Deformation brittle fracture

The cause of brittle fracture in polymers is the inability of the material to quickly dissipate by molecular relaxation processes the internal stresses generated as a result of the imposed deformation. Brittle fracture occurs when the time to failure is the same order of magnitude (or faster) than the speed of the relaxation process that dominates the mechanical behavior in the temperature range of interest. The relevant relaxation processes are the first T < Tg secondary relaxation (p or y). A qualitative criterion for determining whether the relaxation... 

Hiestand Tableting Indices Likelihood of failure during decompression depends on the abihty of the material to relieve elastic-stress by plastic deformation without undergoing brittle fracture, and this is time dependent. Those which relieve stress rapidly are less... 

R. W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th edition, 1996. B. R. Lawn and T. R. Wilshaw, Fracture of Brittle Solids, Cambridge University Press, 1975, Chap. 3. 

The lengths of the molecular chains dominate large strain behavior and crack propagation in contrast to their minimal influence at small strain levels. Consequently, thermosets are characterized by small deformation zones (Fig. 8.2) and brittle fracture. 

The term fracture implies fragmentation of a solid body into two or more bodies under the action of stress. Two main types of fracture mode are observed in solids. The first is ductile fracture which is the failure of a material after it has undergone a considerable amount of plastic deformation. The other is brittle fracture which is the failure of a material without undergoing practically any plastic deformation. The type of failure which occurs depends largely on the nature of the material and its condition however, failure is also affected by... 

This result indicates that the stress necessary to cause brittle fracture is lower, the longer the existing crack and the smaller the energy, P, expended in plastic deformation. The quantity Of represents the smallest tensile stress that would be able to propagate the crack of length 2 L. The term Of (tt L)°5 is generally denoted by the symbol K and is known as the stress-intensity factor (for a sharp elastic crack in an infinitely wide plate). Fracture occurs when the product of the nominal applied stress and the stress concentration factor of a flaw attains a value equal to that of the cohesive stress. 

Fig. 17 Athy-Heckel plots (a) material undergoing plastic deformation (b) material undergoing brittle fracture.

Slope AB = function of elastic deformation Slopes BC and DE = function of plastic deformation and brittle fracture... 

One should note that BC represents a highly elastic material as little plastic deformation or brittle fracture has occurred. Also, sharp differences between the slope CD and DE are indicative of weak, or failed, tablet structures. The RDWF estimated from these plots can provide a good indication of the ejection force. More detailed treatments of such studies are now in the open literature, to which the interested reader is referred [118-120],... 

In addition to plastic deformation, materials may fail by either brittle fracture or ductile fracture fracture being the separation of a body into two or more parts. 

Just as metals can be ductile or brittle, so can organic materials. The Brittle Fracture Index is a measure of the brittleness of a material. It is a measure of the ability of a compact of material to relieve stress by plastic deformation. The Brittle Fracture Index (BFI) is determined [29,31] by comparing the tensile strength of a compact, stress concentrator) in it, o-T0, using the tensile test we have described. A hole in the center of the compact generally weakens a tablet. If a material is very brittle, theoretical considerations show that the tensile strength of a tablet with a hole in it will be about one-third that of a solid tablet. If, however, the material can relieve stress by plastic deformation, then the strength of the compact with a hole in it will approach that of a compact with no hole. The Brittle Fracture... 

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