Page 34 - MetalForming February 2020

Cutting Edge
By Eren Billur, Ph.D.
Digital Image Correlation:
How It Changed the Tensile Test
For metal formers, one of the sim- plest and oldest tests is the tensile test. In the simplest form, a ten- sile test machine must record elonga- tion of the test specimen (􏰀l) and the reaction force (F) multiple times during the experiment. The recorded data then are used to calculate material properties such as elastic modulus E, also known as Young’s Modulus), yield or proof strength (YS, 􏰁Y, Rp0.2, or RE); and ulti- mate tensile strength (UTS, 􏰁UTS, Rm). A tensile test also can determine total elongation (TE percent, A percent, A80, A50 and similar), where the numbers indicate the initial gauge length in mm, but its measurement/calculation depends on the test equipment.
Although the tensile test has been around for many decades and its basic principles haven’t changed, there have been dramatic improvements in data- acquisition systems and controls.1 In addition, metal formers would like to obtain more information from the sim- ple tensile test. While measuring and recording force is relatively simple using load cells, measuring elongation can present challenges. Older machines may still use crosshead displacement
Eren Billur is the founder of Billur Metal Form, a consulting, engineering and training company in Ankara, Turkey. He stud- ied at Baskent University and Virginia Common- wealth University, received a PhD in mechanical engineering from The Ohio State Uni-
versity, and worked as a researcher at the Center for Precision Forming. His areas of expertise include material characterization, sheet metal forming processes, and finite element simulations. He has authored/co-authored more than 20 scien- tific papers (including proceedings) and con- tributed to four books, including “Hot Stamping of Ultra High Strength Steels,” published in 2018. Eren Billur
Billur Metal Form, Founder eren@billur.com.tr
True Stress-Strain curves of BH220 steel 2 determined with mechanical extensometer and DIC. Image courtesy of Atılım University Metal Forming Center of Excellence in Ankara, Turkey—The author thanks research engineer Türkay Muratoglu for his support.
700
600
500
400
300
200
100
100 80 60 40 20
Unifo
rm
Total elongation
DIC
True strain ~0.25
True strain ~0.30
True strain ~0.81
(extensometer)
(extensometer)
00 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
True Strain [ ]
for calculating yield and tensile strength values that do not require pre- cise elongation data. However, these machines cannot precisely calculate elastic modulus. For total elongation (elongation at break), one can mark the initial gauge length and then meas- ure these marks.
Improvements to tensile test machines started with the introduction of an axial extensometer, a delicate clip-on sensor, with knives on the edges, designed to provide precise measurement of the extension. Instead of relying on crosshead displacement— where frame deflection and slipping of the test specimen can impact accu- racy—an extensometer measures elon- gation of the gauge. Adding an exten- someter allowed engineers to precisely measure a material’s elastic modulus and uniform elongation (Ag).
Note: Extensometers are relatively delicate—it is typically advised to remove them before fracture.
By the 1940s, after earing and deep- draw problems had been studied and researchers developed what we now know as r-values (plastic anisotropy coefficients or Lankford parameters),
engineers understood that material tests require the simultaneous meas- urement of length increase and width decrease—achieved by transverse strain measuring. Some extensometers can measure width strain, and, there- fore, enable calculation of r-value and Poisson’s ratio.
The 1980s welcomed the introduc- tion of video extensometers for tensile testing. With a tensile specimen marked with a dot pattern, a camera records movement of the dots during the test, and calculates the average length and width strains. This allowed researchers to measure total elongation without user intervention.
Even with the video extensometer, true stress and strain could only be determined until necking. Enter the noncontact measuring technique called digital image correlation (DIC), which, by using a camera setup and software, allows engineers to measure strain in both directions (longitudinal and width) for small facets. Using DIC, we can develop the stress-strain curve after necking. And, contrary to other systems, DIC simplifies strain testing but complicates stress calculation.
32 MetalForming/February 2020
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True Stress [MPa]
True Stress [ksi]