Surface and Thin Volumetric Inspections with EMAT
Borja Lopez1, Syed Ali, 1, Victor Garcia2
1Innerspec Technologies, Inc, Lynchburg, VA, USA
2Innerspec Technologies Europe, Madrid, Spain
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Abstract: Guided waves are widely ud in non-destructive testing to cover long distances from one source point and to inspect hard-to-reach or inaccessible areas. Using reflection and attenuation methods, ultrasonic guided waves permit locating tight cracks, defects under coatings, and near surface defects that are undetecta-ble with other NDT methods. While Conventional Ultrasonic Testing (UT) techniques require liquid or pres-sure coupling, Electro Magnetic Acoustic Transducer (EMAT) is a non-contact UT technique that generates ultrasonic waves on metallic materials using electromagnetic induction, thus circumventing the need to u liquid couplant. EMAT systems can be integrated in-line to inspect materials at extreme temperatures and fast production speeds.
Keywords: Electromagnetic Acoustic Transducer (EMAT); surface waves; guided waves; integrated sys
tems.
1.Introduction军士长是什么军衔
Detection of surface defects and near-surface defects can be of utmost importance on finished and mi-finished products. Surface defects on finished products can affect the look and per-formance of the finished part. On mi-finished products, a emingly small surface or near-surface problem can increa in size or verity in subquent process with important eco-nomic conquences. The most common non-destructive inspection techniques for detection of surface and near-surface defects include vi-sion (VT) –human or machine-, magnetic parti-cle (MP), dye penetrant (PT) and eddy current testing (ET). All of them have advantages and disadvantages and are lectively ud bad on the requirements and environment of the appli-cation. MP and PT are typically ud in manual or mi-manual operation on stationary objects or at low speeds, and only applicable to surface breaking defects. EC can detect surface and some sub-surface defects at high-speeds in au-tomated environments, but it requires complete coverage of the area of interest, which can po difficulties for integration. It also has limitations detecting long defects (along the longitudinal axis of the part), and flat type defects (lamina-tions). Machine vision systems have evolved greatly in the past few years and can be ud to detect defects at high-speeds in production envi-ronments, but ar
e limited to visible defects and cannot detect tight cracks. Ultrasonic guided waves have also en many developments and incread u for many applications requiring high nsitivity to surface and near-surface de-fects.
2.Ultrasonic Guided Waves
Unlike more conventional bulk waves, guided waves propagate along a part while guided by boundaries, which directly affect the direction and mode of propagation. The boundaries can be a surface of a part or any elongated and rela-tively thin structure such as a rod, tube, plate or rail.
The most common types of guided waves are Rayleigh, Lamb, and Shear Horizontal (SH) waves.
Both Rayleigh and Lamb modes follow an ellip-tical pattern with vertical and horizontal particle motion. However, while a Rayleigh or surface wave has most of the energy concentrated on the surface and subsurface region within one wavelength, Lamb modes can penetrate veral wavelengths and provide a complete volumetric inspection of the material. Both Rayleigh and Lamb waves can travel through long distances with strong particle motion. However, due to the motion in the vertical plane, they can also be attenuated by liquids or coatings surrounding
the boundaries of the material subject of the in-spection. Rayleigh and Lamb waves are the most common types of guided waves, and are frequently ud for new products and in-rvice inspections.
Shear waves propagate perpendicularly to the wave direction, with different polarization de-pending on how they are generated. Piezoelec-tric transducers rely on refraction of longitudi-nal energy to generate Shear Vertical waves which are polarized at 90º from the entry plane. Using high-pressure coupling or electromagnet-ic induction (EMAT) it is possible to generate Shear Horizontal (SH) waves that travel parallel to the entry plane. As guided waves, SH modes are many times the only option for the inspec-tion of pipelines, tank floors, and other struc-tures where liquids, clamps or coatings would attenuate other wave modes with out-of-plane particle motion.
2.1.Guided Wave Equations
Although bulk and guided waves are fundamen-tally different, they are actually governed by the same t of differential wave equations [1]. Mathematically, the principal difference is that, for bulk waves, there are no boundary condi-tions that need to be satisfied by the propod solution. In contrast, the solution to a guided wave problem must satisfy the governing equa-tions as well as some physical boundary condi-tions
For Rayleigh-Lamb modes, the frequency equa-tions can be written as:
for symmetric modes
for anti-symmetric modes
Here p and q are given by
() and ()
The wavenumber k is numerically equal to
⁄
where C p is the pha velocity of the Lamb wave mode and ω is the circular frequency. The pha velocity is related to the wavelength by the simple relation
(⁄)
The Shear Wave equations can easily be solved for phad array C p in terms of the frequency thickness product fd(where d = 2h and ω = 2πf). The results is:
{
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When n = 0(corresponding to the zero-order symmetric SH mode) we have C p= C T, a dis-persion-less wave propagating at the shear wave speed C T. All other SH modes (i.e., for all n ≠ 0) are dispersive.
3.Piezoelectric Vs. EMAT
The prevalent technique for generation of ultra-sound is the piezoelectric transducer. While highly efficient and versatile, piezoelectric transducers need to be coupled to the part in-spected either with high pressure, which limits the scanning ability, or with a liquid medium, which limits the deployment and can produce undesired interferences with the propagation of the wave.
EMAT or Electro Magnetic Acoustic Trans-ducer is an Ultrasonic Testing (UT) technique that generates the sound in the part inspected instead of the transducer.
An EMAT induces ultrasonic waves into a test object with two interacting magnetic fields. A relatively high frequency (RF) field generated by electrical coils interacts with a low frequen-cy or static field generated by magnets to gen-erate a Lorentz force in a manner similar to an electric motor. This disturbance is transferred to
the lattice of the material, producing an elastic wave. In a reciprocal process, the interaction of elastic waves in the prence of a magnetic field induces currents in the receiving EMAT coil circuit. For ferromagnetic conductors, magnetostriction produces additional stress that enhance the signals to much higher levels than could be obtained by the Lorentz force alone. Various types of waves can be generated using different combinations of RF Coils and Magnets.
Becau the sound is generated in the part in-spected instead of the transducer, EMAT has the following advantages over more conven-tional piezoelectric transducers:
∙Dry inspection. EMAT does not require couplant for transmitting sound, which makes it very well suited for inspection of very hot and very cold parts, and integration in automated environments.
∙Impervious to surface conditions. EMAT can inspect through coatings and are not af-fected by pollutants, oxidation, or rough-ness.
∙Easier nsor deployment. Not having wedges or couplant, Snell’s law of refra c-tion does not apply, and the angle of the nsor does not affect the direction of prop-agation. This makes EMAT transducers easier to control and deploy.
∙Ability to generate SH modes. EMAT is the only practical means for generating shear waves with horizontal polarization (SH waves) without high mechanical pressure or low-density couplants that impede scanning of the part.
∙Mode lectivity. The antenna-type con-struction of the EMAT coil combined with
a multi-cycle excitation provides great spec-
ificity in the frequency domain, thus the ability to precily lect the wave mode of interest, which is of great importance for guided wave generation and interpretation. 4.Practical Applications
There are many well-known applications for in-rvice inspections using guided waves such as LRUT –Long Range UT (stationary ring in-spection with tubular waves), and MRUT - Me-dium Range UT (circumferential and axial scanning with plate waves). There are also very successful applications with hundreds of instal-lations using EMAT-generated guided waves for inspection of thin welds [2].
In this paper we will concentrate on more recent applications of EMAT for surface and volumet-ric inspections in factory environments.
4.1.Surface Inspections
4.1.1.Inspection of Round Billets
This application involved the inspection of round billets at the exit of an annealing oven with a surface temperature of 350ºC. The re-quirements included the detection of longitudi-nal, surface breaking cracks propagated from the inside of the billet, and folds on the surface of the billet caud by the rolling process. A ro-tary eddy current system was dismisd due to the temperature and large dimensions of the bil-lets, which would have made the system too costly and cumbersome.
龙山文化遗址入耳式耳机正确戴法The final solution required only two surface-wave EMAT transducers located at different locations of the billet and nding sound around its circumference. The system ud a wave-length of 4mm and was capable of detecting cracks and folds as small as 0.4mm in depth.
Figure 1. Surface Inspection of Hot Billets
4.1.2.Inspection of Copper and Brass Plates The application required the inspection of flat copper an
d brass plates before their introduction in a rolling mill. The two- meter-wide raw plates are fed into a scalper that removes the top-most layer of the plate to eliminate surface imperfections prior to rolling. The customer had installed a machine vision system after the scalper to determine if all the imperfections had been removed, but the system proved inade-quate to detect tight cracks, and produced a lot of fal positives due to the rough texture left by the scalping process.
The project included on-site tests using a porta-ble system to fine-tune the equipment and de-termine the capabilities of the technique. After successful proof-of-principle tests, the customer installed an automated integrated system. Figure 2. Preliminary Installation with Portable
Equipment
The final system included 4 nsors on top and 4 nsors on the bottom of the plate. The system inspected the top 4mm on each side of the plate and was able to detect defects 0.4mm deep and 10mm long. It also included an edge-tracking algorithm that automatically adjusted the gates to compensate for lateral movement of the strip.
Figure 3. Final Installation for Plate Inspec-
tionrnai
4.2.Volumetric Inspections
4.2.1.Inspection of Multi-Layered Clad Prod-
ucts
Multi-layered composites are widely ud to enhance the structural capabilities of different materials. In composites created by cladding or adhesive bonding, delamination is an ever-prent risk that can compromi the quality of the final product. Timely detection of
potential
delamination is very important to save produc-tion cost and prevent conquent failure [3]. The product in this ca included single and three layer combinations of brass/copper/brass, nickel/copper/nickel and other materials ud for the manufacture of coin stock. The custom-er had previously tried EC and normal beam
UT to detect delaminations without success. The lected EMAT technique included two transmitters on one side of the strip nding Lamb waves to receivers on the other side of strip. The receivers measured signal amplitude and Time-Of-Flight variations caud by de-laminations and other imperfections in the strip. After extensive modeling and empirical tests on the three-layer composite, it was discovered that the respons followed a cyclic behavior related to the dimensions of the lamination. At the delaminated region, the incident guided wave mode decompod into wave modes in two sub-systems, one at the delaminated layer, and the other on the other two. At the end tip of the delamination, the two waves converted back to the wave modes in a three layered structure. Since the EMAT receiver has an effective mode lection, the amplitude of the receiving signal was strongly affected by the amount of energy converted back to the incident mode.
The equipment included a double frequency technique to take into account and compensate for this cyclic behavior.
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Figure 4. Coin Stock Inspection System The final system was able to detect laminations as small as 1cm2 at over 1m/s. It was sub-quently replicated with different integration patterns at other coin stock manufacturers with similar success.
5.Conclusions
There are many techniques for surface and near-surface inspection of parts with their own ad-vantages and limitations.
Ultrasonic guided waves are growing in popu-larity due to their high nsitivity to surface and internal defects, and their ability to cover large spans at a distance and with a limited number of nsors. While guided waves are well docu-mented for in-rvice inspections, their u in factory environments is less known by the NDT community.
In this paper we emphasize the unique ad-vantages of Electromagnetic Acoustic Trans-ducer (EMAT) for generation of guided waves, and integration in production environments.
To conclude, we introduce a new generation of NDT systems for surface inspection of long products and volumetric inspection of single and multi-layered strip in factory environments. The systems fo
llow the success of guided waves for thin-weld inspections, and underscore the incread role of guided waves for inspec-tions in factory environments.
References
[1]J. Ro, “Ultrasonic Waves in Solid Media” pp.101-
245.
时光依旧[2] B. Lopez, “Inspection Trends June 2004”.
[3]Gao et al, “Delamination Detection in Composite
Clad Products Using Ultrasonic Guided Wave EMATs”. QNDE Conference 2008.
