Tuesday, 4 October 2016

Data presentation in ultrasonic inspection.

Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are know in the NDT world as A-scan,  B-scan and C-scan presentations. Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning system can display data in all three presentation forms simultaneously.
A-scan presentation:-The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and the elapsed time (Which may be related to the sound travel time within the material) is displayed along the horizontal axis. Most instruments with an A-scan display allow the signal to be displayed in its natural radio frequency form(RF),as a fully rectified RF signal, or as either the positive or negative half of the RF signal. In the A-scan presentation, relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep. In the illustration of the A-scan presentation to the right, the initial pulse generated by the transducer is represented by the signal IP, which is near time zero.As the transducer is scanned along the surface of the part, four other signals are likely to appear at different times on the screen. When the transducer is in its far left position, only the IP signal and signal A,the sound energy reflecting from surface A,will be seen on the trace. As the transducer is scanned to the right, a signal from the Blackwall BW will appear later in time, showing that the sound has traveled farther to reach this surface. When the transducer is over flaw B,signal B will appear at a point on the time scale that is approximately halfway between the IP signal and the BW signal. Since the IP signal corresponds to the front surface of the material, this indicates that flaw B is about halfway between the front and back surfaces of the sample. When the transducer is moved over flaw C, signal C will appear in time since the sound travel path is shorter and signal B will disappear since sound will no longer be reflecting from it.