LIFE EXTENSION TECHNIQUE FOR WELDED STRUCTURE USING HFMI/PIT: A REVIEW ON PAST AND CURRENT RESEARCHES WITH APPLICATIONS

In this paper, High Frequency Mechanical Impact (HFMI) using Pneumatic Impact Treatment (PIT) which can be applied for new or aging welded structure towards asset integrity will be discussed. The technology HFMI/PIT which falls under post weld treatment process is primarily aimed to enhance fatigue life and to strengthen welded joint. At first, the basic principle on fatigue of welded structure based on the IIW Recommendation will be briefly described. Further, various investigations conducted by prominent research universities or institutions and various industrial applications in European countries will be reviewed and discussed. Lastly, the current research on application of HFMI/PIT carried out under Advanced Manufacturing Technology Excellence Centre (AMTEx) at Faculty of Mechanical Engineering UiTM Shah Alam will be presented. As conclusion, it is stated that HFMI/PIT can be applied for extending the structural life and also for design optimization

Fatigue Strength of HFMI-treated High-strength Steel Joints under Constant and Variable Amplitude Block Loading

AbstractLightweight-design of welded high-strength steel structures in cyclic service necessitates the use of post-treatment methods like the high frequency mechanical impact treatment (HFMI). Service loads during operation mostly consist of variable amplitudes, whereat recommendations are only available for the as-welded condition. Therefore, this paper deals with the effect of variable amplitude block loading on the fatigue strength of HFMI-treated T-joints. An evaluation of the real damage sum exhibits characteristic distinctions to constant amplitude test results in regard to the base material strength. The application of an equivalent stress range method by nominal and effective notch stress approach is finally presented

LIFE EXTENSION TECHNIQUE FOR WELDED STRUCTURE USING HFMI/PIT: A REVIEW ON PAST AND CURRENT RESEARCHES WITH APPLICATIONS

In this paper, High Frequency Mechanical Impact (HFMI) using Pneumatic Impact Treatment (PIT) which can be applied for new or aging welded structure towards asset integrity will be discussed. The technology HFMI/PIT which falls under post weld treatment process is primarily aimed to enhance fatigue life and to strengthen welded joint. At first, the basic principle on fatigue of welded structure based on the IIW Recommendation will be briefly described. Further, various investigations conducted by prominent research universities or institutions and various industrial applications in European countries will be reviewed and discussed. Lastly, the current research on application of HFMI/PIT carried out under Advanced Manufacturing Technology Excellence Centre (AMTEx) at Faculty of Mechanical Engineering UiTM Shah Alam will be presented. As conclusion, it is stated that HFMI/PIT can be applied for extending the structural life and also for design optimization

Fatigue life extension of existing welded structures via high frequency mechanical impact (HFMI) treatment

High-Frequency Mechanical Impact (HFMI) is one of the post-weld treatment methods. In this study, comparative axial fatigue tests were conducted on as-welded and HFMI-treated welded transverse attachment details. The test results demonstrated the efficiency of HFMI-treatment in fatigue life extension of cracked welded structures, providing that the existing crack size is less than 1.2 mm. Cracks were created in some specimens through fatigue testing before HFMI-treatment, while other specimens were not subjected to any fatigue loading prior to treatment. Many of the treated specimens ran out after 10 million cycles of loading when tested at a stress range of 150 MPa. Therefore, the stress range was increased to 180 MPa or 210 MPa. No remarkable difference was found between the fatigue strength of the crack-free and the cracked treated specimens. It was found that the induced compressive residual stress can exceed the material yield limit, and reach a depth larger than 1.5 mm in most cases. The induced compressive residual stress, the local material hardening, the increase in weld toe radius, the change in crack orientation and the shallowness of the crack size were the causatives of the obtained long fatigue lives of the HFMI-treated specimens. Besides, linear elastic fracture mechanics calculations were conducted to predict the fatigue lives of as-welded and HFMI-treated details. The results were in agreement with the experiment. Moreover, the calculations showed that the initial crack size, the clamping stress and the induced compressive residual stress were the main factors behind the scatter in fatigue lives

Fatigue improvement of steel bridges with high-frequency mechanical impact treatment

This thesis investigates the performance of fatigue-improved welds with high-frequency mechanical impact (HFMI) for application on new bridges. Fatigue strength improvement with HFMI can enable lightweight design of bridges and allow the utilisation of the benefits of high-strength steels. Studies of various bridge types were performed in this thesis showing that 20% material saving is possible in the main load-carrying members through post-weld treatment and the use of increased steel grades (fy > 355 MPa) where necessary. Limitations of the application of HFMI treatment on bridges were also identified, related to the degree of improvement and choice of steel grade.Experimental work of HFMI-treated joints with thick main plates relevant for bridges is scarce in the literature and comprehensive studies on the thickness effect are few. Therefore, the thickness effect was studied based on an established database of 582 fatigue test results of different types of HFMI-treated joints, collected from 28 studies. It was shown that the thickness effect becomes weaker than what is recommended for as-welded joints as a result of HMFI treatment. Fatigue experiments were conducted on a typical fatigue-prone detail in steel bridges with load-carrying plates of 40 and 60 mm which showed a significant fatigue strength improvement after HFMI treatment, exceeding recommended fatigue strengths given by the International Institute of Welding. Based on the fatigue experiments, a weak thickness effect was derived for non-load-carrying transverse attachment joints where the attachment and weld sizes are kept constant.The performance of HFMI-treated welds in composite steel and concrete road bridges was studied through a state-of-the-art review and simulations of variable amplitude in-service stresses in four case-study bridges in Sweden. It was shown that, in such bridges, very high and varying stress ratios are present due to a high portion self-weight stresses, which constitute up to 50% of the highest total stresses. Furthermore, it was revealed that the fatigue-critical locations in HFMI-treated bridges remain unchanged compared with conventional bridges and that compressive overloads pose no detrimental effect that requires additional attention in the fatigue assessment. Variable amplitude experiments with a bridge spectrum load from the case studies were conducted, including both low and high mean stress tests. The low mean stress tests performed equally or better than the constant amplitude fatigue strength, confirming that bridge loads do not pose any additional damaging effect for non-load-carrying transverse attachment specimens. The high mean stress tests clearly reflected the detrimental effect of high tensile self-weight stresses and enabled verification and development of approaches to consider these effects in design

Fatigue analysis of as-welded and HFMI-treated steel joints by local approaches

analisi a fatica di giunti saldati in condizioni as-welded e trattati HFMI con approcci locali basati sulla meccanica della frattura. Proposta di due nuove curve di progettazione a fatica in termini di tensione equivalente di picco sulla base del peak stress method (PSM)

Application of high-frequency mechanical impact treatment for fatigue strength improvement of new and existing bridges

This thesis investigates the application of High-Frequency Mechanical Impact (HFMI) treatment for fatigue strength improvement of weldments in existing and new bridges. In the former case, the welds have already been subjected to fatigue loading and accumulated damage before treatment. A fatigue testing program is set up, comprising welded specimens subjected to fatigue loading before HFMI treatment to investigate the efficiency of HFMI treatment on existing structures. Moreover, additional fatigue test results are collected from the literature and analyzed. HFMI treatment is found to be very efficient in extending the fatigue lives of existing structures regardless of the accumulated fatigue damage prior to treatment, given that any surface cracks, if exist, have not grown more than 2.25 mm in depth. For practical applications, HFMI treatment is only recommended if the critical details are verified to be free of any surface cracks. Remelting the surface with a tungsten electrode before HFMI treatment is another solution which has rarely been studied on existing structures. Therefore, several experimental investigations are conducted including fatigue testing, measurement of residual stress, hardness testing and scanning the welds topography to study the effect of combining these two post-weld treatment techniques. The combined treatment is found to be efficient as it induces higher and deeper compressive residual stress and local hardening. These aspects are all considered in numerical simulations conducted to investigate the fatigue behaviour of new and existing weldments treated using this combination. The results verify the superiority of the combined treatment to both individual treatments (TIG & HFMI). Nonetheless, because of the complexity associated with TIG remelting, the combination is only suggested for existing structures containing shallow fatigue cracks which can be fused by a tungsten electrode. One major hindrance to applying HFMI treatment on weldments in steel bridges is the lack of design rules and recommendations such as consideration of stress ratio (mean stress) and overloads.\ua0 Therefore, a correction factor (λHFMI) to account for the mean stress effect is derived. This factor is used to magnify the design stress range for fatigue verification of HFMI-treated welded details existing in road and railway bridges. λHFMI is calibrated using measured traffic data that includes millions of vehicles and hundreds of trains. In addition, the characteristic load combination associated with the serviceability limit state is found to be the most appropriate for verifying the maximum stresses in road bridges. Based on the work conducted in this thesis, a complete methodology is proposed for the design and assessment of HFMI-treated welded details in new and existing steel bridges. Finally, the effect of corrosion on the performance of HFMI-treated weldments is studied by analyzing collected test results. Despite the observed reduction in fatigue endurance of HFMI-treated details due to the removal of top layers improved by residual stresses, the obtained fatigue lives are still longer than the design lives assigned for new welded details even in extreme corrosion conditions. However, corrosion protection and removal of sharp HFMI groove edges via light grinding are still necessary to reduce the susceptibility of weldments to corrosion

Fatigue life extension in existing steel bridges. High-Frequency Mechanical Impact treatment and Tungsten Inert Gas remelting in life extension and fatigue crack repair of welded steel structures

This thesis investigates the performance of improved welds with two post-weld treatment methods for application on existing structures. High-Frequency Mechanical Impact (HFMI) treatment and Tungsten Inert Gas (TIG) remelting were used for fatigue life extension of welded structures. Axial fatigue testing was conducted on transversal non-load-carrying attachment treated via the investigated methods. Furthermore, more than 250 test results on different treated welded details were collected, sorted and analysed. HFMI-treatment was found to give a significant fatigue life extension even with the presence of cracks up to 2.25 mm. On the other hand, the efficiency of TIG-remelting was also proven when the crack was completely eliminated after remelting. Even if a small part of the crack remains after remelting, fair fatigue life could be expected. However, it is recommended to use HFMI-treatment or TIG-remelting only when the crack inspection is negative before and after treatment respectively.Complimentary studies showed that the investigated methods induced compressive residual stress, increased the smoothness of the weld toe, increased the local hardness and changed the angular distortion status locally. Moreover, TIG-remelting changed the microstructure in both the fusion zone and the heat-affected zone. HFMI-treatment changed the crack orientation, induced compressive plasticity at the crack tip and caused crack narrowing or even closure. However, these effects were less significant for deeper cracks. Moreover, some practical aspects of the treatment application were investigated. Unlike treating new structures, TIG-electrode should be placed at the weld toe to secure that the maximum fusion depth corresponds to the crack plane. On the other hand, HFMI-indentor should be slanted more toward the base metal than the weld to avoid unintentional crack opening. Moreover, the IIW recommendations for both HFMI-treatment inclination and indentation depth could be extended to cracked structures. The aforementioned investigated parameters (i.e. residual stress, distortions, local hardness and toe\u27s smoothness) were incorporated in fatigue life predictions for both treatment methods. The base metal S-N curve was used to predict the life of specimens treated via TIG-remelting, while Paris law was used to track the crack propagation of HFMI-treated details. The results corresponded well with fatigue test results. Combining TIG-remelting with HFMI-treatment resulted in welds with higher fatigue strength because of the combined effects of crack closure via TIG-remelting and compressive plasticity via HFMI-treatment

Fatigue life estimation of welded structures enhanced by combined thermo-mechanical treatment methods

Different post-weld treatment methods are used to strengthen welded joints that are subjected to cyclic loading. Combining High-Frequency Mechanical Impact (HFMI) treatment with Tungsten Inert Gas (TIG) remelting is rather a new concept. In this paper, the fatigue lives of welded transverse attachments treated by HFMI-treatment, TIG-remelting, or the combination of both are estimated using fatigue damage modelling and finite element deletion. The change in local topography and residual stresses due to treatment are evaluated numerically and incorporated in the analysis. The local hardness is measured by a Vickers tester and incorporated by increasing the elemental ultimate strength. The analysis demonstrates the superiority of the combined treatment because of the introduced compressive residual stress and the improvement in topography. The analysis also shows that the damage is less distributed after the combined treatment than both individual treatments. Besides, the capability of the TIG-HFMI combination in treating existing welded structures with remaining embedded fatigue crack is proven. Besides, available fatigue test results on combined TIG-HFMI treatment shows that this combination gives always longer fatigue life than the characteristic fatigue lives of the treated details by any of the treatment methods. However, many aspects such as TIG arc and HFMI indenter positioning, and indentation and fusion depth should be taken into consideration when the combined treatment is to be applied to existing structures