Guida definitiva ai test di trazione ASTM D412 degli elastomeri

Come eseguire un test di resistenza alla trazione sugli elastomeri secondo la norma ASTM D412 

Scritto da Kayla Thackeray

ASTM D412 è la norma più comune per determinare le proprietà a trazione della gomma vulcanizzata (termoindurente) e degli elastomeri termoplastici. I composti di questa famiglia sono usati per creare una vasta gamma di beni di consumo come pneumatici, palloni da calcio ed elastici. Questa famiglia produce anche materiali altamente specializzati, come gli O-ring delle navette spaziali che devono funzionare in modo affidabile in condizioni ambientali estreme.

 

Questa guida è progettata per introdurvi agli elementi di base di un test di trazione ASTM D412 e fornirà una panoramica delle apparecchiature di test dei materiali, del software e dei campioni di trazione necessari. Tuttavia, chiunque abbia intenzione di condurre test ASTM D412 non dovrebbe considerare questa guida un sostituto adeguato per leggere la norma completa.


Cosa misura?

ASTM D412 misura l' elasticità di un materiale mentre è sotto sollecitazione di trazione, così come il suo comportamento dopo il test quando il materiale non è più sotto stress. ASTM D412 è condotto su una macchina di test universale (chiamata anche macchina per test di trazione) a una velocità di 500 ± 50 mm/min finché il provino non cede. Anche se ASTM D412 misura molte diverse proprietà a trazione, le seguenti sono le più comuni:

  • Resistenza alla trazione: lo stress di trazione massima applicato nello stiramento di un provino fino alla rottura.
  • Stress di trazione a un determinato allungamento: lo stress richiesto per allungare la sezione trasversale uniforme di un provino sino a un determinato allungamento.
  • Allungamento finale: l'allungamento al quale si verifica la rottura nell'applicazione di uno stress di trazione continua.
  • Set di trazione: l'estensione che rimane dopo che un provino è stato allungato e lasciato ritrarsi in un determinato modo espresso come percentuale della lunghezza originale.

Test ASTM D412 su sistemi di test universali Instron

Si prega di notare che ASTM D412 non si applica ai test su elastomeri duri e a basso allungamento come l'ebanite. Chi ha bisogno di testare l'ebanite e le materie plastiche dure deve fare riferimento ad ASTM D638.

 Rapporto sul test ASTM D412

Sistema di test di trazione

La maggior parte dei test ASTM D412 viene eseguita su una macchina di test universale da tavolo con una varietà di accessori che possono essere configurati per eseguire correttamente un test di resistenza alla trazione ASTM D412 per elastomeri e gomma.

Un pacchetto standard deve includere un sistema di test della serie 3400, un estensimetro a corsa lunga XL e un paio di morse a rullo manuali. Questo sistema è eccellente per aziende di produzione di elastomeri di piccole e medie dimensioni o per coloro che acquistano il loro primo sistema D412. Un sistema più avanzato ed efficiente è il sistema di test Serie 6800 con un estensimetro video senza contatto AVE 2 e una coppia di prese pneumatiche ad azione laterale della Serie 2712 con un kit di aria avanzato. Questo sistema rappresenta la norma mondiale per i produttori di pneumatici e le grandi multinazionali di elastomeri. Per i clienti che richiedono un’alta produttività, i sistemi di test multi stazioneInstron sono la risposta e possono testare fino a cinque campioni contemporaneamente. Un esempio di impostazione del test ASTM D412 è illustrato qui sotto.

ASTM D412 test setup

ASTM D412 Test Setup

1. Instron 6800

2. Bluehill Universal Dashboard (2490-696)

3. 2580 Series Load Cell

4. 5 kN Pneumatic Side-Action Grips (2712-045)

5. 2603-080 Long Travel Extensometer

 

 

 

Software

Bluehill Universal comes with a free application module upon purchase. Inside each application module are pre-configured test methods and supporting documentation to assist with testing to various standards, including ASTM D412 for those who choose the Elastomer module.

Bluehill Universal software

Specimens

ASTM D412 describes two methods for testing, depending on specimen type. Please be aware that these two testing methods do not produce identical results. Test Method A makes use of “dumbbell” or “dogbone” shaped specimens, while test Method B is designed for ring-shaped specimens. Test Method A is more commonly used than Test Method B. Dogbone specimens must be die cut from an injection molded sheet or plaque. The direction in which the sheet is extruded is important as it produces a subtle grain pattern in the material. For consistency, all specimens should be cut with the lengthwise portion parallel to the grain.

Specimen Measurement

There are six allowable types of dogbone specimens for ASTM D412 testing and two allowable types of cut ring specimens. The most commonly tested shape is a die cut C dogbone specimen, which calls for an overall length of 115 mm (the gauge length being 25 mm), width of 6 mm, and thickness of 3 mm. Thickness of the specimen must be measured three times, with the median value being the accepted value.

To ensure conformance with the standard, all specimens used for ASTM D412 testing must be measured with a micrometer in accordance with ASTM D3767. The Automatic Specimen Measuring Device feature in Bluehill® Universal allows operators to connect up to two micrometers or measurement devices to the computer and input the data directly into the software. This eliminates the chances of operator input errors and increases efficiency.

Specimen Alignment

In order to test properly, specimens must be held perpendicular to the jaw faces and not tilted on an angle. Specimen misalignment can cause major variations in test results, and proper care should be taken to ensure that the specimens are all aligned consistently for each test.

One way of addressing misalignment is by using a jaw face that is close to the same width as the specimen, making it relatively easy to visually adjust alignment. The easiest way to prevent misalignment is to use a specimen alignment device which mounts directly onto the grip bodies. This is a simple bar that provides an adjustable stopping point so operators can easily see that their specimen has been aligned correctly.

Grips

Once grips are tightened onto the elastomeric specimens in preparation for running a test, unwanted compressive forces are frequently applied. These forces, although minute, can interfere with test results if not treated properly. It is important that they not be balanced after the specimen is inserted, as this will cause an offset in results. Bluehill Universal materials testing software can be programmed to normalize forces across multiple specimens and remove any slack or compressive force, ensuring consistent results between specimens. Specimen Protect, which is available on the 6800 Series universal testing machines, is designed to prevent damage to the specimen or system during the set-up phase of a test, before a test’s operational limits are defined. When turned on, Specimen Protect automatically adjusts the crosshead to keep any unwanted forces under a certain limit.


As elastomer samples stretch, their thickness decreases. For this reason, ASTM D412 recommends grips that tighten automatically. A consistent and uniform pressure across the jaw faces ensures that thinning samples do not slip out of the grips during testing. Side action pneumatic grips with serrated faces are highly effective for testing elastomers. With pneumatic grips, the inlet air pressure adjusts automatically, enabling the gripping force to remain constant even if the specimen thickness changes significantly during a test. Another effective option is to use self-tightening roller grips, which use a spring to put constant pressure on the sample during testing. For those using ASTM D12 test Method B, O-ring specimens can be tested with our O-Ring Fixture. These grips prevent local stress concentrations by using rotating shafts on the upper and lower fixtures.

10 kN Model | 2712-046
2613-002
5 kN Model | 2713-002

Extensometer

Although extensometers are not required by the standard, a strain measurement is recommended for the most accurate and repeatable results. Strain measurement based on crosshead travel can be less accurate due to elongation of the specimen outside the gauge length. The XL Long Travel extensometers are designed to work for elastomeric materials and can also be used through specimen break. The adjustable clamping force on the knife edges reduces premature failure of the specimen. This extensometer can measure strain up to 3,000% from a gauge length of only one inch.

Long-Travel Extensometer

A non-contacting extensometer like the Standard or Advanced Video Extensometer 2 (SVE 2 or AVE 2) is the most ideal for elastomeric materials. It is especially suited for specimens that are fragile or require an environmental chamber for testing. Non-contacting video extensometers offer the added benefit of reducing results variance due to operator interference and increasing efficiency in labs with high throughput needs.

Automatic Video Extensometer

Environmental Chamber

Elastomers tested under ASTM D412 are often destined for future use under non-ambient conditions. Because ambient conditions have a great impact on the tensile properties of elastomers, care must be taken to ensure that test conditions mimic those of the intended end-use application. Test speed, temperature, humidity, specimen dimensions, and pretest conditions all have notable effects on test results and must be controlled in order for the test to produce useful data. To ensure that these elements simulate the material's end-use application, ASTM D412 is frequently performed inside an environmental chamber where heating or cooling (LN2 or CO2) can be used.

Instron's 600 Series environmental chambers enable operators to monitor the temperature inside the chamber throughout the duration of the test. In addition, soaking time and temperature can be set up within Bluehill Universal to ensure that all specimens are conditioned appropriately before the start of the test.

 Environmental Chamber

Throughput

For labs looking to increase their throughput, several modifications to the system setup can be made. Automatic specimen measuring devices, pneumatic grips, and specimen alignment devices all increase test efficiency by reducing the amount of manual input needed from the test operator. However, testing time for ASTM D412 depends on the expected elongation of the specimen and cannot be minimized. For materials with high ductility, the duration of the test consumes the majority of the overall test cycle. In these cases, multi-station test frames may provide the best throughput because the operator can run up to 5 tests simultaneously. 

Fully automated test systems are also available and are designed to incorporate specimen measurement, specimen loading, testing, and removal. These systems can run for hours without requiring any operator interaction. In addition, these systems help reduce variability due to human error.

 
 

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