PSI - Issue 42

Miloslav Kepka jr. et al. / Procedia Structural Integrity 42 (2022) 762–768 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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(HiFiT), Pneumatic impact treatment (PIT) and Ultrasonic needle peening (UNP). Common to all named processes is the use of a roller-shaped tool or a set of rollers with different diameters and tip shapes, made of high-strength steels, which serve as indenters oscillating at a high frequency. The main advantage of the HFMI method compared to the above-mentioned procedures is the higher efficiency of this method for high-cycle fatigue and, at the same time, high repeatability, better controllability of the quality of execution, and less demanding operation. The service life of welded joints is an important topic in many areas of research. Standards or recommendations are often used to assess welded joints for durability. This is case of fatigue design of rail vehicles. There are catalogues of welded details with the relevant FAT classes for welded details in the IIW recommendation (Hobbacher, 2016) and in the standards such as Eurocode 3. The FAT classes are not dependent on the base and additional material used. For each detail there is just one FAT class for steel and one FAT class for aluminium. A curve with a fixed slope m = 3 is defined for each class. FAT classes are given by default for loading R = 0.5. The FAT classes are not dependent on the base and additional material used. There is one FAT class for steel and one for aluminium for each welded detail. Thanks to the higher frequencies of modern devices, the distance between the individual impressions of the indentors is smaller and the resulting groove in the butt of the weld is smoother after processing, which affects the quality and efficiency of the processing. The method is suitable for both butt and corner weld adjustments. A significant advantage of this method is the possibility of using it both for new structures and for increasing the fatigue life of existing steel structures, if they have not yet initiated a fatigue crack. The HFMI method is a very progressive and user-friendly method of increasing the fatigue life of welded steel structures. It is also suitable for common types of steel, but its advantages are most evident when welding high-strength steels. For harmonic loads, this improvement is defined in the already cited IIW recommendation.

Fig. 1. Maximum increase in the number of FAT classes (Marquis, 2016).

According to this recommendation (see Fig. 1), there will be an increase in the contractual fatigue strength and a shift of the fatigue S-N curve towards a higher service life (into the category of a weld with a higher FAT fatigue strength). For steel with a yield strength of less than 355 MPa, the HFMI treatment is reflected in an improvement of up to 4 FAT classes. For steels with yield strength above 355 MPa, the application of HFMI will result in an improvement of one additional FAT class for every 200 MPa increase in yield strength, see figure taken from IIW recommendation. Also, to calculate the fatigue life, you can count on a milder slope of SN curves m = 5 instead of m = 3, as is usual for normal welds under harmonic loading without modifications, or modified by means of hammering or teming.