In Conventional ultrasonic non-destructive condition monitoring the testing personnel utilize his testing experience to interpret defects while analyzing the ultrasonic echo. Due to the coarse material structure, the ultrasonic wave attenuate more so sometimes it gives false impression of defect and provides unreliable results. The conventional inspection method is too subjective and highly relies on the effectiveness of testing personnel so the inspection reliability is low. To overcome these limitations a multi-interrogation ultrasonic technique and correlation of several parameters such as attenuation, acoustic velocity and grain size with material features is suggested in this paper.
Non Destructive Testing (NDT) is the testing of materials, for surface or internal flaws or metallurgical condition, without rendering it unfit for service.
There are various NDT methods of inspecting the internal defects of complex surface parts, e.g. radiograph, eddy flow, ultrasonics, etc. Amongst these testing techniques, ultrasonic testing has an important role due to its strong penetration, good direction, high sensitivity, overall comparative low cost, and being harmless to the human body and the material of the parts.
Ultrasonic [1, 2, 3, 4] is a highly effective non destructive condition monitoring method which can detect even incipient faults. So Mean Time between Failure (MTBF) increases due to proactiveness and reduce Mean Time to Repair (MTTR) by improving trouble shooting capabilities. Ultrasonic nondestructive testing methods  play an important role in the physical characterization of engineering materials and in the assessment of their quality and serviceability in structures. In a wide range of different NDT methods, ultrasonic velocity approaches have been used extensively to evaluate the density and the elastic constants of metals and alloys.
Ultrasonic evaluation enables us to determine  elastic modulus and acoustic properties. Not one technology can cover everything. Each condition monitoring techniques have some advantages and some disadvantages. The recommendation is to incorporate two or more technology so as to assure reliable results.
The predominant factor in attenuation measurement is the relationship between the ultrasonic wavelength and the grain size [7, 8, 9, and 10]. These are related as follows:
High attenuation occurs when λ < D,
Where λ = wavelength, D = grain diameter
Low attenuation occurs when λ > D
Also high attenuation is associated to high level of damage and low attenuation to low level of damage. It means if wavelength of ultrasonic wave if not properly selected than it give false impression of presence of defect in material. So if we know the grain size (Using SEM) of material to be tested than we can easily fix the suitable wavelength for testing.
Using SEM grain size of specimen can be accurately determined. The scanning electron microscope  is a very effective and accurate method. It...