Defect Characterization in Thick Composites by Ultrasound

Abstract

In today's application of composites, thick composites are beginning to be used for load bearing structural members. nondestructive evaluation of composites relies heavily on ultrasound as The the probing field, but the ultrasonic NDE of thick composites poses challenges. First, to penetrate a large thickness of composite one must new use ultrasound of low frequencies, but the long wavelengths at low frequencies afford only poor resolution. Secondly, with increasing thickness, the anisotropy of the material assumes a greater importance and certain simplifying assumptions acceptable in thin composites are no longer valid. Moreover, in pulsed ultrasonic measurement of thick composites, the high total attenuation associated with the large thickness and its frequency dependence often changes the spectral content of the pulses considerably [1). The main objectives of this paper are to use ultrasonic techniques to evaluate the porosity content and to measure delamination sizes in thick composites. Porosity has been known to degrade the mechanical properties of composites; porosity occurring between the plies is particularly detrimental to the interlaminar shear strength of laminates. A method developed previously for estimating porosity content in thin laminates [2,3), in which the void content was slope of the attenuation versus frequency curve (da/df, or the was related linearly to the "attenuation slope".) content in thick composites is The utility of this method for quantifying void now explored.