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, and refracted. Ultrasonic waves are transmitted by high frequency particle vibrations, and can be transmitted through air, water, and solids such as steel. These waves are transmitted in homogenous solid objects much like pointing 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 material is 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 reflected waves 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 generate an audible note - with electricity, typically between 50 and 1000 Volts - 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 and generate voltages that are measurable by data acquisition hardware. When operating in through-transmission mode, two ultrasonic transducers are used; one transducer generates the wave and the 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 microseconds, then the system listens for the echos. The system listens on the order of 10-15 milliseconds. Even in the most advanced systems, the transducers are pulsed every 500 microseconds.
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 than those further away from the transducer. The figure below depicts the TOF display from the previous example.
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 the ultrasonic wave is traveling 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 of the 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 of penetration, and grain size of the material. Materials made up of fine grained material, such as metals, permit deep penetration by ultrasonic waves of all frequencies. However, coarse-grained material, including many plastics, scatter high frequency ultrasonic waves. The higher the frequency, the smaller flaws the system will detect, 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 the material 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 waves through 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 are produced.
Ultrasonic beam refraction and mode conversion is 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 the side? 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 a block 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.
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 the transducer 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 is approximately 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 the ultrasonic 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 between two 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 to as 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 the ability to look at surfaces that are not parallel to the front surface, such as welds.
L-wave and S-wave refraction angles are calculated using Snell’s Law. Snell’s Law also can be used to determine the first critical angle for any combination of materials.
θR = angle of the refracted beam in the UUT
θI = incident angle from normal of beam in the wedge or liquid
VI = velocity of incident beam in the liquid or wedge
VR = velocity of refracted beam in the UUT
For example, calculate the first critical angle for a transducer on a plastic wedge that is examining aluminum.
VI = 0.267 cm/µs (for L-waves in plastic)
VR = 0.625 cm/µs (for L-waves in aluminum)
θR = 90° (angle of L-wave for first critical angle)
θI = unknown
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 will be reflected by the UUT.
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 one medium 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 acoustical impedance ratio.
Z = ρV
Z = acoustical impedance
ρ = density
V = 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 second material 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 to 1; whereas, when ultrasonic waves are passed from air to steel, the acoustical impedance is approximately 100,000 to 1. Almost 100% of the ultrasonic energy will be reflected when passing ultrasonic waves from air to a solid such as steel, making air a very poor ultrasonic couplant.