Isometric curves

As indicated above, the stress-strain presentation of the data in isochronous curves is a format which is very familiar to engineers. Hence in design situations it is quite common to use these curves and obtain a secant modulus (see Section 1.4.1, Fig. 1.6) at an appropriate strain. Strictly speaking this will be different to the creep modulus or the relaxation modulus referred to above since the secant modulus relates to a situation where both stress and strain are changing. In practice the values are quite similar and as will be shown in the following sections, the values will coincide at equivalent values of strain and time. That is, a 2% secant modulus taken from a 1 year isochronous curve will be the same as a 1 year relaxation modulus taken from a 2% isometric curve. 

This is a stress relaxation problem and strictly speaking stress relaxation data should be used. However, for most purposes isometric curves obtained from the creep curves are sufficiently accurate. By considering the 1.5% isometric curve shown in Fig. 2.8 it may be seen that the initial stress is 16 MN/m2 and the stress after 1 week is 7 MN/m2. 

It may be seen from Fig. 2.71 that in most cases where the failure is ductile the isometric curves are approximately parallel to the fracture curve, suggesting that this type of failure is primarily strain dominated. However, the brittle... 

This is a stress relaxation problem but the isometric curves may be used. 

From the 0.417% isometric curve, the stress after 1 year is 1.45 MN/m ... 

Isometric curves are obtained by plotting stress vs. time for a constant strain isochronous curves are obtained by plotting stress vs. strain for a constant time of loading. These curves may be obtained from the creep curves by taking a constant-strain section and a constant-time section, respectively, through the creep curves and replotting the data, as shown in Eigure 3.17. 

An isometric curve provides an indication of the relaxation of stress in the material when the strain is kept constant. Since stress relaxation is a less common experimental procedure than creep testing, an isometric curve, derived like the preceding curves from creep curves, is often used as a good approximation of this property. 

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... 

A conventional creep curve as exhibited by most materials is illustrated in Fig. 2.25 although many engineers present the data using log axes to produce a graph of the form shown in Fig. 2.26. Data from families of strain-time curves at various values of constant stress are used to produce isochronous stress-strain curves (Fig. 2.27). These are obtained by cross-plotting stresses and strains at various times from the commencement of loading. The results of creep tests can also be used to derive constant strain, or isometric, curves of stress versus time, also as illustrated in Fig. 2.27. 

Fig. 8.14. Creep data for PES at 150 °C replotted (a) as isochrorious curves and (b) as isometric curves (after ICI pic).

This is a stress relaxation problem but the isometric curves may be used. From 2% isometric, after 10 seconds, = = 837.5 MN/m ... 

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