Extreme Plastic Flow

Most unusual and initially most surprising was the amount of plastic strain measured in impacted RDX crystals. RDX is usually regarded as a brittle crystal which fractures at about 10 % strain. Impact on 50 to 100 mg of poly-crystalline RDX samples by a 10 kg free falling drop weight released from a height of about. 2 m formed disc shaped samples about 15 mm in diameter and. 1 mm thick. The surfaces of the impactor and the anvil on which the sample was placed were 

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As two surfaces are brought together, the pressure is extremely large at the initial few points of contact, and deformation immediately occurs to allow more and more to develop. This plastic flow continues until there is a total area of contact such that the local pressure has fallen to a characteristic yield pressure of the softer material. 

The science of the deformation and flow of matter the study of a range of phenomena extending from the plastic flow of solids to the behaviour of fluids under extreme conditions. Rheometer... 

Since an understanding of the importance of any one process contributing to the failure in thermoplastics and the control over these processes is only partly attainable, a knowledge and understanding of the nature of endurance Hmits is of extreme importance for successful use of plastics, in particular engineered thermoplastics [27]. In terms of the failure type, polymer fracture may occur as a rapid extension of an initial defect, plastic flow of the matter and the thermally activated flow of the macromolecules. In all these cases, however, fracture is a localized phenomenon characterized by a large inhomogeneity of deformations. 

Highly oriented fibres produce ESR signals imder tensile deformation in the ranges of 8 to 16% strain and 500 to 900 MN/m stress. They do not undergo yield and plastic flow for the simple reason that the molecules are already highly aligned in manufacture and their capacity for plastic flow has already been exhaused. Such fibres therefore represent one extreme of the various conditions in which polymeric solids are obtainable. 

At room temperature plastic flow of amorphous metals occurs in the form of highly localized shear deformation bands. Multiple irregularly spaced shear bands appear in the deformed region. Fig. 46 gives an example of an almost completely back bent ribbon. Similar observation can be made in uniaxial compression and after rolling (Davies, 1978). Shear bands are less numerous in traction tests even when observed after fracture (Fig. 48a). Since these shear bands are extremely thin, TEM observations indicate a thickness of 5 to 20 nm (Masumoto and Maddin, 1971 Sethi et al., 1978 Donovan and Stobbs, 1981), and the surface step heights are on the order of micrometers, shear strains comparable to superplastic metals occur in this very small volume of the band. Pampillo... 

It is generally accepted that the most effective dissipative processes are those involving large plastic deformation before crack initiation. These large plastic deformations take place in shear banding and crazing. However, extreme localization of plastic deformations into small volumes leads to macroscopic failure initiated from these areas of large plastic flow. This is even more pronounced in the... 

When the elastic limit of a metal has been exceeded, it will undergo plastic flow beginning at the yield point. It is this property of metals that is exploited for cold and hot working into desired shapes. When a metal is deformed permanently from the tension force, it exhibits a property known as ductility. By comparison, the term malleability refers to the permanent deformation of a metal under a compression force e.g., hammering, cold-rolling, etc.). Although most ductile metals are also malleable, the reverse is not always true. For example, lead is extremely malleable... 

Another class of materials (nonmetals) is capable of extremely great elastic deformations. These are called elastomeric materials. Usually, materials exhibiting plastic deformation under stress are the more desirable for structures. Ductility is desirable so that accidental stresses beyond design values can be redistributed to safer levels by means of plastic flow. 

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