GB549140A - Materials testing apparatus - Google Patents

Abstract

549,140. Strain-gauges. SIMMONS, jun., E. E. March 5, 1941, No. 2962. Convention date, Feb. 23, 1940. [Class 40 (i)] [Also in Group XXXVI] A strain-gauge comprises a filament of resistance wire associated with a body under test so as to undergo changes of strain and therefore of resistance proportional to those of the body. The invention is applicable in impact dynamometers and pressure gauges and to measuring the strain at various points in a gun barrel. In the case of an impact dynamometer, the indications are given by a momentarily operated cathode-ray oscillograph and are photographed. The oscillograph may also give an indication of the extension of the specimen. In other cases the resistance variations may be indicated by a galvanometer. Construction of resistance-wire elements. These may consist of a filament of " Constantan " or " Advance " woven into a mat with a warp of fine silk threads or wound round displaceable or removable studs on a jig, Figs. 28, 29, and subsequently stuck to a paper sheet by a varnish such as that sold under the Registered Trade Mark " Glyptal." In another arrangement, the jig has a convex surface and in others the filament is wound spirally, helically and as shown in Fig. 32. Construction of dynamometers. In an impact dynamometer of known design, the specimen 6 is secured between a transverse member 5, designed to be struck by a weight 4, and a member 9 carrying a resistance wire element 19 which is stuck over its whole surface to the member 9 as by " Glyptal " cement. The member 5 carries on a flexible strip 57 a contact point 59 which engages an interrupter drum 45 built up from alternate layers of aluminium foil and celluloid and so interrupts a circuit as the specimen is extended. The drum is rotatable so as to expose a new surface to the contact needle after use. The invention may also be applied to machines in which a projectile is fired into a hollow specimen, the wire element being similar to that on the machine just described, Figs. 19, 22 (not shown), or comprising a wire wound helically on and cemented to a washer 82, Fig. 21, which is compressed on the impact of the projectile. In another arrangement, Fig. 23, the specimen 83 is subjected to a torsional impact and sensitive wire elements 84 are secured to the arms of an S-shaped support 83a. Pressure gauge. As shown in Fig. 10, a bulb 71 of quartz or of a steel, copper or aluminium alloy having good elastic properties or of glass, has a resistance-wire filament wound helically on and cemented to, its surface and is connected to the source of fluid pressure to be measured. If desired, the bulb may be sealed within a container communicating with another source of pressure. Cathode-ray oscillograph. In the arrangement shown in Fig. 18 a condenser 202 is normally charged and gives the horizontal deflector plates the full deflecting potential. The tube however remains dark on account of a biassing voltage developed across a resistance 213 by rectifying a high-frequency current normally generated by an oscillator 206. When a test is to be made, a condenser 69 is charged by momentarily operating a switch 67 and the weight 4 of the dynamometer is released. The transverse member 5, Fig. 3, carries a displaceable pin 65 of solder or other inelastic conductor over which the condenser 69 is discharged when the weight 4 strikes the pin. The resulting potential drop in a resistance 70 causes an amplifier valve 216 to give an impulse to fire a hot cathode gas discharge tube 200 over which the deflector condenser 202 is discharged. This stops the oscillator 206 to start the cathode beam and allows the latter to swing to its starting position. The beam is then deflected horizontally at a rate determined by the value of resistance 201 over which the condenser 202 is charged and vertically to an extent determined by the variation of the dynamometer resistance. The vertical deflecting potential may be obtained by rectifying amplified high-frequency oscillators from a bridge circuit, unbalanced more or less by the resistance element. Alternatively the circuit shown in Fig. 14 may be used in which a brief impulse is passed on at impact via a condenser 32 from a circuit comprising a battery 29, a ballast resistance 31 or a constant current vacuum tube or a filter 31 for compensating for the frequency characteristic of the dynamometer resistance 30. At the same time the trace is interrupted a number of times by the device 59, 45 (controlled as previously mentioned by the extension of the specimen) which intermittently gives high voltage paralysing impulses via an amplifier and an isolating diode tube 217 to the grid of the cathode ray tube. A switch 221 enables the interrupter to be replaced by a resistance or by a source 222 of alternating current for giving a time scale. For calibration purposes, a normally shortcircuited variable resistance is provided. If the short-circuit is removed and a horizontal sweep is made, a trace such as 90, Fig. 17, is recorded and indicates the effect of a known change of resistance. In this figure, 92 shows the time scale and 62 the test trace interrupted at 63 by the elongation of the specimen. Other circuits may be associated by means of isolating diodes such as 217 with the oscillograph tube.