  The rheological parameters of materials are determined in the industry
according to international standards established generally on the basis of
widespread techniques and robust methods of estimation. Concerning solid
polymers and the determination of Young's modulus in tensile tests, ISO 527-1
or ASTM D638 standards rely on protocols with poor scientific content: the
determination of the slope of conventionally defined straight lines fitted to
stress-strain curves in a given range of elongations. This paper describes the
approach allowing for a correct measurement of the instantaneous elastic
modulus of polymers in a tensile test. It is based on the use of an appropriate
reduced model to describe the behavior of the material. The model comes a
thermodynamical framework and allows to reproduce the behavior of an HDPE
Polymer until large strains, covering the elastoviscoplastic and hardening
regimes. Well-established principles of parameter estimation in engineering
science are used to found the identification procedure. It will be shown that
three parameters only are necessary to model experimental tensile signals: the
instantaneous ('Young's') modulus, the maximum relaxation time of a linear
distribution (described with a universal shape) and a strain hardening modulus
to describe the 'relaxed' state. The paper ends with an assessment of the
methodology. Our results of instantaneous modulus measurements are compared
with those obtained with other physical experiments operating at different
temporal and length scales.
