## 2 AE Monitoring at the 105-C Reactor Facility
### Results and Discussion
#### 2.3.3 Damage Classification Using Acoustic Emission  
To provide a means for interpretation of the data, Intensity Analysis graphs were developed at each AE monitoring location. The method was first introduced by [PERSON] and others [26] for the evaluation of fiber reinforced polymer vessels and is based entirely on signal strength. Intensity Analysis is a graphical method that differs from many other forms of acoustic emission assessments in the sense that it is focused on trends in the AE data as opposed to individual events. Intensity Analysis uses two parameters, which are both based on signal strength: (a) historic index (plotted on the horizontal axis) and (b) severity (plotted on the vertical axis). Historic index and severity can be calculated using Equations (1) and (2) where \(N\) is the number of hits up to time (\(t\)), \(S_{oi}\) is the signal strength of the \(i\)-th event, and \(K\) is an empirically derived factor that varies with the number of hits. The value of K has been previously selected in one case as:  
Figure 8: Vertical column to roof interface: (**a**) Signal strength contour plot at elevation face sensors, (**b**) source location at elevation face sensors, and (**c**) signal strength contour plot at the side face sensor. Figure 9: Signal strength contour plot: (**a**) Horizontal beam location and (**b**) control location. (a) \(N/A\) if \(N\leq 50\), (b) \(K=N-30\) if \(51\leq N\leq 200\), (c) \(K=0.85N\) if \(201\leq N\leq 500\), and (d) \(K=N-75\) if \(N\geq 501\)[4]. \[H(\ t)=\frac{N}{N-K}\frac{\sum_{i=K+1}^{N}S_{oi}}{\sum_{i=1}^{N}S_{oi}} \tag{1}\]  
\[S_{r}=\frac{1}{50}\sum_{i=1}^{i=50}S_{oi} \tag{2}\]  
Historic index, \(H(t)\), is a form of trend analysis that incorporates historical data in the current measurement. It is sensitive to changes of slope in cumulative signal strength versus time and compares the signal strength of the most recent hits to a value of cumulative hits. Severity, \(S_{r}\), is defined as the average signal strength for the 50 hits with the highest numerical value of signal strength. The intensity analysis method has been widely used for the assessment of structural systems during load testing including reinforced concrete systems [12; 27; 28] and has been extended to the case of corrosion damage in pre-stressed concrete specimens [2; 3; 4; 22]. By plotting the maximum historic index and severity values obtained over the duration of the test, an Intensity Analysis plot is generated. Due to the relationship between AE signal strength and damage growth, points that plot upward and to the right are associated with higher levels of damage. Because Intensity Analysis (IA) uses historical information, an initial point on the Intensity Analysis plot must be chosen. This may be approached through visual inspection, numerical modeling, electrochemical measurements (in the case of corrosion damage), coring and petrographic examination, and other methods including suitable nondestructive evaluation techniques. Only a visual inspection was practical for the monitored locations within 105-C. Therefore, the initial point was chosen based on visual inspection. The values of historic index and severity for the initial point were considered to account for pre-existing damage such that the historic index value at any time cannot be less than that for the initial point. For the severity, the distribution of the highest 50 signal strength values collected during the monitoring period in terms of their scattering from the mean value was used to develop the other 50 signal strength values with the same distribution but have a mean value equal to the severity of the initial point. Then the highest 50 numerical values from the collective set of 100 signal strengths, 50 from the monitoring period, and 50 developed from the initial point are used to calculate an updated severity value that takes into account the pre-existing condition.