Creep data

BET24909 European collaboration in creep data development for welds weld[Pg.936]

The relaxation time (arbitrarily defined as the time taken for the stress to relax to half its original value) can be calculated from the power-law creep data as follows. Consider a bolt which is tightened onto a rigid component so that the initial stress in its shank is CTj. In this geometry (Fig. 17.3(c)) the length of the shank must remain constant - that is, the total strain in the shank e must remain constant. But creep strain can rqjiace elastic strain e - , causing the stress to relax. At any time t... 

The hoop stress in the tube under the working pressure of 50 bar (5 MPa) is 5 MPa X 50 mm/5 mm = 50 MPa. Creep data indicate that, at 900°C and 50 MPa, the steel should fail after only 15 minutes or so. In all probability, then, the failure occurred by creep rupture during a short temperature excursion to at least 870°C. 

Major polymer manufacturers can usually supply creep data on those of their polymers which are likely to be subjected to long-term loads. A typical set of such curves is given in Figure 9.9. Sections through the creep curves at constant... 

Figure 9.10. Presentation of creep data sections through the creep curves at constant time and constant strain give curves of isochronous stress-strain, isometric stress-log (time) and creep modulus-log (time). (From ICI Technical Service Note PES 101, reproduced by permission of ICI...

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. 

It is apparent therefore that the Superposition Principle is a convenient method of analysing complex stress systems. However, it should not be forgotten that the principle is based on the assumption of linear viscoelasticity which is quite inapplicable at the higher stress levels and the accuracy of the predictions will reflect the accuracy with which the equation for modulus (equation (2.33)) fits the experimental creep data for the material. In Examples (2.13) and (2.14) a simple equation for modulus was selected in order to illustrate the method of solution. More accurate predictions could have been made if the modulus equation for the combined Maxwell/Kelvin model or the Standard Linear Solid had been used. 

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. ... 

A polypropylene pipe of inside diameter 10 mm and outside diameter 12 mm is pushed on to a rigid metal tube of outside diameter 10.16 mm. If the polypropylene pipe is in contact with the metal tube over a distance of 15 mm, calculate the axi force necessary to separate the two pipes (a) immediately after they are connected (b) 1 year after connection. The coefficient of friction between the two materials is 0.3 and the creep data in Fig. 2.5 may be used. 

During a test on a polymer which is to have its viscoelastic behaviour described by the Kelvin model the following creep data was obtained when a stress of 2 MN/m was applied to it. 

Note < 0.5% so no correction to tensile creep data is needed). (2.10) Critical Stress,... 

This is a stress relaxation problem, but the question states that creep data may used... 

So from text using the fact that for a strain of 0.4%, ctq — 5.5 MN/m (from creep data)... 

Creep data for periods at 100 and 1000 hours (or more, if available) covering stress and temperature conditions closely comparable to those of product application. 

Viscoelastic creep data are usually presented in one of two ways. In the first, the total strain experienced by the material under the applied stress is plotted as a function of time. Families of such curves may be presented at each temperature of interest, each curve representing the creep behavior of the material at a different level of applied stress. Below a critical stress, viscoelastic materials may exhibit linear viscoelasticity that is, the total strain at a given time is proportional to the applied stress. Above this critical stress, the creep rate becomes disproportionately faster. In the second, the apparent creep modulus is plotted as a function of time. 

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. 

Creep data applications are generally limited to the identical plastic, temperature, stress level, atmospheric conditions, and type of test. Data of a relatively short duration of 1000 h can be extrapolated to long... 

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. 

Creep rupture. Creep-rupture data are obtained in the same way as creep data except that higher stresses are used and the time is measured to failure (Figs. 2-28 and 29). The strains are sometimes recorded, but this is not necessary for creep rupture. The results are generally plotted as the log stress versus log time to failure (110). In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. General technical literature and product data sheets seldom provide a complete description of a material s behavior prior to rupture. It should include the development of any crazing and stress whitening, its strain-time... 

If the same stress level prevails for 200 hours, the total strain will be the sum of the initial strain plus the strain due to time. This total strain can be obtained from a creep-data curve. If, for example, the total deformation under a tension load for 200 hours is 0.02 in. 

Similarly, Ea can then be determined for one year. Extrapolating from the creep-data curve, which is in fact a straight line, gives a deformation of 0.025 in. per in. Thus,... 

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. 

Some examples of creep data presented in different formats are given in Figs. 2-35... 

Table 2-7 Flexural creep data of reinforced plastics at 23°C...

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. 

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. 

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. 

Master curves are important since they give directly the response to be expected at other times at that temperature. In addition, such curves are required to calculate the distribution of relaxation times as discussed earlier. Master curves can be made from stress relaxation data, dynamic mechanical data, or creep data (and, though less straightforwardly, from constant-strain-rate data and from dielectric response data). Figure 9 shows master curves for the compliance of poly(n. v-isoprene) of different molecular weights. The master curves were constructed from creep curves such as those shown in Figure 10 (32). The reference temperature 7, for the... 

This result can also be derived from Eqs. 10, 108 and 109 with j1=I1Q.kBT. Figure 68 shows a good agreement between the observed and calculated creep data of PpPTA fibres [7]. 

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