Engineering topics

Duke, J. C., Jr.

Publications and source records attributed to Duke, J. C., Jr..

16 records

NASA NTRS1989

A theoretical basis for the acousto-ultrasonic evaluation of composite laminates

The application of the acousto-ultrasonic (AU) technique to composite laminates involves imparting a mechanical excitation to the surface of a composite plate to create a mechanical disturbance within the material. At the same time, a transducer is used at another location on the same or another surface of the material to sense vibrations (ultrasonic waves) caused by the disturbance. Experimental results are presented and related to concepts of through-thickness-transverse resonance and plate wave theory.

Kiernan, M. T.↗

NASA NTRS1989

A study of the stress wave factor technique for evaluation of composite materials

The acousto-ultrasonic approach for nondestructive evaluation provides a measurement procedure for quantifying the integrated effect of globally distributed damage characteristic of fiber reinforced composite materials. The evaluation procedure provides a stress wave factor that correlates closely with several material performance parameters. The procedure was investigated for a variety of materials including advanced composites, hybrid structure bonds, adhesive bonds, wood products, and wire rope. The research program focused primarily on development of fundamental understanding and applications advancements of acousto-ultrasonics for materials characterization. This involves characterization of materials for which detection, location, and identification of imperfections cannot at present be analyzed satisfactorily with mechanical performance prediction models. In addition to presenting definitive studies on application potentials, the understanding of the acousto-ultrasonic method as applied to advanced composites is reviewed.

Duke, J. C., Jr.↗

NASA NTRS1989

Impact damage development in damaged composite materials

A procedure for predicting the nature of impact damage development based on the measured acousto-ultrasonic (AU) response of fiber reinforced crossply laminates with or without damage is described. Results of AU evaluation as well as penetrant enhanced radiographs of damaged laminates are presented.

Duke, J. C., Jr.↗

NASA NTRS1988

PC analysis of an acousto-ultrasonic signal

An introduction is given to PC software developed to analyze a digitized signal. The specific way in which the software was implemented and the relative ease with which the same software can be implemented in different systems for various applications are discussed. The basic equations and related theory used in the software are furnished. Specifically, mention is made of signal digitization, dc biasing, Fourier analysis, moment analysis, digital filtering, and transfer functions. Examples of calculations are given to indicate the physical significance of variables calculated from the frequency domain via moment equations.

Kiernan, M. T.↗

NASA NTRS1988

Acousto-ultrasonics as a monitor of material anisotropy

This paper discusses experimental results obtained by performing the acousto-ultrasonic (AU) method at various azimuthal angles on the surface of fiber-reinforced composite plates. The use of an IBM-PC/data-acquisition board to obtain a digitized AU signal to be analyzed by specially developed software is described. An introduction is given to the use of AU parameters to quantify information evidenced in amplitude/frequency plots. A description of how the parameters are obtained by calculating various spectral moments and the area under particular ranges of the spectral density curve is presented. Results are given from voltage/time plots, amplitude/frequency plots, and plots showing the variation of calculated AU parameters with azimuthal angle. Finally, how the variation of AU parameters with azimuthal angle may be related to variation in material properties with azimuthal angle is discussed.

Duke, J. C., Jr.↗

NASA NTRS1986

A study of the stress wave factor technique for nondestructive evaluation of composite materials

The acousto-ultrasonic method of nondestructive evaluation is an extremely sensitive means of assessing material response. Efforts continue to complete the understanding of this method. In order to achieve the full sensitivity of the technique, extreme care must be taken in its performance. This report provides an update of the efforts to advance the understanding of this method and to increase its application to the nondestructive evaluation of composite materials. Included are descriptions of a novel optical system that is capable of measuring in-plane and out-of-plane displacements, an IBM PC-based data acquisition system, an extensive data analysis software package, the azimuthal variation of acousto-ultrasonic behavior in graphite/epoxy laminates, and preliminary examination of processing variation in graphite-aluminum tubes.

Sarrafzadeh-Khoee, A.↗

NASA NTRS1986

Measure of displacement around holes in composite plates subjected to quasi-static compression

Contour maps of thickness changes were obtained for three quasi-isotropic graphite-epoxy plates with central holes, loaded in compression. Thickness changes were determined for six load increments from nearly zero to within a few percent of the failure load. The largest change of thickness occurred near the hole but not at the boundary of the hole. Below 90 percent of the failure load, the thickness changes were nearly proportional to load. Irregularities of thickness changes occurred in zones of compressive stresses and they were attributed to localized fiber buckling. A new optical technique was developed to measure thickness changes with high sensitivity. It utilizes a comparatively simple means of holographic interferometry on both sides of the specimen, followed by additive moire to obtain thickness changes as the sum of the out-of-plane displacements. Sensitivity was 12.5 x 10 to the -6 power in. per fringe order. The fringe patterns represent thickness changes uniquely, even when specimen warpage and consequent out-of-plane displacements are very large.

Duke, J. C., Jr.↗

NASA NTRS1986

Ultrasonic stress wave characterization of composite materials

The work reported covers three simultaneous projects. The first project was concerned with: (1) establishing the sensitivity of the acousto-ultrasonic method for evaluating subtle forms of damage development in cyclically loaded composite materials, (2) establishing the ability of the acousto-ultrasonic method for detecting initial material imperfections that lead to localized damage growth and final specimen failure, and (3) characteristics of the NBS/Proctor sensor/receiver for acousto-ultrasonic evaluation of laminated composite materials. The second project was concerned with examining the nature of the wave propagation that occurs during acoustic-ultrasonic evaluation of composite laminates and demonstrating the role of Lamb or plate wave modes and their utilization for characterizing composite laminates. The third project was concerned with the replacement of contact-type receiving piezotransducers with noncontacting laser-optical sensors for acousto-ultrasonic signal acquisition.

Duke, J. C., Jr.↗

NASA NTRS1986

Analytical ultrasonics for evaluation of composite materials response. Part 1: Physical interpretation

The phenomena associated with the propagation of elastic waves in anisotropic materials are discussed. Wave modes propagating in general directions relative to the material coordinate system are not purely longitudinal nor transverse. Hence the generation of ultrasonic waves by common piezoelectric transducers will generate multiple modes to some extent. The received signals will likely be a combination of different modes. When using two transducers to send and receive ultrasonic waves, deviation of the energy flux vector may reduce the apparent value of the received signal unless the proper orientation of the two transducers with respect to one another is taken into account. And application of reflection from plane boundaries for the purposes of making certain measurements may lead to misinterpretation of results unless one is aware of the differences in multiple mode generation and critical angle phenomena between isotropic and anisotropic materials. When studies or characterizations of composite materials by ultrasonics are to be performed, these phenomena must be taken into consideration so that proper and correct application and interpretation of the measurements can be made. Finally, attention must be drawn again to the fact that composite materials are heterogeneous by definition. The results discussed here have been determined for homogeneous materials only. While the assumption of homogeneity appears to be valid for certain wavelength ranges in composites, future work must continue to study the phenomena of wave propagation in anisotropic, nonhomogeneous materials.

Henneke, E. G., II↗

NASA NTRS1986

Analytical ultrasonics for evaluation of composite materials response. Part 2: Generation and detection

To evaluate the response of composite materials, it is imperative that the input excitation as well as the observed output be well characterized. This characterization ideally should be in terms of displacements as a function of time with high spatial resolution. Additionally, the ability to prescribe these features for the excitation is highly desirable. Various methods for generating and detecting ultrasound in advanced composite materials are examined. Characterization and tailoring of input excitation is considered for contact and noncontact, mechanical, and electromechanical devices. Type of response as well as temporal and spatial resolution of detection methods are discussed as well. Results of investigations at Virginia Tech in application of these techniques to characterizing the response of advanced composites are presented.

Duke, J. C., Jr.↗

NASA NTRS1986

Factors influencing the ultrasonic stress wave factor evaluation of composite material structures

To demonstrate that the finite-element model can be used to investigate some of the factors influencing the ultrasonic stress wave evaluation of materials, a hypothetical case was studied in which classical vibration theory was used. Vibration analysis and experiments for the undamaged case were conducted on an isotropic aluminum plate and a unidirectional graphite-epoxy plate, using a point source to excite the plates. The finite-element solution correlated within eight percent with the exact method. The frequencies predicted by the finite-element model were observed in the experiments in both plates, although in the composite plate, additional frequencies were observed which could not be accounted for. Damaged isotropic plates were also considered. The effects of increasing damage severity with constant damage area, and increasing damage area with constant severity on the resonant frequencies were analyzed.

Rebello, C. J.↗

NASA NTRS1985

A study of the stress wave factor technique for the characterization of composite materials

This study has investigated the potential of the Stress Wave Factor as an NDT technique for thin composite laminates. The conventional SWF and an alternate method for quantifying the SWF were investigated. Agreement between the initial SWF number, ultrasonic C-scan, inplane displacements as obtained by full field moire interferometry, and the failure location have been observed. The SWF number was observed to be the highest when measured along the fiber direction and the lowest when measured across the fibers. The alternate method for quantifying the SWF used square root of the zeroth moment (square root of M sub o) of the frequency spectrum of the received signal as a quantitative parameter. From this study it therefore appears that the stress wave factor has an excellent potential to monitor damage development in thin composite laminates.

Govada, A. K.↗

NASA NTRS1984

Nondestructive evaluation of composite materials - A design philosophy

Efficient and reliable structural design utilizing fiber reinforced composite materials may only be accomplished if the materials used may be nondestructively evaluated. There are two major reasons for this requirement: (1) composite materials are formed at the time the structure is fabricated and (2) at practical strain levels damage, changes in the condition of the material, that influence the structure's mechanical performance is present. The fundamental basis of such a nondestructive evaluation capability is presented. A discussion of means of assessing nondestructively the material condition as well as a damage mechanics theory that interprets the material condition in terms of its influence on the mechanical response, stiffness, strength and life is provided.

Duke, J. C., Jr.↗

NASA NTRS1983

A study of the stress wave factor technique for the characterization of composite materials

A testing program was undertaken to provide an independent investigation and evaluation of the stress wave factor for characterizing the mechanical behavior of composite laminates. Some of the data which was obtained after performing a very large number of tests to determine the reproducibility of the SWF measurement is presented. It was determined that, with some optimizing of experimental parameters, the SWF value can be reproduced to within + or - 10%. Results are also given which show that, after careful calibration procedures, the lowest SWF value along the length of a specimen will correlate very closely to the site of final failure when the specimen is loaded in tension. Finally, using a moire interferometry technique, it was found that local regions having the highest in plane strains under tensile loading also had the lowest SWF values.

Henneke, E. G., II↗

NASA NTRS1983

Characterization of composite materials by means of the ultrasonic stress wave factor

The usual approach to nondestructively evaluating a composite structure involves inspection and mechanical analysis of the inspection results. Such an approach has met with only limited success. On the other hand, the ultrasonic stress wave factor technique directly evaluates the material. Despite requiring access to only one surface of the material, the technique interrogates the material in the directions of applied load. Using the stress wave factor technique it is possible to determine the failure location in the material. The correlation of the stress wave factor with stiffness is shown. In addition, the use of the technique for determining the strength or life of composite material structures is discussed.

Duke, J. C., Jr.↗

NASA NTRS1982

Measurements of displacement around holes in composite plates subjected to quasi-static compression

Attempts to develop a whole-field, high sensitivity optical technque for measurement of load induced changes of thickness of composite plates are described. Graphite-epoxy plates of quasi-isotropic layup were used as test specimens. Changes of thickness of three plates, each with a central hole of different size, were measured as a function of applied compressive loads. The first test specimen showed localized zones where no holographic fringes were present, surrounded by zones of low fringe contrast. This was thought to be a result of localized motion or creep. Subsequent tests with an increased time interval between specimen loading and holographic exposure demonstrated no localized zones of absent fringes. No plausible explanation for radical differences in the quality of fringe patterns was found, although the use of specimens having a high quality mirrorized surface provided superior change of thickness patterns. Recommendations for further investigations and changes in test procedure are presented.

Duke, J. C., Jr.↗