What do you need to measure?

The platforms described in the previous chapter access a range of states via a number of thermodynamic loading paths taken by a material as it deforms. Some load to the elastic limit, some up to the finis extremis where electronic bonding changes its nature, and some beyond that. What follows will concern loading from the elastic limit to the point at which ambient descriptions of strength cease to apply. A few of the loading paths necessary to define an equation of state for a material are shown in the schematic of Figure 4.1. There are a range of outputs which may be sensed to give insight into the response of materials under load. Experiments should aim to map their states beyond the yield point statically and dynamically. In the first case they induce an ideal stress state to define operating mechanisms represented in suitable models, which are later tested against other loading down more complex paths. Thus shock experiments map out Hugoniot curves but can also yield information that allows one to deduce compression isotherms and isentropes. Isotherms are generally measured using static compression experiments at some fixed temperature in the diamond anvil cell (DAC). To briefly recap, the isentrope generally lies between the isotherm and Hugoniot curves and is in fact tangent to the Hugoniot at the common starting state. Although shock experiments generally yield only a final P–V state on the Hugoniot, an ideal isentropic compression experiment (ICE) yields a continuous locus of points along a different loading path. Although not precisely following the isentrope, it is certainly possible to load more slowly and avoid the adiabatic conditions of shock, and so this is better dubbed shockless loading. To record this data demands sensors capable of acquiring pressure, density and temperature as a function of time, which requires sub-nanosecond data collection under the fastest loadings. To measure deviatoric quantities entails measures of the stress state in the target which is itself directional. Thus a series of accurate, time-resolved sensors has been developed to make such measurements in these experiments. Another means of recording the data is to use a quantitative imaging technique (such as X-rays) to deduce state parameters from the flow. Imaging itself allows the visualisation of geometries changing under load whist offering non-invasive measurements of flow parameters.