Abstract : The behaviour of materials subjected to high-energy dynamic loadings (impacts, blasts \dots) is usually sensitive to strain-rate (viscoplastic) and/or damage. Conventional procedures for the characterization of corresponding models of behaviour use statically determined tests requiring restrictive hypotheses. So, it is impossible to deal with heterogeneous mechanical fields and the exploitation of tests is limited to small levels of strain. Moreover, several tests have to be performed, at constant strain rate, to characterize viscoplasticity. However, these limitations do not allow to take advantage of the large amount of information available thanks to full-field measurements. One solution is to use statically undetermined tests to deal with heterogeneous fields. Among available tools, the Virtual Fields Method (VFM) has undeniable advantages compared to classic FEMU methods. This study focuses on the development of the VFM for the characterization of Johnson-Cook's viscoplastic model of behaviour. An asset of the VFM is that it makes possible the characterization of the viscoplastic part of the model with only one testing, under dynamic conditions, thanks to a statically undetermined exploitation of heterogeneous strain and strain-rate fields. A short-term prospect is to use the VFM to identify parameters of elastoplastic models of behaviour coupled with damage (e.g. Lemaitre). The feasibility was demonstrated for numerical data.