194 
ME. A. A. GRIFFITH ON 
10. Methods of Increasing the Strength of Materials. 
The most obvious means of making the theoretical molecular tenacity available for 
technical purposes is to break up the molecular sheet-formation and so eliminate the 
“ flaws.” In the case of crystalline material this has the further advantage of eliminating 
yield and probably also fatigue failure. 
In materials which normally have curved sheets, the molecular fields of force are 
presumably asymmetrical, and the process indicated above would lead of necessity 
to a random arrangement, which might be unstable. It has been seen that in glass 
and fused silica it is actually unstable, except in the case of the finest fibres. 
As regards crystalline materials, however, in which the fields of force must have 
some sort of symmetry, there seems to be no reason why there should not be possible 
a very fine grained stable configuration, which could be derived from the ordinary 
crystalline form by appropriate rotations of certain molecules to new positions of stable 
equilibrium, in such a way as to break up the gliding planes. The grain of such a 
structure need be but a few molecules long, and its strength would approximate 
to the theoretical value corresponding with the heat of vaporisation. 
There is some evidence that mild steel which has been put into the amorphous 
condition by over-strain tends, under certain conditions, to take up a stable fine-grained 
formation of this kind, in preference to resuming its original coarse crystalline configura¬ 
tion, in that a temperature of 0° C. appears to prevent recovery from tensile over¬ 
strain.* 
These considerations suggest that if a piece of metal were rendered completely 
amorphous by cold-working, and then suitably heat-treated, its molecules might take 
up the stable strong configuration already described. The theory indicates, however, 
that over-straining tends to set up tensile stresses in the unchanged parts of the crystals 
which may start cracks long before decrystallisation is complete. Such cracking could 
be prevented if the over-straining were carried out under a sufficiently great hydrostatic 
pressure, and this line of research seems to be well worth following up. It might, of 
course, be found that the requisite pressure was so enormous as to render the method 
unworkable, but if the theory is sound there seems to be no other reason why definite 
results should not be obtained. 
The problem may be attacked in another way. As has been seen, the theory suggests 
that the drop in stress at the initiation of yield is due to the surface energy of the inter¬ 
crystal boundaries. Thus the yield point may be raised by “ refining ” the metal, 
i.e., so heat-treating it as to reduce the size of the crystals. The limit of refinement is, 
doubtless, reached when each “ crystal ” contains but a single molecule and the material 
is then in the strong stable state already described. 
Refining is also of great value in connection with resistance to fatigue failure. Suppose, 
in accordance with the foregoing theory of fatigue, that one crystal has been fractured, 
* Coker, ‘ Phys. Rev.,’ 15, August, 1902. 
