·Table of Contents
·Materials Characterization and testing
Material Characterisation ProbeHarri Jeskanen, Pentti Kauppinen, Jorma Pitkänen and Seppo Tähtinen
VTT Manufacturing Technology
|Fig 1: Principle of measurement with low-frequency wide-aperture transducers.|
Possible cracks opening to the surface or lying slightly below the surface will be detected based onthe disappearance of signal of leaky-Rayleigh-waves (Fig. 1c).
Fig 2: Principle of construction of low-frequency wide-aperture transducers.
The result of transducer characterization performed by utilizing an alumina plate containing indentations made with a Vickers hardness tester is shown in Fig. 3. The dimensions of the beam are estimated to be 1 x 4 mm (width x length). The penetration depth is approximatelly l.
|Fig 3a:||Fig 3b:|
|Fig 3: (a) Drawing showing the alumina plate (20 x 30 mm) used for characterization of the beam. The positions and type of the 6 indentations made with the Vickers hardness tester are shown.(b) Ultrasonic C-scan image demonstrating the dimensions of the beam on the surface of the plate.|
Measurements based on the velocity of Rayleigh-waves
|Fig 4a:||Fig 4b:|
|Fig 4: (a) Time-of-flight C-scan image of a stainless steel block coated with layers (600, 400 and 200 mm) of thermally sprayed alumina. The image shows the relative sound velocities in the coating. (b) The velocity in the substrate affects the velocity measured on the surface of the coating.|
The same principle can be used to measure the variations in the velocities of Rayleigh-waves which correlate e.g. with the porosity and density of the material.
Measurements based on the attenuation of Rayleigh-waves
The measurements shown in Fig. 5 (b) and (c) have been performed by using the contact transducers shown in Fig. 2b.
|Fig 5: (a) A gas turbine blade containing electrodischarged notches of various depths. (b) Ultrasonic B-scan image of the blade measured in pitch-and-catch mode. (c) Ultrasonic B-scan image of the same blade measured with the same transducer in pulse-echo mode.|
Fig 6: (a) Small scale prototype of ITER (International Thermonuclear Experimental Reactor)
first wall after thermal fatigue test at 5 MWm 2 heat flux for 1000 cycles with 15 seconds dwell time.
(b) Cross section of the copper layer showing cracks initiated from the copper surface. Ultrasonic
C-scan images of the copper interface before (c) and after (d) the high heat flux test.
| Fig 7: (a) Test block containing a 0,5 mm deep artificial defect and (b) a laminar disbonding.
(c) Detection of surface opening defects by monitoring the leaky-Rayleigh wave. (d) Disbonding of
the brazing shown in the C-scan image produced with longitudinal waves.
By proper design of the transducer both laminar defects and surface defects can be detected with the same transducer.
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