Mechanical properties and mechanical behaviour of thermoplastic polymers

Thermoplastic polymers and the plastics (polymers once reinforced, formulated and/or filled) on which they are based, like all structural materials, must have mechanical characteristics that comply with a set of specifications, which in turn are dictated by the structure's conditions of use.

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, elasto-plastic etc. – used in structural design software.

These two levels of conceptualization are not disjoint, and in both cases, the user implicitly places himself within the framework of continuum mechanics. He is thus seeking to represent the relationship 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 measured quantities may depend on the material, and the associated analysis should be adapted to plastics 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 elementary deformation processes confer linear elastic behavior on the material, and determining an apparent plasticity threshold (or yield point) only makes full physical sense if 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