ASTM D412 弹性体拉伸测试权威指南

如何按照 ASTM D412 执行弹性体抗拉强度测试

作者:Kayla Thackeray

ASTM D412 是用于测定硫化(热固性)橡胶和热塑性弹性体拉伸性能的最常见的标准。此类化合物可用于制造各种消费品,如轮胎、足球和橡皮筋。此类化合物也可以生产高度专业化材料,如航天飞机上的 O 形环,其必须在极端环境条件下可靠运行。

 

本指南旨在向您介绍 ASTM D412 拉伸测试的基本要素,并概述所需的材料测试设备、软件和试样。但是,计划进行 ASTM D412 测试的任何人员都不应认为参阅本指南后便不必再阅读完整标准


该标准测量什么?

ASTM D412 测量材料在拉伸应变下的弹性,以及材料在测试结束后不再受压时的行为特性。ASTM D412 采用通用测试机(又称拉伸测试机)以 500 ± 50 mm/min 的速度执行,直至试样失效。ASTM D412 可以测量多种不同的拉伸性能,以下几种最为常见:

  • 抗拉强度 – 将试样拉伸至断裂所用的最大拉伸应力。
  • 给定伸长率下的拉伸应力 – 将试样的等截面拉伸至给定伸长率所需的应力。
  • 极限伸长率 – 连续施加拉伸应力期间发生断裂时的伸长率。
  • 拉伸形变 – 试样完成拉伸后且能够以指定方法缩回时保持的延伸长度,表示为原始长度的百分比。

基于 Instron 通用测试系统的 ASTM D412 测试

请注意,ASTM D412 不适用于测试硬质低伸长率弹性体,如硬橡胶。如需测试硬橡胶和硬塑料,请参考 ASTM D638

 ASTM D412 测试报告

拉伸测试系统

大多数 ASTM D412 测试采用带各种配件的台式通用测试机执行,可通过配置按照 ASTM D412 标准正确执行弹性体和橡胶抗拉强度测试。

标准包中包含 3400 系列测试系统、XL 长行程引伸计和一对手动辊夹具。此系统非常适合中小型弹性体制造企业,以及首次购买 D412 系统的企业。6800 系列测试系统带有 AVE 2 非接触式视频引伸计和一对 2712 系列气动侧动夹具及高级散热套件,是一种更先进、更高效的系统。此系统是轮胎制造商和大型跨国弹性体制造商采用的国际标准。对于需要高测试量的客户,Instron 的多工位测试系统可满足这一需求,它能够同时测试多达五种试样。下图为 ASTM D412 测试设置示例。

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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