Mechanical Properties Laboratory

Tensile properties

The tensile test is, probably, the most extended method for the mechanical characterization of materials. The fundamentals of the test is to tensile the specimen from its ends until break continuously recording the applied force and the elongation produced in the sample, determining the stress-strain curve characteristic of each material. The basic parameters determined in this test are the elastic modulus, yield stress and yield strain (if exists) and tensile strength and elongation.

There are three equipments in the LATEP facilities for tensile testing, two of which are equipped with temperature chambers for tests between -100 and 360 °C.

Strain Hardening (SH) test

Through the SH hardening modulus determination on polyethylene resins used for pipe application is possible (in a quick way and using a low amount of material) to estimate the Slow Crack Growth (SCG) resistance of the resin and, in this way, to rank materials according to its SCG resistance. The test is carried out using a tensile machine equipped with a temperature chamber and a special video-extensometer for an accurate measurement of the strain process during the tensile test. LATEP carries out SH test following the international ISO/PRF 18488 standard and also other alternative methodologies developed by the laboratory during the last years.

LATEP is accredited according to UNE-EN ISO/IEC 17025:2017 for testing "Determination of the Strain Hardening Modulus in relation to slow crack growth (SH)" ISO 18488:15 (accreditation no. 1486/LE2715).

Dynamic fatigue tests - Cracked Round Bar Test

Material fatigue is a phenomenon that occurs under cyclic dynamic loads, causing the material to collapse below its static load limit. The process of rupture due to mechanical fatigue develops without apparent plastic deformation, presenting the fracture a characteristic appearance, with an area of ​​progressive rupture (cracking) and another of sudden rupture (fragility). To determine the resistance of materials under the action of dynamic loads, they are subjected to repeated constant or variable forces of specified magnitude. The cycles or alternations of efforts that suffers the material are counted until failure of the specimen, therefore, the fatigue test is classified as a dynamic, destructive test.

LATEP has an INSTRON Electropuls E3000 equipment to perform high-frequency dynamic fatigue tests and low-speed static tests. It is also valid for the Crack Round Bar Test, according to ISO 18489.

Flexural properties

The flexural test is mainly used as method for the stiffness determination. This test is almost as usual in hard polymeric materials as the tensile test, and has the advantages of simplifying the specimen machining and avoiding the problems associated with the use of clamps. The most important parameter obtained in this test is the flexural modulus.

The LATEP have three universal machines for conducting both tensile and flexural tests, one of them equipped with temperature chamber for performing tests in the range between -100 and 360 °C.

Impact resistance

In the Impact resistance test a high velocity stress is applied to the specimen. These tests are very useful because impacts are common events during the life in service of materials; hence different methodologies have been developed through the years to assess the resistance of the materials to impact.

The LATEP carries out two different impact tests, both based on a pendulum: Charpy impact test and Izod impact test. In both cases it is possible to use plain or notched specimens with prismatic geometry, determining the absorbed energy during the break of the material.

LATEP has an instrumented Impact pendulum INSTRON/CEAST of last generation for testing from -70 to 130 °C.

Dynamic Mechanical Analysis of Polymers

Dynamic Mechanical Analysis (DMA) is one of the most used tools to study the viscoelastic properties of polymers by measuring its elastic modulus and its damping after applying a sinusoidal strain over the tested specimen.

Elastic modulus can be determined because of the relation between the oscillation amplitude and the applied force. Meanwhile, the damping coefficient of the polymer is determined by the gap between the force and the displacement. The variation in the viscoelastic properties of the materials with temperature allows identifying the different transitions of the material.

LATEP has a Dynamic Mechanical Analyzer DMA Q800 (TA Instruments) with working capabilities in a frequency range of 0.01 to 200 Hz and temperatures from -150 to 600 °C.

Hardness tests

The hardness of a material is the resistance to be penetrated by a harder body (indenter) with a specific geometry, its value depends on the elastic modulus and viscoelastic properties of the material.

Depending on the type of indenter, load used and the speed of application of the load, the hardness test has different names. LATEP conducts Shore (scales A and D) hardness tests. Shore hardness, the most common in polymeric materials, an indenter with truncated cone geometry is used, and the applied forces are 10 N in Shore A scale, and 50 N in the Shore D scale used for harder polymeric materials.