Frequency
Frequency selection involves a trade-off between penetration, small flaw detect ability and sensitivity. Lowering frequencies increase penetration and raising the frequency increases the ability to detect small flaws. By increasing the bandwidth of a transducer, penetration
can usually be increased without sacrificing resolution. Generally, flaws as small as one-half wavelength can be reliably detected.
Element Size
The best aim is to select the smallest element size that is consistent with the frequency/beam spread characteristics that are compatible with your scan rate requirements. In flat faced transducers, element size indicates the width of material that can be inspected with one pass. In focused immersion transducers, the
element size will be relative to the ‘depth of field’ of the focused unit. In low frequency transducers a very small element diameter will cause excessive beam divergence. In any given element size, these effects of divergence
can be lessened by increasing the frequency.
Bandwidth
Performing over a large frequency range, broadband highly damped (shock wave) transducers are responsive to frequencies extending above and below their nominal values. Their advantage lies in the inspection of materials
which have large acoustical absorption or scattering effects, or wherever high resolution flaw testing is a prime consideration. Generally used for high resolution thickness gauging of thin materials while utilizing contact, delay – line and immersion testing techniques, broadband
transducers afford maximum resolution in detecting flaws near the front and far surfaces of test materials.
Broadband highly damped transducers exhibit critically damped pulse characteristics which are essential for error-free thickness gauging and high resolution flaw detection. Narrowband, moderately damped, transducers provide maximum material penetration and sensitivity.
Recommended for the majority of flaw detection applications these transducers are ideal where known frequency specifications exist. | | Since the sensitivity bandwidth is limited in a narrowband transducer, it has greater output at the centre frequency. Narrowband transducers generally contain tuning networks as an integral part of the transducer assembly
and this optimizes the transducer frequency characteristics of the flaw detector, maximizing bandwidth sensitivity. Sonatest narrowband transducers are tuned to within ± 10% of the nominal frequency.
Lens Configuration
To give optimum and reliable performance on a range of testing materials under a range temperatures, Sonatest provide transducer lens configurations.
Contact transducers have flat aluminium oxide wear surfaces to enable resistance to abrasion. Some models feature removable membranes to increase coupling on rough surfaces. The epoxy covering on angle beam transducers allows an improved acoustical match into
the lucite wedge for added sensitivity. Delay line transducers have either fixed or removable delay tips made of polystyrene or special high temperature resistant materials that retard wear. To match surface curvatures and maximize test reliability, the surface of these delay
tips maybe contoured. In immersion testing the transducer lens configuration determines whether the beam will focus to a single spot or line configuration in the test material. Choosing an optimal focal length and shape (line or spot) while considering their relationship to element size and ‘depth of field’ is crucial to proper immersion transducer selection. |
