Stainless steel tubing (or any other variety of rigid metallic tubing) can be difficult to test in tension for two reasons. The first is that standard methods of gripping will cause the tubing to collapse and lead to pre-mature failure of the specimen. The second is strain measurement is usually not possible with traditional clip-on extensometers since the knife-edges lead to high-stress concentrations and consequently failure of the thin specimen wall. The purpose of this testing was to recommend a gripping solution and evaluate the use of the Advanced Video Extensometer (AVE)
for accurate, non-contacting strain measurement.
For this test we used a 5582 tensile testing system
configured with 30kN wedge grips
with Vee-serrated faces, and an AVE with 200 mm Field of View (FOV) lens. We inserted solid mandrels, that matched the inner diameter of the specimens, into the gripped ends to encourage failure within the gauge area. Additionally, we used a white paint pen to mark the specimen gauge area for strain measurement with the AVE. The AVE set up window in our Bluehill® 2 Software
demonstrates the relationship between the marks on the specimen and the marks captured by the AVE before the start of a test.
In conclusion, these results show that the gripping and marking techniques were able to accurately measure tensile load and axial strain through failure in each specimen.