Fracture of Solids' 



By J. E. Field 

 Cavendish Laboratory, Cambridge, England 



[With 4 platesl 



The failure of a solid by fracture is an experience common to all, 

 whether it be the breaking of a cup, the shattering of a car windscreen, 

 or the event leading to disaster with an aircraft. The nature of the 

 initiation, subsequent path, and speed of development of fracture often 

 appear unpredictable. The result of fracture is frequently cata- 

 strophic. It is this aspect of finality which creates the greatest prob- 

 lems for the engineer who at present only overcomes them by clever 

 design and the use of large safety factors. 



It would, of course, be difficult and undesirable to avoid using brittle 

 solids since they combine so many useful properties with their brittle- 

 ness. Glass as the prime example of a brittle solid has, in one or 

 other of its forms, high hardness, good resistance to chemical reaction 

 and thermal shock as well as its most valuable of properties, trans- 

 parency. Further, one of the many modern requirements is for solids 

 which remain strong at high temperatures. Above about 1,000° C. 

 the solids w^hich still retain some degree of strength are frequently 

 those that exhibit brittleness at room temperature. 



Fracture is, however, not only a calamity to be avoided ; it is fre- 

 quently the best way of dividing a solid. The energy required to 

 cleave a diamond or split a log is far lower than that needed by any 

 sawing process. The surfaces of cleaved materials are frequently 

 smooth and plane ; properties which have many scientific uses besides 

 their importance in jewel stones. 



TYPES OF FRACTURE 



If a solid is pulled hard enough it will eventually fracture. On the 

 atomic scale this is the stage where the binding forces between the 

 atoms are finally overcome by the tensile stress we have applied. The 



1 Reprinted by permission from The Times Science Review (London), No. 51, Spring 

 1964. 



431 



