Sizing Canted Flaws in Weldments Using Low-Frequency Emats

Techniques for detecting and sizing flaws with electromagnetic-acoustic transducers (EMATs), previously used successfully for normal planar flaws, were applied to canted flaws in steel plates. Comparisons were made between metallographic and ultrasonic measurements on specially prepared welds. Results indicated a high probability of detecting canted flaws (\u3e 0.5-mm deep) with EMATs. The EMAT sizing was highly repeatable and, for the most part, very accurate. Some, as yet unexplained, inaccuracies did show up, however, in some weld sections. There is a possibility that the calibration curve may be more complex for canted flaws than for normal flaws

Artillery Projectile Inspection with EMAT\u27s

A technique for ultrasonically inspecting 135 mm artillery projectiles has been demonstrated. Noncontact EMAT\u27s placed on the outside of the projectile excited 2.25 MHz surface waves and 1.8 MHz angle shear waves which were used to detect EDM slots on both the inner and outer surfaces. The defect locations and sizes, 0.051 em (0.020 in.) deep and 0.508 em (0.200 in.) long were chosen on the basis of fracture mechanics analysis. Each was detected with signal-to-noise ratios in excess of 30 dB. These results indicate that it is feasible to construct a system for full inspection of projectiles at normal production line rates. Among the desirable features of such a system are the absence of an ultrasonic couplant, the resulting potential for high speed operation, and the convenience with which multiple transducers can be incorporated to achieve high coverage and to gain additional information for defect characterization

Pulsed Electromagnets for EMATs

In many industrial NDE applications pulsed electromagnets may be more desirable than large static electromagnets or permanent magnets for magnetic biasing of electromagnetic acoustic transducers (EMAT\u27s). Since electromagnetic acoustic transduction is confined to one skin depth at the operating frequency of the EMAT, the transduction efficiency can be enhanced by the dynamic concentration of the magnetic flux near the surface. This paper describes a number of physical phenomena associated with EMAT generation under pulsed-magnetic-field bias. In particular, it is observed that for maximum transduction efficiencies the ultrasonic pulse must be retarded relative to the initiation of the current pulse to the electromagnet windings. A second maximum in the transduction efficiency is observed when the pulsed magnet-EMAT system is operated on ferromagnetic steel (1020). The second maximum is associated with magnetostrictive effects. Operation of pulsed magnet-EMAT systems at elevated temperatures (400°C) is demonstrated on aluminum

UT with SH-Waves and Electromagnetic Ultrasonic (EMUS) -Transducers

In the ultrasonic testing practice of today SV- and longitudinal waves are exclusively used because these wave types can be excited by piezoelectric ultrasonic transducers introduced a long time ago

Weld Inspection with Shear Horizontal Acoustic Waves Gnerated by EMATs

The potential advantages of using electromagnetic acoustic transducers (EMATs) for nondestructive evaluation of metal parts have been known for some time. Recently a generically new EMAT has been perfected which can generate and receive horizontally polarized shear (SH) wave andle beams. SH waves offer considerable advantages over SV waves for inspecting metal parts of complex shape: 1) they reflect specularly from planes containing the direction of particle displacement, 2) they can be generated in any direction lying· in the saggital plane with equal efficiency, and 3) SH wave transducers inherently discriminate against Raleigh, L and SV waves. These advantages make SH waves particularly useful for weldment inspection. A brassboard system was assembled for locating natural and simulated flaws in thick MIG welds, and a new technology for placing controllable defects in weld deposits was developed. It was then shown that the SH-wave inspection system was capable of producing an accurate map of the controllable defects introduced into the weld deposit. The ultrasonic map compared well with the notes taken by the welder and contained considerably more detail than the radiographic map. The inspection was performed at 1.7 MHz with the SH wave beam axis inclined at approximately 38° with respect to surface normal. Tungsten and alumina rod inclusions as small as 3/32 inch in diameter with localized within the weld deposit with signal-to-noise ratios of better than 10 dB and the inspection was performed on an as-welded sample without surfaces preparation through surface grinding or polishing

Optimization of Electromagnetic Transducer Systems

Electromagnetic transducers have a number of inherent advantages, some of which have been touched on by other speakers. Historically, their major disadvantage has been their high insertion loss. We have undertaken a project which has been designed to explore techniques for optimizing transducer efficiencies and increasing the dynamic response of ultrasonic systems which use electromagnetic transducers. Today, we would like to report the results of that project

Advanced EMAT Inspection Systems: Projectiles and Welds

EMAT\u27s appear particularly suitable for automated inspection systems because of their ability to operate at high temperatures, at high speed, and without couplant. This paper reviews the progress in two important areas: the high speed inspection of artillery projectiles and high temperature inspection of MIG welds. In each case, material is presented illustrating the system concept and the ease of detection of appropriate flaws. Included is a discussion of the operational characteristics of such systems using SH and SV waves for inspection. The paper also describes the advantages of employing SH waves for volumetric inspection of very thick sections of complex geometries