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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Hi-Life (Damage mechanism of High Frequency Mechanical Impact (HFMI) Treated Welded Structures under Service Loading to Increase the Fatigue Life for Lightweight Design)

Teaser

Summary

The use of high-strength steel (HSS) grades leads to significant weight reduction and increased service life, especially in complex-welded structures. However, their implementation is limited in practice due to the fatigue issues at welds, welds which have the same fatigue life as those in lower strength steels. High-frequency mechanical impact ) treatment allows for important fatigue life improvement based on the induced compressive residual stress (RS), improved weld shape and cold-worked surface region. In current drafted guidelines, these factors were derived according to the weld toe failures obtained under constant amplitude loading (CAL) and also verified by a limited
dataset under variable amplitude loading (VAL), the latter being more realistic of service loadings. Current knowledge on HFMI-improved welds has also shown that fatigue failures may also initiate at other regions, rather than weld toe. In spite of the fact that relaxation of induced RS state has been claimed to be the main reason of different damage mechanisms resulting in the failure location change, scientific questions such as: why, how and under what conditions this effect occurs or what damage mechanisms play a dominant role, remain unanswered. Based on the above context, the objective of this proposed project aims to solve the damage mechanisms of HFMI-treated welds under service loading by considering fatigue tests, investigating the microstructures and developing analytical approaches. The proposed scientific approach includes investigations on the development of grain structure size/orientation through the depth of HFMI groove by neutron scattering, and on the relation of grain-orientation-dependent RS state under service loading. This training will also allow transferring of the obtained knowledge obtained to the industrial partner for the implementation of this novel treatment technique by utilizing HSS in the shipyard.

Work performed

The researcher, Dr. Halid Can YÄ±ldÄ±rÄ±m, was employed as assistant professor at Chalmers University of Technology in Sweden when the time he received the MSCA IF grant. He then took two years of leave from his position to carry out this project. Dr. YÄ±ldÄ±rÄ±m spent 16 months on the project Hi-Life at EPFL- Ã‰cole Polytechnique FÃ©dÃ©rale de Lausanne in Switzerland. The training also included several collaborations and research visits to other European Organizations as described in the Annex 1 of the grant agreement. These collaborators were Aalto University in Finland, Heinz Maier-Leibnitz Zentrum (MLZ) in Germany, Meyer Turku Shipyard in Finland, and SONATS SAS in France.
During his MSCA training, Dr. YÄ±ldÄ±rÄ±m involved also in teaching of CIVIL 526 Steel Structures (selected topics). He gave 12 hours of lectures in each semester at EPFL Civil Engineering master students.
The project Hi-Life was terminated on 31.12.2018 (8 months before its originally planned end date) since Dr. YÄ±ldÄ±rÄ±m decided to pursue his academic carrier as a tenured professor at Aarhus University in Denmark.
Dr. YÄ±ldÄ±rÄ±m believes that MSCA IF has boosted his career and he has reached one of his goals as he has started his new position on 01.01.2019.
Since the project was terminated before the original end date, it has achieved some of its objectives and milestones. The tasks that were not achieved or partially available are given the Periodic Report.

Final results

This proposed research program leads to deeper understanding and quantification of the damage mechanism of HFMI treatment and its effect on the fatigue properties of welds under service loading, and thus, the possibility for a better utilisation of the HSS grades in complex heavy-duty welded structures, e.g. decks, bridges. The experienced researcher will provide the scientific evidence needed for the applicability of HFMI-treated welded details under VAL and grain-orientation-depended residual stress of improved and deformed welds. These scientific investigations are done by using the neutron diffraction facilities. The development of the damage mechanism leads to understand the application limits of the treatment method under such loading conditions. This understanding is not available for HFMI welds yet. Brief information of the expected outcomes on scientific and industrial sectors is explained below.
Expected outcome for the scientific questions, scientific added value and innovative tools developments:
(1) The ER investigates the behaviour of HFMI-induced compressive (beneficial) residual stresses to determine damage mechanism under the effect of mechanical loading. This will help to develop an accurate assessment of the realistic service life of a component by assessing the magnitude of expected stress relaxation during service,
(2) The ER develops an empirical relation for damage mechanisms of HFMI welds based on grain-orientation-depended residual stress state and grain size, which is not ready yet. This will lead to identify the application limits of the treatment method precisely by providing e.g. the maximum load allowed in a VAL to drive crack growth
Expected outcome for the industries, manufactures and end-users in inter/multidisciplinary environment:
(1) The optimisation or application limits of HFMI method at service loading for selected HSS grades will be ready. This avoids end-users from over (or unnecessary place) treatment of the weld toe
(2) The development of new improvement factors (e.g. for VAL) for the drafted design rules will be introduced and validated. This enables the applicability of improvement factors for various industrial sectors like lifting devices, bridges, and bulk transport vehicles.