Vision-based measurement of crack generation and evolution during static testing of concrete sleepers

In this paper, vision-based measurement system is developed to monitor the crack generation during the static test of concrete sleepers. In particular three point bending test described in the European Standard, EN 13230-2 [1] is considered. This test is the acceptance tests for concrete sleepers, which are declared in the mentioned Standard as “safety critical components for railway applications”. In this ambit, the validation and the check of the reliability of crack opening technique is a crucial point for the safety of passengers and infrastructure. The mentioned Standard imposes to measure the width of the main crack appearing on the sleeper and track it during the test, but it does not prescribe the measuring technique to be used or the maximum uncertainty that can be accepted. Crack width is often estimated manually, with the help of a handheld microscope. The measuring uncertainty is strongly related to the experience of the operator. In this work, a vision-based measuring technique, based on digital image correlation (DIC) technique, is proposed and validated. In the proposed approach, the crack path is first detected automatically, relying on the strain map generated by DIC, and then the crack width is estimated, relying on the displacement map. Finally, the proposed measurement approach is compared with the state-of-the-art technique based on the manual analysis of images acquired with a handheld microscope. The results show that the crack width measured with the proposed approach is compatible with the data obtained by the manual technique. Thanks to the proposed full-field measurement, the crack width can be estimated not only in a few manually detected points along the crack, but for the full length of the crack with an automatic procedure. Moreover, the proposed technique can be applied in an unmanned way, since the camera does not need to be manually moved along the sleeper side to detect the crack: it is mounted in a fixed position and the image processing software automatically detects the crack path and width. The operator is therefore not requested to work close to the sleeper during the loading, with advantages also for the safety.