256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



Lastly, close observation will show some sharp cleavage edges, 

 representing profiles of fractures on other crystal planes. The most 

 prominent of these has been found to be a set also forming a pyramid, 

 like the twins, but about twice as high. The story of deformation and 

 fracture for this metal is thus written into the subtle markings on its 

 fracture facets. 



THE FRACTURE OF STEEL SHIPS 



From such observations of the path of fracture through the individ- 

 ual grains in a metal all these deductions can be made, and many more. 

 A particularly important instance has to do with a problem involving 

 both the loss of material and human lives. 



During the recent war, more than 40 of the welded steel ships made 

 in this country fractured completely in two, and there were more than 

 4,000 reported cases of lesser fractures. The problem is one of an 

 elusive property, simply called toughness, whose identification remains 

 a great challenge in current metallurgical research. 



Two steels, identical in virtually every respect so far as common 

 analysis is concerned, will behave so differently when placed in service, 

 such as that of deck plate, as to cause shipwreck in one case and no 

 trouble whatsoever in the other. Extensive researches conducted in 

 many laboratories about the country, principally under sponsorship 

 of the United States Navy, are now showing that the temperature 

 range in which this change occurs is radically different for different 

 steels. The fundamental reason for this difference remains unknown. 



Nevertheless, fractographic study — as a new tool applied to the 

 problem — has recently been shown to disclose a clear distinction 

 between steel that will fail and steel that will not fail in service in 

 a given range of temperature. Plate 2, figure 1, is a fractograph of 

 a steel that is known to be tough. At a magnification of 1,000 diam- 

 eters, an individual grain shov^^s a pattern reminiscent of coral. The 

 grain itself is very small — only a tiny fraction of the size of the 

 bismuth crystal in the previous plate 1, figure 2 — and there is no 

 flatness anywhere in the fracture field. When this steel fractured, 

 here due to a hammer blow at —196° C, the separation was continu- 

 ally impeded by the observed minute roughness as it traveled through 

 the metal. The fractograph shows this pattern of roughness visually, 

 which can therefore be interpreted as a pattern of toughness. 



A sharply contrasting fracture facet is shown in plate 2, figure 2, 

 for steel that is of similar composition to that shown in the previous 

 figure, but is known by much mechanical testing to be inferior with 

 respect to toughness. The magnification is the same as before, 1,000 

 diameters; and the facets are seen to be about equal in size. A 

 marked difference, however, lies in the comparative smoothness of 



