Isochron

Isochronous governor is a floating-action governor that controls for constant speed. It is equipped with a dashpot or buffer to give momentary regulation for a speed-input change. 

Strain and constant time can give respectively isometric stress-log time curves and isochronous stress-strain curves Figure 9.10). Whilst not providing any new information, such alternative presentations of the data may be preferred for certain purposes. 

ISOCHRONOUS STRESS - STRAIN CURVE CREEP MODULUS - TIME CURVE... 

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...

Long-term deformation such as shown by creep curves and/or the derived isochronous stress-strain and isometric stress-time curves, and also by studies of recovery for deformation. 

Design Methods for Plastics using Deformation Data Isochronous and Isometric Graphs... 

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. 

Quite often isochronous data is presented on log-log scales. One of the reasons for this is that on linear scales any slight, but possibly important, non-linearity between stress and strain may go unnoticed whereas the use of log-log scales will usually give a straight-line graph, the slope of which is an indication of the linearity of the material. If it is perfectly linear the slope will be 45°. If the material is non-linear the slope will be less than this. 

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. 

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. 

From the 3 year isochronous curve for polypropylene, the initial modulus, E =... 

The maximum stress or strain is not specified so an iterative approach is needed. From the 1 year isochronous for PP the initial modulus is 370 MN/m2... 

From the 1 year isochronous curve, the initial modulus = 370 MN/m2... 

From the 1 day isochronous curve, the maximum stress at which the material is linear is 4 MN/m. TTiis may be converted to an equivalent shear stress by the relation... 

Iso-chrom, -chromatisch, a. isochromatic. -chron, -chronisch, a. isochronal, isochronous, isochronio. 

The influence of mechanical deformation on LRO-kinetics was investigated during isochronal and isothermal temperature treatment. 

Figure 1. Relative change of resislivily during isochronal annealing (AT=10K, At=10min) of completely recrystallized CujAu Tlie LRO-parameter as calculated by Rossiter formalism (A=0 5) is given on second axis ( ) increasing, (0) decreasing temperature (o) plateau values of small step annealing procedure.

That the first stage of ordering (resistivity decrease) is correlated with excess vacancies not being in thermal equilibrium can be seen from measurement during isochronally lowering the temperature from the disordered state (0), which shows that atomic mobility is frozen below 280°C. 

In Figure 1 in addition to isochronal measurements plateau values of a small step armealing treatment (order-order relaxations) are given for a comparison (o). These points reflect a variation of equilibrium state of LRO-parameter with temperature. 

Figure 2 Relative change of resistivity during isochronal annealing (AT=10K, At=10min) of deformed samples Deformed in disordered state 40% (A) and 80% reduction ( ) deformed in ordered state 30% reduction ( )...

Figure 3 gives a comparison of changes in microhardness during isochronal armealing of a recrystallized sample as well as samples cold-rolled in the disordered (40% and 80%) and the ordered state (30%), respectively. 

From a comparison of the evolution of hardness of all samples during isochronal armealing it can be concluded that for high deformation in the disordered state and deformation in the ordered state, recovery and recrystallization is prevented up to T, in the sample deformed to 40% reduction in the disordered state recovery and recrystallization processes seem to start as soon as atomic mobility is enabled (260°C). 

Figure 2. Resistivity change versus temperature for isochronal annealing (AT=I0K, At=l 5min) of Au-5at%Fe in the recrystallized (7) and deformed state ( ) (cold-rolled to about 80% thickness reduction).

Figure 4. Comparison of SRO-kinetics of Ag-23at%Zn for different states of defect atmealing. ( ) as-rolled state A (21% and 65% thickness reduction) (A), (0) isochronal defect annealing to 553K (state C) and 843K (state D), respectively, (A), 3h recrystallization treatment at 843K (state E). For more details see .

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