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 Solution:

• 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.