436 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



polymers and certain crystalline solids. High-speed photographic 

 records show that the formation of the bands is complete before the 

 plate specimen starts to bend and that only at a later time do the long 

 radial fractures produced by the bending start to develop. 



FRACTURE VELOCITY 



Once a fracture is initiated the question arises as to how fast it can 

 travel. The answer appears to be that a fracture can have any 

 velocity up to a certain maximum. It is reasonable that a maximum 

 velocity exists since it would not be expected that a fracture velocity 

 would exceed stress wave velocities, since in the one case a rupture of 

 atomic bands occurs and in the other merely a transmission of stress. 



The measurement of fracture velocities is usually achieved by the 

 use of high-speed photography or ultrasonic techniques, although 

 markings on the fracture surfaces often give extra information. Ex- 

 amples of these markings include faint lines called "rib" marks which 

 are formed when a fracture pauses, and other lines ("river" patterns) 

 w^hich denote the direction of travel of the fracture for each part of 

 the surface. These "river" patterns occur on glasses, and both metallic 

 and nonmetallic crystals and are formed when the fracture advances 

 simultaneously on slightly different levels. The most important mark- 

 ings for velocity determinations are Wallner lines (named after H. 

 Wallner who first explained them) and an example is shown in plate 3, 

 fig. 2. These were photographed on the fracture surface of a glass 

 plate. The lines are formed by the interaction of the fracture front 

 with transverse stress pulses started when the fracture passes through 

 an imperfection, usually at the edge of the specimen. If the fracture 

 origin is known and also the transverse wave velocity for the solid, 

 the fracture velocity can be determined. This idea has recently been 

 extended and an ultrasonic beam of waves of frequency about 5 mega- 

 cycles per second is passed through the solid as the fracture advances. 

 The resultant fracture surface shows a series of fine ripples, and the 

 spacing of these, since the time interval is accurately known, gives a 

 direct measure of the fracture velocity. 



High-speed photography is a technique which can measure fracture 

 velocities accurately (to about 1 percent) provided the camera is 

 capable of giving accurate synchronization and framing rates in ex- 

 cess of 10" per second (i.e., the order of 1 microsecond between pic- 

 tures). A sequence showing the fracture of a toughened glass 

 specimen is given in plate 4, fig. 1. This is the type of glass frequently 

 used in car windowscreens. The glass is about five times stronger than 

 plate glass, and is made from plate glass by a heat treatment process 

 which puts a thin outer layer into compression. However, the treat- 

 ment leaves the inner layers in tension and if a crack grows through 

 the outer layer the fracture propagates catastrophically. It is clear 



