Seminar report for ultra sonic detecion for hiddemn corrotion detection

Ultrasonic techniques for hidden corrosion detection in aircraft wing skin
INTRODUCTION

Ultrasonic inspection is a non-destructive method in which beams of high frequency sound waves that are introduced in to the material are used to detect surface and sub surface flaws. The sound waves travel through the material with some attend loss of energy and are reflected at interfaces .the reflected beams is detected and analyzed to define the presence and location of flaws. Different ultrasonic techniques are there for finding the flaws or defect in which guided waves demonstrate an attractive solution where conventional ultrasonic techniques are less sensitive to defects such as corrosion/disbands. In thin multilayered wing skin structures and hidden exfoliation under wing skin fasteners. Guided lamb waves have been used for long range or large area corrosion detection and the evaluation of adhesively bonded structure. Ultrasonic guide waves are promising but require procedure development to ensure high sensitivity and reliable transducer coupling.

ULTRASONIC TESTING
Basic principle and general application
Most commercial ultrasonic testing is done at frequencies between 1 and 25 MHz. The elastic wave travel in to the material with some loss of energy due to attention and are reflected at the interfaces. The reflected beam is analyzed to detect and locals the defect and for their quantitative evaluation. These waves are completely reflected at solid-gas interfaces, partial reflection at solid-liquid or solid-solid interfaces. The reflected energy depends mainly on acoustic impedance of the material at the interfaces and the acoustic impedance of a given material is the product of density and velocity. This widely used NDT method has a lot of applications like defining bond characteristics, measurement of thickness of the components, estimation of corrosion and determination of physical properties, structure, grain size and elastic constant. In the case of austenitic stainless steel welds because of the problems associated with high scattering and attenuation ultrasonic waves at low frequencies are employed. For successful application of ultrasonic inspection, the testing must be suitable for type of inspection being done and the operator must be successfully trained and experienced.

TYPES OF ULTRA SONIC WAVES
On the basics of the mode of vibration or particle displacement ultrasonic waves are classified as longitudinal waves, transverse waves, surface waves and lamb waves
Longitudinal waves
Some times called compression waves are the type of ultrasonic waves most widely used in the inspection of metals. They travel through metal as a series of alternate compression and rarefaction in which the particle transmitting the wave vibrate back and forth in the direction of the travel of the waves. Because of the easy generation and detection, this type of ultra sonic waves is most widely used in ultra sonic testing. These types of waves can propagate in solids, liquids and gas.
Transverse waves.
This type of ultrasonic waves is called transverse wave or shear wave because the direction of particle displacement is at right angles to the direction of propagation. Unlike longitudinal waves transverse wave cannot be supplied by the elastic collision of adjacent molecular or atomic particles. For the propagation of transverse wave it is necessary that each particle exhibits a strong force of attraction to its neighbors so that as the part moves back and forth its pull its neighbors with in thus causing sound to move through the material with the velocity associated with the transverse waves which is about 50% of the longitudinal wave for the same material
Surface waves
They are another type of ultrasonic waves used in the inspection of metals. These waves travel along the flat or curved surface of relatively thick solid parts. In the surface waves, particle vibration generally follows an elliptical orbit. Surface waves are subjected to