GE Inspection Technologies, Probes

Definitions

Description	Explanation
Element size D or a x b	Diameter D or length x width a x b of the transducer element. The size of the element strongly effects the shape of the transmitted sound field. Slight deviations, e. g. imperfect shape or positions with reduced radiation due to poor bonding, cause considerable evaluation errors, even when calibrated to a reference flaw! We grind the elements to precise dimensions in order to guarantee narrow tolerances and do not, for example, break them from larger elements for cost reasons.
Nominal frequency f	The mean frequency of all probes of the same type. The frequency has a great influence on the evaluation of reflectors. Even the shape of the sound field and the reflection behavour of angled reflectors are strongly dependent on the frequency. With increasing frequency, the echo height from non-vertically positioned reflectors to the sound beam decreases. This is why each probe is checked by our Quality Control to see if its frequency coincides with the nominal frequency, according to the identification label, within very narrow tolerances. This is entered into the probe data sheet.
Bandwidth B	The range of frequencies in the echo pulse whose amplitude, at the most, is 6 dB less than the nominal frequency. fo = upper, fu = lower frequency limit for a 6 dB drop in the amplitude. With B = 100%, a 4 MHz probe for example has an fo of 6 MHz and an fu of 2 MHz. Large bandwidths mean shorter echo pulses, which mean high resolution and a good penetration power, because the lower frequencies of the pulse become less attenuated than the nominal frequency. At high attenuation, the frequency of reflected signals decreases, compared to the nominal frequency, as the distance increases. This must be taken into account with flaw evaluation. The bandwidth of each probe is therefore checked and must, within narrow tolerances, coincide with the mean value of all probes.
Operating range AB and AB6/1	AB: Distance range in which the echo of a flat bottom hole, having a diameter of 2 mm in steel with low sound attenuation, can be clearly detected. AB6/1: Distance range in which the echo of a 1 mm flat bottom hole, in the immersion technique, drops to half the height of the maximum indication.
Focal distance F Near field length N	F: The distance of a small reflector from the probe producing the highest possible echo. Probes are focused in order to detect small reflectors and produce a high echo amplitude. Focusing is only possible within the near field of the probe. The near field length N is the focal distance of the unfocused probe which constitutes the sound pressure maximum at the largest distance from the probe. N is determined by D, c and f. λ = wave length c = sound velocity D_eff= effective element diameter Focal point and near field length are the distances with the best sound concentration and reflector recognition. Therefore when a probe is selected for a critical test, the flaw expectancy range must be in the focal area or near field length. The data in the tables refer to steel with the exception of immersion testing in water.
Echo width EB	Practical measure for the far resolution of a probe. EB is the width of a backwall echo with a screen height of 80 %, at a near field length or focus, read off at a screen height of 20 %.
Focal diameter FD₆	Diameter of the sound field in the focal distance or near field length with a 6 dB drop of the echo indication.
Near resolution FBB and ZYB	Detectability of a flat bottom hole (FBB) or cylindrical bore hole (ZYB) with specified diameter (... Ø) under the surface (in ... mm). With the immersion technique, the surface of the test object is positioned in the near field length or in the focal point. For example FBB0.4Ø in 1.0 means, that a flat bottom hole of 0.4 mm diameter is resolved at a depth of 1 mm.
Beam shape	The shape of the probe sound field for a 6 dB drop of echo height. The corresponding numbers in the tables relate to the forms shown in the pages 37 to 39.
Pulse shape	The presentation of signals, as they are at the instrument input coming from plane reflectors.
Spectrum	Display of all the frequencies in the echo pulse. The frequency amplitudes are shown over the frequency.
Beam angle ß	The angle between the main beam and the normal axis of the test surface.
Gain reserve Vr	Possible gain increase from a backwall echo within the near field up to the noise generated in the system instrument/probe. The higher the gain reserve, the greater the sound penetration depth, and the smaller the reflectors detectable.
Size of contact face	Diameter resp. length × width (L×W) of probe contact face.

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