Ultrasonic methods impedance testing

C. Kleesattel, "The Ultrasonic Contact Impedance Testing Method," The ECHO, Vol. 27, Krautkramer GmbH. 

Figure 17.94 Ultrasonic C-scan of cfrp laminate showing an area of impact induced delamination white indicates negligible damage and black bad damage. Source Reprinted from Cawley P, The sensitivity of the mechanical impedance method of nondestructive testing, NDT Int, 30(4), 209-215, Aug 1987.

In order to reduce the occurrence of rail-defect-related failures, various methods are used to detect rail defects. These methods include visual inspection by the track maintenance staff, ultrasonic defect detection, eddy-current testing, ground-penetrating radar, impedance spectroscopy, and electromagnetic acoustic transducers [16,17]. 

However, it appears that none of the non-destructive tests currently employed directly correlate with any critical failure property. Most industrial test techniques such as through-transmission and pulse-echo ultrasonics, sonic vibration techniques. X-ray radiography, thermal inspection methods, holography, liquid penetrants, etc. basically attempt to find defects in the joint. Such defects may arise from several sources. Some defects arise from porosity, cracks or voids in the adhesive layer or at the interface and are typically filled with air they will simply be referred to as voids in the present discussions. However, during the service life of the joint such voids may fill with water which makes them far more difficult to detect since, for example, water has a much higher acoustic, impedance than air. Also, zero-volume voids, or debonds, may occur when the adhesive and substrate are in contact but no... 

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