Properties and mechanical behavior of thermoplastic polymers

Thermoplastic polymers and the reinforced materials (formulated and/or filled) on which they are based must have mechanical characteristics that comply with a set of specifications, itself dictated by the conditions of use of the structure to which they contribute.

These characteristics can be expressed in terms of what are traditionally defined as "mechanical properties" – such as modulus of elasticity, apparent threshold of plasticity or fracture quantities – or in terms of models (or model parameters) of behavior – elastic, elastoplastic, etc. – used in dimensioning software.

These two levels of conceptualization are not disjoint, and in both cases, the user implicitly places himself within the framework of continuum mechanics. In this way, he or she seeks to represent the existing relationships between the deformation of a continuous medium, assumed to represent the material, and the stress developed in it.

However, the physical processes involved in deformation depend, of course, on the nature of the material and its microstructure (organization of the material's constituent elements, crystallization and/or morphology of the mixtures). As a result, the representativeness of the measured quantities may depend on the material, and the associated analysis must be adapted to plastic materials and their specific features.

Despite their apparent greater simplicity, and the reassuring existence of identification standards, the notions of "mechanical properties" also make an a priori assumption about the nature of behavior. Thus, identifying a modulus of elasticity (or Young's modulus) presupposes that the elementary deformation processes impart linear elastic behavior to the material, and determining an apparent plasticity threshold (or yield point) only makes full physical sense if the irreversible deformation processes are governed by a stress threshold.

At the same time, the mechanical behavior of plastics is characterized by great diversity. In fact, this family of materials coexists in a large number of behavioral types, often referred to, out of habit, as viscoelastic, viscoplastic, hyperelastic, hardening, softening or damageable. Specialists sometimes give these terms more precise definitions, but their "common" meaning, adopted in this article, is recalled in the glossary (§ 6