GB427989A - Improvements in fatigue testing machines - Google Patents

Abstract

427,989. Testing fatigue of materials. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, JOLLEY, L. B. W., and THOMPSON, J. S., c /o General Electric Co., Wembley, Middlesex. Dec. 22, 1933, No. 36167. [Class 106 (ii)] The machine comprises a self-oscillating electrical circuit which serves to maintain the specimen in vibration in one of its natural modes, the specimen itself forming the frequency determining element, the oscillations being recorded and a frequency meter provided to mark the departure from the starting frequency. The specimen bar 1 rests on knife edges 2, 2, a magnet 3 under each end being provided with two windings, a direct-current winding 4 and an alternating-current winding 5 of which the latter maintains the " free-free " vibrations. The windings 5 are supplied from the secondary of a transformer 6 fed from an oscillating generator gas-filled triode 7 having the primary of the transformer 6 in the anode circuit. A condenser 8 is shunted across the transformer winding, while the cathode is connected to the direct current supply through a resistance 9. A choke 10 and circuit breaker 11 are located in the supply circuit. The grid is connected to the supply side of the resistance 9 through a high resistance 12, the secondary of a transformer 13 and a resistance 14. The primary of the transformer 13 is connected to a small " pick-up " 15. The frequency of the circuit is adjusted until it equals the natural frequency of the specimen. The windings 4 are supplied through an adjustable resistance 16 and a choke 17. A stretched steel band 18 is tuned to a frequency slightly lower than that of the specimen and an electromagnet 19 placed close to the band is supplied from the transformer 6. On the specimen developing a slight crack its frequency drops slightly, setting the band 18 to vibrate until the contact 20 is periodically reached thus shorting the resistance 14. The ensuing heavy current through the triode trips the circuit-breaker 11 and stops the test. A plurality of such units may be arranged to form a single apparatus testing several specimens at different amplitudes. The amplitude of vibration of the specimen may be measured by attaching a small gold bead to its end and causing the bead to give an image of a light forming a fine line whose length can be measured on the graduated eyepiece of a microscope. Alternatively, it can be measured by a micrometer provided with an electrical contact, by microscope or by attaching a trapezoidal piece of paper to the end of the bar and noting the horizontal position of the apparent point of intersection of the extreme images of the opposite sides. To record the number of oscillations a stationary marker is held in contact with a chart on a rotating drum against the influence of a spring by an electromagnet which is de-energized when the contact breaker is operated. If the angular displacement of the drum is related to the number of vibrations of the specimen, the length of the line record indicates the number of cycles performed by the specimen. To obtain a complete fatigue curve the relationship between the displacement of the drum and the number of vibrations should be logarithmic. One form of such drive consists in rotating the drum by a spring which is controlled by an escapement wheel 22 and a reciprocating pallet 23 each movement up or down of which allows the drum to rotate one step. The pallet is reciprocated by a bar 24 moved transversely to its length. The bar is provided with a series of pins 25 each engaged by a roller 28 on the end of a bell-crank 26, the other end of which engages a cam 29 having five lifts 30. Each lift raises and lowers the bar 24 to advance the drum 21 one step in each movement. The final lift 30 is made higher and rocks the lever 26 out of engagement with its pin. The next cam which is driven at one third of the speed of the first then takes up the drive and so on. Specification 397,167 is referred to.