Mechanical testing of plastics - Instantaneous characteristics

With annual growth rates higher than the industry average, plastics (or polymers) have seen their field of application expand, particularly in environments where reliability and cost are paramount. The current trend is to integrate high-performance design and quality assurance tools into production circuits, even going as far as computer-aided engineering (CAE), which, thanks to modeling, reduces or even eliminates the need for prototype parts to go through the test bench.

However, these tools can only be fully effective if the characterization and behavior data for the materials used to produce the parts are available, and fit in perfectly with the intended conditions of application, so that they can be fed into computerized databases to help design structures or serve as a reference in a production control plan.

In this modern industrial context of part design, solid-state mechanical testing takes on even greater importance, as it defines the relevance of the data on which manufacturers rely.

These tests specify all the experimental means used to measure certain properties under well-defined mechanical stress conditions, with, if necessary, the combined action of environmental parameters (temperature, humidity, liquid, gas). Their aim is to determine :

• complete mechanical behaviour laws for materials, general functions linking a cause (e.g. a field of forces) to an effect (e.g. a field of deformations) and presenting certain thresholds beyond which the material no longer retains its functionality;

• elementary mechanical characteristics, singular points of behavior laws (moduli, elongations and stresses at critical thresholds such as nonlinearities, flow, fracture).

This is in fact a characterization principle that is much more complex in experimental reality, particularly for polymers, where the law of loading as a function of time must always be perfectly specified due to their pronounced viscoelastic character. It should be borne in mind that with certain test methods borrowed from metals, the time factor is not explicitly taken into account, which can lead to erroneous interpretations.

Depending on the case and the objective being pursued, the following can be achieved (figure 1 ):

• instant tests ;

• long-term tests [AM 3 511] ;

• fracture mechanics tests...