FAILURE UNDER "FATIGUE" 63 



the crystals have accommodated themselves to 

 the load, and on some of the surfaces actual reversal 

 of the slip takes place each time the load is reversed. 

 Now at each time of slipping there is, as we have 

 already seen, a considerable disturbance of the 

 crystalline arrangement near the surface of slip, 

 and when this slipping is continually reversed this 

 disturbance becomes an actual process of attrition. 

 The whole process has been watched under the 

 microscope, and what happens after a certain time 

 is that the surface of continual slipping developes 

 into an actual fissure or crack. The next step is 

 that, since this particular crystal has been thrown 

 entirely out of action, a greater proportion of the 

 total load is thrown upon its immediate neighbours 

 and the process of slip-reversal and attrition takes 

 place in them. In this fashion the flaw or crack 

 started within perhaps a single crystal, works 

 its way through the entire piece of metal with 

 increasing rapidity until fracture results. This 

 fracture naturally shows the crystal-facets upon 

 which the slip-reversals have taken place and 

 thus exhibits the typical "crystalline" appearance 

 of such fatigue fractures. 



This explanation of the process of failure by 

 "fatigue" is in itself simply a further step in the 

 scientific explanation of phenomena met with in 

 practice; it is the third or fourth step in a chain 

 of purely scientific discoveries, and it is only at 

 this stage that practical application becomes 



