ISO 6892 Tensile Testing of Metallic Materials

 

One of the greatest evolutions of ISO 6892-1 has been largely related to test control methods, which can pose a significant challenge in metals tensile testing. This evolution was mainly led by the work conducted as part of the TENSTAND project, where it was identified that the test rates between different machines testing to the same standard will yield different results due to materials' strain rate sensitivity. The 2009 version introduced testing rates based on strain rate (Method A), which became the preferred method. The traditional test method inherited from EN10002:2001 was based on maintaining a stress rate during the elastic region, which was needed for manually-operated machines. This original method was also retained, and became 'Method B' in the 2009 version of the standard.

The introduction of Method A caused confusion, as many users thought Method A was only achievable by using equipment capable of closed-loop strain control, where instead it could also be accomplished by using a consistent crosshead speed. To clarify this situation, ISO 6892-1 was revised again in the second edition, ISO 6892-1:2016. The 2016 version includes three test methods, A1, A2, and B, where the former Method A is split into two different clearly-defined test methods, Method A1 (closed-loop strain control) and Method A2 (consistent crosshead speed) while Method B continues to be based on maintaining a stress rate during the elastic region. A note was added to Method B to clarify the range of the test where a stress control must be maintained. The video below discusses Method A1 in greater detail.

ISO 6892-1 measures the tensile properties of metallic materials in any form at an ambient temperature. Tests carried out under controlled conditions must be made at a temperature of 23 degrees Celsius plus or minus 5 degrees. For testing at elevated temperatures, please refer to ISO 6892-2. ISO 6892-1 measures many different tensile properties, with the following being the most common:

Yield Strength - The stress at which a material becomes permanently deformed. ISO 6892-1 determines both upper and lower yield strength: depending on the yielding phenomena, ISO 6892-1 specifies both upper and lower yield strength requirements for discontinuously yielding material and the offset yield method for continuously-yielding material. 

Yield Point Elongation - Only suitable for discontinuously yielding material, the Yield Point Elongation is the difference between the elongation of the specimen at the start and at the finish of discontinuous yielding (the area in which an increase in strain occurs without an increase in stress).

Tensile Strength - The maximum force or stress that a material is capable of sustaining during a tensile test.

Reduction of Area - A measurement of the ductility of a material. This is the difference between the original cross sectional area of a specimen and the area of its smallest cross section after testing, usually expressed as a percentage decrease in original cross section. The smallest cross section can be measured at or after fracture.

ISO 6892-1 accommodates a wide variety of specimen types due to the huge range of applications that metallic materials are used for. Primary specimen types include sheets, plates, wires, bars, and tubes. Full details on specimen preparation and measuring can be found in the annexes:

Annex B: Types of test pieces to be used for thin products: sheets, strips, and flats between 0, 1, and 3 mm thick.

Annex C: Types of test pieces to be used for wire, bars, and sections with a diameter or thickness of less than 4 mm.

Annex D: Types of test pieces to be used for sheets and flats of thickness equal to or greater than 3 mm, and wire, bars, and sections of diameter or thickness equal to or greater than 4 mm.

Because ISO 6892-1 testing is conducted on a wide variety of metals, system force requirements can differ greatly. The force measuring system of the testing machine must be in accordance with ISO 7500-1, class 1, or better. The Instron® 6800 Series offers test frames suitable for testing sheet metal (10kN) all the way up to steel plate (600kN). The 6800 Series provides a superior load frame including pre-loaded bearings, precision ball screws, an extremely stiff crosshead and base beam, and low-stretch drive belts. These features contribute to an overall advanced performance, producing highly accurate results. The features also help to minimize the energy stored during a test, which is especially evident when testing high-strength metallic materials to ISO 6892-1.

There are many different gripping technologies suitable for ISO 6892-1 testing (wedge, hydraulic, pneumatic, etc.) they can all be classified as either proportional or non-proportional according to the way in which they exert clamping force on the specimen.

With proportional grips, the force exerted on the specimen is proportional to the tensile load being applied. As tensile load increases during a test, so does the gripping force on the specimen. Wedge grips are a popular option for proportional gripping and come in manual, pneumatic, and hydraulic varieties to suit a wide range of testing applications. The shape of a wedge grip is what allows it to exert proportional pressure: as tensile force is applied to the specimen, the specimen is pulled more tightly into the narrowest area of the wedge, increasing the gripping pressure.

With non-proportional grips, the clamping force on the specimen remains consistent and is independent of the tensile load being applied. This is typical of side-acting grips and fatigue-rated hydraulic wedge grips where the clamping force is generated by a power source that is not directly associated with the tension loading of the specimen. This source is typically a high pressure (210 bar/3000 psi or higher) hydraulic supply. One benefit of non-proportional grips is that the clamping force is typically more adjustable, which offers more potential application advantages. For example, when testing non-machined specimens, fine adjustments can help users achieve optimal gripping pressure while minimizing stress concentrations that could cause premature failure.

There are three different types of extensometers that are typically used for ISO 6892-1 testing: clip-on devices, non-contacting devices, and automatic contacting extensometers. Depending on the calculations needed, extensometers must be in accordance with ISO 9513 class 1 or 2. Clip-on extensometers such as the 2630 Series are the most common type used. These devices can provide incredibly accurate and stable strain data and are typically cheaper to purchase than other types. They need to be robust enough to survive high-throughput testing labs and absorb any shock from the breaking of high-capacity metallic specimens if not removed during the test.

Automatic contacting devices such as the AutoX750 offer the benefit of repeatable clamping forces and placement, which can reduce variances between different operators placing clip-on extensometers manually. Automatic contacting devices can also adapt to multiple gauge lengths, which can be cost effective for users who need to test a variety of specimen types. The AutoX is designed to be robust enough to remain on throughout the test through failure. However, when combined with Bluehill® Universal software, the AutoX750 can be set up to remove itself automatically just before specimen failure in order to prevent any excessive wear to the knife edges.

Non-contacting devices such as the AVE 2 Automatic Video Extensometer offer the benefit of removing any influence incurred by the extensometer making physical contact with the specimen. For instance, if a test specimen is very thin, such as with packaging metals, the weight of a clip-on device can alter the results considerably. The knife edges used to affix the device to a fragile specimen can also damage the specimen and cause premature failure. Furthermore, because the AVE does not contact the material, there is no potential for the extensometer to be damaged or worn when testing high-capacity materials.

Nearly all modern testing machines come with pre-installed software, and it is important that the calculations in the testing software are in compliance with ISO 6892-1 and match with existing data. Not all software packages are created equal, and it is important to know that the platform you choose provides reliable results.

Thousands of customers around the globe rely on Bluehill Universal® to test their material to ISO 6892-1. All calculations required in ISO 6892-1 testing are already pre-configured into Bluehill Universal, but for those who prefer to start from scratch and build their own method, the interface makes it easy for users to manually input their own calculations. The metals methods package also provides pre-built methods to all of the following standards: ASTM E8 / E8M, ASTM A370, ASTM A615, ASTM E646, ASTM E517, EN10002, ISO10113 & ISO10275.

Most labs testing to ISO 6892-1 need to test a high volume of specimens on a regular basis. For this reason, anything that can be done to increase throughput is advantageous. Fortunately, there are many options for increasing a lab's testing throughput. Small software modifications can reduce repetitive tasks, and some grips and extensometers can reduce setup time and increase repeatability, which will reduce the need to run retests. Finally, there is the option to fully automate the entire testing process, which allows testing to run for several hours without the need for any operator interaction.