by Tom Nelligan
Ultrasonic nondestructive testing is a versatile technique that can be applied to a wide variety of material analysis applications. While ultrasonic NDT is perhaps better known in its more common applications for thickness gauging, flaw detection, and acoustic imaging, high frequency sound waves can also be used to discriminate and quantify some basic mechanical, structural, or compositional properties of solids and liquids. Ultrasonic material analysis is based on a simple principle of physics: the motion of any wave will be affected by the medium through which it travels. Thus, changes in one or more of four easily measurable parameters associated with the passage of a high frequency sound wave through a material-transit time, attenuation, scattering, and frequency content-can often be correlated with changes in physical properties such as hardness, elastic modulus, density, homogeneity, or grain structure.

Principles Ultrasonic NDT utilizes the range of frequencies from approximately 20 KHz to over 100 MHz, with most work being performed between 500 KHz and 20 MHz. Both longitudinal and shear (transverse) modes of vibration are commonly employed, as well as surface (Rayleigh) waves and plate (Lamb) waves in some specialized cases. Because shorter wavelengths are more responsive to changes in the medium through which they pass, many material analysis applications will benefit from using the highest frequency that the test piece will support. Sound pulses are normally generated and received by piezoelectric transducers that have been acoustically coupled to the test material. In most cases a single transducer coupled to one side of the test piece serves as both transmitter and receiver (pulse/echo mode), although in some situations involving highly attenuating or scattering materials separate transmitting and receiving transducers on opposite sides of the part are used (through transmission mode). A sound wave is launched by exciting the transducer with either a voltage spike or a continuous wave impulse. The sound wave travels through the test material, either reflecting off the far side to return to its point of origin (pulse/echo), or being received by another transducer at that point (through transmission). The received signal is then amplified and analyzed. A variety of commercial instrumentation is available for this purpose, utilizing both analog and digital signal processing.

A significant advantage of ultrasonic testing over other material analysis methods is that it can often be performed in-process or on-line. High frequency sound waves can often be successfully transmitted into and out of moving materials without direct contact, through the use of a water bath or water stream as a coupling medium. Measurements can also be performed within closed containers by coupling sound energy through the wall. Because sound waves penetrate through the test specimen, material properties are measured in bulk rather than just on the surface. It is sometimes even possible, through the use of selective gating, to analyze just one layer of a multi-layer, multi-material fabrication.
The relevant measurement parameters will typically be one or more of the following:
1. Sound velocity/pulse transit time: Sound velocity is usually the easiest ultrasonic parameter to measure. The speed of sound in a homogenous medium is directly related to both elastic modulus and density; thus changes in either elasticity or density will affect pulse transit time through a sample of a given thickness. Additionally, varying degrees of nonhomogeneity may have an effect on sound velocity.
2. Attenuation: Sound energy is absorbed or attenuated at different rates in different materials, governed in a complex fashion by interactive effects of density, hardness, viscosity, and molecular structure. Attenuation normally increases with frequency in a given material.
3. Scattering: Sound waves reflect from boundaries between dissimilar materials. Changes in grain structure, fiber orientation, porosity, particle concentration, and other microstructural variations can affect the amplitude, direction, and frequency content of scattered signals. Scatter effects can also be monitored indirectly by looking at changes in the amplitude of a backwall echo or a through-transmission signal.
4. Frequency (Spectrum) content: All materials tend to act to some degree as a low pass filter, attenuating or scattering the higher frequency components of a broadband sound wave more than the lower frequency components. Thus, analysis of changes in the remaining frequency content of a selected broadband pulse that has passed through the test material can track the combined effects of attenuation and scattering as described above.
In some applications ultrasonic data such as velocity can be directly used to calculate properties such as elastic modulus. In other cases, ultrasonic testing is a comparative technique, where in order to establish a test protocol in a given application it will be necessary to experimentally evaluate reference standards representing the range of material conditions being quantified. From such standards it will be possible to record how sound transmission parameters vary with changes in specific material properties, and then from this baseline information it will be possible to identify or predict similar changes in test samples.

Equipment: A wide variety of ultrasonic instrumentation can be used in material analysis applications. Sound velocity can be measured with simple hand-held ultrasonic thickness gauges, while velocity, attenuation, and scattering effects can all be observed with modern digital flaw detectors. Pulser/receivers with appropriate auxiliary equipment and ultrasonic imaging systems with appropriate software can be used to quantify all of these properties, and to perform spectrum analysis (frequency content) testing as well. For information on both instrumentation and transducer recommendations for specific tests, contact us.

Applications: The following is a summary of some specific material analysis applications where ultrasonic techniques have been used and documented. Extensive discussion, as well as bibliographies on the subject, can be found in the texts by ASNT1 and Lynnworth2. Both books are recommended as a source of further detailed information regarding both test procedures and specific instrumentation requirements.

Elastic moduli: Young's modulus and shear modulus in homogenous, nondispersive materials can be calculated from longitudinal wave and shear wave velocity (along with material density). Use of waveguides often permits measurement at high temperatures.

Nodularity in cast iron: Both the concentration of graphite in cast iron and its shape and form can be quantified through velocity measurements.

Cure rate in epoxies and concrete: The speed of sound in these materials changes as they harden; thus sound velocity measurements can be correlated to the degree of curing. Concrete testing usually requires access to both sides for through-transmission coupling.

Liquid concentrations: The mixture ratio of two liquids with dissimilar sound velocities can be correlated to the sound velocity of the solution at a given temperature.

Density of slurries: The liquid/solid mix ratio of slurries such as drilling mud and paper slurry at a given temperature can be correlated to sound velocity and/or attenuation.

Density in ceramics: Uniformity of density in both green and fired ceramics can be verified by means of sound velocity measurements.
Food products: A wide variety of tests have been reported, including age of eggs and potatoes, ripeness of fruits, fat content in beef, and percent of solids in milk. Generally these tests are both nondestructive and non-contaminating.

Polymerization in plastics: In plastics and other polymers, variations in molecular structure such as length or orientation of polymer chains will often result in corresponding changes in sound velocity and/or attenuation.

Particle or porosity size and distribution: Changes in the size or distribution of particles or porosity in a solid or liquid medium will affect the amplitude and frequency of scattered ultrasound.
Grain size in metals: Changes in grain size or orientation in steel, cast iron, titanium, and other metals will cause changes in the amplitude, direction, and/or frequency content of scattered ultrasound.

Anisotropy in solids: Variations in sound velocity, scattering, and/or attenuation across different axes of a solid can be used to identify and quantify anisotropy.

Case hardening depth in steel: High frequency shear wave backscatter techniques can be used to measure the depth of case hardening.

Temperature measurement: Ultrasonic thermometry has been used to measure very high temperatures (over 3,000 degrees Celsius) by monitoring changes in sound velocity in a reference medium.

For Further Reading:
1) American Society for Nondestructive Testing, Nondestructive Testing Handbook, Volume 7, Ultrasonic Testing (ASNT, 1991)
2) Lynnworth, Lawrence C., Ultrasonic Measurements for Process Control (Academic Press, 1989)