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Designing with creep data

The factors that affect being able to design with creep data include a number of considerations. [Pg.65]

Designing with creep data. The factors that affect being able to design with creep data include a number of considerations. First, the strain readings of a creep test can be more accessible to a designer if they are presented as a creep modulus. In a viscoelastic material the strain continues to increase with time while the stress level remains constant. Since the creep modulus equals stress divided by strain, we thus have the appearance of a changing modulus. [Pg.77]

Fig. 3-20 compares the flexural modulus versus temperatures for four 30% GRTP s. Because modulus is a frequently appearing property in mechanical design equations, creep data often are plotted as apparent or creep modulus. These data are shown in Table 3-6 for GRTP s. As can be seen, the apparent creep modulus improves with glass reinforcement. Generally, the creep modulus of the reinforced thermoplastics decreases as stress and temperature are increased. However, the creep modulus data for reinforced nylon, acetal, polyester, polysulfone, and polyvinyl chloride appear to be less dependent on stress under the conditions of this particular test. When creep modulus data at different stresses coincide—a phenomenon known as the Boltzman superposition—there is an obvious reduction in the amount of testing required. However, such a relationship is both temperature and stress dependent and must be confirmed at the conditions of interest for the specific material involved. Other techniques, such as time-temperature superposition and other empirical correlations, also have been devised to simplify the time-dependent response of plastics ... [Pg.64]

Real polymers require more elaborate systems of springs and dash-pots to describe them. This approach of polymer rheology can be developed to provide criteria for design with structural polymers. At present, this is rarely done instead, graphical data (showing the creep extension after time t at stress a and temperature T) are used to provide an estimate of the likely deformation during the life of the structure. [Pg.194]

Third, creep data application is generally limited to the identical material, temperature use, stress level, atmospheric conditions, and type of test (that is tensile, flexural, or compressive) with a tolerance of 10%. Only rarely do product requirement conditions coincide with those of a test or, for that matter, are creep data available for all the grades of materials that may be selected by a designer. In such cases a creep test of relatively short duration, say 1,000 hours, can be instigated, and the information be extrapolated to long-... [Pg.77]

Failure can be considered as an actual rupture (stress-rupture) or an excessive creep deformation. Correlation of stress relaxation and creep data has been covered as well as a brief treatment of the equivalent elastic problem. The method of the equivalent elastic problem is of major assistance to designers of plastic products since, by knowing the elastic solution to a problem, the viscoelastic solution can be readily deduced by simply replacing elastic physical constants with viscoelastic constants. [Pg.113]

The tensile modulus is an important property that provides the designer with information for a comparative evaluation of plastic material and also provides a basis for predicting the short-term behavior of a loaded product. Care must be used in applying the tensile modulus data to short-term loads to be sure that the conditions of the test are comparable to those in use. The longer-term modulus is treated under the creep test (Chapter 2). [Pg.310]

In conclusion regarding creep testing, it can be stated that creep data and a stress-strain diagram indicate whether plain plastic properties can lead to practical product dimensions or whether a RP has to be substituted to keep the design within the desired proportions. For long-term product use under continuous load, plastic materials have to consider creep with much greater care than would be the case with metals. [Pg.318]

You are designing a plastic chair with the aid of a finite-element program, and have chosen a toughened polypropylene for the application. Creep data for the material at 23°C are available in graphical form for space reasons, they are given here by the equation ... [Pg.393]

In order to minimise as far as possible the influence of processing variables, studies have been carried out using tensile creep tests on carefully prepared compression moulded specimens. It must be realised that, vv ith injection moulded articles, the creep properties will also be subject to variation with the amount and direction of residual flow orientation, while, with crystalline polymers such as the polyolefins, the creep effects will also be influenced by variations in density caused by a combination of flow orientation, compressive packing, and cooling effects. Stresses will generally be complex and will often involve compressive and flexural components. However, articles should normally be designed to limit the strains occurring to quite low levels, where a reasonable correlation can be expected between tensile, compressive, and flexural creep data. [Pg.519]

For the designer there is generally a less-pronounced curvature when creep and relaxation data are plotted log-log. Predictions can be made on creep behavior based on creep and relaxation data. This usual approach makes it easier to extrapolate, particularly with creep modulus and creep-rupture data. [Pg.182]

Creep data in designing products has been used for over a century particularly since the 1940s. Unfortunately there is never enough data especially with the new plastics that are produced. However, relationships of the old and new are made successfully with a minor amount of testing. [Pg.189]


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See also in sourсe #XX -- [ Pg.197 , Pg.198 ]

See also in sourсe #XX -- [ Pg.175 ]




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