BEHAVIOUR OF MATERIALS IN TESTING MACHINE 179 
stress, A is the range of stress, and is a coefficient depending on the 
“material, For ductile iron and ductile steel the average value of n is 
t 1:5. For the harder and more brittle qualities, » may be as high 
as 2°2. In estimating the range of stress, if a tension is taken as positive, 
then a compression must be taken as negative. 
The following are examples of the application of the foregoing formula 
to a bar of mild steel having a static tenacity of 26 tons per square inch. 
m= 15. 
(1) Range of stress from fix. t00. A=fonox. 
~ Fras. = Max, + (262-15 x 26fmax, Hence Jingx. = 15°75 tons per sq. in. 
q (2) Range of stress from finx. t0 dfmax. A=Fhnax. 
; eas = 2 ranx. + ,/26?-1°5 x 26 x dfx, Hence finax. = 21°43 tons per sq. in. 
_ __ (3) Range of stress from fina. tO —Sinax. A = %Foaax. 
Frat. =Smax. + [262 — 1°5 x 26 x 2fnax, Hence fnax,= 8°67 tons per sq- in. 
4 The safe working stress is obtained by dividing /,,,,. by a factor of 
safety. 
Other empirical formule for fluctuating load stresses are given on 
252. 
> 170. Fatigue of Metals.—The loss of strength which occurs when 
a metal is subjected to a fluctuating load for a considerable time is 
frequently said to be due to fatigue. It is necessary, however, to dis- 
» ish between deterioration of strength due to mere fluctuation of 
"stress and deterioration due to shocks. It is well known that a crane 
chain, if kept in use for a long time, may fracture abruptly when carrying 
a load less than that which it has been in the habit of carrying. This 
_ deterioration of strength is, however, probably due to a slow accumula- 
tion of permanent set produced by shocks due to the sudden starting 
_ or stopping of the load when raising or lowering (see Art. 88, p. 69, and. 
_ Art. 168, p. 177), and the chain becomes in consequence less able to 
resist shocks. It has been found that the power of the chain to resist 
_ shocks is restored by annealing, and it is a common practice to anneal 
 erane chains at frequent intervals, say, once or twice a year: Unwin has 
to restrict the term fatigne to deterioration due to shocks which 
vable by annealing. 
— “I71. Mechanical Properties of Steel after Heat Treatment.— 
__ The sixth report to the alloys research committee of the Institution of 
Mechanical Engineers by the late Sir William ‘C. Roberts-Austen and 
_ Professor William Gowland, relates to the heat treatment of steel.* This 
voluminous report contains a record of a large amount of research on the 
»& oe of various kinds of heat treatment on the mechanical properties of 
. a of steel containing different amounts of carbon. A few of the 
ts will be given here. 
4 _ The table on page 181 gives the maximum stress and the elonga- 
____ tion obtained in tensile tests of the bars as received from the rolls, and 
____ also after the different kinds of heat treatment described. Additional 
 * Proceedings of the Institution of Mechanical Engineers, 1904. 
