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

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

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

Ultrasonic Inspection of Rubber Sonar Dome Windows

By using a rubber window acoustically matched to sea water, the Sonar system on a destroyer can be made considerably more sensitive. However, if the layered construction of the window develops delaminations while in service, the hydrodynamic characteristics of the structure may become modified and acoustic noise can be generated during high-speed operations. In order to inspect for these delaminations while the ship is tied up to its dock, a pulse-echo ultrasonic scan performed by a diver using a sea-water coupled transducer would appear to be ideal. However, the choice of acoustic parameters suitable for inspecting rubber were unknown. Laboratory studies sho1~ed that by utilizing very short time duration pulses whose center frequencies lie between 0.5 and 1.0 Mhz, it was possible to detect water-filled pockets within 0.2 inches of the outer surface of the window. A prototype instrument suitable for shipboard and dry dock operation has now been constructed. This instrument features optimized pulse excitation of low-frequency broad band transducers when attached to 50 to 100 feet of coaxial cable. A signal light is also incorporated to provide the diver with information on the condition of the rubber window under his transducer

Nondestructive Testing of “Thick” Aerospace Honeycomb Structures Using Through-Transmitted Ultrasonic Guided Waves

The idea of using guided elastic waves for the purpose of assessing the fitness for service of aerospace composite structural materials is not new. “Pure” longitudinal or shear waves cannot exist in layers whose thickness dimension is of the order of an ultrasonic wavelength