2.3.2. Evaluation of Data at the Crane Maintenance Area

Figure 7a,b show the cumulative signal strength (abbreviated as CSS) at each monitored location for resonant and broadband sensors, respectively. Signal strength of an AE hit is a measure of the area under the recorded signal envelope (sometimes referred to as MARSE, Measured Area under the Rectified Signal Envelope) [13]. Higher levels of signal strength are associated with higher levels of energy release due to crack growth events.

**Figure 7.** Cumulative signal strength (CSS) of: (**a**) Resonant sensor and (**b**) broadband sensors.

While the signal strength of an AE hit is related to the intensity of damage growth at a particular instant in time, cumulative signal strength is related to increases in damage growth rates over a particular testing period. Rapid increases in the cumulative signal strength curve are related to rapid increases in damage growth. The relationship between rapid changes in the cumulative signal strength curve and damage growth has been utilized to assess damage in different structural systems [26] including reinforced concrete bridges [12] and corrosion damage in reinforced concrete laboratory specimens [2,3].

In both Figure 7a,b, it is apparent that sharp changes in the slope of cumulative signal strength that indicate sharp increases in damage progression related to the vertical column to roof interface location occurred in several different instances. For example, a sharp increase in damage growth is noticed at the end of November, between 4 March 2015 and 13 March 2015, and between 8 April 2015 and 15 April 2015. These sharp increases were noticed for both the resonant and broadband sensor types. As expected, the broadband sensors exhibit slightly lower values of cumulative signal strength due to the relatively low sensitivity of this sensor type.

The highest change of slope for resonant and broadband sensors at the vertical column to roof interface occurred at the end of November in 2014. This sudden increase in cumulative signal strength was accompanied by localized spalling of concrete, which may have caused the detachment of two sensors previously mentioned. This spalling supports the findings that significant damage occurred during this time period.

To allow for the comparison of AE activity from each sensor, the response of broadband sensors was normalized to that of resonant sensors. The normalization was determined based on the application of a simulated source [25] applied at both resonant and broadband sensor locations on the reactor concrete block. Pencil lead breaks (PLBs) were applied at different angles around a resonant sensor (0, 45, 90, 135, and 180 degrees) at distances of 3 inches and 6 inches in each direction. Three PLBs were applied at each distance. The CSS recorded from PLBs applied at each distance was calculated separately. The same procedure was repeated for a broadband sensor. The ratio of CSS detected from the resonant sensor to the CSS from the broadband sensor was calculated for the cases of 3 inches and 6 inches from the sensor. The average of the ratios achieved at the two distances was found to be approximately equal to 10. Thus, cumulative signal strength detected from WDI sensors was normalized using a factor of 10.

Figure 8a is a visual representation of the intensity of damage at each sensor location using a contour plot. The plot is based on cumulative signal strength results (units of pico-Volt seconds) where high cumulative signal strength is plotted in red, which indicates high damage while low cumulative signal strength is plotted in blue. This indicates lower damage. The contour plots show relative intensity of AE activity.

As seen in the plot, the highest normalized cumulative signal strength values were detected at the top left of the elevation face sensors and at sensor 9 at the side of the vertical column. The 2D source location results (for the data detected from the five sensors at the same plane) show that most AE events were also detected at the top left of the sensor grid, which suggests that damage is progressing at this location (Figure 8b). Figure 8c likewise indicates very high damage progression in the vicinity of sensor 9 with the highest value of normalized CSS detected at sensor 9.

Figure 9 shows similar contour plots at the horizontal beam and control locations. Similar to the vertical column to roof interface location, normalized data was used to generate the plot. The same contour scale seen in Figure 8 was used to generate the plots. As seen in Figure 9, lower damage occurred at the horizontal beam location (Figure 9a) and the control location (Figure 9b) when compared to vertical column-to-roof interface location.

**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.
