# Microcrack Detection Using Spectral Response Data Alone
## 3 Crack Inspection with Simple Specim  
To verify the theory, an impact experiment was conducted using a simple rectangular specimen, one end of which (40 mm) was clamped using a rigid body jig, as shown in Figure 3. Two simple specimens were prepared for the impulse test and the material of the responsible specimens was SS400 (POSCO, Pohang, South Korea); they differed in that one specimen had a microcrack along its middle at one face. The response data for these simple specimens were obtained using attached unidirectional accelerometers (#1-#6, Model: 3225F2, Dytran, Chatsworth, CA, USA), and that for the rigid body jig was obtained using another accelerometer (#5), as shown in Figure 3. Three acceleration sensors (#1, #2, and #5) were attached at the front planes (X-Y), and the two sensors (#3, #4) were attached at the rear planes (X-Y) as shown in Figure 3. The other two acceleration sensors (#5, #6) were attached at orthogonal planes (X-Z). Because the microcrack was located at node #1, as shown in Figure 4a, the response at node #1 was the nearest to the crack position (see Figure 5) and that at node #2 was far from the cracked position on the same face of the specimen. The other specimen had no crack at node #1, as shown in Figure 4b. Nodes #3 and #4 were at a similar distance from the cracked location in a manner corresponding to nodes #1 and #2, respectively, except that the face of the measured specimen was opposite to the cracked one. Because the specimen had a square cross section and the clamping area (40 mm \(\times\) 19 mm) was perpendicular to the two measured faces of the specimen, the boundary conditions for the two faces were assumed to be similar except for the existence of the crack. Figure 3: _Cont._  
Figure 4: Attachment location of sensor #1: (**a**) specimen with crack and (**b**) specimen without crack. Figure 3: Configuration of simple specimen with one end clamped and sensor locations: A: 40 mm, B: 150 mm, and C: 9.5 mm. Before approaching the microcrack detection, the experimental modal analysis was preliminarily conducted for two rectangular simple specimens with and without microcrack as shown in Figure 4. The impact hammer (Model: 5800B3, Dytran, Chatsworth, CA, USA) was used to assign impact force at #1, and response acceleration data were obtained at seven locations from #1 to #7, respectively. The acquisition of FRFs was done with TEST.LAB (Siemens, Munich, Germany), and the identification of modal parameters, both the resonance frequency and the modal damping coefficient, were conducted with PolyMax algorithm in TEST.LAB software. Maximum frequency was set to 3200 Hz, and 1024 spectral lines were used during the modal parameter identification. In case of uncracked specimen case, the first bending mode was found at 1145 Hz (with 0.38% damping coefficient) and the X-Y plane was in-plain mode, whereas the first mode of cracked specimen was 1090 Hz (0.51% damping coefficient, in-plain for X-Z plane) and the second bending mode could be identified at 1150 Hz (1.15% damping coefficient, in-plain for X-Y plain). The modal assurance criterion (MAC) value between two bending mode showed 0.57 owing to the orthogonality between X-Z plane and X-Y plane. The third peak of the cracked specimen was also found at 1185 Hz but that mode was not considered because MAC value between the second bending mode and the third one was 92.3 so that two peaks seemed to be very similar each other. The resonance frequency of the rectangular simple specimen was decreased from 1145 Hz to 1090 Hz, and sideband frequencies were also initiated under the presence of microcrack at the simple specimen. The results of two rectangular simple specimen of the modal analysis are summarized in Table 1.