FRACTURE OF SOLIDS — FIELD 



437 



Table 1. — Fracture and Stress Wave Velocities {In feet per second) 



Material f 



Fracture 

 velocity (V) 



Longitudinal 

 wave velocity (Ci) 



Transverse wave 

 velocity (Cj) 



Vf/Ci 



Soda glass _ , 

 Fused silica 



Sapphire 



Diamond- - 



5,000 



7,000 



14, 500 



24, 000 



18, 000 



19, 500 

 36, 000 

 60, 000 



11,000 

 12, 500 

 21, 000 

 40, 000 



0. 28 



0. 36 



0. 4 



0. 4 



from the picture that the fractures all travel at the same velocity. The 

 change of appearance in the fracture pattern after frame 8 (fourth in 

 row 2) is caused by the interaction of the reflected longitudinal pulse, 

 now a tension, with the advancing front. 



Recent fracture velocity and stress wave velocity measurements 

 made at Cambridge from sequences such as in plate 1, fig. 1; plate 2, 

 fig. 2; and plate 4, fig. 2, are shown in table 1 above. The fracture 

 velocities are all approximately one-third of the longitudinal stress 

 wave velocity. Fracture velocities in metals are usually a lower frac- 

 tion of the stress wave velocities. This is mainly because much of the 

 fracture energy is lost in doing plastic work. The smaller value of 

 the ratio for glass (0.28 as compared with 0.4 for sapphire and 

 diamond) may also be significant in showing that glass itself does 

 not behave in a completely brittle fashion. (Indentation experiments 

 have also indicated this.) 



REMOVAL OF SURFACE DEFECTS 



It appears that the key to the strength of solids lies in the existence 

 of microcracks and other imperfections. Once these are removed 

 higher strengths ensue. Several materials have already been produced 

 in fiber and whisker form with high strengths. Experiment shows that 

 glass has most of its flaws located at the surface and is therefore amen- 

 able to surface treatments such as toughening, ion exchange (in which 

 the sodium atoms at the surface are replaced by larger ones, thus put- 

 ting the surface layers into compression) , and etching. In the etching 

 process hydrofluoric acid acts partly by removing the flawed layer and 

 partly by romiding off the flaw tips. The effect of removing a few 

 microns (10"* cm.) of glass, and greatly improving the strength, is 

 illustrated by the hnpaot mark in plate 4, fig. 2, in which the lower half 

 only of the specimen was etched. Impact was by a liquid jet on the 

 dividing line between the treated and mitreated regions (see also plate 

 2, fig. 1) . Improvements of strength by etching of up to 500,000 p.s.i. 

 have so far been reported. Materials such as hard polymers and 

 ceramics have flaws distributed throughout the bulk, so a surface treat- 

 ment alone does not have such a marked effect (their initial practical 

 strengths may, of course, be higher) . 



766-74G--C5 ?.3 



