ASTM D412:弹性体拉伸试验权威指南

如何依照ASTM D412对弹性体实施拉伸强度试验

作者 Kayla Thackeray

ASTM D412是测定硫化(热固性)橡胶和热塑性弹性体拉伸性能时最常用的标准。该系列产品中的复合物可用于制造丰富多样的消费品,如轮胎、足球、橡皮圈等。该系列还能打造一些必须在极端环境条件下有可靠表现的高度专业化材料,比如航天飞机上的O型圈。

ASTM D412 夹具和引伸计

本指南旨在帮助您了解ASTM D412拉伸试验的基本信息,并概括介绍所需要的材料试验设备、软件及样品。但对于打算实施ASTM D412试验的人来说,不应认为本指南足以代替阅读全部标准。.

测试哪些属性?

ASTM D412可测试某一材料在拉伸应变下的的弹性,以及试验结束后材料不再受力时的特性。ASTM D412试验在一台万能试验机(也称为拉伸试验机)上进行,速度为500±50 mm/分钟,直至样品失效。尽管ASTM D412能够测试多种不同的拉伸特性,但最常见的是下面几种:

  • 拉伸强度——拉断一件样品所需要施加的最大拉应力。
  • 既定延伸率下的拉应力——将一件试样的均匀横截面拉伸至既定延伸率所需要的应力。
  • 极限延伸率——施加持续拉应力情况下发生断裂时的延伸率。
  • 拉伸永久变形——一件样品经过拉伸并待其以指定方式恢复后的伸长情况,以原长度百分比表示。

请注意,ASTM D412并不适用于测试硬质的低延伸率弹性体,如硬橡胶。如果是测试硬橡胶和硬塑料,应参照ASTM D638

 ASTM D412试验报告

拉伸试验系统

大多数ASTM D412试验都是在桌面式万能试验机上完成的,同时还要配备各种所需配件,以保证对弹性体和橡胶正确实施ASTM D412拉伸强度试验。

Bluehill Universal ASTM D412预设试验

一个标准产品包将包含一套3300系列试验系统、一个XL长行程引伸计和一对手动辊式夹具。对于中小规模的弹性体制造企业或首次购买D412系统的企业来说,该系统是绝佳的选择。

英斯特朗3300系列试验机

一个更加高级和有效率的系统包括5900系列试验系统是更先进、更高效的系统。它带有一个AVE 2非接触式视频引伸计和一对2712系列平推式气动夹具(含高级气动工具包)。对于轮胎厂商和大型跨国弹性体厂商而言,该系统则属世界标准。

 5900系列试验系统(带AVE2视频引伸计和气动夹具)

对于有大批量要求的用户,英斯特朗提供了多工位试验系统,它能同时测试多达5个样品。

多工位试验机

试样

根据试样类型的不同,ASTM D412介绍了两种试验方法。请注意,这两种试验方法会得到不同的结果。A方法用于“哑铃形”和“狗骨形”试样,而B方法用于环形试样。A方法比B方法更为常用。要获得狗骨形试样,需要在注塑成型板上进行冲切。板子挤压方向很重要,因为它会在材料上产生细微的颗粒纹理。要想获得最精准的结果,应顶着颗粒纹理方向切割一半的试样,而另一半则顺着颗粒纹理方向切。

试样测试

对于ASTM D412试验,总共有六种狗骨形试样和两种环形试样是允许的。最常见的测试形状是冲切C狗骨形试样,它的要求是总长度为115 mm(标距长度25 mm),宽度6 mm,厚度3 mm。试样的厚度必须测三次,然后取中间值。

为确保符合标准,所有用于ASTM D412试验的试样都要依照ASTM D3767标准采用测微计测试。Bluehill® Universal中的“试样尺寸自动测试装置”使得操作员可以将最多两个测微计或测试装置连接至电脑,并将数据直接输入到软件中。这样可避免发生操作员人为的数据输入错误并提高效率。

试样对中

试样必须保持垂直于夹面,而无歪斜,以确保正确测试。试样错位可能导致试验结果出现较大的差异,因此应注意确保每一次试验都能始终如一地做到试样对准。

解决未对中的一个方法是使用宽度与试样接近的夹面,这样更便于进行目测调节。最简便的防未对中的方法是使用一个直接安接到夹具本体上的试样对准装置。它其实就是一个简便的杆子,能提供可调节的止动点,让操作员轻松看到试样对准情况。

试样对中

夹具

当我们为了准备试验而将塑料试样固定到夹具上后,就很有可能产生向试样施加的压缩力。如果得不到正确处理,这些力(尽管很微小)可能会影响试验结果。试样在固定夹持后是否保持平衡很重要,若不平衡会造成结果的偏差。用户可以对Bluehill Universal材料测试软件进行设置,以便在多个试样之间对力的大小进行规范并去除任何松弛力或压缩力,确保在试样之间取得一致的结果。在5900系列万能试验机上有试样保护功能可供选用,它能防止试样或系统在试验装设阶段受到损坏(在设定试验的操作极限以前)。开启后,试样保护功能会自动调整“横梁”位置,将不需要的力控制在一定限度内。

随着弹性体样品拉伸,其厚度会减小。为此,ASTM D412推荐采用能自动夹紧的夹具。在不同夹面中保证一致而均匀的压力能确保试验过程中,变薄的样品不会从夹具中滑出。带有锯齿面的气动平推夹具对于测试弹性体来说非常有效。气动平推夹具的进气压力就可自动调整,即使试样厚度在试验过程中出现较大变动,夹紧力也能维持不变。另一种有效方法是选用自夹紧的辊式夹具,试验过程中,它通过一个弹簧对试样施加恒定压力。如果采用的是ASTM D12 B方法,可借助我们的O型圈工装对O型圈试样进行测试。通过上/下工装上的旋转轴,这些夹具可防止出现局部应力集中。

夹具

引伸计

尽管标准中并未要求使用引伸计,但建议使用应变测量测试,以获得最精准且可重复的结果。由于试样伸长超出标距长度范围,基于横梁运行的应变测试可能不会太精准。XL长行程引伸计针对弹性材料而设计,还可在试样断裂中使用。刀刃上的可调节的夹紧力有助于避免试样过早失效。这款引伸计能通过一英寸标距长度测试高达3,000%的应变。

长行程引伸计

非接触式引伸计,如标准或高级视频引伸计2(SVE 2AVE 2),对于弹性材料而言是最理想选择。它尤其适合那些容易受损或需要环境试验箱的试样。此外,非接触式视频引伸计还能减少因操作员人为干扰而出现的结果差异情况,并帮助有大批量需求的实验室提高效率。

自动视频引伸计

环境试验箱

依照ASTM D412测试的弹性体往往旨在日后于非常温条件下使用。由于常温条件会给弹性体的拉伸性能带来很大影响,因此必须注意确保试验条件模拟最终用户的预期应用条件。试验速度、温度、湿度、试样尺寸以及试验前条件都对试验结果有着显著的影响。为保证试验获得有效数据,必须对这些因素予以控制。应在环境试验箱(允许进行加热或冷却(LN2或CO2)控制)进行符合ASTM D412标准的测试,确保这些因素能模拟材料的最终应用状况。

英斯特朗600系列环境试验箱使操作员能够在整个试验期间监测试验箱内的温度。此外,可在Bluehill Universal内设置均热时间和温度,以确保试验开始前,所有试样均得到合理处理。

 环境试验箱

测试量

对于想要增加测试量的实验室,可在系统设置方面作多处改动。试样尺寸自动测试装置、气动夹具以及试样对准装置都能减少操作员的手动输入量,从而提高试验效率。但是,ASTM D412的试验时间需视试样的预期延伸率而定,无法一味地缩减。对于延展性好的材料,试验时长会占据整个试验周期的大部分时间。这种情况下,多工位试验架可提供最佳处理量,因为操作员能同时开展多达5次试验。

此外,还可选择全自动试验系统,它集成了试样测试、试样加载、测试及移除等功能。系统可运行数小时,而无需操作员干预。而且还有助于减少因人为错误而产生的差异情况。

测试量


相关内容


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.

 
 

Related Content

6800 Series Premier Testing Systems Brochure

Instron 6800 Series Universal Testing Systems provide unparalleled accuracy and reliability. Built on a patent-pending Operator Protect system architecture with an all-new Smart-Close Air Kit and Collision Mitigation features, the 6800 Series makes materials testing simpler, smarter, and safer than ever before.

Bluehill Universal Brochure

Bluehill Universal Software is built from the ground-up for touch interaction and an intuitive user experience. Discover simpler and smarter testing with features such as pre-loaded test methods, QuickTest in seconds, enhanced data exporting: and Instron Connect – a new feature that provides a direct communication link to Service. Users of previous versions of software such as Bluehill 2 and Bluehill 3 can easily upgrade to the newest version of Bluehill.

3400 Series - Affordable Testing Solutions

Instron 3400 Series Universal Testing Systems for Tensile, Compression, Bend, and other material property tests.

2603-080 Long Travel Extensometer

The Instron® model XL (long travel) extensometer is a precise device for measuring strain in highly extensible materials such as elastomers, semi-rigid plastics and films.

2712-04X Series Pneumatic Side-Action Grips

Pneumatic side-action grips offer a versatile gripping solution for a wide range of materials. Rated capacities between 1 and 10 kN.

2713-007 Series Self-Tightening Roller Grips

The self-tightening grips are primarily designed for the precision testing of elastomeric and other flexible materials exhibiting a large reduction in cross-section area under load.