Deformation of plastics

Linear viscoelasticity Linear viscoelastic theory and its application to static stress analysis is now developed. According to this theory, material is linearly viscoelastic if, when it is stressed below some limiting stress (about half the short-time yield stress), small strains are at any time almost linearly proportional to the imposed stresses. Portions of the creep data typify such behavior and furnish the basis for fairly accurate predictions concerning the deformation of plastics when subjected to loads over long periods of time. It should be noted that linear behavior, as defined, does not always persist throughout the time span over which the data are acquired i.e., the theory is not valid in nonlinear regions and other prediction methods must be used in such cases. 

Owing to the deformability of plastics - in particular of thermoplastics - mechanical pretreatment methods are applicable only to a very limited extent. So for example, if jet pressure is too high the blasting abrasive can be shot into the surface. For polyethylene and polypropylene for instance, the SACO-mefhod described in Section 7.1.2.1 has proven its worth. It develops a surface suitable for the formation of adhesive forces by means of chemically modified blasting abrasives (silication). 

As described above, the deformation of plastics compared to metal is viscoelastic and viscous. The temperature-time dependence of the properties of plastics (and... 

Dean, G. Wright, L., (2003) An evaluation of the use of finite element analysis for predicting the deformation of plastics imder impact loading. Polymer Testing, V01.22, pp.625-631... 

ASTM D 621 Standard Test Methods for Deformation of Plastics Under Load. 

Fillers are simple or complex substances of mineral, animal or vegetable origin, which are used for altering polymer properties and lowering cost. These kinds of additives may reduce crack formation and deformation of plastic material, and also improve the surface characteristics, abrasion, temperature and humidity resistance. 

D 621 DEFORMATION OF PLASTICS UNDER LOAD D 635 RATE OF BURNING IN HORIZONTAL POSITION D 637 SURFACE IRREGULARITIES IN TRANSPARENT SHEETS D 638 TENSILE PROPERTIES OF PLASTICS... 

Uddin, M.Z., Yamaguchi, M., Watanabe, M., Shirai, H., Hirai, T. Electrically induced creeping and bending deformation of plasticized poly (vinyl chloride). Chemistry Letters 30, 360-361 (2001)... 

Strapping is used where the heat could fluctuate, as in a railroad boxcar or storage room, the steel at high temperature could expand and cause what it contained to become loose. The elastic deformation of plastics lets them retain their tension with temperature change, however. 

Van der Waals forces increasing after deformation of plastic particle surfaces,... 

Ashby, M. F. (1970), The deformation of plastically nonhomogeneous materials. Philosophical Magazine 21, 399-424. 

Figure 26.2 Deformation of plastic around threads for thread-forming screw.

M.F. Ashby, Deformation of plastically non-homogeneous materials. Phil. Mag. 21(170),... 

In addition, on the basis of analogous specimens, the accumulation of damage and plastic deformation of metal structure were simulated. These results provide the possibility to obtain the prediction charts of the metal work s residual resource. 

Whereas in the scope of plastic deformations differences are observed Arc welding of pipes <6 32 mm, wall thickness 6,5 mm has caused own tensile stress of 260 MPa in the jont, relief at 720°C during 4 hours, has caused a lowering of stress to 60 MPa. 

The calibration graph for the probe using a strength machine, has been shown in Fig. 7 It can be observed that the dependence of indications of the device of Wirotest type on the loading is linear within the proportionality limit scope. After unloading the indications do not return to zero, but show own stress caused in effect of plastic deformation of the tested sample... 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

Another aspect of plasticity is the time dependent progressive deformation under constant load, known as creep. This process occurs when a fiber is loaded above the yield value and continues over several logarithmic decades of time. The extension under fixed load, or creep, is analogous to the relaxation of stress under fixed extension. Stress relaxation is the process whereby the stress that is generated as a result of a deformation is dissipated as a function of time. Both of these time dependent processes are reflections of plastic flow resulting from various molecular motions in the fiber. As a direct consequence of creep and stress relaxation, the shape of a stress—strain curve is in many cases strongly dependent on the rate of deformation, as is illustrated in Figure 6. 

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

Two approaches have been taken to produce metal-matrix composites (qv) incorporation of fibers into a matrix by mechanical means and in situ preparation of a two-phase fibrous or lamellar material by controlled solidification or heat treatment. The principles of strengthening for alloys prepared by the former technique are well estabUshed (24), primarily because yielding and even fracture of these materials occurs while the reinforcing phase is elastically deformed. Under these conditions both strength and modulus increase linearly with volume fraction of reinforcement. However, the deformation of in situ, ie, eutectic, eutectoid, peritectic, or peritectoid, composites usually involves some plastic deformation of the reinforcing phase, and this presents many complexities in analysis and prediction of properties. 

Some materials that are atomically ordered also develop a sHp-iaduced anisotropy as a result of plastic deformation. The origin is thought to be identical to that of thermomagnetic anisotropy, ie, short-range directional order, except that the order is brought on by deformation rather than by heat treatment ia a field (3,4). 

A series of events can take place in response to the thermal stresses (/) plastic deformation of the ductile metal matrix (sHp, twinning, cavitation, grain boundary sliding, and/or migration) (2) cracking and failure of the brittle fiber (5) an adverse reaction at the interface and (4) failure of the fiber—matrix interface (17—20). 

Fig. 9. Effect on fatigue strength of the plastic deformation of a carburized steel surface by shot peening (B) as compared to nitriding (A) and heat treating...

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. 

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