Ultrasonic Non-destructive Testing

Ultrasonic Testing (UT) uses sound waves having frequencies usually in the mega hertzs range. Two basic methods in UT are pulse echo and through transmission. While the former method makes use of a single transducer, the later makes use of two. In the pulse echo method a transducer, made of piezoelectric material, transmits a pulse of mechanical energy into the material. The energy passes into the material, reflects from the back surface, and is detected by the same transducer, yielding a signal on an oscilloscope with a time base. The oscilloscope normally shows the original pulse of the ultrasonic transducer (front surface echo), the back reflection and any extra blip indicating a reflection from a defect in the material. From the oscilloscope timing, the depth of the defect below the surface can be determined. Alternatively, in the transmission method, two transducers are placed on opposite sides of the material and any reduced intensity sensed by the receiving transducer indicates defect shadowing part of the ultrasonic energy. The location of defect can not be obtained. Both pulse echo reflection and transmission methods are in use and their selection depends on the accessibility of the component.

UT can detect defects oriented both in the plane of and normal to the surface of components using normal beam or angle beam transducers. By suitable design of ultrasonic transducer, ultrasonic beams can be introduced into a material at almost any angle. There are several forms of ultrasonic waves, the most widely used in NDT being compressional (longitudinal) and transverse (shear) waves. In a specific application of tube testing for detecting defects normal to the wall, the beam is converted to a shear wave which is propagated around the circumference or along the axis. In order to detect defects efficiently by UT, it is necessary to make the wave length comparable to or smaller than the expected defect size. Hence, for detection and assessment of smaller defects, it is necessary to use high frequency. The UT data can be displayed in three modes viz. A-scan, B-scan and C-scan.

Ultrasonic NDT methods are also widely used for detection and characterisation of defects in plates, castings, forgings, welds, structures etc. A few important areas in ultrasonic testing are

SAFT
Phased-arrays
C-Scan Imaging
Spectral Analysis
TOFD
EMATs
Signal Processing
Tomography
AI / Expert / Knowldege Systems
Non-linear Ultrasonics
Laser-based Ultrasonics

Attenuation of ultrasonic energy can be used for quantitative evaluation of material properties. Empirical correlations have been obtained between ultrasonic attenuation and the impact strength, fracture toughness, grain size and tensile strength of steels. Similarly, ultrasonic velocity measurements can be used to estimate residual stresses in materials. This methodology uses shear waves polarised in two mutually perpendicular directions. These waves have slightly different velocities and so interfere, so that as the transducer is rotated, the interference vanishes when the polarising planes are parallel and perpendicular to the stress axis. Once this axis is known, the actual stress can be computed from the velocities. Stress measurements using ultrasonic technique are also dependent on the acousto elastic effect i.e. strain induced ultrasonic wave velocity variations in materials. By precise measurement of ultrasonic velocity, information about stress can be obtained. Using a new method, accuracy of transit time measurements of the order of 0.2 nano second has been achieved.This method has been effectively used for characterisation of microstuctures in a variety of steels.

Ultrasonic examination of austenitic stainless steel welds is complicated by beam skewing, distortion, and deflection due to anisotropy and coarse grained microstructure in the weld regions. One ultrasonic wave mode that is not prone to beam skewing and distorion is shear horizontal (SH) wave. SH waves can be generated only electromagnetic acoustic transducers (EMATs). Phase EMATs capable of generating angle beam SH waves in any angle of incidence are popularly used for NDE of stainless steel welds. Artificial neural network based method has been developed for quantitative characterisation and classification of defects in stainless steel welds.