Design creep data

The creep strength of steels is a factor limiting the maximum temperatures for such high-pressure equipment as shells and stirrers of high temperature reactors. Table 3.10 presents creep data for temperatures ranging from 400 to 600°C. The stress for 1% creep in 100,000 hours (which is a design criterion) is accepted to be not less than two-thirds of the creep stresses. 

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. 

An underground polypropylene storage tank is a sphere of diameter 1.4 m. If it is to be designed to resist an external pressure of 20 kN/m for at least 3 years, estimate a suitable value for the wall thickness. Tensile creep data may be used and the density of the polypropylene is 904 kg/m. ... 

Different viscoelastic materials may have considerably different creep behavior at the same temperature. A given viscoelastic material may have considerably different creep behavior at different temperatures. Viscoelastic creep data are necessary and extremely important in designing products that must bear long-term loads. It is inappropriate to use an instantaneous (short load) modulus of elasticity to design such structures because they do not reflect the effects of creep. Viscoelastic creep modulus, on the other hand, allows one to estimate the total material strain that will result from a given applied stress acting for a given time at the anticipated use temperature of the structure. 

Basics Creep data can be very useful to the designer. In the interest of sound design-procedure, the necessary long-term creep information should be obtained on the perspective specific plastic, under the conditions of product usage (Chapter 5, MECHANICAL PROPERTY, Long-Term Stress Relaxation/Creep). In addition to the creep data, a stress-strain diagram under similar conditions should be obtained. The combined information will provide the basis for calculating the predictability of the plastic performance. 

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

The tests are performed under carefully controlled stress (load), temperature, time, and creep (elongation) conditions. To save time, tests for different constant loads are performed simultaneously on different specimens of the same material. Creep tests may be rather extensively conducted, as for example when developing creep data prior to the design and fabrication of the first all-plastic airplane (41). The usual procedure is to plot the creep versus time curve, but other combinations are possible. 

Extensive amount of these type data has been plotted but unfortunately most of it is privately owned. Creep data available from material suppliers, college and government projects, etc. can provide guidelines. However where the product has to meet critical requirements that usually include safety of people and data from previous work does not exist, creep test have to be conducted and properly applied by the designer. 

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. 

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-... 

Certain conclusions can now be developed, based on creep-data test results First, for practical design purposes, the data accumulated for up to 100 hours of creep are of no real benefit. There is usually too much variation during this test period, which is of a relatively short duration. 

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. 

Creep information is not as readily available as short-term property data sheets are. From a designer s viewpoint, it is important to have creep data available for products subjected to a constant load for... 

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. 

In the category of deformation properties the phenomena of stress-strain behaviour, modulus and yield, stress relaxation and creep have been discussed already in Chap. 13. Here we want to give special attention to the long-term deformation properties. For a good design we need sufficiently reliable creep data (or stress-strain curves as a function of time and temperature). 

It is actually the creep strength of a grouted soil (either in unconfined compression or triaxial compression, depending on the specific application) that should be used for design purposes, not the unconfined compressive strength. In the absence of specific creep data, the value of one-fourth to one-half of the unconfined strength may be used for applications. A suitable safety factor must be applied to these suggested values. 

This property is an important consideration in the design of parts from polytetrafluoroethylene. PTFE deforms substantially overtime when it is subjected to load. Metals similarly deform at elevated temperatures. Creep is defined as the total deformation under stress after a period of time, beyond the instantaneous deformation upon load application. Significant variables that affect creep are load, time under load, and temperature. Creep data under various conditions in tensile, compressive, and torsional modes can be found in Figs. 3.12 through 3.19. 

The long-term stress-strain behavior of polymers is generally more important than shortterm properties where the product is expected to sustain a stress or strain in service. Creep is dearly the most relevant where the product or component is to be subjected to a more or less constant stress. This is the case for a great many uses of rigid plastics and for such products as rubber mountings. Hence, creep data is often an essential design factor for plastics but is only used for other polymer types when particular applications are in mind. 

Products are used at different temperatures, and each has a unique stress distribution. Consequently, design requires creep data under a wide variety of conditions. However the test programme to generate such data is excessively long. For most plastics there will be tensile creep data, for times up to... 

Despite the attractiveness of time-temperature superposition and the potential saving in time, the method has not in fact been widely used to obtain creep data for design. One good reason for doubt about the precision of the method is the existence of physical ageing (see Section 4.4.1). Nevertheless, general points well worth retaining in the mind are (i) creep deformation processes are speeded up at higher temperatures (ii) the effective time at a temperature Tg is t/a-p, where r is the time for the same mechanical effect at another temperature 7. 

For ease of reference, the creep data are usually replotted in one or more different ways, as Ulustrated in Figures 8.14(a) and (b). Isochronous stress-strain curves (Figure 8.14(a)), which are discussed in Chapter 4, are included in most discussions of creep characteristics. From isochronous curves of this type, the engineer can determine the secant modulus of the pofymer at any given strain or applied stress and time under load. This creep modulus E(creep compliance at the appropriate stress and time creep data is in the form of isometric curves, as shown in Figure 8.14(b), which are helpful in designing plastic components to a... 

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 ... 

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