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 plastic strain ratio (r) 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 using an axial and transverse extensometers. Where the true plastic width strain against true plastic length strain is homogeneous, a single data point method can be used, with inhomogeneous deformation a statistical method can be used. These calculations can be done automatically using Bluehill ® Universal 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, plastic strain ratio ('r' value) and the strain hardening exponent ('n' value). These calculations place a high physical demand on traditional contacting extensometers for measuring axial and transverse strain to allow for enough travel to test the specimens through break, but small enough gauge lengths to ensure high accuracy in measurements. Additionally, relevant ASTM and ISO testing standards have accuracy requirements that must be achieved.

The Japanese Standards Association promotes JIS test standards for sheet metal testing applications which require calculations such as yield strength, yield point elongation, ultimate tensile strength, plastic strain ratio ('r' value) and the strain hardening exponent ('n' value). These calculations place a high physical demand on traditional contacting extensometers for measuring axial and transverse strain to allow for enough travel to test the specimens through break, but small enough gauge lengths to ensure high accuracy in measurements. Additionally, relevant JIS, ASTM and ISO testing standards have accuracy requirements that must be achieved.

Video extensometry has not traditionally been used for this application, as most customers in the metals industry use contacting extensometers and trust the reliability of the results. Developments in the technology of extensometry offer metals testing customers other options, such as Instron's Advanced Video Extensometer 2 (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 also increases productivity and simplifies testing.

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 10113:2006, ASTM E517 – 00(2010), and JIS Z 2254:2008 in order to fully understand the test setup, procedure, and results requirements.