Abstract
Although the combined effect of strain rate and temperature on the behaviour of metals is widely recognized, no universally accepted viewpoints are available about the physical phenomena. Experiments on a highly ductile A2-70 steel, performed at moderate dynamic rates (10 s-1) and different initial temperatures (20 to 150 °C), are firstly aimed here at assessing whether the thermal softening previously verified at static rates on the same steel is also suitable for describing now the mixed effect of dynamic rates and consequent variable temperatures, or further contributions to the thermal softening are necessary for describing such mixed effects. A general multiplicative model of the dynamic hardening is proposed, based on a static flow curve at room temperature to be increased by the dynamic amplification and to be decreased by the thermal softening, the latter incorporating the known “static component” depending on both strain and constant temperatures, together with a new “dynamic component” incorporating the dependence on the temperature variation and promoted by fast straining. The dynamic amplification of the stress is then obtained from another series of dynamic tests ran at initial room temperature and four nominal strain rates between 1 and 1800 s-1. The trend obtained is compatible with the seizing of the strain rate effect beyond necking onset, already found for other metals in previous works. All the experiments are based on the acquisition of the current load (by load cells for the testing machine and by strain gauges for the Hopkinson bar) and of the current cross section through optical diameter measurements by a fast camera; then, the effective current values of true stress-true strain-true strain rate are measured on a semi-local basis over the neck section at different instants during the test.
Highlights
The characterization and modelling of the behaviour of metals at high temperatures and high strain rates have been extensively studied in the literature, but there are still open questions regarding the combined effects of strain, strain rate and temperature
ΣEq is the flow stress incorporating dynamics and thermal effects, σEqSt is the reference flow stress, R is the strain rate amplification and SST + SDYN is the thermal softening, with the first term derived from static tests at constant temperature and the second term complementing the first one in case of dynamic tests with variable temperature
Dynamic tests at initial room temperature have been conducted by means of an Split Hopkinson Tension Bar (SHTB) setup at high strain rates and by a hydraulic machine at intermediate strain rates
Summary
The characterization and modelling of the behaviour of metals at high temperatures and high strain rates have been extensively studied in the literature, but there are still open questions regarding the combined effects of strain, strain rate and temperature. In order to correctly identify the above combined effects, the equivalent stress-strain curves of the material at different strain rates and temperatures must be evaluated Regarding this aspect, Peroni et al [1] developed a neck-profile-based inverse method while Sasso et al [2] proposed a novel. In Kapoor & Nemat-Nasser [6] and Walley et al [7], it is demonstrated that the plastic work is almost completely converted into heat, while Jovic et al [8] and Rittel et al [9] calculated that lower fractions of plastic work can be converted into heat for the same material depending on the stress state conditions Another relevant aspect of the dynamic hardening is the interaction between temperature, strain rate and plastic instability. All tests are supported by camera acquisitions for measuring the effective evolving neck sections; the experimental true stress-true strain curves are processed for deriving the flow curves extracting and separating the functions of thermal softening and dynamic amplification
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