Specimens Of Different Thickness Research Articles

The magnetic stability of face-cooled composite tape superconductors

The concept of face-cooled composite tape is proposed to stabilize high critical current densities in Nb/sub 3/Sn, V/sub 3/Ga, and oxides. The stability is promoted by the combined action of the dynamic stability and the transient cryogenic stability. The expression of the limiting current density of stability is derived as a function of pertinent parameters of the superconductor, stabilizing normal metal, and coolant. An experimental proof of the analysis is given for an Nb/sub 3/Sn-Cu system. The specimens are made by alternately stacking strips of Nb/sub 3/Sn and copper. Stability is estimated by observing flux jumps for specimens of different thicknesses of copper. The experimental results agree well with the analysis. and an overall current density of about 500 A/mm/sup 2/ is stabilized. The concept of face-cooled composite tape will offer a useful basic criterion in designing conductors of high-T/sub c/ oxides.

THE SIZE EFFECT OF MICROSTRUCTURAL IMPLICATIONS OF THE WEAKEST LINK MODEL

Abstract— CT specimens made of a reactor pressure vessel steel were loaded at—40°C until final failure occurred by cleavage fracture. The samples of Jlcl values obtained in these tests are analysed using the weakest link model. The size effect observed with specimens of different thicknesses is compared with the predictions of the weakest link model. A formula is derived for the distribution of the locations of fracture origins which have been determined for almost all specimens with a scanning electron microscope. The distribution of the size of the “weak spots” is calculated from the distribution of the fracture origins using two different models for the stress field ahead of the crack tip. These fractographic results and the Jlcl data confirm the basic ideas of the weakest link model. The deviations observed between the quantitative predictions of the weakest link model and the data can partly be explained by the change in the stress state ahead of the crack tip caused by a change in the specimen thickness.

高温間欠過大荷重下における裂伝ぱの加速

Acceleration of crack growth under intermittent overloading was studied at 650°C by using two types of center notched plate specimens of different thickness made of a SUS 304 stainless steel. When the hold time of overload was very short (20 seconds), the crack growth rate was significantly accelerated to be about 20-50 times as large as that of static creep cracks, and the fracture surface morphology was different from that under static loading, indicating extremely ductile transgranular fracture by glide plane decohesion or microvoid coalescence. The acceleration in growth rate could be ascribed to recovery of the material during the low stress period. In the thinner plate specimens, the crack growth rate under intermittent loading was correlated well with modified J-integral J and agreed with the growth rate of static creep cracks in J-da/dt diagram. In the thicker plate specimens, however, the crack growth rate under intermittent loading did not agree with that of the thinner plate specimens in J-da/dt diagram, being about 1/5 times. Transgranular fatigue type crack growth appeared in the low growth rate region, and the growth rate was different from that of creep cracks.

A study of the delamination resistance of IM6/PEEK composites

This paper describes the delamination behaviour of unidirectional carbon fibre/PEEK (poly(ether ether ketone)) composites. Tests have been performed under mode I and mode II loading conditions, using double cantilever beam (DCB) and end-notched flexure (ENF) specimens. Mode I tests on specimens of different thicknesses indicated that propagation values of G Ic were dependent on thickness, higher values being obtained or 5 mm than for 3 mm. This was due to increased fibre bridging and multiple crack formation in thicker specimens and may explain scatter in previously published values. Fibre distribution was inhomogeneous in IM6/PEEK specimens, with clusters of fibres forming. The delamination resistance of these specimens was independent of cooling rate over a range from 50 down to 0·3°C/min. The behaviour of IM6/PEEK composites is compared with that reported for AS4/PEEK materials.

Manifestation of electrification effects in study of radiation optical properties of glasses

The change in optical transmission of specimens of K-208 glass under the action of an electron flux is studied experimentally as a function of electrification conditions. The kinetics of darkening of specimens of different thicknesses and the darkening distribution over the surface indicate the effect of radiation electrification of the glass on motion of moderate energy electron fluxes. A model is proposed for a dielectric electrified by irradiation, which explains the dependence of the change in optical properties of the glasses studied on irradiation conditions.

Transferability of results of small scale experiments to real structures

Numerical evaluations in combination with experiments on the basis of the J-integral methods are a necessary step in the chain of transferability from small specimens to real structures. For three cases, single-edge notched specimens of different thicknesses, flat plates under tension containing two through-cracks and side-grooved compact specimens of various steels, both finite element calculations including crack growth and experiments using the partial unloading technique were performed. The results show a good agreement of the experimental and numerical J-values and confirm the experimental procedure to evaluate J from the work done on the specimen. Moreover, for the single-edge notched specimen the strong influence of the angular stiffness of the loading system on the specimen behaviour is demonstrated.

Brittle-to-ductile transition in steels and the critical transition temperature

Specimens of different thickness made of a low-alloy steel were tested to determine fracture toughness under static and impact loading at room and low temperatures. It was found that for the specimen of each thickness there was a specific brittle-to-ductile transition temperature Tps defined as the upper temperature boundary up to which plane-strain conditions were valid at fracture. The temperature Tee, as well as the brittleness transition temperature Ts0 (determined via the 50% ductile component of the fracture surface), rose with the increase in the thickness B of a specimen. The value of Tps was found to be in a linear dependence on lgB for both static and impact loading.

Determination of stable crack growth resistance of ductile material using an rdcb specimen

A method to determine the plane stress fracture resistance curve for the stable crack growth of ductile material is presented. A reinforced double cantilever beam (RDCB) specimen of thin grooved section is used to determine the COD and JR of aluminium 2024-T3 material which exhibits substantial stable crack growth. The method makes use of the BEF and Dugdale models to determine the load-deflection behaviour of the specimen. The derivation is based on the experimentally determined load and crack extension values. The crack growth resistance for specimens of different thicknesses are deduced using the proposed method and are compared with those of the compliance method as well as the finite element method.

Adhesive fracture mechanics

Recently Williams, Malyshev and Salganik and others have applied the concepts of fracture mechanics to predict adhesive fracture. The specific adhesive fracture energy, γa, is defined as the energy released per unit of new surface created on separation of dissimilar materials. Williams has elucidated the similarity between adhesive and cohesive fracture from the standpoint of a Griffith energy balance analysis. One finds that for either cohesive or adhesive fracture, crack instability (where the crack has a characteristic dimension,a) is predicted by a general equation of the form $$\sigma _{cr} = k\sqrt {E\gamma /a}$$ wherek=f [geometry and loading] and includes all loading and geometric factors,E and γ are Young's modulus and specific fracture energy, respectively, and σcr is the applied load at incipient failure. Within the fracture mechanics interpretation the adhesive fracture energy γa is viewed as a fundamental property of the adhesive system. It is important to note, however, that it may depend on surface preparation, curing conditions, absorbed monolayers, etc. It is, therefore, essential that if γa is used to predict adhesive fracture for different geometries, then the surface preparation must be identical with that for the test specimen. If γa is a system parameter, then it would be possible to predict fracture by conducting an energy balance analysis of the configuration, utilizing values of γa, Young's modulus, and Poisson's ratio as determined from separate simple test specimens. There is, however, a need to establish that γa is a system parameter which is independent of geometry. One can in principle perform a number of tests on several specimen configurations or, more effectively, several tests on a single specimen which changes configuration between successive tests. In this latter case the surface and dissimilar materials remain constant. A specimen which is suitable for our purpose was developed by Williams and Jones by incorporating aspects of tests first suggested by Dannenburg and Salganik and Malyshev. The test method considers a disk or plate which has been bonded to a substrate material except for a central portion of radiusa. When pressure,p, is injected into the unbonded region, the plate lifts off the substrate and forms a blister whose radius stays fixed until a critical pressure,p cr, is reached. At this critical value the radius of the blister increases in size, signifying an adhesive failure along the interface. An energy balance analysis is available for the circular blister specimen in the two limiting cases of a thick plate (Williams) or a very thick medium (Mossakovskii), each with an infinite outer radius. Having established the utility of this general test method, we have considered it necessary to extend the analysis and test calibration capability to other thicknesses for more general engineering applications, as for example, very thin membranes which are used in paint coatings. An axisymmetric finite element numerical analysis was, therefore, conducted for specimens of different thickness and debond radii to establish the energy balance for the various arbitrary thicknesses. A continuous curve for arbitrary specimen thickness was then produced on a dimensionless plot ofp 2 a/Eγ versush/a whereh is the specimen thickness. The region ofh/a over which the limiting case equations are valid was also established within the accuracy of the numerical analysis. Since many, if not most, bond geometries are not readily analyzed in closed form, the numerical procedures for energy balance analysis is included and may be used for analyzing geometries other than the blister test specimens. Experiments were conducted over a broad range ofh/a and found to agree with the analytical elasticity solutions which assumed a constant (for given surface preparation, temperature, loading conditions, strain rate independence, etc.) value for γa. These experiments confirm that for this system at least γa is a constant independent of geometry.

Accuracy of thermocouples in radiant-heat testing

Results of an investigation of the accuracy of iron-constantan thermocouples in measuring transient surface temperatures produced by radiant heating on metallic specimens are given. The criterion of thermocouple accuracy was derived from the theory of heat conduction. The thermocouples were tested on metallic specimens of different thicknesses at varying heating rates. Comparison of these results with analytical criteria permitted quantitative determination of thermocouple errors.

Absorption Correction in Precision Determination of Lattice Parameters

X-ray powder and rotation patterns of six cubic substances (W, Si, Cr, TiO, Al, and V) using specimens of different thickness were made; the patterns were measured, the lattice constants calculated and plotted against the θ angle or against the Nelson and Riley function. Such curves for thin mounts, semitransparent to x-rays (below 0.2 mm in diam), consist of two parts: a region of high slope (due to absorption) and one approaching a zero slope at high Bragg angles. The latter, varying in extent with the size and nature of the mount, indicates that due to the rapid decrease in absorption, the lines in this region are shifted, little if any, from their correct positions. Hence, lattice constants calculated from the reflections of the high θ angles lying on the horizontal portion of the curve need not be corrected for absorption. The constant, with negligible error, can be calculated from a single last strong α1 or β reflection. If an extrapolation is applied, the curve must be traced through points in the high back reflection region. Otherwise any extrapolation, disregarding the horizontal part, would lead to lattice constants too high in value. The amount of line displacement due to absorption in thin samples (below 0.2 mm in diam) agrees with the geometrical concept of Hadding and Buerger.

Electron Bombardment Effects in Thin Dielectric Layers

Dielectric layers of mica, Pyrex, arsenic sulphide and antimony sulphide, between 0.7 and 4μ thick, have been investigated under electron bombardment with electron energies up to 50keV. The specimens carry evaporated Al electrodes, which are practically transparent to the primary electrons. The results include transmission curves for the primaries; the range values are in agreement with the Thomson-Whiddington law. With both electrodes at the same potential a steady secondary current begins to flow to the bottom electrode when the bombarding electrons have penetrated about half the specimen. Experimental values of primary and secondary currents for specimens of different thickness and dielectric fall on common curves when plotted against a suitable independent variable. The secondary current is shown to depend on the absorbed energy. Results for mica with applied fields up to 106 vcm-1 across the specimen are included. An interpretation of the secondary current curves is given in terms of the displacement of charge carriers in the space-charge field within the dielectric

Birefringence of Viscacelle

DR. N. H. HARTSHORNE, of University College, Swansea, writes: “With reference to my letter on the birefringence of ‘Viscacelle’ in the issue of NATURE of February 16 (p. 269), my attention has been directed to a note on the birefringence of ‘Cellophane’ contributed by Mr. Arthur M. Grundy to Watson's Microscope Record in 1931 (Sept., p. 22). The name ‘Cellophane’ is commonly, though mistakenly, used as a general term for artificial cellulose sheet, and as Mr. Grundy did not state the source of his material, I am uncertain as to its exact nature. It was, however, doubtless very similar to, if not identical with ‘Viscacelle’. He noted the uniformity of its optical character, and the fact that specimens of different thicknesses gave appropriately different polarisation colours, but he gave no data for the relative retardations associated with different thicknesses. He expressed the view that the optical uniformity of the material ruled out stress as a cause of its double refraction. As regards ‘Viscacelle’, I do not think that the double refraction is due in any degree to stress in the finished material, but, as I indicated in my letter, the stress involved in the spinning process (that is while the material is being drawn from the spinning slot into the coagulating bath) almost certainly causes an orientation of the cellulose molecules, and this accounts for the observed direction of ‘slow’ vibration. I regret that I did not see Mr. Grundy's note earlier, and I gladly acknowledge his prior discovery of the double refraction of artificial cellulose sheet.”