Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Patent
1996-06-24
1998-02-24
Oda, Christine K.
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
G01N 2904
Patent
active
057213798
DESCRIPTION:
BRIEF SUMMARY
This invention relates to electromagnetic acoustic transducers.
Such transducers are used for generating and detecting ultrasound waves, for example shear waves, where the vibration direction is parallel to the wavefront. The transducers can generate acoustic waves in an electrically conducting sample without needing to be in contact with it or an acoustic couplant liquid, and so can be used to measure the thickness or surface properties of the sample.
An electromagnetic acoustic transducer normally has a permanent magnet or electromagnet, to create a static magnetic field, and a coil wound perpendicular to the static field direction. If an input current is pulsed through the coil when the transducer is close to a conductor, an eddy current is induced. A Lorentz force interaction between the eddy current and the static magnetic field results in a dynamic stress in a direction mutually perpendicular to the directions of the static field and eddy current. The dynamic stress acts as an ultrasound source. The transducer can also act as a detector of ultrasound waves vibrating predominantly in the same direction as the dynamic stress. In this case the ultrasound wave interacts with the static field to produce an eddy current which creates a dynamic magnetic field which in turn induces output current pulses in a transducer coil; either that of the original transducer, or a separate transducer. The input current pulses are created by discharging a capacitor, while the output pulses are passed via a preamplifier to a recorder such as an oscilloscope.
Electromagnetic acoustic transducers are normally operated in a resonant mode, at relatively low frequencies, below 4 MHz. The frequency is chosen in accordance with the material of the sample being investigated. The generating transducer is driven with a toneburst current, and any separate detecting transducer is tuned to the same frequency as the generating transducer. This arrangement has a good signal-to-noise ratio, but has the disadvantage that the ultrasound waves, and the output current pulses are long and resonant. The resonant detecting transducer further increases the pulse length. It is then difficult to measure accurately the time between one output pulse and the next, so that accurate measurement of the thickness of very thin samples, or detection of some near surface defects, is virtually impossible.
According to a first aspect of the present invention, an electromagnetic acoustic transducer for generating ultrasound waves in an electrically conducting sample comprises magnet means for producing a static magnetic field, and a coil through which brief current pulses are passed to create a dynamic magnetic field, with input means for producing the brief current pulses, the interaction between the fields and the conducting sample generating ultrasound waves, the arrangement being such that the frequency content of the ultrasound generated is broadband, and is determined by the characteristics of the current pulses.
It will be appreciated that a transducer able to operate over a broad band of frequencies is not tuned, and it has been found, quite surprisingly, that it operates satisfactorily. The advantage of the transducer is that the output pulses produced are also brief, being substantially of the same duration as the input pulses, so that it is relatively easy to measure accurately the interval between one pulse and the next. This makes it possible to measure accurately the thickness of very thin samples, and to detect near surface defects.
The frequency content of the ultrasound ranges from DC to 20 MHz. It may be varied by altering the characteristics of the input current pulses. The rise time of the input current pulses is preferably less than 100 nanoseconds. The current may be of the order of 50 amps.
The input pulses may be generated in an input circuit having a high voltage DC supply charging a capacitor through a resistor, the discharge of the capacitor to the coil being controlled by a fast switch. The coil has a low inductance, and the inducta
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Al-Kassim Adil
Edwards Christopher
Palmer Stuart B.
Oda Christine K.
The University of Warwick
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