FRACTURE OF SOLIDS — FIELD 433 



these microcracks are not easily observable. However, since 1920, 

 decoration and etching techniques coupled with fracture experiments 

 have built up a considerable body of evidence which largely substan- 

 tiates the idea of microcracks. Other sources of weakness can also 

 occur such as inclusions, voids, notches, and growth steps. All of 

 these can act so as to increase the stress concentration at a point in the 

 solid. 



Crystalline materials (and this includes metallic crystals) may or 

 may not contain microcracks initially, but they will usually contain 

 defects of structure (dislocations) which will allow the planes of atoms 

 to slide relative to each other without separation (plastic deformation) . 

 If the movement of the dislocations is blocked (tliis could be caused by 

 the inclusion of a foreign particle) the dislocations build up causing a 

 high-stress concentration with the possible formation of a microcrack. 

 This crack could then initiate bulk fracture. 



Materials without defects, such as carefully produced whiskers or 

 fibers, exhibit high strengths approaching the theoretical values. This 

 tends to confirm the importance of defects and indicates a possible, 

 albeit difficult, way of obtaining high-strength solids. 



TRANSMISSION OF STRESS 



When a stress is applied to a body the disturbance is not experienced 

 instantaneously throughout the whole body, but is transmitted by stress 

 waves which travel with a definite velocity. The effect is very similar 

 to that when ripples traverse the surface of a pond. In a solid whose 

 properties are independent of direction, a disturbance travels through 

 the body of the solid in two waves — a longitudinal (dilatational) wave 

 in which the particle motions are in the direction of propagation, and 

 transverse (distortional waves in which the particle motions normal to 

 the wave front. The velocities of the wave depend on the elastic con- 

 stants of the solid. These constants are themselves related to the 

 elastic moduli (i.e., ratio of stress to strain produced) . For glass, the 

 longitudinal and transverse wave velocities are about 18,000 and 11,000 

 feet per second respectively, but for diamond, a material with very 

 high elastic constants, the velocities are higher, having values of about 

 60,000 and 40,000 feet per second. Physically, the more rigid the 

 atomic structure the faster the waves pass and vice versa. 



Plate 1, fig. 1, shows pictures taken from a sequence of high-speed 

 photographs of stress waves propagating in a Perspex specimen of 

 dimensions 2 in.X2 in.X%2 in. The waves were initiated by the 

 detonation of a small charge of explosive at the midpoint of the top 

 edge, and were made visible by the insertion of crossed polaroids into 

 the optical system of the camera. Both waves are seen ; the velocity of 

 the fastest wave, the longitudinal, is about twice that of the transverse 

 waves. When the waves reach the boundaries of the Perspex they 



