Ultrasonic Testing

Introduction

Basic Principles of Sound

• The measurement of sound waves from crest to crest determines its wavelength (λ).
• The time is takes a sound wave to travel a distance of one complete wavelength is the same amount of time it takes the source to execute one complete vibration.
• The sound wavelengthis inversely proportional to its frequency. (λ = 1/f)
• Several wave modes of vibration are used in ultrasonic inspection.The most common arelongitudinal, shear, andRayleigh (surface) waves.
• Ultrasonic waves are very similar to light waves in that they can be reflected, refracted, and focused.
• Reflection and refraction occurs when sound waves interact with interfaces of differing acoustic properties.
• In solid materials, the vibrational energy can be split into different wave modes when the wave encounters an interface at an angle other than 90 degrees.
• Ultrasonic reflections from the presence of discontinuities or geometric features enables detection and location.
• The velocity of sound in a given material is constant and can only be altered by a change in the mode of energy.
• Sound is produced by a vibrating body and travels in the form of a wave.
• Sound waves travel through materials by vibrating the particles that make up the material.
• The pitch of the soundis determined by the frequency of the wave (vibrations or cycles completed in a certain period of time).
• Ultrasound is soundwith a pitch too highto be detected by the human ear.

Principles of Ultrasonic Inspection

• Ultrasonic waves are introduced into a material where they travel in a straight line and at a constant speed until they encounter a surface.
• At surface interfaces some of the wave energy is reflected and some is transmitted.
• The amount of reflected or transmitted energy can be detected and provides information about the size of the reflector.
• The travel time of the sound can be measured and this provides information on the distance that the sound has traveled.

Test Techniques

• Ultrasonic testing is a very versatile inspection method, and inspections can be accomplished in a number of different ways.
• Ultrasonic inspection techniques are commonly divided into three primary classifications.
  • Pulse-echo and Through Transmission (Relates to whether reflected or transmitted energy is used)
  • Normal Beam and Angle Beam(Relates to the angle that the sound energy enters the test article)
  • Contact and Immersion(Relates to the method of coupling the transducer to the test article)

Test Techniques – Pulse-Echo

• Digital display showing signal generated from sound reflecting off back surface.
• Digital display showing the presence of a reflector midway through material, with lower amplitude back surface reflector.
• The pulse-echo technique allows testing when access to only one side of the material is possible, and it allows the location of reflectors to be precisely determined.

Test Techniques – Through-Transmission

• Two transducers located on opposing sides of the test specimen are used. One transducer acts as a transmitter, the other as a receiver.
• Discontinuities in the sound path will result in a partial or total loss of sound being transmitted and be indicated by a decrease in the received signal amplitude.
• Through transmission is useful in detecting discontinuities that are not good reflectors, and when signal strength is weak. It does not provide depth information.