SQUIDs

Simulations about Eddy Current Distributions and Crack Detection Algorithms for a SQUID Based NDE System. 

A SQUID [2] provides two basic advantages for measuring small variations in the magnetic field caused by cracks [3-7]. First, its unsurpassed field sensitivity is independent of frequency and thus dc and ac fields can be measured with an resolution of better than IpT/VHz. Secondly, the operation of the SQUID in a flux locked loop can provide a more than sufficient dynamic range of up to 160 dB/VHz in a shielded environment, and about 140 dB/>/Hz in unshielded environment [8]. 

In this paper we present simulations and measurements of several types of excitation coils, which match the special requirements for a SQUID based eddy current NDE system. We note however that all calculations presented here on penetration depths, current distributions and crack-detecting algorithms are also useful for conventional eddy current testing systems. 

In order to realise such a high dynamic range, either a local compensation coil at the location of the SQUID [9] or a gradiometric excitation coil like the double-D coil have to be used. In case of the electronic compensation, the excitation field and the response of the conducting sample is compensated by a phase shifted current in an additional coil situated close to the SQUID-sensor. Due to the small size of this compensation coil (in our case, the diameter of the coil is about 1 mm), the test object is not affected by it. 

The geometric compensation by means of a gradiometric coil is realised by placing the SQUID exactly between the two halfs of the coil, in order to detect only the response of the sample. In both cases we could achieve a reduction of the excitation field at the location of the SQUID of up to 1000. Electronic and geometric compensation together leads to an improvement of six orders of magnitude in the dynamic range, compared to a system without excitation field compensation. 

Abstract An Eddy current method applying a High Temperature Superconductor ( HTS ) DC SQUID sensor operating at Uquid nitrogen temperature (77K) is presented. The method is developed for the detection of surface or surface near defects. We compare the performance of the SQUID system with the performance gained from a commercial Eddy current system, while using identical probes. The experimental data are obtained on defects in gas turbine blades. The advantage of planar conformable probes for the use with the SQUID is discussed. 

Lew frequency Eddy current probing For frequencies below some 100 Hz the SQUID is coupled with a completely superconducting flux antenna. This antenna has to be within the cryogenic vessel. The Eddy current excitation is done in a conventional way. But care must be taken, that interference between the excitation field and the flux anteima and SQUID is... 

While the conventional Eddy current read out uses induced voltage and voltage amplification, the SQUID read out operates the probe as a magnetic flux transformer. This demands to minimize the resistivity of the pick up loop. 

We realized an Eddy current SQUID system of the high frequency type a room temperature Eddy current probe is connected to a SQUID sensor at hquid nitrogen temperature. Fig.3 gives an overview over the components of the system, fig, 5 shows a schematic diagram of the electronics. 

Several types of Eddy current probes were used with the SQUID system and the commercial system as well. High inductance wire wound probes with a ferritie eore and low induetance planar thick frhn coils were applied. The wire wound probe is the commonly used probe for high resolution conventional testing. The low inductance planar cod is more suited to be apphed in combination with the SQUID system. It is well adapted for surfaee defects and shallow defects. 

T/cm s at 1 MHz. Further SQUID development will allow to improve these data by one order of magnitude. 

In order to maximize the excitation, precautions have to be taken to avoid cross-talk between excitation and signal. Therefore differential probes are commonly used with a SQUID system Nevertheless, for the discussed defects the SQUID system has a lower excitation field by a factor of about 100 compared with the commereial system This we must keep in mind, when we compare measured signal to noise ratios. There is a potential to improve for small defeets, when eross-talk is managed very well. 

As stated above the SQUID amplifier demands a low inductance Eddy current probe in order to be able to amplify signals up to 1 MHz. Low inductance Eddy current probes can be obtained by reducing the number of turns and by loosing the magnetic coupling between the tums. So magnetic cores should be avoided as well as tight wounded tums. For this purpose planar coils are the best... 

A niobium planar SQUID gradiometer operarting in an unshielded environment Supercond. Sci. Technol. 9 (1996) A109-A111... 

HTS SQUID System with Joule-Thomson Cryocooler for Eddy Current Nondestructive... 

M. V. Kreutzbruck, J. TroU, M. Miick, C. Heiden, Y. Zhang Experiments on Eddy Current NDE with HTS rf SQUIDs Apphed Superconductivity Conference 1996,... 

To realize an automatic evaluation system, it would be desirable to also suppress geometrically caused signals as well, so that only the actual defect signals are obtained. Several approaches have already been made which are also to be implemented as part of a SQUID research project (SQUID = Super Conducting Quantum Interference Device). 

First Routine Aircraft NDT with SQUID Gradiometer. 

With the availability of such a SQUID system the question arises what it brings to nondestructive testing. One has to check if existing methods can be improved by using this systems. And, one has to check if this system allows for powerful new NDT methods. 

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