FRACTURE OF SOLIDS — FIELD 435 



the fracturing is more severe. The reason for this is that at low 

 impact velocities the time of impact is relatively long and the stress 

 waves, with their high velocity, have time to distribute information 

 about the stress to all parts of the body during the impact time. Since 

 the stress distribution quickly approaches that of the static case, the 

 pattern of fracture for a low velocity impact is similar to that for 

 static indentation. An example of this is window glass broken by a 

 stone ; the long radial fractures and the displaced cone of glass are the 

 main features of the impact. 



For very high impact velocities the duration of the impact becomes 

 short compared with the time taken by the stress waves to pass through 

 the body. Thus a point in the solid no longer receives a long train 

 of stress waves which gradually build up the stress, but rather a con- 

 centrated pulse of stress of short duration. Very intense pulses last- 

 ing only 1 or 2 millionths of a second can be produced by a variety 

 of methods, one of which is the detonation of a small quantity of ex- 

 plosive on the surface of a solid, or, as has been shown recently at Cam- 

 bridge, when a jet of liquid strikes a solid at high velocity. (This re- 

 sult has practical significance when aircraft pass through rain.) An 

 example of the fracturing caused by the impact of a cylinder of liquid 

 water of diameter 3 mm. at 2,400 feet per second on plate glass is shown 

 in plate 2, fig. 1. The diameter of the large ring fracture corresponds 

 closely with the size of the head of the cylindrical jet. This ring 

 fracture and central area closely resemble the static case illustrated in 

 plate 1, fig. 2, except that the main ring crack is made up of several 

 fractures rather than one continuous crack. The additional features 

 are the short circumferential fractures. These are entirely of stress 

 wave origin, and are formed when the sharp pulse reaches a micro- 

 crack capable of giving a stress concentration sufficient for fracture. 

 The fractures remain short and develop as separate events since the 

 stress waves are themselves of short duration. The stress wave which 

 causes these particular fractures is the Rayleigh Surface wave. Their 

 formation is illustrated in plate 2, fig. 2. The pictures, separated by 

 only 2 microseconds, show a lead slug impacting against the top edge 

 of a 3 in. by 3 in. by I/4 in. glass specimen at about 600 feet per second. 

 The point at which fresh fractures appear moves out from the center 

 at the Rayleigh wave velocity of approximately 10,000 feet per second. 



When thin plates of glass are loaded by intense short duration pulses 

 extra "bands" of fracture occur as seen in Plate 3, fig. 1. This shows 

 the result of the impact of a cylinder of water at a velocity of approxi- 

 mately 4,000 feet per second on i/^-inch-thick glass. The circular bands 

 of fracture are again of stress wave origin, and occur only on the front 

 surface. They are formed when the Rayleigh Surface wave is rein- 

 forced by tensile components from the stress waves reflected at the 

 back surface of the glass. Similar bands have been produced on hard 



