Halid Can Yildirim

Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed fatigue assessment guidelines

In the past decade, high-frequency mechanical impact (HFMI) has significantly developed as a reliable, effective, and user-friendly method for post-weld fatigue strength improvement technique for welded structures. During this time, period 46 documents on HFMI technology or fatigue improvements have been presented within Commission XIII of the International Institute of Welding. This paper presents one possible approach to fatigue assessment for HFMI-improved joints. Stress analysis methods based on nominal stress, structural hot spot stress, and effective notch stress are all discussed. The document considered the observed extra benefit that has been experimentally observed for HFMI-treated high-strength steels. Some observations and proposals on the effect of loading conditions like high mean stress fatigue cycles, variable amplitude loading, and large amplitude/low cycle fatigue cycles are given. Special considerations for low stress concentration details are also given. Several fatigue assessment examples are provided in an appendix. A companion paper has also been prepared concerning HFMI equipment, proper procedures, safety, training, quality control measures, and documentation has also been prepared. It is hoped that these guidelines will provide stimulus to researchers working in the field to test and constructively criticize the proposals made with the goal of developing international guidelines relevant to a variety of HFMI technologies and applicable to many industrial sectors. The proposal can also be used as a means of verifying the effectiveness of new equipment as it comes to the market.

A round robin study of high-frequency mechanical impact (HFMI)-treated welded joints subjected to variable amplitude loading

High-frequency mechanical impact (HFMI) treatment has been significantly developed as a reliable, effective and user-friendly method for post-weld fatigue strength improvement technique for welded structures. The development of an International Institute of Welding best practice guideline for implementing HFMI has been hindered by the lack of directly comparable experimental data for numerous HFMI methods. In this study, nominally identical longitudinal attachments in high-strength steel were manufactured in one welding workshop and distributed to four HFMI equipment manufacturers for treatment. Specimens were fatigue tested on a machine using identical variable amplitude loading histories. HFMI groove measurements were done for each specimen and X-ray diffraction-based residual stress measurements were performed on 10 specimens. The HFMI groove dimensions and the residual stress states showed similarity in general, however small changes were observed. Experimental results indicate that all of the HFMI-improved welds from the HFMI equipment manufacturers satisfied the previously proposed characteristic S–N line based on both the yield strength and the specimen geometry. Results of the study are valuable and promising with respect to the development of a future guideline. The goal of the study has not been to compare treatments, so specific data points are not associated specific HFMI equipment manufacturers.

Overview of Fatigue Data for High Frequency Mechanical Impact Treated Welded Joints

Abstractpaper provides an overview of published experimental data on the fatigue strength of welded joints by high frequency mechanical impact (HFMI) treatment methods, In total, 414 data points from four specimen types are available,tests were performed using constant amplitude R = 0.1 axial tension fatigue, but some data for other R rations, variable amplitude testing and bending fatigue are also reported. An S-N slope of m = 5 gives a very good description of both individual data sets and of the composite data Design curve recommendations for the four joint types and for the structural stress-based design curve are given. HFMI treated specimens generally follow the same trend as experimental data for hammer peened specimens, but the degree of improvement is better. Data for large structures, at stress ratios other than R=0.1 and for variable amplitude loading are still needed in order to update the IIW guideline for post-weld improvement. There is a general trend for increasing fatigue strength improvement as a function of steel yield strength but this influence needs further study in order to develop guidelines. Quality assurance measures for HFMI treatment methods must also be defined.

Fatigue strength assessment of HFMI-treated butt welds by the effective notch stress method

Experimental fatigue data for high-frequency mechanical impact (HFMI)-treated butt welds with low stress concentrations have been collected and analysed using the effective notch stress (ENS) method given by the International Institute of Welding (IIW). The aim of this study is to suggest a more reliable fatigue assessment procedure for HFMI-improved butt welds with low stress concentration factors. In total, 165 published test results for butt welds subject to R = 0.1 and R = 0.5 axial loading are presented. Kn, min values for the ENS approach are suggested in order to avoid computational problems due to low stress concentrations at the weld toe. Minimum notch stress concentration values are suggested based on the material yield strength (fy). All the data is presented with respect to previously proposed and verified material fy correction method. For HFMI-treated butt welds in the ENS system, the use of the Kn, min values with the fy correction representing one fatigue class (approximately 12.5 %) increase in strength for every 200-MPa increase in fy has been proposed and verified.

Experimental verification of HFMI treatment of large structures

Design recommendations for high frequency mechanical impact (HFMI)-treated welds have been proposed based on available experimental fatigue data of axially-loaded high strength steel specimens which include longitudinal, cruciform and butt welds. Test specimens were of a size appropriate for laboratory study. However, in reality, structures in civil, offshore and ship industries generally include large-scale and more complicated components, such as bridges, cranes, platforms, excavators etc. This paper presents a further validation of the design proposals by considering fatigue data sets which are obtained from large-scale components. The extracted fatigue data from the available literature includes bridge, crane and beam like components. In total, 65 published test results of weld details with various yield strengths (250 ≤ fy ≤ 725 MPa) and stress ratios (-1 ≤ R ≤ 0.56) are presented. All the data are found to be in good agreement with the previously-shown design curves.