434 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



reflect and return through the block. This reflection always causes 

 a change of phase and the longitudinal wave, for example, which passes 

 out as a compression returns as a wave of tension. In thin plates of 

 brittle material this effect can lead to failure causing a scab of material 

 to become detached from the rear surface. The brittle solid fails in 

 this manner since although it is strong in compression it is compara- 

 tively weak in tension. Instances of this so-called "scabbing" fracture 

 were frequent in the last war when thin sheets of armor plate were 

 struck by fast projectiles. Eeinforcement by two waves, of either the 

 same or different type, can also lead to localized fracture. 



On the surface of a solid a third type of wave, the Rayleigh Surface 

 wave, is developed. This wave travels at a velocity about 90 percent 

 of the transverse wave. Since it exists only in a thin layer at the 

 surface it loses energy in two dimensions, wliereas the body waves do 

 so in three. When transmitted over large distances this wave retains 

 its intensity to a greater degree and is usually the main component of 

 the disturbance from earthquakes. As will be seen below it is also 

 important in explaining certain fracture phenomena. 



MODE OF FRACTURE 



The way in which the stress is applied to a solid greatly influences the 

 final form of fracture. Starting at the one extreme of "static" loading 

 this can perhaps best be represented by the example of a steel ball 

 pressed with increasing force against the surface of a solid. With a 

 brittle solid such as glass, the first form of failure is the formation of a 

 "ring" crack which closely follows the edge of the contact area where 

 the maximum tensile forces exist. The fracture usually starts at 

 one point and then travels round, keeping at right angles to the maxi- 

 mum tensile stress, until the full circle is complete. The point of 

 initiation may be slightly away from the contact area since it will 

 depend on the location of the microcrack which gives the greatest 

 stress concentration. If the stress is increased still further a second 

 ring crack forms while the initial fracture develops into the solid 

 forming a conical surface of fracture. Plate 1, fig. 2, shows this stage ; 

 the faint circular bands around the ring cracks are interference fringes 

 formed in the gap between the fracture planes. In thin glass the 

 fracture may reach the back surface giving a perfect cone of material. 

 The cone angle is usually about 140°. (This form of failure is not to 

 be confused with the scabbing failure mentioned briefly above in con- 

 nection with stress waves.) Thin glass will also bend causing large 

 tensile forces at the rear surface which result in long radial fractures 

 growing from a point opposite the loaded area. 



If the steel ball impacts against the glass stress waves have to be con- 

 sidered. At relatively low impact velocities the general appearance 

 of the fracture does not greatly alter from the static case except that 



