Metallurgy; an introduction to the study of physical metallurgy . ould be of Uttle more value than the power ofenduring a few hundred thousand reversals only ; failure wouldoccur in both ceises, and the advantage of a shghtly longerlife would be small. If, however, the true safe range is oncedetermined, then shafts and other parts can be so designed aato remain well within this range, and failure from fatiguewill thereby be entirely excluded. At first sight it wouldseem difficult to determine the true safe range in any simplemaimer, but research has shown that this determination isreally a fai
Metallurgy; an introduction to the study of physical metallurgy . ould be of Uttle more value than the power ofenduring a few hundred thousand reversals only ; failure wouldoccur in both ceises, and the advantage of a shghtly longerlife would be small. If, however, the true safe range is oncedetermined, then shafts and other parts can be so designed aato remain well within this range, and failure from fatiguewill thereby be entirely excluded. At first sight it wouldseem difficult to determine the true safe range in any simplemaimer, but research has shown that this determination isreally a fairly simple matter. It is only necessary to carryout a Wohler or similar alternating stress test under at leastthree, but preferably four, different intensities of loading. THE MECHANICAL TESTING OF METALS 227 BO chosen that under the heaviest loading the test-bar breaksafter a few thousand reversals, the subsequent loadings beingdiminished in such a way that under the last—and lightest—loading the bar either does not break at all or only does so after (J 1^. /234S6783/0/INumber of Reversals Endured before Fracture, in Hundred-Thousands. Fig. 92.—^Alternating Stress Test Cuives for two Materials. several miUion reversals. The results thus obtained are thenplotted with the number of reversals endured as abscissae andthe intensity of the stress as ordinate. It is found that thepoints so plotted lie on a curve of parabolic form which tendsto become parallel to the axis as the intensity of the stressdiminishes. The horizontal asymptote of this parabola is q2 228 STUDY OF PHYSICAL METALLURGY the limit of the safe range of stress to which the material maybe exposed indefinitely under reversal, without risk of failurethrough fatigue. In Fig. 92 are shown the plotted curves fortwo materials having different limits of safety in this respect;it will be seen that the two parabolic curves cross one another,so that, if compared by their behaviour under a somewhatsevere loading, the wor
Size: 1333px × 1874px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjectmetals, bookyear1922
