Mechanical tests are performed generally in order to describe the behavior of a material which is subject to deformation due to application of forces.

This concept, simple in principle, give raise to a huge number of different tests and measurements, depending on many variables.

Speaking from an experimental point of view, a first distinction must be done between the tests with 'slow' deformation rate(like creep, tensile, flexions, etc.) and the test with 'fast' deformation rate (impacts, for example).

A second distinction we can do between the 'universal type' tests (for example performed with a tensile dynamometer) and the 'specific type' tests (for example we can think to a specific apparatus for testing resistance of a shoe).

Generally speaking WE have to choose what we need.

NOTE of general character: in order to obtain numbers on which discuss and that have to be compared, a special attention must be taken in the preparation of the specimen.

This aspect represent from 50% to 99% of the test and is often underevaluated.

-          Tensile, compression and flexural tests

-          Preparation and conditioning of the specimens

A lot of international standard are present in this field; generally speaking we can distinguish two possibilities: specimens which can be prepared simply by cold pressing with a metal preform (plasticized specimens, low thicknes specimens) and specimens which have to be prepared with hot press and cutting or with injection moulding.

The way for preparing the specimens is different and depends on the material. The experience is a great advantage in this phase.

The specimen MUST be conditioned at least 24 hours in standard condition before the test.

-          Apparatus

For these purposes we need a dynamometer prepared for our measurement needs.

A dynamometer is constituted of (at least):

1) The frame, which must be overdimensioned in respect to our needs.

2) A load cell which measure the forces, with adequate dimension and precision

3) A measure of the deformation (directly on the frame or, better, with extensometers on the specimen)

4) An elaboration system (PC)

and, obviously: 5) a micrometer in order to measure the thickness of the specimens.

Other accessories can be added, which can be specific (for flexion test, for measures at different temperatures, for compression tests, different load cells, and so on)

-          Measure

What we measure:

                Elastic modulus

                Yield stress

                Yield elongation              

                Stress at break

                Elongation at break

                .......

The measure MUST be followed directly, in order to see all what happens: a broken specimen can be a font of important informations and a good stereo-microscope can help you a lot.

-          Creep

We will describe briefly these type of tests, because, generally speaking, before to think at a creep test, we have to be sufficiently 'specialist'.

Sometimes there is the need to measure the deformation of a material which is subject to a constant stress for very long times: creep test go in this direction.

These tests are however time expensive test, sometimes years long, and obviously with proportionally high costs: they request an absolute knowledge of the variables which play a significant role: a tensile test we can reply if there is something wrong but the replying of a creep test give a retardation of months or more in the response.

-          Preparation and conditioning of the specimens

These variables are FUNDAMENTAL for a creep test. The considerations are similar to that for a tensile test.

-          Apparatus

Specific apparatus are possible for a creep test. The conditioning in temperature and humidity is very important for the entire duration of the test (sometimes years!)

-          Measurements

The specimens are stressed with a constant stress in time (we can think at a fixed weight on a specimen). What is measured is the deformation as a function of the time.

From these measurements we can have a lot of informations on the characteristics of the materials.

-          Impact tests

The name is auto-explaining. With these tests we want to understand how the material reacts when is subjected to a hidden stress for a very short time (some milliseconds usually): we are in a range completely opposite to that of the creep tests. The international standards are usually used in these tests.

-          Preparation and conditioning of the specimens

Adding to the same consideration which are needed for the tensile tests, one of the fundamental aspect for the impact tests is the NOTCHING of the specimen, where needed. This operation must be done with great care. In this case too a stereomicroscope is needed. If the notching is not needed, the cure for the external form of the specimens is fundamental before to do the tests. The conditioning of the specimens is fundamental on the same level.

-Apparatus

The machines are different, depending on the needs:

- impact tensile tests with pendulum

- 'Charpy' pendulum

- 'Izod' pendulum

- impact dards

- specific tests

Generally, is a good thing to adequate our measurements to the international standards for the material under evaluation.

-          Measurements

Usually the specimen is subjected to impact with a known energy and the energy which is absorbed from the specimen during the impact is measured.

The specimen is analyzed, which may break completely or partially or simply deform. Sometime the term 'resilience' is used in these tests, sometime not properly used.

The analysis of the specimen after beeing subjected to impact can give useful information on the type of the break and on the propagation of the fracture.

-          Abrasion tests

The resistance to abrasion is a complex phenomenon which is linked with the nature of the surface of the material; due to this complexity, the tests give often comparative values, rarely absolute values. There are however apparatus which respond to specific standards and which allow to obtain sufficiently reproducible values.

-          Preparation and conditioning of the specimens

As for all the mechanical tests, this point is fundamental. Particularly, for the abrasion, cure must be taken in the aspect of the surface of the specimens.

-          Apparatus

There are apparatus responding to international standards, characterized from the fact that the abrasion element must have the same efficiency during the time. There are machine with specimens which rotates under calibrated abrasion elements and machines in which the abrasion element is automatically periodically substituted.

-          Measurements

Usually is measured the loss in weight of the specimen which is subjected to the test for a definite time. With microscope it is possible to analyze the surface before and after the abrasion cycles.

There are no standard samples for calibration; the preferred conditions are:

a) abrasion element continously substituted and which operates with identical cycles.

b) periodic control of the abrasion efficiency with a known material and with identical specimens.

-          Fatigue tests

-          Preparation and conditioning of the specimens

The fatigue tests are specific for every application. Usually the time request is high (sometimes weeks), so the consideration on the correct preparation and conditioning of the specimens are very important.

Adding to this importance, there is the fact that, if no good specimen are prepared, it will be difficult or impossible to know it at the end, so we will have lost a lot of time, without results.

If possible, it is better to characterize mechanically, for example with tensile or impact test, some specimen before to carry out fatigue tests, in order to verify eventual anomalies.

-          Apparatus

The apparatus are very different and the conditions can be very different.

It is not useful to give here a list: for every sector there is a indication for a specific fatigue test.

Generally speaking, considered the fact that these apparatus have mechanical parts in movement for a long time, the stability and the reliability of the apparatus must be granted.

If you will need to test the material at different conditions (temperatures, humidities, and so on), before to choose an apparatus, please verify that it will function with the needed conditions.

-          Measurements

The fatigue test is not exactly a measurement: we must set which variables are interesting (mechanical, optical, chemical, and so on) and measure them before and after the test.

It is a must to carry out the test on a large number of specimens.

-          Hardness measurements

-          Preparation and conditioning of the specimens

It represent the 98% of the test. Do not measure the hardness if the specimens are not good prepared and conditioned. The measure is usually very simple and give a number to compare.

-          Apparatus

A specific apparatus must be chosen for your needs and for your range of hardness. Usually the costs of the apparatus are not very high and can give a lot of information if utilized “cum grano salis”. The calibration of the apparatus is of fundamental importance.

-          Measurements

There are different 'grades' of hardness: Rockwell, Shore, IRHD, and so on.

You have to consider your materials and choose consequently: the measure is normally simple and takes few minutes.

-          Other mechanical tests

There are other different mechanical characterization, like the resistance and the behaviour to explosion, to perforation, to scratching, and so on. The term 'mechanical tests' include many tests, sometimes specific for the application.

Dr. Maurizio Veronelli - RDLab137 srl

Specialist in Polymer Science