Design aspects of high strength steel welded structures improved by high frequency mechanical impact (HFMI) treatment

Abstract

This doctoral study is concerned with the fatigue strength of welded steel structures which are improved by high frequency mechanical impact (HFMI) treatment. A comprehensive evaluation of 417 HFMI test data obtained from the literature and 24 HFMI fatigue data tested as a part of this work are studied. According to the statistical analyses an S-N slope of five (5) is proposed. A yield strength correction procedure which relates the material yield strength (fy) to fatigue is presented and verified based on the constant amplitude R = 0.1 axial tension fatigue data. The fy correction method significantly reduced the observed scatter in the data with respect to data without any fy correction. Fatigue strength evaluations are done based on the nominal stress (NS), the structural hot spot stress (SHSS) and the effective notch stress (ENS) methods. By defining a reference fy at 355 MPa, an increase in strength of approximately 12.5% for every 200 MPa increase in fy above the reference fy is found. For the NS and SHSS systems, this study gives HFMI design recommendations including a five (5) fatigue class increase in strength with respect to the NS and SHSS fatigue classes for the same weld detail in the as-welded condition. In the case of the ENS, a four (4) fatigue class improvement is proposed and verified. For HFMI welds with fy > 950 MPa, the proposals are extended to represent a stepwise increase up to an eight (8) fatigue class improvement for the NS and the SHSS methods whereas the ENS method leads to a seven (7) fatigue class improvement. All the proposed characteristic curves in this study are conservative with respect to available fatigue test data. In the experimental study case, longitudinal non-load carrying high strength steel attachments were considered. Specimens were manufactured by a robot using an identical weld procedure and afterwards they were sent to four different HFMI tool manufacturers for post-weld treatment. All improved specimens were tested using the same variable amplitude loading history. Experimental test results indicate that all of the HFMI-improved welds from the four different HFMI equipment manufacturers satisfied the previously-proposed characteristic S-N line based on both the material fy and the specimen geometry. In addition, detailed specimen alignment, weld profile and HFMI groove measurements were done for each specimen. Residual stress measurements were performed on some of the specimens using the X-ray diffraction method. While clear differences were observed, the HFMI groove dimensions and the resulting residual stress state following treatment were generally similar. The goal of the round robin study was to verify that a single guidance could be developed for different HFMI technologies.