ND T abstracts

For over twenty years thermography has been under development as a nondestructive evaluation (NDE) technique for composite materials. Several techniques are in use in laboratory, manufacturing quality control, and field inspection applications. The p o ~ of this paper is to present the capabilities of the externally applied thermal field (EATF) thennographic NDE method, to discuss the applications for which it seems best suited, to describe its limitations, and to highlight directions of further developmenL

40201 Bakis, C.E.; Reifsnider, K.L. Nondes t ruc t ive evaluat ion of f iber composite laminates by thermoelas t ic emission Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. Vol. 7B, pp. 1109-1116. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

During the rapid cyclic loading of a structure (> .5 Hz), the cyclic variation of surface temperature can be measured with infrared radiometry. The temperature variation is related to the reversible, adiabatic deformation of the material. Adiabatic thermography differs from dissipative thermography, where temperature variations are associated primarily with dissipated energy rather than stored energy. The radiometer system used in the present investigation is capable of resolving local stresses that reflect global stress distributions and macroscopic variations in material properties, including flaws and damage. The present paper emphasizes fundamental aspects of the technique for the nondestructive evaluation of fiber-reinforced composite laminates.

40200 Russell, S.S.; Lin, S.S.; Henneki, E.G. V i b r o t h e r m o g r a p h i c NDE of f iberous composites Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. Vol. 7B, pp. 1101-1108. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

Vibrothermography refers to the process of mapping the temperatures on the surface of a component while the component is undergoing forced mechanical vibration. Damaged zones in a polymeric fiber reinforced composite more efficiently convert kinetic energy to heat. Hence, at certain frequencies, delaminations and other damage appear to a scanning infrared camera as hot zones. This discussion of Vibrothermography will highlight the capabilities, advantages and limitations of using Vibrothermography as a nondestructive evaluation tool for fiber reinforced composites. Also, the frequency dependence of exciting delaminations will be examined within the framework of a Raleigh-Ritz model of a delamination. The predictions of this model are compared to laboratory results. Several trial investigations are presented.

38874 Tivanov, G.G. Analysis of the ma themat ica l models used in t he rma l quali ty cont ro l of composite mater ia l s Soviet Journal of Nondestructive Testing, Vol. 23, No. 5, pp. 343-345 (Jan. 1988)

This article contains a brief analysis of the temperature contrasts outlined with the use of isotropic and anisotropic models in studying the nondestructive testing of composite materials by thermal control. The applicability of the parabolic equations of heat conduction are discussed.

38872 Lin Shiangshin Frequency dependent heat generat ion du r ing v i b r o t h e r m o g r a p h i c test ing of composite mater ia ls Dissertation Abstracts International, Vol. 48, No. 11, p. 3335 (May 1988)

This investigation concerns the frequency dependent heat generation behavior and the heat generation mechanisms for the thermal patterns of delamination in fiber reinforced composites during a vibrothermographic test, which uses real time thermography as a nondestructive evaluation of a structure or a component excited with mechanical vibration. A local resonance model was proposed in the past to describe the frequency dependent heat generation behavior during a vibrothermographic test, and this model was used as a basis for writing software for calculating the natural frequencies of plate with the size of delamination. Vibrothermographic tests were performed on three glass-epoxy panels that each contained four different sized simulated delaminations. Comparison between the observed vibrothermal peak frequencies and the natural frequencies predicted by the local resonance model, and investigations of the thermoelastic emission field in the delamination region using SPATE, were made to determine the validity of the local resonance model.

38869 Shelley, T. Flashes show defects in depth Eureka, Vol. 8, No. 6, pp. 25-28 (Jun. 1988)

Accurate fast (much less than one second) three dimensional defect location can be achieved using a new technique called Pulse Video Thennography. Specimens are irradiated with poises of light from xenon flash lamps and then their surfaces are observed with an infra-red camera. This method can be used for metals composites and some coated structures and results ate conveniently displayed on video monitors. Image enhancement can be incorporated as part of an on-line quality verification system.

37962 Potter, R.T. Stress analysis in l amina ted f ibre composi tes by thermoelas t ic emission Stress Analysis by Thermoelastic Techniques, Proceedings of a Conference, London (United Kingdom), 17-18 Feb. 1987. pp. 110-120. Edited by B.C. Gasper. Proceedings of SPIE - The International Society for Optical Engineering, Vol. 731 (1987)

The equations relating the output of the SPATE 8000 thermoelastic stress analysis sytem to the strains in multidirectional laminated fibre composites have been developed. The system has been calibrated for use with a particular carbon fibre/epoxy resin material. For multidirectional laminates under a generalised state of stress, it has been observed that the sensitivity of the SPATE output to the various strain components differs significantly such that even the qualitative interpretation of SPATE scans is difficult. However, there are a number of special cases which are applicable to practical structural laminates for which the interpretation of scan is greatly simplified.

37960 Oliver, D.E.; Jaeger, P. SPATE Applicat ions in Nor th Amer ica : r epor t on U.S. SPATE users group Stress Analysis by Thermoelastic Techniques, Proceedings of a Conference, London (United Kingdom), 17-18 Feb. 1987. pp. 213-226. Edited by B.C. Gasper. Proceedings of SPIE - The International Society for Optical Engineering, Vol. 731 (1987)

This paper offers an overview of SPATE activities in the U.S.A. Examples of industrial applications range from small nero-engine turbine blades tested at excitation frequencies in excess of 20 kHz to composite pressure vessels loaded at 0.5 Hz. Pioneering research at several U.S. Universities is also described.

37953 Owens, R J-/. Applicat ion of the thermoelas t ic effect to typical aerospace composite mater ia ls Stress Analysis by Thermoelastic Techniques, Proceedings of a Conference, London (United Kingdom), 17-18 Feb. 1987. pp. 74-86. Edited by B.C. Gasper. Proceedings of SPIE - The International Society for Optical Engineering, Vol. 731 (1987)

This paper describes a series of laboratory tests undertaken to demonstrate the application of thermoelastic measurements for use on typical woven carbon epoxy composites used in the aerospace industry. The tests undertaken showed that the woven carbon epoxy produced linear thermoelastic output with increasing load. No significant changes in thermoelastic output were observed with changes in warp direction relative to load direction. A carbon aluminium honeycomb specimen was examined and clear differences in the stress distribution were seen between the two surfaces of the honeycomb. These are attributed to local geometry changes resulting from the manufacturing methods. The results demonstrate the suitability of the thermoelastic effect to assess non-homogeneous anisotropic materials. Considerable time and cost savings could be made by the application of tbermoelastic techniques to existing composite fatigue and mechanical testing programmes.

37949 Deom, A.; Boscher, D.; Baleagas, D.; Gardette, G. NDT of carbon-epoxy composites by a pulsed pho to the rma l method and t ransposi t ion to s t imula ted in f ra red t h e r m o g r a p h y (In French) Qualite Revue Pratique de Controle Industriel, No. 147, pp. 58-63 (Oct. 1987)

A pulsed photothermal method has been developed by which delaminations in a carbon-epoxy composite can be detected and characterised. It involves no mechanical contact and requires access only to one surface of the test piece. A heat-flux pulse is applied to the test piece and the variation of its surface temperature is measured with an infrared detector. Any delamination causes an anomaly in this temperature curve, from which its depth and size can be calculated. This point method is

106 NDT&E International Volume 26 Number 2 1993