# Wideband Vibro-Acoustic Modulation for Crack Detection in Wind Turbine Blades
## 3 Laboratory Investigation of Damage Accumulation in Steel Due to Cyclic Loading  
An experimental study was conducted in the lab to examine the use of wideband VAM for detecting cracks, using the experiment outlined in reference in [58] and the harmonics outlined in [50,59]. The sample used in the experiment was a 10-inch A36 bar with a cross-section of 1 \(\times\) 1/s inches. In order to manage the expected position of the fatigue crack, a central hole with a 1/4 inch diameter was created. Two piezoelectric disks were affixed to the sample to transmit and receive signals. We used piezoceramic with a thickness of 1 mm and a diameter of 10 mm produced by PI Ceramic (www.piceramic.com (accessed on 16 August 2023)). The specimen was installed in the material test system (MTS). The experimental arrangement is depicted in Figure 13.  
Two conditions were imposed on the specimen: assessment and damaging until it completely split. The MTS was used to apply the cyclic tensile load, aiming for a setpoint of 10.5 kN and a magnitude of 10 kN. While evaluating with VAM, the material test system (MTS) was utilized to stimulate the specimen using pumping waves at a frequency of 10 Hz and an amplitude of 0.5 kN. Together, a wideband noise signal was transmitted into the sample through one of the piezoelectric discs. An NIUSB 6361 data acquisition (DAQ) board was utilized to activate the pump wave, and its signal was increased 50 times using a piezo driver power amplifier. As a result, the notch location witnessed an interaction between the noise signal and the pump wave, which occurred in the presence of a defect. The other piezoelectric disc received the modulated signal, which was then directed to a bandpass filter with a frequency range of 110-220 kHz before being received by the DAQ board. Finally, the spectral modulated signal was analyzed using a computer. In this experiment, two types of signals were sent to the sample during the damaging process: (1) harmonic signals and (2) wideband noise signals. Figure 12: The dependence of the SNR for the wideband VAM DEMON spectrum, demonstrating the increase in the SNR with time window of analysis for the 52-m blade (Case 2). For comparison between the analysis of the VAMs of the noise signals and the sinusoidal signals, a harmonic high frequency of \(f=189\) kHz and a low frequency of \(F=10\) Hz were selected. Figure 14 shows the modulated signal spectra of both the intact sample and the same sample after developing a crack. Figure 14: Spectral analysis of the modulated signal for a probe wave at 189 kHz: (**a**) after a 5000-cycle load; (**b**) after a 48,004-cycle load [58]. Figure 13: The experimental setup for VAM using a wideband probe wave [58]. In the traditional VAM method, the MI is used to assess the non-linearity. When a crack occurs in the sample, an increase in the amplitude is observed in the two sidebands with frequencies \(F+f\) and \(F-f\). This MI measurement is defined in Equation (13). As seen in Figure 14a, when the sample is at the 5000-cycle point (undamaged state), the sidebands exhibit low amplitude, resulting in a low MI. Conversely, in a 48,004-cycle load (damaged state), as illustrated in Figure 14b, the high sideband amplitude results in a high MI measurement. To evaluate the MI for each load point for this examined steel sample, we present the normalized cycle fatigue lifetime versus MI measurement in Figure 15. In undamaged conditions, the MI remains relatively low and begins to rise when minor cracking occurs in the sample due to the tensile cycle load produced by the MTS machine. MI measurement is a conventional method used to assess cracks in materials. To compare this method to the previous example, we propose a new wideband signal, with the frequency band of 150-200 kHz incorporated into the VAM method analyzed via the DEMON technique. In the normalized cycle fatigue lifetime versus SNR, the same cycle load point at MI (5000-cycle load) displays a low SNR value, while a high SNR level is observed at the 48,004-cycle load.