Plastic Strain Ratio (r)
ISO 10113, ASTM E517, and JIS Z 2254 specify methods for determining the plastic strain ratio (commonly called the r-value) of flat metallic materials, typically sheet and strip. The r-value of sheet metal is its ability to resist thinning or thickening when subjected to a tensile or compressive force.
To calculate the r-value, a test is performed to ISO 6892 and requires the use of axial and transverse extensometers. Where the behavior of true plastic width strain versus true plastic length strain is homogeneous, a single point calculation can be used. For material that exhibits inhomogeneous deformation, a regression method is recommended, and ISO 10113:2020 strongly recommends measuring transverse strain at multiple locations evenly distributed along the axial gauge length, to be averaged into one value. The AverEdge32™ feature can provide this capability, and these calculations can be done automatically in real time using the Bluehill® Universal materials testing software.
An ISO 10113, ASTM E517, or JIS Z 2254 test is typically performed at the same time as ISO 10275, ASTM E646, or JIS Z 2253, the tensile strain hardening exponent (n-value). Sheet metal testing applications require calculations such as yield strength, yield point elongation, ultimate tensile strength, r-value and n-value. These calculations place a high physical demand on traditional contacting extensometers for measuring axial and transverse strain. They need to allow for enough travel to test the specimens through break, but small gauge lengths make it more challenging to ensure high accuracy of the measurements. Additionally, relevant ASTM and ISO testing standards have accuracy requirements that must be achieved.
Historically, video extensometry is not frequently used in these applications, as most customers in the metals industry prefer traditional contacting extensometers and trust the reliability of the results. Significant advances in the technology of video extensometry offer metals testing customers other options, such as our Advanced Video Extensometer (AVE 2). Not only does this solution allow for simultaneous collection of axial and transverse strain data, and reduce the maintenance required for wear and tear on knife-edges, it has been shown to increase productivity and simplify testing. In addition, the AverEdge32™ feature has shown substantial improvement in reducing transverse strain measurement variability.
Instron’s AutoX Biaxial automatic contacting extensometer is another excellent solution for biaxial strain measurement, as it provides repeatable r-value results that are not influenced by operator input as traditional extensometers are.
The challenges of testing to this standard are:
- Biaxial strain measurement
- r-value calculation
- Instron’s AVE 2 and AutoX Biaxial extensometers provide transverse and axial strain measurement that meets the standards’ requirements
- Bluehill Universal allows for easy calculation of r-value within the software, and even allows live plotting of r-value
It is important to review ISO 10113, ASTM E517, and JIS Z 2254 in order to fully understand the test setup, procedure, and results requirements.
AverEdge32 is an optional advanced feature of Instron’s AVE 2 non-contacting extensometer. Standard extensometers with a single local transverse strain measurement can exhibit stepped changes in transverse strain data, making it challenging to obtain an accurate value. AverEdge32 measures transverse strain at multiple locations along the specimen edges and averages them to produce a smooth and repeatable value, which is essential when calculating the r-value of sheet metal.
- 1.3 MB
The second generation Advanced Video Extensometer (AVE 2) utilizes patented measurement technology in the fastest, most accurate non-contacting strain measurement device commercially available.
- 1.4 MB
The AutoX 750 is a high-resolution, long-travel automatic contacting extensometer. It can be mounted onto any electromechanical 3300, 5500, or 5900 table top and floor model systems, as well as LX, DX, HDX, and KN static hydraulic testing systems. It is well suited for applications involving plastics, metals, biomedical, composites, elastomers, and more. The AutoX has a maximum travel of 750 mm and accuracy of ± 1 µm.
- 1.4 MB
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.
- 3.2 MB
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.
- 4.4 MB