Creep diagram

Fig. 11.3. Schematic creep diagram. At different temperatures, the stress that causes fracture after a certain time is plotted. Each point on the curve corresponds to one experiment. Rm/t/T is the stress in a specimen that fails after a time t at temperature T. For example i m/ioo 000/550 is the failure stress after 10 h at 550°C...

Figure 1.50 Changing aging mechanisms Left Schematic creep diagram Right Resuiting service iife curve 104]...

For each load and with the selected maximum dimensions (thicknesses) the resulting strains are determined. The individual service live sections are determined via creep diagrams using media and temperature effects as parameters. The sum of individual sections then has to he < 1 according to Eq. 1.65, if Eq. 1.66 is to apply ... 

For the internal pressure creep resistance, lifetime at constant internal pressure load is determined. Subsequently, internal pressure creep resistance is plotted in the internal pressure creep diagram as a function of lifetime (s. Section 2.5.4). If the test is performed in air as the medium, information about thermal oxidative resistance is obtained. 

C. The first creep exposure of the SIM procedure is a conventional creep test in the sense that the creep test specimen does not have a history of creep loading. However, the second and subsequent creep exposures are complicated by having thermal histories of the previous steps this complexity defines the SIM procedure. The strain results of a conceptual SIM test performed at 40% of ultimate tensile strength are plotted as a creep diagram in Fig. 9.13(a). 

A typical stress—strain curve generated by a tensile tester is shown in Eigure 41. Creep and stress—relaxation results are essentially the same as those described above. Regarding stress—strain diagrams and from the standpoint of measuring viscoelastic properties, the early part of the curve, ie, the region... 

Times-to-failure are normally presented as creep-rupture diagrams (Fig. 17.9). Their application is obvious if you know the stress and temperature you can read off the life if you wish to design for a certain life at a certain temperature, you can read off the design stress. 

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

The predicted strain variation is shown in Fig. 2.43(b). The constant strain rates predicted in this diagram are a result of the Maxwell model used in this example to illustrate the use of the superposition principle. Of course superposition is not restricted to this simple model. It can be applied to any type of model or directly to the creep curves. The method also lends itself to a graphical solution as follows. If a stress is applied at zero time, then the creep curve will be the time dependent strain response predicted by equation (2.54). When a second stress, 0 2 is added then the new creep curve will be obtained by adding the creep due to 02 to the anticipated creep if stress a had remained... 

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. 

Creep modeling A stress-strain diagram is a significant source of data for a material. In metals, for example, most of the needed data for mechanical property considerations are obtained from a stress-strain diagram. In plastic, however, the viscoelasticity causes an initial deformation at a specific load and temperature and is followed by a continuous increase in strain under identical test conditions until the product is either dimensionally out of tolerance or fails in rupture as a result of excessive deformation. This type of an occurrence can be explained with the aid of the Maxwell model shown in Fig. 2-24. 

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. 

The duration of testing is not specified, but ISO 11403-1 [36] proposes that the loads should be 20%, 40%, 60% and 80% of the maximum load for the respective temperature and that strains should be tabulated after 1,10,100,1,000 and 10,000 h (10,000 h equals 13.7 months). This data will enable creep strain curves and an isochronous diagram to be prepared (load plotted against strain for each duration) with sufficient accuracy for design. 

The problem with this procedure is that there is considerable variation between specimens in the initial strain which, coupled with the small slope of the creep-rupture diagram, can lead to large errors in shifting. 

Finally, Figure 4.11(d) shows that glass fibre reinforcement is an efficient means to reach more suitable creep moduli. It should be noted that the modulus scale is four times that of the previous diagrams and that the load is ten times higher. 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

Stress relaxation master curve. For the poly-a-methylstyrene stress relaxation data in Fig. 1.33 [8], create a master creep curve at Tg (204°C). Identify the glassy, rubbery, viscous and viscoelastic regions of the master curve. Identify each region with a spring-dashpot diagram. Develop a plot of the shift factor, log (ax) versus T, used to create your master curve log (ot) is the horizontal distance that the curve at temperature T was slid to coincide with the master curve. What is the relaxation time of the polymer at the glass transition temperature ... 

For practical applications empirically determined creep data are being used, such as D(t) or, more often, E(t) curves at various levels of stress and temperature. The most often used way of representing creep data is, however, the bundle of creep isochrones, derived from actual creep curves by intersecting them with lines of constant (log) time (see Figure 7.7). These cr-e-curves should be carefully distinguished from the stress-strain diagram discussed before, as generated in a simple tensile test ... 

Because all tissues are viscoelastic this means that their mechanical properties are time dependent and their behavior is characterized both by properties of elastic solids and those of viscous liquids. The classic method to characterize a viscoelastic material is to observe the decay of the stress required to hold a sample at a fixed strain (stress relaxation) or by the increasing strain required to hold a sample at a fixed stress (creep) as diagrammed in Figure 7.1 and explained further in Figure 7.2. Viscoelastic materials undergo processes that both store (elastic) and dissipate (viscous)... 

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