Isochronous graph

These latter curves are particularly important when they are obtained experimentally because they are less time consuming and require less specimen preparation than creep curves. Isochronous graphs at several time intervals can also be used to build up creep curves and indicate areas where the main experimental creep programme could be most profitably concentrated. They are also popular as evaluations of deformational behaviour because the data presentation is similar to the conventional tensile test data referred to in Section 2.3. It is interesting to note that the isochronous test method only differs from that of a conventional incremental loading tensile test in that (a) the presence of creep is recognised, and (b) the memory which the material has for its stress history is accounted for by the recovery periods. 

The only unknown on the right hand side is a value for modulus E. For the plastic this is time-dependent but a suitable value may be obtained by reference to the creep curves in Fig. 2.5. A section across these curves at the service life of 1 year gives the isochronous graph shown in Fig. 2.13. The maximum strain is recommended as 1.5% so a secant modulus may be taken at this value and is found to be 347 MN/m. This is then used in the above equation. 

Whereas the abundance of Sr in rubidium rich rocks changes over time due to the radioactive 3 decay of Rb as a function of the primordial rubidium concentration and the age of the mineral, the abundance of the stable Sr isotope and consequently the Sr/ Sr is constant in nature. The constant Sr/ Sr isotope ratio is often used for internal standardization (mass bias correction) during strontium isotope ratio measurements of Sr/ Sr. In the rubidium-strontium age dating method, the isotope ratios Sr/ Sr and Rb/ Sr are measured mass spectrometrically (mainly by TIMS or nowadays by ICP-MS) and the primordial strontium ratio ( Sr/ Sr)o at t = 0 and the age t of the rock can be derived from the isochrone (graph of measured Sr/ Sr isotope ratios (represented on the ordinate) as a function of the Rb/ Sr ratio (on the abscissa) in several minerals with different primordial Rb concentrations). The age of the minerals will be determined from the slope of the isochrone (e — 1), and the primordial isotope ratio ( Sr/ Sr)o from the point of intersection with the ordinate (see Figure 8.9). Rb-Sr age dating is today an... 

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