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Fundamentals of Ultrasonic Imaging and FlawDetection

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Overview

Ultrasonics refers to any study or application of sound waves higher in frequency than the human audible range. Music and common sounds that are considered pleasant are typically 12 kHz orless, while some humans can hear frequencies up to 20 kHz. Ultrasonic waves consist of frequencies greater than 20 kHz and exist in excess of 25 MHz. They are used in many applicationsincluding plastic welding, medicine, jewelry cleaning, and nondestructive test. Within nondestructive test, ultrasonic waves give you the ability to"see through" solid/opaque material and detectsurface or internal flaws without affecting the material adversely.

Table of Contents

Basics of Ultrasonic TestUltrasonic Wave ModesSnell’s LawAcoustic Impedance

Basics of Ultrasonic Test

Ultrasonic wavelengths are on the same order of magnitude as visible light, giving them many of the same properties of light. For example, ultrasonic wavelengths can be focused, reflected, andrefracted. Ultrasonic waves are transmitted through air, water, and solids such as steel by high-frequency particle vibrations. These waves are transmitted in homogenous solid objects much likepointing a flashlight around a room with various objects that reflect light. The directed energy in an ultrasonic wave is reflected by boundaries between materials regardless of whether the materialis gas, liquid, or solid. Ultrasonic waves are also reflected by any cracks or voids in solid materials. These reflected waves, which are caused by internal defects, can be compared to the reflectedwaves from the external surfaces, enabling the size and severity of internal defects to be identified.

Generating and detecting ultrasonic waves requires an ultrasonic transducer. Piezoelectric ceramics within ultrasonic transducers are "struck" – similar to the way tuning forks are struck to generatean audible note – with electricity, typically between 50 and 1000 V – to produce the ultrasonic wave. The ultrasonic wave is carried from the transducer to the unit under test (UUT) by a couplant –typically water, oil, or gel – and is reflected back to the transducer by both external surfaces and internal defects.

When operating in pulse-echo mode, ultrasonic transducers act as both emitters and receivers. The reflected ultrasonic waves vibrate the piezoelectric crystal within the ultrasonic transducer andgenerate voltages that are measurable by data acquisition hardware. When operating in through-transmission mode, two ultrasonic transducers are used; one transducer generates the wave andthe other receives the wave.

In a typical application, the ultrasonic transducer is struck with a high-voltage pulse, which lasts on the order of 5 µs, and then the system listens for the echoes. The system listens on the order of10 to 15 µs. Even in the most advanced systems, the transducers are pulsed every 500 µs.

The most primitive method to analyze the reflected ultrasonic signals is time-of-flight (TOF) display, or A-scan. Discontinuities that are closer to the ultrasonic transducer are received sooner thanthose further away from the transducer. The figure below depicts the TOF display from the previous example.

: Document Type Tutorial : Yes NI Supported: Feb 11, 2010 Publish Date

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The x-axis on the A-scan is not typically units of time but is converted to distance. This conversion is accomplished by measuring, or looking up, the speed of sound through the material that theultrasonic wave is traveling through and performing the conversion. Although there are a few exceptions, the speed of sound through a material is governed largely by the density and elasticity ofthe material. For most materials, the speed of sound within homogenous material is easy to research and find.

Most ultrasonic nondestructive test applications range from 400 kHz to 25 MHz. The frequency of the ultrasonic sensor is chosen based on several factors including detectable flaw size, depth ofpenetration, and grain size of the material. Materials made of fine-grained material, such as metals, permit deep penetration by ultrasonic waves of all frequencies. However, coarse-grainedmaterial, including many plastics, scatter high-frequency ultrasonic waves. The higher the frequency, the smaller the flaws the system detects, but the depth of penetration decreases.

Ultrasonic Wave Modes

Two predominant types of waves, or wave modes, are generated within a material with ultrasonic waves: longitudinal and shear. Longitudinal waves (L-waves) compress and decompress thematerial in the direction of motion, much like sound waves in air. Shear waves (S-waves) vibrate particles at right angles compared to the motion of the ultrasonic wave. The velocity of shear wavesthrough a material is approximately half that of the longitudinal waves. The angle in which the ultrasonic wave enters the material determines whether longitudinal, shear, or both waves areproduced.

Ultrasonic beam refraction and mode conversion are comparable to light as it passes from one medium to another. Remember how the straw in the glass of water looks broken if observed from theside? The same phenomenon occurs with ultrasonic waves as they are passed into a UUT. The figure below depicts an ultrasonic transducer that transmits an ultrasonic wave through water into ablock of steel. Because the direction of the ultrasonic wave is at a 90-degree angle with the surface of the steel block, no refraction occurs and the L-wave is preserved.

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As the angle of the ultrasonic transducer is altered, refraction and mode conversion occur. In the figure below, the ultrasonic transducer has been rotated 5 degrees. The longitudinal wave from thetransducer is converted into two modes, longitudinal and shear, and both wave modes are refracted. Notice that the waves are refracted at different angles. In this example, the L-wave isapproximately four times the transducer angle and the S-wave is just over two times the transducer angle. Angles that create two wave modes are not appropriate because they cause theultrasonic transducer to receive multiple echoes, making it difficult to analyze the data.

Refraction and mode conversion occur because of the change in L-wave velocity as it passes the boundary from one medium to another. The higher the difference in the velocity of sound betweentwo materials, the larger the resulting angle of refraction. L-waves and S-waves have different angles of refraction because they have dissimilar velocities within the same material.

As the angle of the ultrasonic transducer continues to increase, L-waves move closer to the surface of the UUT. The angle at which the L-wave is parallel with the surface of the UUT is referred toas the first critical angle. This angle is useful for two reasons. Only one wave mode is echoed back to the transducer, making it easy to interpret the data. Also, this angle gives the test system theability to look at surfaces that are not parallel to the front surface, such as welds.

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Snell’s Law

L-wave and S-wave refraction angles are calculated using Snell’s law. You also can use this law to determine the first critical angle for any combination of materials.

Where:θ = angle of the refracted beam in the UUTRθ = incident angle from normal of beam in the wedge or liquidIV = velocity of incident beam in the liquid or wedgeIV = velocity of refracted beam in the UUTR

For example, calculate the first critical angle for a transducer on a plastic wedge that is examining aluminum.

V = 0.267 cm/µs (for L-waves in plastic)IV = 0.625 cm/µs (for L-waves in aluminum)Rθ = 90 degree (angle of L-wave for first critical angle)Rθ = unknownI

The plastic wedge must have a minimum angle of 25.29 degrees to transmit only S-waves into the UUT. When the S-wave angle of refraction is greater than 90 degrees, all ultrasonic energy isreflected by the UUT.

Acoustic Impedance

When performing ultrasonic testing, it is important to understand how effectively ultrasonic waves pass from one medium to another. Generally, when an ultrasonic wave is passed from onemedium to another, some energy is reflected and the remaining energy is transmitted. The factor that describes this relationship is referred to as acoustical impedance and the acousticalimpedance ratio.

Z = ρV

whereZ = acoustical impedanceρ = densityV = velocity of sound through medium

For reference, air has low acoustical impedance, water has higher impedance than air, and steel has higher impedance than water. The acoustical impedance ratio is the impedance of the secondmaterial divided by the first. The higher the ratio, the more energy is reflected. For example, when ultrasonic waves are passed from water to steel, the acoustical impedance is approximately 20 to1; whereas, when ultrasonic waves are passed from air to steel, the acoustical impedance is approximately 100,000 to 1. Almost 100 percent of the ultrasonic energy is reflected when passingultrasonic waves from air to a solid such as steel, making air a very poor ultrasonic couplant.

For more information, see the page.Ultrasonic and Nondestructive Test (NDT)

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