Issue 50

I. Dakanali et alii, Frattura ed Integrità Strutturale, 50 (2019) 370-382; DOI: 10.3221/IGF-ESIS.50.31 372 T HE EXPERIMENTAL TECHNIQUE AND ITS APPLICATION Acoustic Emission hen a structure is under stress and the stress field exceeds the materials’ capacity, an abrupt release of energy occurs producing transient elastic waves that propagate through the material in the form of concentric circles. The elastic waves propagate inside the material and they can be detected by AE sensors attached on the struc- ture’s surface [11]. The acoustic emission technique has three significant advantages. It can locate the damage, provide information for the nature of the acoustic sources and quantify the damage. The accurate source location is an extremely complicated task. The main reasons are that the signal travels in different modes and has a range of velocities. There are also materials that have different velocities in different directions within their volume. Moreover, there are many defects in a material and in most cases the waves are reflected, which causes superpositions or attenuations. Consequently, when the signal is recorded by a sensor it has already been subjected to many alterations [12]. Based on the AE characteristics, useful information can be extracted for the source’s nature. Continuous emissions produced by friction [13] have completely different recordable characteristics compared with the burst signals produced by the initiation and propagation of cracks [11]. The basic advantages of the AE method are its high sensitivity, the early detection of defects and cracks and the real-time monitoring at a relatively low cost [14]. Nowadays, an alternative approach is widely used for the classification of the acoustic emissions’ source based on the relation between the signals’ average frequency with the RA (Rise Time/Amplitude) parameter. The Rise Time RT, is the time interval between the first threshold crossing and the signal peak (Fig.2b). The Rise Time is used for the qualification of signals and as a criterion for noise filter. The amplitude is the greatest measured voltage in a waveform and it is measured in decibels (dB). This is an important parameter in acoustic emission inspection because it determines the detectability of the signal. The signals with high average frequency and low RA parameter are attributed to tensile cracks. Otherwise, signals with low frequency and higher RA are caused by shear or mixed mode cracks. So far, the results of the qualification of the damage by AE parameters have been proved very encouraging [15-17]. Figure 2 : (a) Qualification of the damage by AE parameters [15]; (b) Parameters of an acoustic signal [16]. Experimental procedure Three types (‘classes’) of specimens were prepared. The basic specimens’ configuration consists of a marble block (10cm x 10 cm x 15 cm) with a central through hole (diameter:14mm) (Fig.3a). Half of the predrilled hole is filled with a liquid cement paste (binder and water without any aggregate) (Fig.3b). The other half of the hole remains empty. To ensure that, a soft cloth is inserted into the hole preventing the liquid from escaping till its final curing . Meanwhile, a threaded titanium bar is driven into the paste for an appropriate length of the hole (Fig.4c). After removing the cloth, the bar-paste sub- assemblage is visible into the hole, either at the bottom end of the specimen or half way through its height depending on the specimen’s class, providing an entry point for monitoring the bar’s pull-out. More specifically, the first class of experiments includes the classical pull-out tests (1 st class) [18] with the anchoring length being limited to the upper half of the marble’s hole (Fig.4c). The marble block is restrained by a rigid metallic plate with a hole (diameter= 5cm) to its center, where the titanium bar is inserted and gripped by the frame’s upper jaw (Fig. 4a, b). The plate is supported by 4 stiff threaded steel bars (Fig.4a). (a) (b) W