Thermomechanical Behaviour of Materials under Extreme Conditions Studied with High-Speed Optical and Infrared Imaging

Investigating material behaviour under extreme conditions is fundamentally important for improving material performance in various critical applications, such as a bird crashing into a jet engine, operations inside a fusion reactor, and the atmospheric re-entry of a spaceship. The mechanical behaviour of materials is commonly described by stress-strain plots under given conditions. However, deformation does not always occur uniformly, and many complex phenomena cannot be accurately described by these measurements only. Extensometers and strain gauges describe the average strain over a certain length or area, but they cannot describe complex local phenomena comprising strain localization, e.g. necking, Lüders bands, Portevin–Le Chatelier bands and adiabatic shear bands. Furthermore, the adiabatic heating and thermal softening caused by a temperature increase are typically not measured or analysed in these experiments. In this work, the current state-of-the-art full-field measurements were developed further to address some of the shortcomings described above. At first, synchronized optical and infrared high-speed imaging was used at low temperatures, and later the methods were used for the characterization of material behaviour at temperatures up to 1350 °C at a strain rate of 1600 s−1 in tension. Four advanced steels and a commercially pure titanium were studied in tension at strain rates from 2.5×10−4 up to 1600 s−1 at room temperature, and the titanium was also tested at temperatures up to 1350 °C. The uncertainties of the high-temperature measurements were evaluated in terms of strain, displacement, temperature resolution, epipolar and desynchronization errors. The errors remained reasonable and considerably lower than any of the quantities of interest measured under the selected extreme conditions. At elevated temperatures, the necking onset of commercially pure titanium under tension occurred almost immediately after yielding, and a considerably higher strain localization was observed with much less adiabatic heating in comparison with the tests at room temperature. It was also possible to deduce from the full-field strain/temperature data that the strain localization first evolved at a constant rate but suddenly accelerated at a critical strain, leading to an increase in the material temperature. The full-field approach allowed much more data to be gathered from the high temperature tests than with only traditional testing methods. The compression response of titanium, iron, copper, tin and a CoCrFeMnNi high-entropy alloy was studied at room temperature and at strain rates from 3×10−4 up to 3100 s−1. The Taylor–Quinney coefficients (β) determined for these materials were initially notably lower than the typically assumed 0.9. For the high-entropy alloy, the Taylor–Quinney coefficient did not change much during deformation and remained close to a constant value of 0.5. For the other metals, the Taylor–Quinney coefficient was lower at the onset of plastic deformation but increased to close to 0.9 at higher strains. The interpretation of the strain hardening behaviour and β suggests that a faster evolving microstructure is characterized by a lower β and higher strain hardening rate. The methods presented in this work provide a robust framework for investigating the thermomechanical behaviour of materials under extreme conditions. The data acquired from these measurements are also useful for constitutive modelling, inverse modelling approaches and validation of finite element models.