A new loading-constraining device for mechanical testing with misalignment auto-compensation

The mechanical characterization of materials is often performed by expensive experimentations: for this reason the research is often developed with the collaboration of several laboratories, finally collating al the results. At this stage problems may arise due to data scattering: previous studies showed that it is often due to lateral or angular misalignments between the fixed and the moving machine crossheads. The object of this work is to design, manufacture and experimentally validate a novel loading-constraining device for misalignment compensation: conceptual design is applied to determine the optimal technical solution. The developed device, which can be mounted on different testing machines, achieves misalignment reduction with the use of a thrust bearing with sphered housing washers. The bearing is mounted beneath one of the specimen heads: the inverted orientation of the upper ring, allowing relative lateral displacement between the two rings, and the spherical shape of the housing compensate for offset and angular misalignments. Experimental results showed that such misalignments are respectively reduced to less than 0.02 mm and 0.05°, while the bending strain is lowered to just 6% of the nominal one, so that the requirements in different standards for mechanical testing can be fulfilled.