64 MODERN SCIENCE OF METALS 



apparent. An insight into the mechanism of 

 fatigue failures at once opens up the possibility of 

 avoiding their disastrous incidence. Fatigue frac- 

 ture results from reversals of slip in individual 

 crystals of a piece of metal ; if we avoid the occur- 

 rence of such slip in the first instance, the whole 

 chain of events leading to ultimate fracture is 

 prevented. From this follows the simple practical 

 rule that if any piece of metal is to resist the 

 alternating application of a load for an indefinite 

 number of times, this load must not be large 

 enough to produce even a slight amount of slip 

 in any of the minute constituent crystals of the 

 metal. Now slip occurs so soon as the metal 

 undergoes even the slightest plastic or permanent 

 change of shape; expressed in technical terms 

 this fact is stated by saying that slip occurs so 

 soon as the elastic limit is passed. Up to this 

 "elastic limit," when the load is removed the 

 metal returns to its original shape the metal has 

 merely been elastically deformed; but so soon 

 as this limit is passed, the metal retains a per- 

 manent although possibly very minute change 

 of shape. The problem is thus reduced to deter- 

 mining with great care and accuracy the intensity 

 of loading which a given metal can bear without 

 passing this elastic limit and then so arranging 

 matters that the loads applied in service never 

 exceed this value. To that extent our discovery 

 leads to a practical engineering rule of the highest 



