# EMD-Shannon Entropy-Based Methodology to Detect Incipient Damages in a Truss Structure
## 4 Experimentation  
### Experimental Setup  
In order to validate the efficiency and usefulness of the proposal, the experimental data of a 9-bay, 162-member truss structure in a bridge style, located at Autonomous University of Queretaro, campus San Juan del Rio, are employed for assessing the healthy condition of the bridge, as well as its damage condition produced by corrosion with different levels of severity. Figure 2 shows the experimental setup. The bridge is made of aluminum and has the following dimensions: 6.4 m length, 0.71 m height, and 0.71 m width. It is subjected to white noise produced by an electrodynamic shaker from Labworks model ET-126B powered by a Labworks Model PA-138 linear power amplifier in order to simulate low-intensity ambient vibrations, i.e., a real-life situation [2], for assessing the bridge's state. A tri-axis accelerometer, Ax, Ay, and Az, from KISTLER model 8395A placed on the four bay is used to measure the vibrational response of the bridge. It measures \(\pm\)10 g with a resolution of 400 mV/g and a bandwidth from 0 to 1000 Hz. The vibration signals measured by the sensor are stored and sent to a PC using a data acquisition system (DAS) from National Instruments model NI-USB-6001 with a 14-bit resolution analog-to-digital converter (ADC). A sampling frequency of 200 Hz is set into  
Figure 1: Schematic diagram of the proposed methodology.  
the DAS, since the excitation presents a bandwidth from 0 to 100 Hz. It is important to mention that ambient vibrations are in this bandwidth or of low frequency [2]. The acquisition time for each test is 20 s, which results in 4000 samples. The experiment is repeated forty times for each condition (healthy and corrosion to different levels of damage i.e., 1 mm, 3 mm, 5 mm, and 8 mm (a total of 1480 tests) in order to generate statistical information.  
Initially, the vibration signals of the healthy bridge are acquired by using the sensor placed in the four-bay of the structure, as shown in Figure 3; then, the proposed methodology is applied to each axis of the vibration signals (Ax, Ay, and Az), where the nonlinear index (SEI), one for each axis and for each test of the healthy structure, are obtained. Next, the different levels of corrosion are applied to the bridge, and the measured vibration signals are proceeded by the proposal to estimate the nonlinear indices that allow us to estimate the structure's condition. In this sense, in the first-bay, one of the elements is replaced by another with damage. By using the same location, the elements with different levels of damage are applied. This procedure is repeated for all the ways of the bridge, and their respective nonlinear indices are obtained. It is important to mention that the location of elements with damage is selected randomly in each bay with the aim of demonstrating that the proposal can detect damage regardless of the location of the damaged element. Figure 3 shows the location of the damaged elements applied to the bridge. It is also important to note that only one sensor is required to detect the damage regardless of the damage location, demonstrating the usefulness and effectiveness of the proposed technique.  
To exemplify the process, Figure 4 illustrates the measured vibration signal for healthy conditions and incipient damage produced in the first-bay (1 mm of reduction in the diameter of the bar element) in the all directions (Ax, Ay, and Ay), respectively. This figure shows that specific differences related to the structure's condition between the healthy and corrosion-damaged segments cannot be readily observed. Hence, a method capable of revealing differences between a healthy and a damaged structure is presented here.  
Figure 3: Location of the bar elements with different damages applied to the bridge.  
Figure 2: Experimental setup.