# Damage Detection Using Modal Rotational Mode Shapes Obtained with a Uniform Rate CSLDV Measurement
## 4 Experimental Validation  
### Experimental Configuration  
The test piece in this paper was a steel plate with the same size and material properties used for numerical analysis. In order to reduce the measurement noise, a reflective sheeting was pasted on the measured surface of the plate to maximize the backscatter of laser light. The plate was clamped at the bottom edge with four M8 bolts and excited via a sound speaker near the top edge. The sound speaker is located close to the plate in order to ensure the plate can be excited. The CSLDV system consists of a Polytec PSV-400 LDV scan head, NI PXI system and an amplifier. The scanning mirrors in the Polytec LDV scan head is controlled by an analogue voltage input and output card in the additional NI PXI system in order to manipulate the continuous measurement of the rectangle area. The vibration signals are also captured by the NI PXI system. A sinusoidal excitation signal was generated by the junction box of Polytec system and was amplified by the amplifier before sending to the sound speaker. The control of all the hardware and signal processing were accomplished by the LabVIEW codes. The test configuration, scan head of LDV and NI PXI system are plotted in Figure 6a.  
In order to facilitate control of the scan path of scanning mirrors, the original position of the laser spot should be located at the center of the plate. Moreover, the distance between the scan head and the laser spot was measured and prepared for calculating rotation angles of the mirrors in order to cover the area of the scanned plate as much as possible. To avoid the control mirrors rotating too much, the distance should be larger than 1.5 m and the corresponding maximum rotated angle is less than 3.5 degrees for the test piece. In order to increase the spatial resolution, the sampling frequency rate is set to be 16,384 Hz. Scan rates of 0.05 Hz along the x-direction and 10.0 Hz along the y-direction were chosen to give a Zigzag trajectory with a good coverage of the objective surface. The scanning boundaries are (\(-\)88, 88) along the x direction and (\(-\)39, 39) along the y direction to avoid the laser spot moving out of the plate. With these parameters, the spatial resolutions along the x and y directions are about 1.7 mm and 0.05 mm, respectively. It only takes 20 s to complete the continuously scanning measurement.  
Three damaged cases, described in Table 1, are considered and discussed to demonstrate the effectiveness of the proposed method. The first case is a single crack at Crack 1 position, similar with the simulation in Section 3.1. The crack length is 13 mm and approximately 16.25% of the width of the plate. The second is a multi-crack case with two different length cracks at different locations, as shown in Figure 6b. The first crack is the same as the first case and a second crack with 22 mm length is added  
Figure 6: The test configuration. (**a**) The test configuration, scan head of laser doppler vibrometer (LDV) and National Instruments (NI) and PCI extensions for Instrumentation (PXI) system. (**b**) Test piece of multi cracks.  
at Crack 2 position, which is more than 27.5% of the width. The last case is a slight crack with only 6 mm length (7.5% of the width) at the position of Case 1 to illustrate the ability to detect a slight damage.  
At the beginning of the test, frequency response functions of several points were measured, and the first four mode frequencies of these damaged plates were obtained. These mode frequencies later were set as the excitation frequencies in uniform-rate scanning CSLDV tests to measure the corresponding mode shapes.