Planar Crack Propagation Research Articles

Crystallographic features of tensile fracture of polycrystalline beryllium

An etch pitting technique has been developed and has been combined with electron fractography to study the crystallographic features of tensile fracture of polycrystalline beryllium. The crack initiation and the plane of crack propagation of a fine-grain forged beryllium, as well as commercial sheet, were determined from room temperature to 800°F. The fracture origin was internal for both forged beryllium and commercial beryllium sheet. An intergranular origin was observed for forged beryllium at ambient and elevated temperatures, with crack propagation occurring by {11 2 0} fracture. Only transgranular cleavage was observed for commercial beryllium sheet, with crack initiation occurring by basal plane cleavage.

Some factors affecting the quality of cropped billets

The status of bar cropping as a production technique is reviewed in the light of recent associated technological advances. On this basis, cropping is restrictively defined in terms both of the metallurgical process involved and also of the relevant aspects of the related hardware. Bar cropping, which is actually a long established technique for metal billet production, is shown to have acquired keen interest in this and in a number of foreign countries as a topic for academic study and commercial redevelopment, the motive being better billets at a given price, or cheaper billets of a given quality. The requirements fundamental to the achievement of this aim are simply defined. Two courses of experimental investigation are briefly described which were designed to supplement current knowledge in the following two aspects of the process: (a) The effect on the billet separation mechanism (which is ideally one of fracture by plane crack propagation) of externally superimposed axial tensile loads, and (b) The effect of forcing the bar stock by means of a compressive axial load against an abutment as a means to continuous production of billets of length less than the characteristic cross sectional dimension in a High-Energy Rate Forming Machine.