NONLINAR ULTRASONIC SPECTROSCOPY OF FIRED ROOF TILES

Paper deals with non-linear interaction between elastic wave and structural defects in fired roof tiles. Present ultrasonic non-destructive testing methods are very sophistic, but they have some limitations for practice use. For example, it is an impossibility integral testing procedure, time demanding or practically infeasible testing of solids with inhomogenous materials or with complicate forms. In such examples it is possible to utilize measuration of non-linear effects of wave propagation and creation of higher harmonic signals in the vicinity of defects. This paper describes measuring set-up method for non-linear effects determination. First, the short summary and comparison of various methods of nonlinear ultrasound spectroscopy is shown. It was chosen the CV method with one pure harmonic exciting signal and with evaluating of 2 and 3 harmonic element as a quantity of nonlinearity. The corresponding measuring set-up is described. This method is practically applied to fried roof tiles because testing of this material is problematic by other NDT method. The selection of exciting frequency by analysis of frequency responses is shown. Measured spectrums for good tiles and for tiles with cracks shows usability of this method for rated aim. Introduction: Among the various non-destructive testing (NDT) techniques, the ultrasonic methods are perhaps the most frequently used. On the other hand, it has some limitations for practice use. For example, it is an impossibility integral testing procedure and on the account resulting elaborateness, time and financial expensiveness or practically infeasible testing of solids with inhomogenous materials. Analogical problems bring ultrasound testing of bodies with complicate forms. These and other problems of ultrasounic NDT leads to searching new methods. New promising non-destructive testing methods are based on the non-linear behaviour of current defects and inhomogeneities regarding the elastic wave propagation processes. In a present paper, the non-linear techniques, know as NEWS (Nonlinear Elastic Wave Spectroscopy) and their effective use are discussed. There are two groups of methods available for application: resonance and non-resonance. Bodies exhibiting strong resonance effects make it possible to study, above all, the non-linear effect of the resonance frequency shift versus exciting signal intensity. These methods are usually referred to as SIMONRAS (Single Mode Nonlinear Resonance Ultrasound or Acoustic Spectroscopy) [2-4]. The resonance methods are rather labour-intensive and require many readings to be taken (frequency response curves for various signal magnitude levels). Therefore, they are not suited for fast in-process measurements. Non-resonance methods are used to study suppressed resonance specimens. These methods analyse the effect of non-linearities on acoustic signals propagating through them. These methods can be split into two groups. In the first group, a single ultrasound harmonic signal is employed. The non-linearity gives rise to additional signals featuring different frequencies according to Fourier expansion. In general, the amplitudes of these additional components decrease with the natural number n: fn= n f1 n = 0, 1, 2..∞ , (1) Nevertheless, among the emerged signals, the third harmonic appears to be most pronounced, see Fig. 1. This is why the third harmonic amplitude is pursued by most researchers, especially in electronics [7].