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Aviation History
1939
1939 - 1028.PDF
T APRIL 6, 1939 FLIGHT. 355 treatment in order to determine its effect upon the strength and other physical qualities of light alloys. In the high- frequency furnace the metal is melted by means of the heating effect of eddy currents induced in it. '' Electroflo '' automatic control is fitted to a number of the furnaces. In the mechanical test room an Avery automatic Brinell machine has been installed, the impression of the test- pieces being made under identical conditions each time, since the operation is electrically controlled. The test- piece is placed on the anvil and raised until it is almost in contact with the ball, and then, 071 pressing the starting button, the load is applied for a period of fifteen or thirty seconds, as selected by previously operating a two-way switch. When the predetermined time period has elapsed the machine stops automatically and the indentation in the test piece is examined under a microscope and measured in the usual way. Also in this room there is a Vickers hardness-testing machine in which the indenting tool is a pyramid-shaped diamond. A system of levers applies the load, which is automatically governed by a trip release. The diameter of the impression is then measured by a microscope with moving shutters. An Avery Izod impact tester is also provided and two tensile testing machines are being installed. One of these is for measuring automatically the linear variations of the test-pieces during the full test period by means of an optically functioning time and elongation recording appara- The staff of the research laboratory. In the centre are Messrs. T. F. Bradbury, H. C. Hall (in charge) and H. Gresham. tus. Long hot duration tests from 200 deg. C. to 700 deg. C. are cairied out in an air-circulating electric furnace with an automatic control which gives a maximum varia tion in temperature of plus or minus 20 deg. C. Owing to the large mag nification of the measuring device employed an elongation as small as 0.00004 in- can be read clearly from the recorded diagram. Particularly well equipped is the X-ray department, with its Philips Metalix Macro 150 mobile unit, which is capable of an output of 150 kilo- volts or 20 milliamps continuous, and by the addition of a further generator can be given an output of 30a kilovolts or 10 milliamps for the examination of steel. The plant has a self-pro tected shockproof water-cooled uni polar type of tube which gives good flexibility in use, and it is particularly useful for the examination of light alloy castings up to a maximum thickness of 4 in. By the use of this plant, oxide inclusions, shrinkage, blow-holes, cracks, segregation and other defects in the castings are shown up with the maximum of detail. X'Ray Protection As some of the castings to be examined are too large to be dealt with in the type of screening stand generally employed for industrial radioscopy, the plant is arranged so that the operator and the controls are screened off in a safety zone from the object to be examined and the source of radiation. The walls of the X-ray room are covered in 4 in. of barium sulphate plaster with a lead equivalent of 9 mm. The ceiling has four tons of lead laid between con crete, and the entrance is through a sliding door of lead carried on ball bearings. The safety zone has a lead screen which has been welded into position, thus avoiding any cracks or screw holes through which leakage might occur, and the bottom of the lead is curved under the concrete foundations of the teak block flooring. An inspection window 2 ft. 6 in. square is composed of four sheets of lead protective glass. The microscopical examination of metals in order to observe their construction is, of course, important, and in the micro room there is a Vickers projection microscope of the inverted type. Magnifications up to 4,400 times are obtained. A Zeiss binocular microscope is also used, and in the same room there is a Hounsfield Tensometer which is used for making tensile and other tests upon small specimens having a cross- sectional area from 1/80 sq. in. to 1/5 sq. in. The advantage of this Tensometer is that small specimens about 1 in. in length may be taken from a number of different places in the same casting or forging, machined to the required dimensions as test-pieces, and then tested. As the specimen is subjected to load the stress is recorded by a mercury column in a tube alongside which is a calibrated scale. At the same time, a rubber- surfaced cylinder is caused to rotate and to it is fixed a prepared chart paper. As the different readings are taken a pointer is moved opposite to the meniscus of the mercury column, and then a stylus is depressed, recording a definite point on the chart so that an autographic record of the stress-strain curve is produced. In the macro room, as its name implies, there is equipment for macro-etching specimens of material so that examination may be made of coarser structural features, such as the crystal- Intermittent immersion tests for corrosion. The test-pieces fixed to the discs, are immersed in the liquids every ten seconds as the discs revolve.
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