MECHANICAL PROPERTIES AND CHARACTERIZATION OF PLASTIC MATERIALS

Why is it important to study the mechanical properties of a material?

The mechanical properties of a material are important for studying its behavior both when subjected to external stresses and during its duration in normal use; it is therefore important to study their characteristics based on use, therefore whether nylon should be applied to climbing ropes or polyethylene from shopping bags should be used.

The mechanical properties must therefore be studied above all to ensure the performance, safety and durability of a component and therefore also to predict/prevent the failure of a material. A failure is any change in the material that causes the deterioration or loss of its structural/functional capabilities.

To understand HOW and WHEN failure occurs it is necessary to determine the mechanical properties of a material through specific tests, for example by studying the deformation or breakage following the application of a load.

Failure can appear on the specimen as deformation or breakage.

The stresses leading to deformation can be time dependent, thus resulting in elastic or plastic deformation, or time dependent such as creep or relaxation.

Breaks can be caused by the application of a static load (brittle or ductile breaks) or dynamic load, therefore by fatigue.

What tool is used?

When we talk about the mechanical properties of a plastic material, those characteristics that are studied through the use of a particular instrument are therefore generally taken into consideration: the dynamometer. The dynamometer is an instrument which, through the use of a load cell, a strain gauge and the movement of the clamps, allows you to traction or compress a sample to study its characteristics.

The tests that are generally performed are therefore mechanical characterization tests representing the intrinsic behavior of the material when subjected to external stresses.

TRACTION

Among the most important tests used to study the properties and behavior of the material is the tensile test, in which a stress is applied to a standardized specimen (bar or cane bone) which is pulled with an ever-increasing force based on to the response that each individual material gives. From this derives a stress curve with respect to the deformation imposed by the traction itself. From here it is possible to obtain a variety of information in mechanical terms including tensile strength, final elongation of the specimen and its elastic modulus.

Elongational stress: F is the force applied perpendicularly to the section of the specimen (A0), i.e. thickness multiplied by width.

Elongation strain: the ratio between ΔL which is the elongation and the initial useful section of the specimen (l0).

Modulus of elasticity or Young’s modulus: expresses the ratio between stress and deformation in the case of uniaxial loading conditions and in the case of “elastic” behavior of the material. It is defined as the ratio between the applied stress and the resulting deformation.

The specimens used are generally standardised according to the method, for example for the ISO 527-1/2 standard dog bone specimens with defined characteristics are used:

BENDING

Another test used to study mechanical properties is bending. The stress that causes it is called bending moment. For simplicity, we can say that a body is subject to a bending stress when, due to the constraints to which it is subjected, it reacts, opposing, a system of forces applied to it that would tend to make it rotate around its own point.

RESILIENCE TEST

The third and final example we will discuss is the Charpy or Izod type resilience or impact test.

This is an impact test, or a dynamic test, in which the material reacts to a fast action and is excellent for studying its performance in “real life” conditions, as an object may frequently experience collisions, falls or impacts of various kinds. The load, during the impact, is applied impulsively, in the order of milliseconds, causing dynamic effects, such as inertial forces, due to the acceleration of the masses and viscoelastic forces proportional to the deformation speed.

Resilience is the resistance of the material to dynamic stresses (shocks). It measures the energy needed to break a sample subjected to an extremely fast stress (0.1 -200m/s)

Materials that have a high resilience are called tough, while those that have a low resilience are called brittle.

So be careful, toughness is not resilience!

We have addressed a hint of the most important mechanical and dynamometric tests, but in our laboratories we can perform these tests and many others, visit our website on laboratory tests!

For further information and insights you can also contact us or sign up for our training course “MECHANICAL CHARACTERISATION OF PLASTIC MATERIALS: TENSILE, COMPRESSION, FLEXURE, ADHESION AND RESILIENCE” in the TRAINING section!

A special thank you goes to Dott.ssa Giulia Atzeni, , part of the Plastlab Team as a laboratory technician, which raises our awareness on this topic so as to provide us with new content for these #laboratory pills!

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