Plastics creep

Mechanical properties of plastics can be determined by short, single-point quaUty control tests and longer, generally multipoint or multiple condition procedures that relate to fundamental polymer properties. Single-point tests iaclude tensile, compressive, flexural, shear, and impact properties of plastics creep, heat aging, creep mpture, and environmental stress-crackiag tests usually result ia multipoint curves or tables for comparison of the original response to post-exposure response. 

Since the tensile test has disadvantages when used for plastics, creep tests have evolved as the best method of measuring the deformation behaviour of... 

For some plastics, particularly nylon, the moisture content can have a significant effect on the creep behaviour. For such plastics, creep curves are normally available in the wet and dry states and a knowledge of the service conditions enables the appropriate data to be used. 

For the investigation of the time and the temperature dependence of the fibre strength it is necessary to have a theoretical description of the viscoelastic tensile behaviour of polymer fibres. Baltussen has shown that the yielding phenomenon, the viscoelastic and the plastic creep of a polymer fibre, can be described by the Eyring reduced time (ERT) model [10]. The shear deformation of a domain brings about a mutual displacement of adjacent chains, the... 

The essential difference between treatments of chemical processes in the solid state and those in the fluid state is (aside from periodicity and anisotropy) the influence of the unique mechanical properties of a solid (such as elasticity, plasticity, creep, and fracture) on the process kinetics. The key to the understanding of most of these properties is the concept of the dislocation which is defined and extensively discussed in Chapter 3. In addition, other important structural defects such as grain boundaries, which are of still higher dimension, exist and are unknown in the fluid state. 

Indentation hardness by Barcol hardness tester for rigid plastics Creep relaxation of gasket materials... 

Consider a continuous fiber-reinforced ceramic as a multiphase system where the individual phases are parallel to one another and to the uniaxial loading direction. The fibers (or fiber bundles), matrix, and interface zone are treated as individual phases. In general, each phase undergoes elastic-plastic (creep) deformation. In the present analysis, the creep rate of each phase, e is assumed to obey a general creep law of the following form... 

Cellulosic Membranes. The first asymmetric membrane for gas separation appeared in 1970 (Table II), and It was not surprising that this membrane was a modified CA membrane of the Loeb-Sourirajan type (17). Gelled CA membranes for water desalination must be stored wet In order to maintain their permeation performance. However, In gas permeation, wet, plasticized membranes tend to lose their properties with time due to plastic creep of the soft material under pressure and due to slow drying during which the microporous sublayer may collapse and thus increase the thickness of the dense skin-layer. Gantzel and Merten (17) dried CA membranes with an acetyl-content of 39.4% by quick-freezing and vacuum sublimation at... 

Tank design is the most challenging for the design engineer. Experience has proven to be the most positive as well as the most expensive for many fabricators of thermoplastics. Much of the design information from which formulas are derived is proprietary and jealously guarded. As can be appreciated, design factors can be voluminous when one considers the available plastics, creep properties of these plastics at various temperatures, all of which must be determined as a function of time. 

Creep tests can be carried out using any mode of deformation, and logically the mode most relevant to the application would be chosen. For plastics, creep in tension is by far the most common, but creep in flexure is also internationally standardized. By contrast, creep of rubbers is most commonly carried out in compression with tests in shear also being standardized. These differences reflect the different applications of these materials where creep is likely to be important. 

Creep performance may be inferred from the solution of simpler, desired product geometry, or a simple model of creep properties may be used. However, any such model must adequately represent the creep properties of the material, as with models for metals where creep contained in FEA software may not be adequate for plastics, particularly unreinforced TPs. Metals creep is usually approximated by its secondary (constant rate) component, whereas plastics creep is essentially primary creep (decreasing creep rate), one consequence of which is that creep strain for metals is usually plotted against time, but for plastics it is plotted against log time. 

Under appropriate conditions, metal design involves plasticity, creep, and geometric nonlinearity. These topics are treated in standard texts and have been put into computer software. However, such complexities are necessarily modeled in a simple technical format. 

If this deformation field does not fulfill the geometrical compatibility, a strain tensor related to stress is generated. The constitutive equation, which represents the mechanical behavior of the material, relates this strain tensor and the stress tensor. Due to the memory effect of wood, this tensor has to be divided into two parts (1) an elastic strain, connected to the actual stress tensor and (2) a memory strain, which includes all the strain due to the history of that point (e can deal with plasticity, creep, mechanosorption, etc.). 

Creep rupture n. The rupture of a plastic under a continuously apphed stress that is less than the short-time strength. This phenomenon is caused by the viscoelastic nature of plastics. Creep-rupture tests are generally conducted over a series of loads ranging from those causing rupture within a few minutes to those requiring several years or more. 

Unfilled polypropylene generally is not recommended for use in load-bearing applications, because the plastic creeps rapidly even under moderate stresses. Glass-reinforced grades, however, can be used in high-load applications. Polypropylene also has good unnotched and excellent notched impact strength. Polypropylene s heat resistance is much superior to that of polyethylene. 

Figure 85 shows the CR spectrum (micro-plasticity) of brittle porcelain at low temperatures compared with its fracture stress Of vs temperature dependence. One can see that the increased micro-plasticity (creep rates) arising at ca. 200 K in porcelain corresponds to a bend in the Of(T) plot towards the higher strength values. Such an effect is in accordance with a physical model for brittle fracture [278,338,339] that takes the role of micro-plasticity (local shear strains in the loaded brittle solid) into account. The latter results in some relaxation of dangerous local overstresses and, therefore, increasing the strength. 

Needleman, A. and Rice, J. R. (1980), Plastic creep flow effects in diffusive cavitation of grain boundaries, Acta Metallurgica 28, 1315-1332. 

Stress relaxation is rarely measured in plastics, creep testing being preferred. There are a number of standardised test methods for rubber as detailed by Brown [24]. 

Temperature Creep stress Plastic creep strain a), %... 

When a plastic material is subjected to a constant load, it deforms quickly to a strain roughly predicted by its stress-strain modulus, and then continues to deform slowly with time indefinitely or until rupture or yielding causes failure (16). This phenomenon of deformation under load with time is called creep. All plastics creep to a certain extent. The degree of creep depends upon several factors, such as type of plastic, amount of load, temperature, and time. 

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