Theoretical and Experimental Study of Composite Materials Using Longitudinal Ultrasonic Spectroscopy

Abstract

INTRODUCfIONComposite materials are nowadays involved in various designs [1] in areas like aeronautics, road, rail or sea transportation, and chemical industries, among other. Using these material brings a number of advantages : the lowering of overall mass, the increase in the applied mechanical loads before breakdown occurs, the resistance to contamination by chemical agents, the ease of material forming using molding techniques, and the matchability of the material to the particular mechanical function required for a given part in a structure. Among composite materials, the carbon-epoxy ones are costly but also very efficient. They find important applications in aeronautics and we will focus our attention here to them.Like for any machined product, defects are induced during the realization of composite material parts. However, the nature of these defects is very specific (delamination, porosities ... ) when compared to metals. The heterogeneous and anisotropic nature [2,3] of these materials make them very difficult to control and specialized testing procedures must be elaborated for this purpose. Specifically, ultrasonic techniques appear well suited for the detection of bulk defects [4]. The classical ultrasonic techniques rely on the observation of the time diagram of the echoes reflected on or transmitted through the defects in the sample under study. The heterogeneous structure of composite materials may however induce extraneous acoustic noise [5] and make the interpretation of the previous echogram very questionnable and often impossible. Spectral analysis [6] appears then as a complementary tool useful for overcoming these difficulties. The study reported in this paper will make use of the two techniques, all the tests being performed with bulk longitudinal ultrasonic waves impiging under normal incidence on unidirectionnal or crossedply carbon-epoxy composites. STUDY OF THE LOW FREQUENCY RESPONSE OF COMPOSITE MATERIALS.Several sets of samples have been studied, whose main characteristics are summarized in table 1. Their response to low frequency longitudinal waves (in the 5 to 30 MHz frequency range) has been first investigated.

Signal-processing methods for analysing the structure of carbon-epoxy-resin composite materials

“…A reasonable fit between the mean ply thickness values obtained on a number of unidirectional and cross-ply composites samples using all the previous methods has been obtained [2,3,5,6]. Some dispersion in the ply thickness has also been found.…”

“…If one is working in ultrasonic continuouswave mode, the elementary reflected waves combine then in amplitude and phase, giving a frequency-dependent global result. This is the principle of the ultrasonic resonancespectroscopy method [1][2][3][4], in which an absorption dip is found in the transmitted spectrum at a frequency…”

“…for a ply, a number (depending on the frequency and attenuation) of reflected waves may be resolved individually [3]. This allows the measurement of the ply thickness and acoustic-reflection coefficient at the boundaries.…”

Characterization of the Ply Boundaries in Carbone-Epoxy Composite Materials Using Longitudinal Ultrasonic Waves

“…The aim of the present paper is to recall the main results of the previous studies in the 5 to 100 MHz frequency range and to present a complementary study on the ultrasonic dispersion phenomena in crossed-ply Carbone epoxy Composite materials. The study of the dispersion of longitudinal waves constitutes a complementary tool for the Control and characterization of the ply boundaries in Carbon-Epoxy Composite materials.…”

“…In recent studies of the propagation of longitudinal ultrasonic waves impinging at normal incidence on such materials [4,5), the finding of absorption frequencies in the transmitted energy spectrum for crossed-ply composite samples has been reported. This phenomenon is attributed to interferences between waves multiply reflected at the ply boundaries, the mechanical mismatch being due to the presence of a very thin residual epoxy layer between each pair of adjacent plies.…”