Monday, 10 October 2016

Normal Beam inspection in ultrasonic inspection

NORMAL BEAM INSPECTION:- Pulse -echo ultrasonic measurement can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the well-known relationship.
     d=vt/2 or v=2d/t
Where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip time.
Precision ultrasonic thickness gages usually operate at frequencies between 500kHz and 100MHz, by means of piezoelectric transducer that generate bursts of sound waves when excited by electrical pulses. A wide variety of transducer with various acoustic characteristics have been developed to meet the needs of industrial applications. Typically, lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, while higher frequencies will be recommended to optimize resolution in thinner, non - attenuating, non - scattering materials.