A high-speed fatigue-tester, and the endurance of metals under alternating stresses of high frequency

Abstract

The deterioration of metals under the action of stress varying rapidly between fixed limits has keen the subject of much experimental investigation. It is found that the important factor in the rate at which this "fatigue" goes on is the algebraic difference of the limits between which the stress varies, usually called the "range of stress"; and that the absolute position of these limits matters little, provided, of course, that the mean stress is not too large. The number of applications of a given range of stress required to fracture the piece increases as the range is diminished, the general nature of the relation between the two being as shown in the curve (fig. 1), which represents the results of a series of tests made by Dr. Stanton on mild steel. In these observations the stress alternated between compression and tension, the ratio of the compression and tension limits being 1·09. The form of the curve suggests that a range of stress not much below 25 tons, which in an average specimen would just cause fracture alter a million reversals, could never break the bar, however often applied. One of the chief objects of the fatigue tests hitherto made has been to discover tins "limiting range." At an early stage in these investigations the question was raised whether the endurance by the material of a given cycle of stress is affected by the rate of repetition of the cycle. Besides its intrinsic interest, tins question is of importance because on the answer to it depends the possibility of reducing the excessive amount of time taken to carry out fatigue tests. The determination within a few per cent, of the limiting range requires several separate tests in which the cycle is repeated at least a million times, and even that number is not always sufficient to give a reasonably close approximation. Wöhler worked with 60 to 80 reversals per minute, and he found that the same wrought iron which could just sustain a million applications of a range of 23 tons broke after 19 million repetitions of a range of 17½ tons. The more recent machines have keen run at much higher speeds, and there is now a machine of the Wöhler type at the National Physical Laboratory which gives 2000 cycles of bending stress per minute. Even at this speed, which I believe is the highest yet reached under conditions admitting of accurate measurement, it takes eight hours to do a million reversals.