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The rheological parameters of materials are determined in the industryaccording to international standards established generally on the basis ofwidespread techniques and robust methods of estimation. Concerning solidpolymers and the determination of Young's modulus in tensile tests, ISO 527-1or ASTM D638 standards rely on protocols with poor scientific content: thedetermination of the slope of conventionally defined straight lines fitted tostress-strain curves in a given range of elongations. This paper describes theapproach allowing for a correct measurement of the instantaneous elasticmodulus of polymers in a tensile test. It is based on the use of an appropriatereduced model to describe the behavior of the material. The model comes athermodynamical framework and allows to reproduce the behavior of an HDPEPolymer until large strains, covering the elastoviscoplastic and hardeningregimes. Well-established principles of parameter estimation in engineeringscience are used to found the identification procedure. It will be shown thatthree parameters only are necessary to model experimental tensile signals: theinstantaneous ('Young's') modulus, the maximum relaxation time of a lineardistribution (described with a universal shape) and a strain hardening modulusto describe the 'relaxed' state. The paper ends with an assessment of themethodology. Our results of instantaneous modulus measurements are comparedwith those obtained with other physical experiments operating at differenttemporal and length scales.

Identification of the true elastic modulus of high density polyethylene  from tensile tests using an appropriate reduced model of the  elastoviscoplastic behavior