PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 



785 



Fig. 15A. — Internal stress raisers in cast iron due to fatigue. Minute serrations in the 

 walls of graphite flakes and the very small tips of the flakes are internal stress raisers of 

 major importance. The path of a fatigue crack is generally from one such structural detail 

 to another. (Magnification 1000 X.) 



Fig. \bB. — Internal stress raiser in cast iron. This is the same as Fig. 15 A but at a magnifi- 

 cation of 3700 X. 



