^ The Collapsed Seongsu Bridge in 1994
Image Courtesy of 최광모 at https://en.wikipedia.org/wiki/File:1994%EC%84%B1%EC%88%98%EB%8C%80%EA%B5%90_%EB%B6%95%EA%B4%B4_%EC%82%AC%EA%B3%A001.jpg
Detection is Better than Rework & Scrapping
They say, prevention is better than cure. We all know that. But, as is usually the case, wisdom is easy to listen than to follow. The world of manufacturing and, particularly, welding is no exception. Recounts of a few catastrophes will illustrate the point.
On April 10, 1989, the Chevron Richmond Refinery in Richmond, California was the scene of a major fire and explosion. While eight firefighters and workers were injured while trying to douse the fire, three workers suffered from third and second degree burns.
After two years and a brief phase of litigation, Chevron agreed, in 1991, to pay $275,000 to the U.S. Department of Labor ‘in full financial settlement’. What caused the disaster – weld failure along an elbow of a pipe carrying hydrogen gas. Fortunately, there were no casualties.
Thirty two people were not so lucky when on October 21, 1994, the Seongsu Bridge over Han River in Seoul, South Korea collapsed. Why – structural failure due to inappropriate welding of the steel trusses of the suspension structure underneath the concrete slab roadway.
Apart from the loss of life, limb, and property, weld defects also dampen progress. One, they slash productivity via reworks and scrapping. And they give cause to avoidable litigation. Both these losses mean diversion of effort from gainful operations.
Electromagnetic Acoustic Transducer (EMAT) & Weld Inspection
Conventional ultrasonic apparatuses employ piezoelectric transducers and a couplant liquid for the non destructive testing (NDT) of welds. Often, the couplant obstructs the waves from making a correct assessment of the weld.
Electromagnetic Acoustic Transducer (EMAT) on the contrary utilizes two overlapping magnetic fields to create the ‘inspector’ sound wave. No couplant needed here.
EMAT is a non-contact sound generation and reception transducer. After decades of research, it is finding application as a method for effective NDT by employing ultrasonic sound waves.
Its main merit is, it does not require a couplant to transmit ultrasonic waves to the workpiece. This feature makes it compatible for inspections in a wide variety of settings and conditions.
Used now for defect detection, thickness measurement, and material property characterization, EMAT is particularly useful in industries such as metal manufacturing and processing, pipeline, automotive, pressure vessel, and boilers.
Operations: EMAT Vs Piezoelectric Transducer
When using piezoelectric transducers for weld NDT, you have to couple them with a liquid or a high pressure medium. While the former can interfere with the inspection, the latter can cut the scanning ability of the waves.
Now, EMAT induces waves inside the workpiece. Piezoelectric transducers, on the contrary, generate waves outside the workpiece that are then transferred to the workpiece. This is the fundamental difference between the two and this is why EMAT does not need a coupling element.
EMAT consists of two main elements, a magnet and an electric coil. The magnet can be a permanent one or an electromagnet. It produces a static or quasi-static i.e. a low-frequency magnetic field.
Alternating Current (AC) of frequency between 20 kHz and 10 MHz is fed into the electric coil to create an alternating magnetic field. This is a relatively high frequency (RF) magnetic field.
As per the application, this AC can be a spike pulse, a continuous wave, or a tone-burst signal. It is the interaction of these two separate magnetic fields that create the ultrasonic waves inside the workpiece placed near the EMAT.
Such interaction also produces a Lorentz Force. This force sets up elastic waves in the workpiece material. And the interaction of these elastic waves with the ambient magnetic field creates an electric current in the EMAT receiver coil circuit.
If the workpiece is a ferromagnetic material (one that can be made into a permanent magnet), Magnetostriction creates additional stresses that amplify the signal in the EMAT receiver coil. Through the use of diverse combinations of magnets and RF, we can create different types of waves.
EMAT uses two mechanisms for transduction i.e. generation of ultrasonic waves inside the workpiece. Lorentz Force generates waves inside workpieces made from electrically conducting materials while magnetostriction does the same in ferromagnetic workpieces.
Lorentz Force is the force exerted on any electrically charged particle moving through a magnetic field. Please note, electric charge in motion is electric current. This force is named after the Dutch physicist Hendrik A. Lorentz.
Extending the same to the macro level, a magnetic field will exert a force on an electric current carrying conductor placed inside it. This is sometimes called Laplace Force.
Magnetostriction is the change in shape of ferromagnetic materials when subjected to a magnetic field. First discovered by James Joule in 1842, it causes frictional heat losses in ferromagnetic cores. The humming sound you hear near transformers is due to magnetostriction.
Applications, Strengths, & Limitations
Typical applications for EMAT include:
- Weld Inspection:
- tubes, coils, and pipe weld review
- austenitic weld checking in the power industry
- laser weld examination of automotive parts
- Detection of Defects in:
- plate lamination
- steel products
- bonded structure lamination
- Inspection of:
- in-service pipelines
Because EMAT induces waves inside the workpiece as opposed to piezoelectric transducers that create waves outside, EMAT comes with the following merits:
- Independent of the Angle of Sensor: for it induces waves inside the workpiece. This also makes EMAT less vulnerable to sensor movement and vibrations
Conventional piezoelectric transducers create waves outside the workpiece and then transmit them inside. You have to align these transducers correctly with the weld geometry in order to transmit the waves correctly
Plus, you have to take refraction into consideration as the waves move from the piezoelectric transducer to the couplant medium and thence to the workpiece
- Broad Distribution of Waves: enables you to inspect a long weld with just one EMAT coil. This is of course in marked contrast to conventional NDT techniques that typically deploy multiple sensors
- Immune to Surface Conditions: such as roughness and contamination as it does not depend on the couplant liquid or high-pressure medium
- Ability to Examine Hot and Cold Workpieces: because it does not use a liquid couplant, EMAT can inspect hot and cold workpieces while also fitting in comfortably with automated inspection systems
- Capable of Inspecting Large and Small Diameter Tubes: as it can produce highly guided waves. These waves fill up the workpiece and thereby enable complete inspection of welded small diameter pipes in a single setting
Couplant-free operation means you can examine welds on large diameter pipes immediately after the welding is complete
But then, there is always the other, inevitable side. EMAT is loaded with the following disadvantages in comparison to piezoelectric transducers:
- Relative Incompatibility with Non-Metallic and Non-Magnetic Materials: such as plastics or ceramics
This is because it relies on Lorentz Force and magnetostriction that are more easily induced in metallic and magnetic materials
- Prone to Interference from Ambient Magnetic Fields
- Generate Lower Power Signals
- Larger Size
Related Technical Details
AC electric current in the EMAT’s electric coil sets up eddy currents in the workpiece. These are superficial, closed loop electric currents that flow in the surface of a current carrying conductor.
Newton’s Third Law of Motion: All Actions Produce Equal & Opposite React
It is these eddy currents that cause the magnetic field to generate the Lorentz Force. Let us see how. According to Faraday’s Laws of Electromagnetic Induction:
- any change in magnetic flux associated with conducting material induces an electro motive force (emf or electric voltage) inside the material
- the magnitude of this emf is directly proportional to the rate of change of magnetic flux
There are two ways in which the magnetic field can change. One, if the coil moves inside a magnetic field of fixed magnitude and/or direction. Second, if you place the coil inside a magnetic field that changes its magnitude and/or direction.
Now, the induced emf creates an induced current, including the eddy current, inside the conducting material. Lenz’s Law speaks on the direction of this induced emf – the direction of the induced emf is such that it opposes the change of magnetic flux that creates it.
Seen from a broader perspective, Lenz’s Law is the extension of the Principle of Conservation of Energy and the Newton’s Third Law of Motion to electromagnetic circuits.
If the induced emf complements the change in magnetic flux that creates it, we would have a situation wherein the induced emf would induce even more emf without any external energy input.
This runs contrary to the time tested conservation of energy principle viz. you can only convert energy from one form to another, you cannot create or destroy it.
Eddy currents do not penetrate deep inside the conducting material. An increase in the conductivity of the material, AC frequency, and permeability makes eddy currents even more superficial.
Distribution of the Lorentz Force is a function of the design of the magnet and the electric coil, the excitation signal provided to the EMAT, relative placement between the workpiece and the EMAT, and the properties of the workpiece.
Ferromagnetic materials will either complement the external magnetic field or contradict it depending on their molecular arrangement. This changes their shape resulting in magnetostriction that, in turn, creates ultrasonic waves inside them.
Research has proved that magnetostriction effect in nickel is stronger than in steel. NDT of steel products therefore uses magnetostriction sensors with nickel patches.
Materials such as quartz, lead zirconate titanate, barium titanate, and Rochelle salt exhibit the Piezoelectric Effect. When subjected to stress, they generate electric charge.
Most such materials also exhibit the reverse effect – generating stress when you apply electric field to them. Piezoelectric transducers used for weld NDT convert such stress to ultrasonic waves.
By creating waves inside the workpiece, EMAT gives better results that conventional piezoelectric transducers that force such waves from the outside. Like they say, the inner self is stronger than outside influences.
To know more of the finer intricacies of weld operations, visit our blog. And to witness the best in marine fabrication services, marine pipe fitting, and large scale custom metal fabrication, contact Kemplon Engineering.