PSI - Issue 57

Kaushik Iyer et al. / Procedia Structural Integrity 57 (2024) 469–477 Kaushik Iyer, et.al. / Structural Integrity Procedia 00 (2019) 000 – 000

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processes, while efficient and cost-effective, can introduce detrimental factors such as residual stresses, distortion, and microstructural changes, compromising the fatigue behavior of welded components. Furthermore, studies show that the increased fatigue properties of high strength steel is typically negated through welding; the reason being the weld toe is the critical area of fatigue failure (Lippardt 2023) (Tsutsumi, et al. 2022) (Caccese, et al. 2006). These factors significantly reduce the fatigue strength, thereby necessitating the implementation of post-weld treatments (PWTs) to enhance the fatigue strength and extend the service life of these structures. High Frequency Mechanical Impact (HFMI) is one of the commonly used PWTs to extend the fatigue life of a welded structure. As the name suggests, this particular treatment involves subjecting the weld geometry to a series of impacts at a very high frequency (90-120 Hz), thereby healing any potential deformations or discontinuities in the weld toe and also inducing compressive residual stresses in the weld toe region. Subjecting the weld toe region to HFMI has proven to significantly increase the fatigue strength of a welded structure (Marquis and Barsoum 2016) (Figure 1) . Moreover, HFMI has proven to be a better PWT treatment in increasing the fatigue life of the weld as compared to other PWTs such as TIG dressing or Burr Grinding. HFMI has also proven to be effective in increasing the fatigue strength of pre-fatigued welds with known discontinuities and cracks. (Al-Karawi et al. 2021) found very minor differences in the fatigue life extension gained by HFMI applied to pre-fatigued welds and newly welded structures.

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Figure 1 (a): HFMI treatment on the weld toe (b) Fatigue class (FAT class) improvement due to HFMI treatment (Marquis and Barsoum 2016)

The introduction of post-weld treatments in welded structures has several advantages including minimizing the structural weight through material selection (using high strength steel) and reducing material usage. The reduction in material usage and the fatigue life extension resulted by the PWT can also influence the Life-cycle Cost (LCC) of a welded structure. A study conducted (Hagnell, et al. 2021) showed the influence of fatigue life on the LCC of a welded component. The study also showed the dominance of the use-phase in the life-cycle cost of the component and the effect of different weld qualities. However, the high tool vibrations and noise can have adverse health effects on the workers while implementing HFMI manually. Therefore, there have been attempts to mechanize the PWT to remove the adverse health effects caused by the vibrations and to significantly decrease the cycle times. However, the introduction of an additional manufacturing step i.e., PWT might have consequences for the manufacturing cost and cycle time of a welded structure. Therefore, it is beneficial from an economic point of view to implement PWTs to increase the fatigue life of the welded structure. Consequently, to evaluate this tradeoff between increased manufacturing costs vs reduced material usage and maintenance, and to effectively motivate the use of PWTs in reducing the economic impacts (i.e. LCC) of a welded structure, a through yet predictive cost modeling methodology is required.