Sunday, 16 October 2016

Distance Amplitude correction (DAC) in ultrasonic inspection.

Acoustic signals from the same reflecting surface will have different amplitudes at different distances from the transducer. Distance Amplitude correction (DAC)  provides a means of establishing a graphic reference level sensitivity as a function of sweep distance on the A - scan display. The use of DAC allows signals reflected from similar discontinuities to be evaluated where signal attenuation as a function of depth has been correlated. Most often DAC will allow for loss in amplitude over material depth  (time), graphically on the A - scan display but can also be done electronically by certain instruments. Because near field length and beam spread vary according to transducer size and frequency, and materials vary in attenuation and velocity, a DAC cure must be established for each different situation. DAC may be employed in both longitudinal and shear modes of operation as well as either contact or immersion inspection techniques.
A distance Amplitude correction curve is constructed from the peak amplitude responses from reflectors of equal area at different distances in the same material. A-scan echoes are displayed at their non - electronically compensated height and peak amplitude of each signal is marked on the flaw detector screen or, preferably, on a transparent plastic sheet attached to the screen. Reference standards which incorporate side drilled holes (SDH), flat bottom holes (FBH),or notches whereby the reflectors are located at varying Depths are commonly used.


Calibration and reference standards for ultrasonic testing come in many shapes and sizes. The type of standard used is dependent on the NDE application and the form and shape of the object being evaluated. The material of the reference standard should be the same as the material being inspected and the artificially induced flaw should closely resemble that of the actual flaw. This second requirements is a major limitations of most standard reference samples. Most use drilled holes and notches that do not closely represent real flaws. In most cases the artificially induced defects in reference standards are better reflectors of sound energy (due to their flatter and smoother surfaces) and produce indications that are larger than those that a similar sized flaw would produce. Producing more realistic defects is cost prohibitive in most cases and, therefore, the inspector can only make an estimate of the flaw size. Computer programs that allows the inspector to creat computer simulated model of the part and flaw may one day lessen this limitation.