Creep design method

Plastics also have the ability to recover when the applied stress is removed and to a first approximation this can often be considered as a reversal of creep. This was illustrated in Fig. 1.8 and will be studied again in Section 2.7. At present it is proposed to consider the design methods for plastics subjected to steady forces. 

Once the limiting strain is known, design methods based on the creep curves are quite straightforward and the approach is illustrated in the following... 

For the past century one successful approach is to plot a secant modulus that is at 1% strain or 0.85% of the initial tangent modulus and noting where they intersect the stress-strain curve (Fig. 2-2). However for many plastics, particularly the crystalline thermoplastics, this method is too restrictive. So in most practical applications the limiting strain is decided based on experience and/or in consultation between the designer and the plastic material manufacturer. Once the limiting strain is known, design methods based on its creep curves become rather straightforward (additional information to follow). 

Once an appropriate value for the maximum strain is chosen, design methods based on creep curves and the classical equations are quite straightforward, as shown in the following examples. 

Creep-fatigue design methods based on elastic analysis... 

A new cree-fatigue design method, which is based on the concept of a generalized elastic follow-up model, is being developed. The elastic followup equations to predict strain magnification and creep relaxation for structural discontinuities are established. 

There are several design methods utilized in the industry for analyzing creep problems. Three of the most prevalent are the Larson-MiUer relation, the Nor-ton-Bailey power law, and the MFC Omega method. The Larson-MiUer relation attempts to extrapolate creep rupture data from experimental results. The Norton-... 

KAERI has contributed to all working groups of Gen-lV VHTR material collaboration graphite, metal and design method, and ceramic and composite. By the end of 2014, 37 technical reports have been uploaded into the Gen-lV materials handbook. In addition, creep test records (45 data) of alloy 617 and tensile test results for the BM, WM, and weld joint of alloy 617 (32 ea) were uploaded into the Gen-lV materials handbook (Generation IV international fomm annual report, 2013). 

T. Asayama, Update and improve subsection NH — alternative simplified creep-fatigue design methods, ASME ST-LLC, STP-NU-041 (2011). 

It should also be noted that in this case the material was loaded in compre-sion whereas the tensile creep curves were used. The vast majority of creep data which is available is for tensile loading mainly because this is the simplest and most convenient test method. However, it should not be forgotten that the material will behave differently under other modes of deformation. In compression the material deforms less than in tension although the efrect is small for strains up to 0.5%. If no compression data is available then the use of tensile data is permissible because the lower modulus in the latter case will provide a conservative design. 

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. 

The method of obtaining creep data and their presentation have been described however, their application is limited to the exact same material, temperature use, stress level, atmospheric conditions, and type of test (tensile, compression, flexure) with a tolerance of 10%. Only rarely do product requirement conditions coincide with those of the test or, for that matter, are creep data available for all grades of material that may be selected by a designer. In those cases a creep test of relatively short duration such as 1000 h can be instigated, and the information can be extrapolated to the long-term needs. It should be noted that reinforced thermoplastics and thermosets display much higher resistance to creep (Chapter 2). 

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