. The strength of materials; a text-book for engineers and architects. t have been fractured byrepetition of stress do not exhibit any weakening that thefatigue idea suggests. The subject is still full of difficulties from the point ofview of a satisfactory explanation of the results. For instance,the effect of overstrain is to cause the material to becomebrittle, and yet the more brittle kinds of steel (the highcarbon steels) show less effect than the mild steel. One explanation, called Fosters Theory, is that themechanical hysteresis (p. 59) causes a very small permanent * Phil. Trans. Roy.
. The strength of materials; a text-book for engineers and architects. t have been fractured byrepetition of stress do not exhibit any weakening that thefatigue idea suggests. The subject is still full of difficulties from the point ofview of a satisfactory explanation of the results. For instance,the effect of overstrain is to cause the material to becomebrittle, and yet the more brittle kinds of steel (the highcarbon steels) show less effect than the mild steel. One explanation, called Fosters Theory, is that themechanical hysteresis (p. 59) causes a very small permanent * Phil. Trans. Roy. Soc, Proc. , 1911. 90 THE STREXGTH OF I\L\TERIALS strain at each repetition and that the effect of these permanentstrains is cumulative so that ultimately the permanent strainbecomes sufficient to cause failure. The appearance of the fracture in these experiments isalways different from that for ordinary static tensile tests;that for mild steel being more like a hard steel. This isprobably due to the effect of overstrain upon the propertiesof the c : c *? Carb en Feyoenta q e Fig, 43.—Repetitions of Stress—Abrupt Change of Section. Effect of Sudden Change of Section upon Results.—The effects of sudden change of section have been investigatedby Dr. Stanton and ^Mr. Bairstow,* who obtained the resultsshown in Fig. 43, in which the limiting stresses for failiure byrepeated loading are plotted against the carbon results may be summarised as follows. 1. The resistance of a screw-cut specimen varied from67 to 70 per cent, of the maximum resistance of the corre-spondmg material, the fracture always taking place at theend of the thread. 2. The resistance of a specimen having a moderately * See Engineering, April 19, 1907. REPETITION OF STRESSES 91 rapid change of section varied from 65 to 72 per cent, of themaximum resistance of the corresponding material. 3. The resistance of a specimen having a sudden change ofsection varied from 47 to 52 per
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