FATIGUE STUDIES OF NON-FERROUS SHEET METALS 589 



The results for alloy "G" brass and high brass show the effect of 

 change in composition on the endurance limit. The greater improve- 

 ment in fatigue endurance of alloy "G" brass due to cold work over 

 high brass shows the superiority of this alloy for spring purposes. 

 This was to be expected on the basis of its physical properties.^- ^ 



Alpha brass hardened by nickel and silicon shows considerable 

 improvement in the endurance limit and also an improvement in the 

 ratio of endurance limit to tensile strength. This offers a means 

 whereby some non-ferrous alloys may be improved in this respect. 



Attention is called to Fig. 6 which gives the stress cycle graphs for 

 nickel silver in three tempers. It will be noted that the curves tend 

 to turn sharply upward for the higher stresses. This indicates the 

 effect of drastically overstressing the metal. The authors have ob- 

 served this effect with other metals on the rotating beam machine. 

 It seems that after a limiting stress value that the number of cycles 

 to failure tends to become constant. 



Conclusion 



From the test results obtained on these non-ferrous metals it is 

 seen that the fatigue endurance limit varies from approximately 12 

 to 36 per cent of the ultimate tensile strength, whereas the commonly 

 accepted ratio of fatigue endurance to tensile strength of steel is in 

 the neighborhood of 40 per cent of its ultimate tensile strength. In 

 other words, it appears that the low endurance limit of these materials 

 emphasizes the need for their careful selection for use as springs. 

 High tensile strength or proportional limit are not sufficient guarantors 

 that the material will perform satisfactorily in service. 



Cold work raises the endurance limit but not in a manner pro- 

 portional to the increase in tensile strength produced by the same 

 cause. There is no correlation between tensile strength and endurance 

 limit except for cold worked metal of a definite composition. For 

 other compositions the correlation is different. 



Precipitation hardening of alpha brass by nickel silicide increases 

 the endurance limit. 



The fatigue failure appears to be a result of a fracture across the 

 crystals of the material. Photomicrographs are given showing in- 

 cipient cracks that were developed in the uniformly stressed areas of 

 the specimen. 



*" Physical Characteristics of Copper and Zinc Alloys," Bassett and Davis. 

 Proc. Inst. Metals Div. A. I. M. M. E., 1928. 



^ The authors are indebted to Mr. L. E. Abbott for his assistance in obtaining 

 laboratory data. 



