444 KEPORT — 1891. 



Launhardt published a formula which applies to the cases in 

 which the stresses are either tensile or compressive, which may 

 be represented as follows : — 



Let h denote the breaking-strength, 

 s „ statical strength, 



'p „ primitive strength. 



Then, h = p -\- (s — ») J ' '' ' 



J max. 



If in the end lattice-bar of a bridge the stress produced by 

 the live load were nine times the dead load, then — 



f min. __ 1 . 



J To ' 



J max. 

 and, using the results given in Table VIII. for wrought-iron, 

 we have — 



h = 13-10 -f (22-8 - 13-1) iV = 14-07 tons ; 

 so that 14-07 tons is the actual breaking-strength of the bar ; 

 and, if the working stress is taken at 4-69 tons per square 

 inch, the factor of safety is, — 



1±}i! = 3 (not-:^ = 4-6). 

 4-69 4-69 ' 



In order to meet the cases which include stresses alter- 

 nating between tension and compression. Professor Weyrauch 

 proposed the following fornmla, in which the vibrating strength 

 is denoted by v : — 



b =^ p - (p - V) — . 



maxBi 



If the greatest tension on a bar is 5 tons, and the greatest 

 compression 10 tons, then — 



max B _ 5 _ j^ . 



TT To ^ ' 



max Bi 

 and, using the same material as before, we have — 



b = 13-10 - (13-10 - 7-15) * = 10-125 tons; 

 and the factor of safety, with a working-stress of 3-375 tons 

 per square inch, is — 



10-125 o , , 22-8 r..s 

 = 3 (not = 6-4). 



3-375 ^ 3-375 



The effect of "fatigue" is considered to be purely local, 

 as it is not possible to discover any change in strength, 

 elasticity, or ductility in material which has been fractured in 

 this way, by retesting it in the ordinary way by means of the 

 testing-machine. 



There is no difference in the results obtained from testing 

 specimens cut from an axle broken in ordinary use, or by means 

 of the drop-test, than would be obtained from testing specimens 



