Stress deformation caused

The performance of a tool material in a given appHcation is dictated by its response to conditions at the tool tip. High temperatures and stresses can cause blunting from the plastic deformation of the tool tip, whereas high stresses alone may lead to catastrophic fracture. In addition to plastic deformation and fracture, the service life of cutting tools is deterrnined by a number of wear processes, some of which are shown in Figure 2. 

Creep the dimensional change of a plastic under load with time followed by the instantaneous elastic or rapid deformation at room temperature permanent deformation caused by prolonged application of stress below the elastic limit. 

There is another result of frozen-in residual stresses that can be equally damaging to the product function and which affects materials that are not in the glassy state. This may affect an impact grade of material or a crystalline plastic even more drastically than a glassy material. The frozen-in stresses are real loads applied to the material and when even slightly elevated temperatures are applied stresses can cause the product to deform severely. 

A real material whose behaviour can be modelled in this way initially undergoes irreversible deformation as the stress is applied. This eventually ceases, and the material then behaves effectively as an elastic solid. Release of the stress will cause a rapid return to a less strained state, corresponding to the spring component of the response, but part of the deformation, arising due to viscous flow in the dashpot will not disappear. 

As tree fruit species are perennial crops, year-to-year influences are often detected. For example, factors in the previous year(s) (e.g. water or nutrient deficiency, hail storm damage, shoot deformation caused by aphids, too high or too low crop load) strongly influence the tree s performance in the next year (Tromp and Wertheim, 2005). Thus, a major objective of agronomic practices used is to buffer the orchard from stress and to keep trees in a balance/equilibrium between vegetative and generative activity. 

Since polymers are viscoelastic solids, combinations of these models are used to demonstrate the deformation resulting from the application of stress to an isotropic solid polymer. Maxwell joined the two models in series to explain the mechanical properties of pitch and tar (Figure 14.2a). He assumed that the contributions of both the spring and dashpot to strain were additive and that the application of stress would cause an instantaneous elongation of the spring, followed by a slow response of the piston in the dashpot. Thus, the relaxation time (t), when the stress and elongation have reached equilibrium, is equal to rj/G. 

In aU wrought processes, the flow of metal is caused by application of an external force or pressure that pushes or pulls a piece of metal or alloy through a metal die. The pressure required to produce plastic flow is determined primarily by the yield stress of the material (cf. Section 5.1.4.3) which, in turn, controls the load capacity of the machinery required to accomplish this desired change in shape. The pressure, P, used to overcome the yield stress and cause plastic deformation is given by... 

Fracture often determines the reliability of a material in its practical applications. Brittle fracture of a material is the reason for a sudden catastrophe. The mechanical property ductile or brittle determines, in essence, whether or not a tool can be made from a given material. Let us identify the imperfections of a crystal and the chemical processes which cause ductility and brittleness. We distinguish two limiting cases of failure 1) A crystal, under external stress, deforms by forming a narrowing neck until eventually ductile rupture occurs. Dislocations are the only imperfections involved in this process of failure. 2) Crystals fracture suddenly. A sharp crack propagates and causes the failure. 

Visco-Ebstic Behavior, Relaxation of Deformation Caused by a Constant Stress o0... 

Deformation is caused by stress from either an external force or an imbalance of internal forces. Quantitatively, a stress a on an area of a specimen is equal to the force applied per unit area. Since a force is a vector with three components, the stress component from the normal component of the force is called normal stress it causes elongation or contraction of the material depending on the direction of the force. The stress components from the two tangential components of the force are called shear stresses they are responsible for the shear deformation. 

The situation for amorphous linear polymers is sketched in Fig. 2.8a. If a polymeric glass is heated, it will begin to soften in the neighbourhood of the glass-rubber transition temperature (Tg) and become quite rubbery. On further heating the elastic behaviour diminishes, but it is only at temperatures more than 50° above the glass-rubber transition temperature that a shear stress will cause viscous flow to predominate over elastic deformation. 

Fig.2 shows the boundary curves for PDMS-MEK system under static and dinamic conditions. One can see that, shear deformation causes the shift of the boundary curves at the low shear rat.e( ) and stress (6) (6=, where 1 -the viscosity of the system) the component compatibility increases that manifests itself in the decrease of Tph, at the high(j) or (3) the shear field causes the increase of Tph testifying to the decrease of the mutual compatibility. So an inversion of the effect that the shear- field has on phase transitions was discovered for this system. 

The presence of residual elastic stresses is inevitable in a surface which contains a plastically deformed zone whose thickness is limited compared with that of the bulk s pecimen. Alternatively, the relief of these stresses may cause distortion of the specimens in cases where the thickness of the two is comparable. Very little work has been done on this important subject and, so far as can be ascertained, none that can be related to the complexities of die plastically-deformed layer. Considerable complications are introduced because the residual stresses may be of thermal as well as mechanical origin and because those of mechanical origin may be altered by the thermal effects. 

Stress-strain properties for unfilled and filled silicon rubbers are studied in the temperature range 150-473 K. In this range, the increase of the modulus with temperature is significantly lower than predicted by the simple statistical theory of rubber elasticity. A moderate increase of the modulus with increasing temperature can be explained by the decrease of the number of adsorption junctions in the elastomer matrix as well as by the decrease of the ability of filler particles to share deformation caused by a weakening of PDMS-Aerosil interactions at higher temperatures. 

---