The origin, nature and utilisation of plastic flow 4/3
4.1 The origin, nature and utilisation of plastic flow
4.1.1 Introductory observations
The initiation of plastic flow owns its origin to a variety of micro-effects associated with the application of an external loading system, the level of which cannot be contained within the elastic recovery capab es of thematerial. The dimensional change (or changes) becomes permanent and leads to either a desirable accommodation of the metal within the confines of the forming pass or, in extreme conditions, to failure resulting in fracture. There are two basic mechanisms of flow. Usually, but not always, the initiation of flow takes place when a sufficiently high shearing stress forces the atoms of a crystal to move toa new position of equilibrium. Since an individual atom cannot change its position without affecting the neighbouring structure, the movement will be possible only if a whole layer of atoms moves along. Slipping of the crystalline structure is thus effected and occurs along the planes of highest atomic density. If the load associated with the shearing stress does not exceed a critical value,return to the original position of equilibrium takes place on its removal and the elastic recovery is complete. Generally, stretching is produced by interfacial slip which, in turn, is independent of the value of the normal stress. Slip of the material is made easier by the presence of dislocations or metal-lattice defects. These are instrumental in producing consecutive atom movement which allowsplastic deformation to occur. Smaller plastic deformations can be produced by twinning or by a shift, of constant atomic distance, in a specified direction. A twinned crystal becomes distorted (in a mirrorlike fashion) along the twinning plane. Although, on the microscopic scale, the mechanism of yielding and the subsequent plastic flow are fairly well understood, it is nevertheless very difficult toextrapolate from the micro-condition to the macro-state with which engineers are constantly concerned. It is for this reason that the concept of continuum of matter is generally adopted and the macroeffects are used as criteria of design and general operational purposes. Even with the acceptance of the continuum approach, it is still clear that a differential in flow is likely to occur. Althoughmost materials are regarded as isotropic, or as continua in which mechanical properties are independent of direction, in a polycrystalline conglomerate lattice faults are usually present and even on this level perfect isotropy is unlikely. Forming of a metal-which takes place prior to the final manufacture of the component-will impose a directionality of flow and a consequent orientation of thegrains. Clearly, therefore, directionality in properties must be expected and will be reflected in the level and type of the material response when the specimen is tested in two mutually perpendicular directions. Anisotropy becomes a recognisable property which has to be accommodated in the computation of the relevant numerical values. A particular case of a micro- rather than macro-anisotropy isthat of the Bauschinger effect which manifests itself in the reduction in the level of the yield strength of the material on reversal of the direction of plastic deformation imposed. The Bauschinger effect is associated with the anistropy of the individual grains which, on application of a specific load, find themselves either in the elastic or plastic state. The subsequent removal of the loadproduces a conglomerate that varies in its properties and remains residually stressed. If the type of stress imposed thereafter corresponds to the prevailing
residual, yielding will occur at a lower stress level than the original condition of the material would suggest. Although the realisation of the presence of these phenomena and the understanding of their effect on plastic deformation are of...