Apparent creep modulus

Creep modulus (apparent modulus) Creep mpture strength Creep strength... 

One of the serious limitations of the earlier creep curves such as the one illustrated in Figure 2-26 was the lack of simplicity of the single-point stress-strain properties such as tensile modulus and flexural strength, especially when one wanted to measure creep as a function of temperature and stress level. Furthermore, the creep data presented in terms of strain were not convenient to use in design or for the purpose of comparing materials. It was obvious that creep curves had to be presented in a more meaningful and convenient way such that they are readily usable. Creep strain curves were easily converted to creep modulus (apparent modulus) by simply dividing the initial applied stress by the creep strain at any time. 

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. 

The creep modulus, also known as the apparent modulus or viscous modulus when graphed on log-log paper, is normally a straight line and lends itself to extrapolation for longer periods of time. 

Apparent creep modulus. The concept of an apparent modulus is a convenient method for expressing creep, because it takes into account the initial strain for an applied stress plus the amount of deformation or strain that occurs over time. Thus, the apparent modulus Ea is calculated in a very simplified approach as ... 

Fig. 2-32 Example of plotting the apparent creep modulus vs. log time.

Second, the creep modulus, also known as the apparent modulus or viscous modulus when graphed on log-log paper, is normally a straight line and lends itself to extrapolation for longer periods of time. The apparent modulus should be differentiated from the modulus given in the data sheets, which is an instantaneous or static value derived from the testing machine, per ASTM D 638. 

The creep behavior of 40% glass-filled polyphenylene sulfide Is summarized In Figure 5 which plots creep modulus against time for three sets of experimental conditions 23°C/5,000 psi, 66°C/5,000 psi, and 121°C/5,000 psi. Table II compares the per cent loss In apparent creep modulus at 1,000 hours and at 10,000... 

LOSS IN APPARENT CREEP MODULUS FOR m GLASS FILLED PPS... 

Test Conditions Loss In Apparent Creep Modulus ... 

The variation of the 100 second tensile creep modulus with 100 second tensile strain is presented in Fig. 16. The behaviour of a specimen cut from an isotropic sheet (which had been subjected to the same thermal cycle as the drawn sheet) is included for comparison. It is apparent that all specimens exhibited non-linear viscoelastic behaviour, but there is little anisotropy of non-linearity. Furthermore the degree of non-linearity exhibited by the specimens from the drawn sheet is similar to that of a specimen from the isotropic sheet. At any chosen creep strain the anisotropy of modulus for the drawn sheet is relatively low. 

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... 

P(2) The creep modulus can be defined from the additional strain exhibited by a material after a given elapsed time. The creep modulus is the apparent stiffness as determined by the total deformation to the time defined. 

Creep measurements involve measuring a constant tensile or flexural load to a respective specimen (as discussed previously) and measuring the strain as a function of time. In a typical creep plot, percentage creep strain is plotted against time. The apparent creep modulus at a particular time can be calculated by dividing the stress by strain at that particular time. Creep compliance is determined by dividing the strain by stress. For a tensile test, the simplest way to measure extension is to make two gauge marks on the tensile specimen and note the distance between the marks at different intervals. However, accurate measurement of extension requires an optical or laser extensometer. In a flexural measurement, the strain is usually calculated with the help of a linear variable differential transformer system. 

At present only special programs are available which take into account the creep performance of statically loaded plastics. The usual method is to use a creep-adjusted apparent modulus of elasticity in a second cut structural analysis to determine whether there will be serious creep effects on the product which must be accounted for in the design. With increased sophistication of the computer programs available it will soon be possible to do a direct computer analysis of the creep effects on a plastics component. In fact, there is current work on programs that produce a time model of the plastics component under load that will show how the stresses affect shape and load capabilities with time. 

The hoop stress can then be obtained by multiplying the appropriate modulus. For high strains, the secand modulus will give the initial stress the apparent or creep modulus should be used for longer-term stresses. The main point is that the maximum strain or stress must be below the value that will produce creep rupture in the material. There is usually a weld line in the hub that can significantly affect the creep-rupture strength of most plastics. 

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 ... 

TABLE 3-6. APPARENT CREEP MODULUS OF GLASS-REINFORCED THERMOPLASTICS ... 

Apparent Creep Modulus of Glass-Reinforced Thermoplastics. 3-88... 

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