PSI - Issue 4

Rainer Boehm et al. / Procedia Structural Integrity 4 (2017) 71–78 Boehm, R. / Structural Integrity Procedia 00 (2017) 000 – 000

76

6

by the large element pitch in the region of the outer diameter of the cone. Caused by the cones design, echoes coming from the grating lobes have low amplitudes (-34 dB) in relation to the main echo and a constant angular distance to the main echo. The number of active elements has an influence on the directivity pattern, the amplitude and the spatial resolution. The amplitude arises with the number of elements lying side by side. However the part of the amplitude, respectively the part of the echo height, becomes smaller towards the edges of the active area and depends on the distance to the middle active element, see fig. 11. a) and b). Fig. 12 shows the arising amplitudes which are not linear to the number of active elements. Additionally, the beam divergence (bw) becomes smaller with a higher number of active elements. The beam divergence as a function of the number of active elements, is shown in fig 12.

0

-9 -6 -3 0

-3

-6

-36 -33 -30 -27 -24 -21 -18 -15 -12

a)

b)

-9

-12

-15

-18 Amplitude per element in dB

Echo height per element in dB

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

Element position in the array

Element position in the array

Fig. 11. Amplitude per element in relation to the middle active element of the array dependent from elements position

17 active el., bw = 1.6 13 active el., bw = 1.8

9 active el., bw = 2.3

Amplitude

5 active el., bw = 3.8

Circumferencial angle position in

Fig. 12. Amplitude and ultrasonic beam width (bw) as a function of the number of active elements

The element length determines the near field length in the plane incidence, this is the axial-radial plane of the axle. In our example the test region is within the near field length, if the element length is larger than 15 mm. that delimitates the maximum sensitivity and the spatial solution in the axis direction, see fig. 13. and fig.14. A possible solution is to focus the sound beam additionally by bending the elements in the plane of incidence.