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The presence of foreign metal bodies and fragments in foodstuff and pharmaceutical products is of major concern to producers. Further, hidden metal objects can pose threats to security. In particular, stainless steel is difficult to detect by conventional coil metal detectors due to its low conductivity. We have employed an HTS SQUID magnetometer for the detection of stainless steel particles which is based on the measurement of the remanent magnetization of the particle. Our aim was to determine the detection limits of HTS SQUID-based remote magnetometry, especially for food inspection purposes, and to make a comparison of this technique to commonly used eddy current coil and x-ray inspection systems. We show that the SQUID system's sensitivity to stainless steel fragments is significantly higher than that of coil systems if the samples are magnetized in a 100 mT magnetic field prior to detection. Further, it has a higher sensitivity than x-ray systems, depending on the density distribution of the product under inspection. A 0.6 mg piece of grade-316 stainless steel (a fragment of a hypodermic needle 0.5 mm long and 0.65 mm diameter) represents the detection limit of our system with a 150 × 150 mm 2 inspection orifice.

In the first part of this work the double refraction observed while measuring the attenuation of ultrasonic transverse waves in heat-treated 4150 steel was investigated. Although the data are not complete, they suggest that this double refraction phenomenon can be utilized to detect the amount of preferred orientation of the crystallites in rolled steel when this orientation is small. Calculated values of fractional velocity differences are given.In the second part the theory of ultrasonic wave propagation in polycrystalline metal is discussed, and the effect of preferential orientation of crystallites on fractional velocity changes is indicated. The use of ultrasonic double refraction data for determining the extent of preferential orientation is discussed. PART I Ultrasonic Double Refraction in 4150 Steel LTRASONIC double refraction was observed while transverse wave attenuation measurements were being made on blocks of hot-rolled 4150 steel which had been austenitized, quenched, and tempered in preparation for tests concerning temper embrittlement. Differences in the velocities of transverse waves polarized parallel to and perpendicular to the rolling direction in the steel resulted in interference patterns (see Fig. 1) such as those observed from irradiated silicon single crystals• and from "compatible" silicon single crystals at different temperatures? The reader is referred to reference 3 for the theoretical treatment of ultrasonic double refraction.A y-cut transducer was bonded to each specimen with phenyl salicylate, and when the polarization direction of the transducer happened to be approximately 45 ø out of alignment with the rolling direction in the steel, the cancellation due to interference became almost complete at some echoes (see Fig. 1). In such cases the echo numbers at which nodes occurred were noted and the fractional differences in velocity were calculated from the m=0 through m=4 branches of the equation (given in reference 3)' /Xv 1 (2m+l)tA+(--1)r•(2to--t•4) v 2fit 4riots4where m is an integer or 0, v the velocity, to a round trip time in the specimen, t• the travel time to the first apparent node, and tT the travel time to the first true node; f is the ultrasonic frequency. Higher branches were excluded since they would have yielded a difference in velocity well above the minimum difference observ-* Author of Part I. able by the pulse-echo method, and no such difference was observed. The values of/Xv/v appear in Table I. When interference patterns occurred the nodes were recorded, but when there was no interference pattern no attempt was made to produce one by reorienting the crystal. As a result the data are incomplete.The possibility that the observed difference in velocity was due to preferential orientation of the grains in the steel due to rolling was explored. If/Xv/v is due to crystal orientation, it is frequency independent (see reference 3). As can be seen from the table, Av/v must be calculated from the m--0 branch of the formulas in reference 3 since this is the only branch gi...

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A theoretical investigation is undertaken into the creep instability of two common shells of revolution, the infinitely long cylinder and the complete sphere-The linear and nonlinear visco-elastic material idealizations are used in the various theoretical procedures. The critical buckling time is greatly influenced by the parameter a/h ratio, magnitude of initial imperfections and material selection. The spherical shell has a lifetime longer than the cylindrical shell having the same a/h ratio, initial imperfections and material constants, while the critical mode numbers are the same.

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