Precision of Measurements of Delayed Slip in Structural High-Strength Assemblies by Contact and Optical Methods

Structural bolted connections of steel plates must fulfill the requirements specified by the standard EN 1090-2:2018 in the European Union. For the slip-resistant joints, the standard establishes the experimental procedure to obtain the delayed slip under a constant loading, which should not exceed 2 μm from the first five minutes to three hours. However, the usual extensometers available in laboratories, like LVDTs, lack the necessary precision to obtain a reliable measurement of such a small displacement because, even if their resolution is 1 μm, the scatter of repeated measurements of the same distance is normally higher. In contrast, the resolution and precision in optical methods like Digital Image Correlation (DIC) can be easily set under 1 μm depending on the number of pixels covering the studied area and the performed subpixel interpolation. This work introduces the use of DIC for delayed slip tests. In comparison with an LVDT, it is verified that this technique is a valid alternative to measure displacements at that scale with a simpler experimental setup.