/epoxy composite laminates is studied through an ultrasonic experiment and theoretical analysis of the recorded waveforms. The specimens are immersed in water, insonified by a beam of acoustic waves from a broadband transducer, and the reflected signals are recorded by a second transducer in a pitch-catch arrangement. The received signals are analyzed by means of a theoretical model in which the composite is assumed to be a transversely isotropic and dissipative medium. A simple model of dissipation is proposed and calculations based on wave propagation in the laminate are carried out. The values of the damping parameters are determined through comparison between the measured and calculated waveforms of the reflected signal. Results are presented for four unidirectional specimens of different thicknesses. The assumed model of attenuation is shown to yield excellent agreement between measured and calculated waveforms in all four cases.

Analysis of Acoustic Pulses Reflected From Fiber-Reinforced Composite Laminates

Abstract: The time histories and spectral characteristics of acoustic pulses reflected from fiberreinforced composite laminates immersed in water are recorded in the laboratory and analyzed through a generalized ray theory and an exact theory. Calculated results for an unidirectional laminate and two angle-ply laminates are compared with measured data. It is shown that for a unidirectional laminate a finite number of mode-converted waves contribute to the overall signal reflected from a thick specimen resulting in certain irregular behavior of the reflected pulses. A phenomenological model of wave attenuation is introduced in the theoretical simulation and the damping parameters are shown to have a strong influence on the amplitude of the reflected pulses. The phase velocity of the guided waves in the laminates are shown to be nearly independent of water loading and material dissipation in a broad frequency range. Agreement between measured and calculated results is found to be excellent to very good in all cases.

Material characterization of composite laminates using low-frequency plate wave dispersion data

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.

Leaky lamb waves for the ultrasonic nondestructive evaluation of adhesive bonds

Abstract: The critical role played by interface zones in the fracture and failure of composites and other bonded materials is well known. However, the existing nondestructive evaluation methods are not capable of yielding useful quantitative information on either elastic or strength related properties of the interface. The authors are investigating the feasibility of applying an ultrasonic method to determine some of the interface properties nondestructively. The method uses guided waves and is based on the fact that the dispersive properties of these waves are strongly affected by the elastic properties of the interface. A coordinated theoretical and experimental program of research has revealed that the correlation between the interfacial properties and the phase velocity of the guided waves is quite strong and is identifiable at least in laboratory specimens. Some recent results of this research are reported in this paper.

Ultrasonic evaluation of adhesive bonding

Abstract: The capabilities of existing ultrasonic nondestructive evaluation (NDE) methods for the strength-related properties of bonds in structural components is reviewed. The severe limitations of conventional NDE methods in yielding quantitative results are indicated. Some recent results of a joint theoretical and experimental program of research using leaky Lamb waves (LLW) in laboratory specimens are presented. The LLW technique is shown to have several advantages over conventional techniques. Potential applications of the technique to determine non-destructively the quality of bonds in a variety of models are discussed.

Characterization of Adhesive Bonding Using Leaky Lamb Waves

Abstract: The performance of adhesive bonds in primary structures strongly depends on the quality of adhesion. Many NDE methods are presently used to detect unbonded areas; however, these methods cannot be used to determine bond properties. In standard ultrasonic techniques, the velocity of bulk wave propagation through the specimen is measured by time-offlight. Unfortunately, the waves reflected from the bonded region cannot be easily identified or analyzed to determine the properties of the adhesive layer.

Characterization of Film Adhesion by Acoustic Microscopy

Abstract: Accurate and reliable information on adhesive bond strength between dissimilar solids is an important factor in joint technology. In spite of considerable efforts to obtain a quantitative measure of film adhesion, this parameter remains elusive, particularly so in cases where non-destructive means of determination are desirable if not mandatory.

Wave Field Produced in a Composite Laminate by a Concentrated Surface Force

Abstract: With the increasing use of advanced composites in a variety of modern applications, it has become necessary to employ reliable and effective nondestructive evaluation (NDE) methods to determine the integrity and serviceability of structural composites. The use of ultrasonic techniques involving guided waves and contact type transducers can provide powerful characterization methods for composites. However, the wave phenomena associated with these methods are, in general, far more complex and less well understood than those associated with conventional techniques, based on longitudinal waves. Realization of the full potential of these newer techniques will require a better understanding of the quantitative features of the wave phenomena than is available at present.