The dynamic excitation method is a special method for vibration excitation, developed by isi-sys. It features many advantages compared to other methods of load, e.g. a very good reproducibility and depth effect.
The procedure uses the fact, that the local eigen-mode of vibration of defects gets visible separately from the modes of vibration of the entire object, already at little excitation power by a shearographic measurement (time average) in a real time measurement.
The vibration states are visible during a frequency sweep of the excitation system in real time (image 1). Usually, this suffices to locate the defects. The subsequent measuring of vibration gradients, -phase and -amplitude allows to determine the mechanic properties of the defect and the eigen-frequency of the object (image 2).
Example:
The example down below shows chosen images of a C/C-Si-C heat shield tile with a delamination in the middle. The frequency sweep went from 1-20 kHz. The analysis is based on the measuring of the light phase of the vibrating delamination (at 20 kHz). Subsequently, phasing and amplitude of the vibrating object can be determined using image processing methods.
Image 1: time average shearograms of chosen modes of vibration from the frequency response of a C/C-Si-C heat shield tile for the spacecraft X38, a rescue vehicle of NASA
Image 2: Automated process of analysis by isi-Studio software using the example of the C/C-Si-C heat shield tile from image 1. The unfiltered, relative phase distribution (a), which was reconstructed from the measured intensity distribution of the shearogram is made stable to a continous distribution (b). This is proportional to the gradients of the deformation w(x,y) of the object.
At first, an automated, patented analysis is conducted to determine the absolute values of phasing. The deformation gradients (dw/dx and dw/dy) of vibration (c) are determined in this manner. Subsequent to this, the deformation gradients are integrated to the vibration amplitudes. (e) is a 3D view of the same vibration amplitude.
a
b
c
d
e
After the apperance of the modes of vibration of the tile at small frequencies one encounters the local eigen-mode of vibration of a delamination from about 20 kHz. Therefore, defects can be identified exactly if the mode of vibration of the object, if regarded as a superposition with the mode of vibration of the entire structure, at the same excitation frequency, is subject to a stronger damping than the local area of the defect.
Does one hence encounter eigen-frequencies of the area, which is local weak in its stiffness by the defect, the defect emerges most clearly -provided that the frequence or its multiple do not accidentally encounter further eigen-modes of the object.
The vibration amplitudes and -phases of the chosen frequency can be analysed automatically using the vibrography-system. The system offers a stroboscopic method as well as a dynamic method for this purpose. Image 2 shows exemplary the automated process of analysis by isi-Studio software using the example of the heat shield tile from image 1.
20180 Hz
Shearography NDT Dynamic excitation method
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Copyright by isi-sys GmbH, T:++49 (0) 561-739798-0, info@isi-sys.com, last update on 11. oct 2011
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