Tensile test with numeric and graphical data overlays just prior to yield. Yield is approximately 740 MPa.
Tensile test with numeric and graphical data overlays just prior to failure. Strain at failure is approximately 30 percent. This image is taken at approximately 25.4 percent strain.
This is the crack on the surface of the specimen after 57,000 cycles. The specimen notch is 17 mm and the surface crack is approximately 27 mm (10 mm from the end of the notch), while the compliance measured crack reads 29.1 mm.
This is the crack on the surface of the specimen after 80,000 cycles. The specimen notch is 17 mm and the surface crack is approximately 32 mm (about 15 mm from the end of the notch), while the compliance measured crack is reads 34.0 mm.
Take a Closer Look at Fatigue and FractureEven the most groundbreaking, innovative ideas are useless if they can’t be shared. In research and development, this often depends on the ability to communicate technical results in a complete, accurate and meaningful way for many audiences: researchers, designers, engineers and test operators, as well as executives making strategic decisions based on test outcomes.
One of the best ways to share results is by combining visual and numeric data. This is especially true when it comes to mechanical testing, and it is this idea that Dr. Erik Schwarzkopf, MTS R&D Engineer and Staff Scientist, presented in Indianapolis, Indiana, at the Sixth Symposium on Automation of Fatigue and Fracture Testing, an event sponsored by ASTM.
Numeric values, such as force, displacement and cycle count, are by definition quantitative. Visual images, however, provide extremely useful qualitative data. By limiting an explanation of test results to one kind of information or the other, researchers run the risk that the results (or a related physical model) will be poorly understood and potentially misused in other applications.
To tell a clearer story, Schwarzkopf recommends overlaying numeric data (in the form of a plot or graph) on video of the test specimen. The combined information helps viewers make more intuitive connections between different features of the specimen deformation (what is shown in the video) and the actual numeric information (how the plot or graph changes over time).
MTS is working with researchers to demonstrate how to embed visual data — including full-motion video and static images — in standard mechanical tests using MTS TestSuite™ Software. Dr. Schwarzkopf presented two practical applications of this concept at the ASTM event.
The first is a basic tensile test performed on a cold rolled steel specimen with a round cross section. While the test was running, a simple consumer video camera on a tripod recorded the gage length of the specimen through necking. After the test was complete, the stress-strain data was combined with the video file using commercial off-the-shelf software. The end result is a video image that clearly shows how the specimen deforms and how the stress-strain graph changes at the moments just before and just after failure.
The second example is fatigue crack growth test performed on a standard CT aluminum specimen. While the test was running, a consumer-grade camera on a tripod was triggered to take a photo whenever the crack grew by 0.05 mm. The surface of the specimen was sanded in the vertical direction, and a simple steel ruler was clamped to the specimen as a length scale for the images. (Note that crack length was not measured in an automated manner from the optical data.) MTS TestSuite software was configured to launch a commercial software program whenever the crack length (as measured by a clip gage) increased by 0.05 mm. The software commanded the camera to take a picture and store it as a JPEG with a file name incorporating the crack length (i.e. “cracklength17.7mm.jpg”).
The benefits of combining optical and numeric data are clear. Overall, this technique helps convey the relationship between different parameters more effectively. With automated sequencing and simple synchronization, the results of tests — and the models they support — become more understandable and accessible, especially to non-experts. In the tensile test, for example, video helps correlate interesting features on the stress-strain curve with necking behavior during the test. In the fatigue crack growth test, visual data can verify other measurement techniques, such as compliance or direct current potential drop (DCPD).
The overlay technique also shows promise for understanding composites and other non-homogeneous specimens. Because these specimens exhibit nonlinear behavior, video imaging can help researchers see exactly how they move when traditional measurement data (which often averages the movement of different parts) does not provide a clear picture.
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