Tensile testing of polysilicon

Tensile specimens of polysilicon are deposited on a silicon wafer; one end remains affixed to the wafer and the other end has a relatively large paddle that can be gripped by an electrostatic probe. The overall length of the specimen is less than 2 mm, but the smooth tensile portion can be as small as 1.5×2m in cross section and 50m long. The specimen is pulled by a computer-controlled translation stage. Force is recorded with a 100-g load cell, whereas displacement is recorded with a capacitance-based transducer. Strain can be measured directly on wider specimens with laser-based interferometry from two small gold markers deposited on the smooth portion of the specimen. The strength of this linear and brittle material is measured with relative ease. Young's modulus measurement is more difficult; it can be determined from either the stress-strain curve, the record of force versus displacement or the comparison of the records of two specimens of different sizes. Specimens of different sizes—thicknesses of 1.5 or 3.5 m, widths from 2 to 50 m and lengths from 50 to 500 m—were tested. The average tensile strength of this polysilicon is 1.45±0.19 GPa (210 ±28 ksi) for the 27 specimens that could be broken with electrostatic gripping. The average Young's modulus from force displacement records of 43 specimens is 162±14 GPa (23.5 ±2.0×10³ ksi). This single value is misleading because the modulus values tend to increase with decreasing specimen width; that is not the case for the strength. The three methods for determining the modulus agree in general, although the scatter can be large.     

Stacks and D-brane bundles

In this paper we describe explicitly how the twisted “bundles” on a D-brane worldvolume in the presence of a nontrivial B-field, can be understood in terms of sheaves on stacks. We also take this opportunity to provide the physics community with a readable introduction to stacks and generalized spaces.     

Processing and properties of polysulfone hollow fibre membranes for gas separation filled with sub‐micron carbon fibres

Polysulfone hollow fibre membranes, of the type produced previously with enhanced gas separation properties, were filled with vapour grown carbon fibres (VGCF) of sub‐micron diameter. The effect of filler content on mechanical and gas permeation characteristics was studied. Hollow fibre membranes filled with up to 2.8 vol. % of VGCF were produced using forced convection dry jet/wet spinning. Fibre stiffness increased with increasing VGCF filler content. The fibres with the highest concentration of filler were about 30% more stiff than unfilled membranes. The addition of VGCF lead to a rise in permeability and some decrease in selectivity.     

A technique for cyclic-plastic notch-strain measurement

The interferometric strain/displacement gage was used to measure local strains of approximately ± 1 percent strain near a central notch in specimens undergoing completely reversed loading. Two notch geometries were tested: a circular hole and a keyhole slot with theoretical stress-concentration factors of 3.1 and 5.9 respectively. Measurements were made at both room and elevated temperatures (149° and 260°C) on three materials having different cyclic properties. This paper describes the experimental techniques for making the notch-strain measurements. The results of this series of tests are used in evaluating Neuber's cyclic rule.     

Temperature and rise-time effects on dynamic strain measurement

Strain pulses in a test specimen were measured over a temperature range of ?73 to +149°C with foil and semiconductor strain gages. These tests were performed to determine if the rise time and amplitude of the gage output change as a function of temperature. The existence of a constant that should be added to the theoretical rise times of resistance strain gages, as suggested by Koshiro Oi, was reexamined. ‘Long’ rise-time strain pulses were produced in the test specimen by an impacting steel ball. The rise times of these pulses were on the order of 7 μs and the amplitudes were approximately 65 μm/m. The results of these tests show that the rise time and amplitude of the gage do not change as a function of temperature. ‘Short’ rise-time strain pulses of approximately 500 μm/m with a rise time of 2 μs were produced in a test specimen by a short pendulum-type hammer apparatus. The results of these tests showed that the amplitude of the gage output was relatively independent of test temperatures but exhibited a slight hysteresis effect. The rise times for these tests remained constant up to a temperature of 93°C, then started to increase. The rise times at 149°C were approximately 100 percent longer than at room temperature. Under optimum conditions, a pulse with a measured rise time of 0.18 μs could be generated. The results of these tests indicated that the theoretical rise-time additive constant of resistance strain gages is 0.05 μs or less. This is one-half the value that Bickle arrived at by reevaluating Oi's data. However, since the real rise time of the pulse was unknown, this additive constant is not necessarily a property of the gage.     

The interferometric strain gage

The interferometric strain gage consists of two very shallow grooves ruled on a highly polished surface. The grooves are cut with a diamond and are 4×10^?5 in. deep and 5×10^?3 in. apart. Coherent, monochromatic light from a He?Ne gas laser incident upon these grooves will produce fringe patterns. A fringe pattern with the fringes parallel to the grooves is formed on each side of the impinging beam. The position of these patterns in space is related to the distance between the two grooves. As this distance changes, the fringes shift. Measurement of these fringe shifts enables one to determine the local strain of the specimen. In this paper, the theory of the measurement is developed first. The strain, ∈, is given by ∈=ΔFλ/d _o sin α _o where ΔF is the average fringe shift of the two patterns, λ is the wavelength of light,d _o is the initial distance between grooves, and α _o is the angle between the incident light beam and the fringe patterns. A procedure for making static measurements with the interferometric strain gage is presented. The sensitivity for these measurements is 0.5 percent strain per fringe shift, and the maximum strain is 4 percent. The method is evaluated by comparing its results with other accepted means of measuring large plastic strain. These other techniques are: post-yield foil gages, a 2-in. clip gage, and an Instron testing machine. The average percent difference among these techniques is less than 0.4 percent based on a full-scale measurement of 4-percent strain. The interferometric strain gage has the following features: a gage integral with the specimen surface, a very short gage length, relatively easy application, and the ability to measure large strains.