Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse waves with centre frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion and erosion. Ultrasonic testing is extensively used to detect flaws in welds.
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Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood and composites, albeit with less resolution. It is used in many industries including steel and aluminium construction, metallurgy, manufacturing, aerospace, automotive and other transportation sectors.
The first efforts to use ultrasonic testing to detect flaws in solid material occurred in the s.[1] On May 27, , U.S. researcher Dr. Floyd Firestone of the University of Michigan applies for a U.S. invention patent for the first practical ultrasonic testing method. The patent is granted on April 21, as U.S. Patent No. 2,280,226, titled "Flaw Detecting Device and Measuring Instrument". Extracts from the first two paragraphs of the patent for this entirely new nondestructive testing method succinctly describe the basics of such ultrasonic testing. "My invention pertains to a device for detecting the presence of inhomogeneities of density or elasticity in materials. For instance, if a casting has a hole or a crack within it, my device allows the presence of the flaw to be detected and its position located, even though the flaw lies entirely within the casting and no portion of it extends out to the surface. ... The general principle of my device consists of sending high frequency vibrations into the part to be inspected and the determination of the time intervals of the arrival of the direct and reflected vibrations at one or more stations on the surface of the part."
James F. McNulty (U.S. radio engineer) of Automation Industries, Inc., then, in El Segundo, California, an early improver of the many foibles and limits of this and other nondestructive testing methods, teaches in further detail on ultrasonic testing in his U.S. Patent 3,260,105 (application filed December 21, , granted July 12, , titled &#;Ultrasonic Testing Apparatus and Method&#;) that &#;Basically ultrasonic testing is performed by applying to a piezoelectric crystal transducer periodic electrical pulses of ultrasonic frequency. The crystal vibrates at the ultrasonic frequency and is mechanically coupled to the surface of the specimen to be tested. This coupling may be effected by immersion of both the transducer and the specimen in a body of liquid or by actual contact through a thin film of liquid such as oil. The ultrasonic vibrations pass through the specimen and are reflected by any discontinuities which may be encountered. The echo pulses that are reflected are received by the same or by a different transducer and are converted into electrical signals which indicate the presence of the defect.&#; To characterize microstructural features in the early stages of fatigue or creep damage, more advanced nonlinear ultrasonic tests should be employed. These nonlinear methods are based on the fact that an intensive ultrasonic wave is getting distorted as it faces micro damages in the material.[2] The intensity of distortion is correlated with the level of damage. This intensity can be quantified by the acoustic nonlinearity parameter (β). β is related to first and second harmonic amplitudes. These amplitudes can be measured by harmonic decomposition of the ultrasonic signal through fast Fourier transformation or wavelet transformation.[3]
In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant [4] such as a gel, oil or water,[1] as in immersion testing. However, when ultrasonic testing is conducted with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required.
There are two methods of receiving the ultrasound waveform: reflection and attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the "sound" is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, representing the arrival time of the reflection. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after travelling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence. Using the couplant increases the efficiency of the process by reducing the losses in the ultrasonic wave energy due to separation between the surfaces.
One of the example that utilize ultrasound for proving material property is the measurement of grain size of specific material. Unlike destructive measurement, ultrasound offers methods to measure grain size in non-destructive way with even higher detection efficiency. Measurement of grain size using ultrasound can be accomplished through evaluating ultrasonic velocities, attenunations, and backscatter feature. Theoretical foundation for scattering attenunation model was developed by Stanke, Kino, and Weaver.
With constant frequency, the scattering attenuation coefficient depends mainly on the grain size; Zeng et al, figured out that in pure Niobium, attenuation is linearly correlated with grain size through grain boundary scattering.[6] This concepts of ultrasonic proving can be used to inversely resolve the grain size in the time domain when the scattering attenuation coefficient is measured from testing data, providing the non-destructive way to predict material's property with rather simple instruments.
(Note: Part of CEN standards in Germany accepted as DIN EN, in Czech Republic as CSN EN.)
Flaw detection is the most commonly used technique among all the applications of industrial ultrasonic testing. Generally, sound waves of high frequency are reflected from flaws and generate clear echo patterns.
Portable instruments record and display these echo patterns. Ultrasonic testing is a safe testing method that is widely used in various service industries and production process, particularly in applications where welds and structural metals are used. The paper gives an overview of the theory, practice and application of ultrasonic flaw detection.
Sound waves are mechanical vibrations that pass through a medium such as liquid, solid or gas. These waves pass through a medium at a particular velocity in an expected direction. When these waves bump into a boundary having a different medium, they are transmitted back. This is the principle behind ultrasonic flaw detection.
Most ultrasonic flaw detection applications use frequencies between 500 KHz and 10 MHz per second. At frequencies in the megahertz range, sound energy travels easily via most common materials and liquids, but does not pass efficiently via air or similar gasses. Also, sound waves of different types travel at different rate of velocities.
Additionally, wavelength refers to the distance between two subsequent points in the wave cycle as it passes via a medium. It is related to velocity and frequency. In ultrasonic flaw detection and ultrasonic thickness gaging, the minimum limit of detection is one-half wavelength and the minimum measurable thickness is one wavelength, respectively.
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In solids, sound waves can be present in different modes of propagation that are characterized by the type of motion involved. The common modes used in ultrasonic flaw detection are shear waves and longitudinal waves.
When compared to soft, heterogeneous or granular materials, hard and homogeneous materials are able to reflect sound waves more efficiently. Three factors, such as beam spreading, attenuation and scattering, control the distance a sound wave will pass in a particular medium.
The amount of reflection coefficient or energy reflected is associated with the relative acoustic hindrance of the two materials. In ultrasonic flaw detection applications, metal and air boundaries are commonly seen, wherein the reflection coefficient reaches 100%. This is the basic principle involved in ultrasonic flaw detection.
At ultrasonic frequencies, sound energy is extremely directional and the sound beams employed for flaw detection are clearly defined. As per the Snell's Law of refraction, sound energy transmitting from one material to another will bend. A beam that is traveling straight will travel in a straight direction; however, a beam that hits a boundary at an angle will bend.
A transducer is an instrument that is capable of converting energy from one state to another. Ultrasonic transducers can transform electrical energy into sound energy and vice versa.
For ultrasonic flaw detection, standard transducers employ an active element that is made of either a polymer, composite, or piezoelectric ceramic. When an electrical pulse of high voltage is applied to this element, it vibrates through a particular spectrum of frequencies and produces sound waves. When an incoming sound wave vibrates this element, it produces an electrical pulse.
Figure 1. Cross section of typical contact transducer
In flaw detection applications, five types of ultrasonic transducers are usually employed. They include contact transducers, immersion transducers, delay line transducers, angle beam transducers, and dual element transducers.
Panametrics-NDT Epoch series are ultrasonic flaw detectors that are compact and portable instruments based on microprocessor. They are ideal for shop and field applications and display an ultrasonic waveform that is easily understood by a trained operator, who detects and classifies the flaws in test pieces. The series comprises a waveform display, an ultrasonic pulser/receiver, a data logging module, and software and hardware for signal capture and analysis. In order to optimize the performance of transducer, pulse amplitude, damping and shape can be controlled. Likewise, in order to signal-to-noise ratios, receiver gain and bandwidth can be modified.
A trained operator can identify particular echo patterns related to the echo response from representative flaws and good parts. This can be done by utilizing correct reference standards and accepted test procedures along with a good knowledge of sound wave propagation. Two calibration standards such as straight beam testing and angle beam testing are used in ultrasonic flaw detection. The latter technique is commonly used in weld inspection.
Figure 2. Typical angle beam assembly
Ultrasonic flaw detection is a comparative method. Although some analog-based flaw detectors are still being produced, most modern instruments employ digital signal processing to promote enhanced stability and accuracy.
This information has been sourced, reviewed and adapted from materials provided by Evident Corporation.
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