AE monitoring

For each field test, a procedure for the AE monitoring is established. 

This header has undergone repair after through-thickness cracking of an assembly weld after 108.000 hours of service, it is presently in operation and periodically submitted to ultrasonic inspection during planned maintenance shutdown. Continuous on-line AE monitoring of the integrity of critical welds is expected to contribute supporting continued and safe operation of the header until its replacement, platmed to take place in 1999. 

Figure 5 provides an example of AE monitoring data from 22.01.1997 to 03,03.1997, in terms of time history of the main plant parameters (fig.Sa), as well as of the AE RMS values (fig. 5b). Normally, very little or no events were recorded, with the exception of the above period, in which an AE activity, very much concentrated in time and space, could be observed a sharp step in cumulative AE events takes place in a short, well defined time interval. A smaller sharp step in EA events had been observed a few days earlier, in the same position. 

The same general comments hold as for Unit 3. Figure 7 provides an example of the AE monitoring data collected from 19.06.97 to 16.07.97, in terms of the main plant parameters vs time (fig. 7a), as well as of the AE RMS values (fig. 7b). 

According to JCMS-IH [2003], such AE parameters as RA value and the average frequency Fa are defined to classify cracks as stated in Chapter 4. From these two parameters, traffic noises could be practically discriminated. In a buried water-pipeline, AE monitoring was conducted (Suzuki and Ohtsu 2004). Because the pipeline was located below road, traffic noises were often detected due to heavy vehicles. Consequently, AE monitoring was conducted when cars and trucks were passing. Results are sununarized in Fig. 9.2. 

An early example of AE monitoring in wood was conducted by DeBaise et al. [1966]. They presented cumulative AE event records for a variety of loading configurations of western white pine. The results they report illu-... 

Schniewind et al. [1996] used AE monitoring to examine issues of temperature and moisture content. Specifically they wanted to see if AE signals could be used as part of a control process for wood drying procedures. They separately examined mode I and mixed mode fracture, and found that mixed mode fracture induced significantly more AE activity. 

While there are many more interesting and relevant applications of AE monitoring to wood properties, we now choose to focus on a specific study to illustrate how AE techniques were evaluated in the context of fiacture mechanics. The study is illustrative on both the value and diffieulties of applying quantitative AE techniques to wood. 

Noguchi M, Fujii Y, Owada M, Imamura Y, Tokoro M, Tooya R (1991) AE Monitoring to Detect Termite Attack on Wood of Commercial Dimension and Posts. Forest Products Journal 41(9) 32-36... 

For AE monitoring of existing concrete structures, it is essential to confirm that any AE signals responsible for the deterioration are not observed under service conditions. In the case that AE signals not of noises but due to deterioration process are detected, the monitoring and the analyses shall be conducted, llie monitoring is performed continuously or routinely, and sometimes temporarily after the disasters. In this case, the selection and classification of AE signals which is closely associated with the deterioration are necessary. 

Shigeishi M, Makizumi T, Jo H, Ueda J (2003) AE monitoring of a reinforced concrete road bridge. Structural faults and repair 11 (CD-ROM) NDTB-SHIG... 

A basic concept for AE monitoring has been proposed as illustrated in Fig. 

Fig. 14.3. AE sensor array and zones of AE monitoring (left direct, right indirect monitoring).

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