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This paper is concerned with the behavior of metallic materials under general complex loadings. A complete experimental investigation is carried out to exhibit the main features of this behavior. Nonproportional loadings are characterized by an additional hardening when compared to proportional loadings. This hardening is observed to be strongly dependent on the loading path and can be recovered when the nonproportionality decreases. It is shown that a classical model of elasto-viscoplasticity fails when applied to the prediction of responses under complex loadings. The examination of the assumptions on which the model is built allows to understand its deficiencies and, on the basis of an experimental program, a simple constitituve model is derived for cyclic elastoviscoplasticity under nonproportional loadings. This model is shown to give good results for the prediction of very complex tests.