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Save time and money using video extensometer for R-Value measurement

03 August 2017

The Imetrum UVX and UVX Flexi materials testing systems can save time and money when testing specimens to ASTM-E517 and measuring R-Value.

Imetrum UXV-Flexi system

Click here to read this article in the digital issue

Testing materials used in sheet metal forming processes is essential for establishing material behaviour to support process design and to determine the quality of incoming batches of material.

Non-contact measurement specialists Imetrum were commisioned to carry out a blind test using its UVX Flexi system against a traditional electromechanical extensometer.

Background to sheet metal testing

Sheet metal forming processes are commonly used in a number of industries to manufacture sheet metal parts, from near flat panels for simple enclosures to complex contoured car body panel profiles.

The key to desigining a successful forming process is a good understanding of the material properties of the available sheet metal. In addition, testing of incoming material can be used to vertify the quality of a batch before use.

Materials are tested to measure their “R-Value” or plastic strain ratio and this test is defined in the ASTM E517 standard. A good description of this can be found in an article in “The Fabricator” magazine.

Traditionally this measurement has been carried out using contacting bi-axial elecromechanical extensometers. Since the specimen is pulled to failure high extensions are required as well as a high degree of accuracy. There is both a risk of damaging specimens with knife edge contact points and a risk of damaging the contacting extensometer when the specimen fails.

UVX Flexi in blind testing

Imetrum were commissioned to test a batch of steel specimens to measure R-Value using the UVX-Flexi video extensometer. This was a blind test against an expert testing laboratory familiar with this test standard and measurement. 

Specimens were received and marked with a specimen marking kit. This stamp and ink kit ensures a safe and consist way of marking specimens. The Imetrum test results were sent for comparison with those measured using a contacting extensometer and feedback was requested to see how the results compared.

Results of testing using UVX Flexi

The results show that the minimum, maximum, average and standards deviations are an excellent match between both types of extensometer.

R-value @ 18% strain





UVX-Flexi video extensometer

Electromechanical extensometer

Batch size












Standard Deviation



The Imetrum UVX-Flexi video extensometer therefore makes a compelling alternative to the contacting extensometer for measuring R-Value.


The blind tests show that Imetrum’s UVX Flexi is an aletrnative to using a traditional contacting bi-axial elecromechanical extensometer, offering traceable, best in class resolution, with proven Video Gauge technology.

Repeatable and accurate measurements are achieved when used in conjunction with Imetrum’s specimen marking kit which eliminates variability in quality often experienced with speckle patterns applied by paint.

The Imetrum system is simple to use, can save time, reduce costs and removes the risk of damaging the test specimen or the measurement device. The Imetrum UVX-Flexi can be calibrated to Class 0.5 / B2 (ISO 9315/ASTM-E83) or better for full tracability to national and international standards. 

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