Abstract:
An experimental adaptation of an air temperature rotating bending-fatigue testing machine has been made for tests on light alloys at temperatures up to 200 deg. C. The machine is shown photographically in Figs. 1 and 2. It consists of a shaft rotating in a pair of self aligning ball bearings, driven by a direct current variable speed motor at one end, and with a chuck at the other end to hold circular-section test-pieces. The latter are held in the chuck by six set-screws. A two-point loading system is adopted and arranged with one scale pan to obviate the possibility of overstressing a test-piece during the application of the weights. Fracture or vibration of the test-piece operates a switch which stops the motor; the number of revolutions are indicated by the usual form of counter. The variable speed control permits critical speeds due to resonances of the test-piece assembly being avoided. For tests at 200 deg. C., a somewhat longer test-piece (see Fig. 3) than the standard air temperature type is used, and the temperature measurements are made by two thermo-couples (ironeureka) secured one to each end of the effective portion of the test-piece. The thermo-couples are connected directly to four insulated terminals on the chuck, which are permanently joined by wires through the middle of the main shaft to the brass slip-rings seen in the photographs. The thermal E.M.F. is picked up from the slip-rings by carbon brushes, so arranged that the contact pressure is obtained by dead-weight loading ; the brushes are only in contact with the rings whilst observations are being made; at other times a cam lifts them clear. The usual potentiometer method, with sensitive galvanometer, is adopted for the E.M.F. determinations, so that troubles due to variation of contact resistance at the brushes are minimised.
