High Frequency Mechanical Impact Research Articles

Fatigue performance of repair-welded and HFMI-treated transverse stiffeners

AbstractLarge portions of infrastructure buildings, for example, highway and railway bridges, are steel constructions and reach the end of their service life due to an increase of traffic volume. Repair welding can restore the current welded constructional detail with a similar fatigue strength. However, due to the increase of fatigue loading (traffic), an increase of fatigue strength is needed in such bridge structures. For this reason, the combination of repair welding and high-frequency mechanical impact (HFMI) treatment was investigated in this study in order to quantify the increase of fatigue life by combining both methods. For this, transverse stiffeners made of steel grade S355J2 + N were subjected to fatigue loading until a pre-determined crack depth was reached. The cracks were detected by non-destructive testing methods. Weld repair was realized by removing the material containing the crack and re-welded by a gas metal arc welding (GMAW) process, following that post weld treated was applied by HFMI-treatment and the specimens were subjected to fatigue loading again. Hardness profiles, weld geometries, and residual stress states were investigated for both the original and the repaired condition. In the repaired condition without additional HFMI treatment, a similar fatigue life than in the original condition is observed for the specimens. The repair-welded and HFMI-treated specimens reach a significant higher fatigue life compared to the repaired ones in the as-welded condition.

Research on the Influence of HMFI and PWHT Treatments on the Properties and Stress States of MAG-Welded S690QL Steel Joints.

The aim of this study was to analyze the effect of the HFMI (high-frequency mechanical impact) treatment of each weld bead on the properties of a butt joint with a ceramic backing welded by robotic method 135 (MAG-metal active gas welding method) and to determine the effect of HMFI on the stress level. This analysis was based on a comparison of three butt joints made of a S690QL plate, in the as-welded condition, with the HFMI of each bead and with the heat treatment carried out with PWHT stress relief annealing. The high-frequency (90 Hz) peening of each weld bead was linked with a stress reduction in the weld via the implementation of compressive stresses into the joint. The HFMI pneumatic hammer was used for this. The correctness of treatment was achieved when 100% of the surface of each bead including the face was treated. As part of the post-welding tests, basic tests were carried out based on the standards for the qualification of welding technology, and as a supplementary test, a stress state analysis using the Barkhausen effect was carried out. The tests carried out showed that the use of high-frequency peening after each pass did not affect the negative results of all the required tests when qualifying the welding technology of S690QL sheet metal compared to the test plates in the as-welded condition and after heat treatment-stress relief annealing. Inter-pass peening of the welded face and HAZ (heat-affected zone) resulted in a reduction in post-weld residual stresses at a distance of 12 mm from the joint axis compared to the stress measurement result for the sample in the as-welded condition. This allowed for a positive assessment of peening in the context of reducing the notch, which is the concentration of tensile stresses in the area of the fusion line and HAZ. The tests carried out showed that the peening process does not reduce the strength properties of welded joints, and the results obtained allow the technology to be qualified based on applicable standards.

Effect of Corrosion and Post-Weld Treatment on the Fatigue Behavior of Multipass Robot GMAW Welds of S700MC Steel

High-strength steel is a candidate material for offshore structures, which are currently being constructed with regular-strength steel. These structures are constantly exposed to harsh environmental conditions and experience cyclic loadings, which can lead to premature failure due to the synergistic effects of corrosion and fatigue. In this regard, the current study aims to investigate the effects of corrosion and High-Frequency Mechanical Impact (HFMI) treatment on the fatigue behavior of welded joints made of S700MC steel. Multipass butt-welded joints were fabricated via the Robot GMAW method at an optimally selected heat input of 0.7405 kJ/mm. The microstructure of the weldments was studied using light optical microscopy. Tensile and Vickers microhardness tests were performed to evaluate the mechanical properties of the welded joints. To simulate marine environment corrosion in the laboratory, the as-welded samples were exposed to salt fog spray for 720 h. Subsequently, specimens were subjected to cyclic loading to evaluate their fatigue strength, while SEM and stereomicroscopy were used to analyze the fractured surfaces, providing a comprehensive understanding of the fracture mode. The findings suggest that although corrosion led to increased surface roughness and the formation of corrosion pits, its influence on the fatigue behavior of the weldments might be less significant compared to other geometrical factors, at least for the exposure time employed in the study.

Fatigue Life Prediction of Prefatigued Welded Q235 Steel Butt-Joints Treated by High Frequency Mechanical Impact

Fatigue Life Prediction of Prefatigued Welded Q235 Steel Butt-Joints Treated by High Frequency Mechanical Impact

Fatigue tests of S690 as-welded and HFMI-treated details with longitudinal and transverse attachments

The complex nature of the fatigue strength of welded steel details, which is influenced by a multitude of factors, can be improved by extending the crack initiation phase of the total fatigue life. This enhancement can be achieved through the use of the High-Frequency Mechanical Impact (HFMI) method. This study discusses the fabrication and laboratory cyclic testing of as-welded and HFMI-treated transversal and longitudinal attachments made from S690QL steel. Assessment of a two-stage model (TSM) for the evaluation of their fatigue life, previously developed by the authors, is also performed. The TSM's estimated fatigue lives of the details under consideration align well with the fatigue test results. Moreover, both the TSM model results and the corresponding fatigue test results demonstrate that the HFMI method significantly prolongs their fatigue life. The fatigue resistance of HFMI-treated details compared to as-welded details increased by 1.67 and 1.91 times for longitudinal and transversal attachments, respectively.

Local relaxation of residual stress in high-strength steel welded joints treated by HFMI

This research studies the influence of high-peak loads on local relaxation of residual stress and fatigue damage in high-strength steel welded joints treated by high-frequency mechanical impact (HFMI) treatment. The joint behavior is simulated with elastic–plastic finite element analyses that account for the combined effect of geometry, residual stress, and material properties. This simulation uses two treated geometry models: with or without surface roughness on HFMI groove, and two material properties: S690QL and AH36 structural steels. The results show that surface roughness and load history, including high-peak loads, significantly influence fatigue response. It is revealed that the model neglecting the surface roughness cannot represent the amount of residual stress change and fatigue damage at less than 100 µm depth from the surface. In addition, the local yield strength in the HFMI-treated zone affects the plasticity behavior near the surface imperfection under the high-peak loads, which provides comparatively different fatigue damage between S690QL and AH36 in some cases. As a result, this study provides the further understanding needed to develop a robust modeling approach to the fatigue life estimation of HFMI-treated welds subjected to high-peak loads.

Fatigue enhancement of welded thin-walled tubular joints made of lean duplex steel

In the present work, the fatigue strength and enhancement techniques of welded thin-walled (wall thickness of t = 2 mm) square hollow section (SHS) joints made of lean duplex stainless steel (grade EN 1.4162) are experimentally and numerically studied. Experimental fatigue tests were carried out on gas metal arc-welded tubular X-joints subjected to fully reversed (applied stress ratio of R = −1) constant amplitude in-plane bending moment. Geometrical improvement techniques were investigated by applying fully penetrating fillet welds and using structural reinforcements. The applications of high-frequency mechanical impact (HFMI) treatment were examined in the context of further enhancing the fatigue strength of the geometrically improved joints. In addition to the experimental work carried out in this study, the fatigue test data of welded thin-walled SHS and rectangular hollow section (RHS) joints were extracted from literature. Structural stress concentration factors were numerically and analytically obtained to validate the fatigue assessment model for welded thin-walled tubular joints using the structural hot-spot stress method. In addition, the effective notch stress (ENS) concept with the reference radius of rref = 0.05 mm was implemented. Compared to the joints in the as-welded state, fatigue strength improvements of 1.53–2.7 were found in the geometrically improved joints. On the other hand, the HFMI treatment did not lead to an additional increase in the fatigue strength capacity of the studied AW joints. The CIDECT guidelines for the structural hot-spot stress method were found conservative for thin-walled joints (t < 3 mm) and could be recommended for fatigue design.

Synergetic Effects of Macro- and Microscopic Residual Stresses Induced by High-Frequency Mechanical Impact Post-weld Treatment on Fatigue Strength Enhancement of S335 Steel T-Weld

Synergetic Effects of Macro- and Microscopic Residual Stresses Induced by High-Frequency Mechanical Impact Post-weld Treatment on Fatigue Strength Enhancement of S335 Steel T-Weld

Investigations on the influence of angular and linear misalignment on the fatigue strength of HFMI-treated structural steel butt joints

The effect of High-Frequency Mechanical Impact (HFMI) treatment on the fatigue resistance of structural steel S355 butt welds with macro geometric imperfections outside of the usual fatigue design limits was investigated. Thin-walled specimens (t = 8 mm) were manufactured with different degrees of angular and linear misalignment and fatigue tested in the as welded and HFMI treated condition. The fatigue test results show that HFMI treatment can compensate the fatigue strength-reducing effects of macro geometric imperfections (ΔσC,HFMI = 127 N/mm2, m = 5), but also that linear and angular misalignment does not reduce the fatigue strength of butt welds as severely as rules and guidelines suggest (ΔσC,AW = 107 N/mm2, m = 3). Underestimating the fatigue strength determined by testing by more than 40%, numerical analysis and strain gauge measurements show that the application of the effective notch stress approach and the structural hot-spot stress approach delivers conservative assessment results, which might not always be representative for the actual fatigue strength benefit that can be gained by HFMI treatment.

Fatigue behaviour of automatically HFMI-treated welds

Due to notches, welds are most critical regarding fatigue failure within cyclic loaded constructions. Therefore, various post-weld-treatment techniques like post-weld treatment by high-frequency mechanical impact (HFMI) treatment have been invented to improve the fatigue strength of welded details. The benefit, resulting from HFMI treatment, has already been proven by numerous studies. Since a manual HFMI treatment must be performed by a skilled and trained person to ensure an acceptable treatment quality, an automated application of HFMI treatment is supposed to result in a more reliable and consistent treatment result, which does not depend on the operator. Furthermore, a robotic application of HFMI treatment enables an economic implementation of HFMI treatment of automated welded constructions like offshore wind energy converters and various mechanical components, as these parts do not have to be taken out of the production chain to manually perform HFMI treatment. This paper focuses on the experimental investigation of the fatigue behaviour of automated HFMI-treated welds, using a developed robotic application of the HiFIT device (specific HFMI tool).

Steel and composite bridges – Enabling sustainable solutions

AbstractSustainable bridges are characterised by three main aspects: saving resources by an efficient construction, allowing for a lifelong use of bridges by optimal detailing in view of fatigue and flexible design concepts for strengthening and adapting to changed user requirements. After a short introduction in the assessment of sustainability for bridges, an overview on recent bridge research in view of sustainability is given. The key point of efficient construction in future is the use of high strength steel (HSS): basic research and adaptation of rules are needed for welding of matching and mismatching connections. Joints and their relevant failure modes are to be investigated before hollow sections of HSS can be used in a wider range. And for steel and composite bridges, optimised plate buckling rules may serve for more efficiency in construction. Bridges are designed for a service life of 100 years. This can easily be achieved and even exceeded if the detailing is realised in view of fatigue. Research on welded details such as butt welds or joints of tubular bracings will be presented. Welds can be improved by post‐weld treatment such as high‐frequency mechanical impact (HFMI) treatment. Fatigue is also a key issue when steel and concrete or timber and concrete are combined for composite bridges. Reinforced composite slabs without prestressing require a fatigue verification for the transverse shear also in areas with cracked concrete. The notch forms an efficient connection in timber‐concrete composite (TCC) bridges, where, however, little has been known up to now on the fatigue behaviour.

HFMI-induced fatigue strength improvement of S355 steel transverse non-load-carrying attachments with lack of fusion in the weld root

The effects of high-frequency mechanical impact (HFMI) treatment and preloading on the fatigue behaviour of the S355 steel transverse non-load-carrying attachments welded with technological defect (‘lack of fusion’) in the weld root are studied. The S355 steel joints were purposely welded with incomplete penetration (a technological defect of 3x3 mm formed over the entire weld root length). As established, this technological defect does not affect the fatigue life since the crack initiation and propagation occur along the weld toe. HFMI leads to a tenfold increase in fatigue life of the joints with technological defects. Preloading the as-welded joints at maximum alternating stress of 260 MPa for 64,000 cycles leads to partial relaxation of tensile residual stresses. HFMI transforms tensile stresses into compressive ones in both non-preloaded and preloaded samples. Compressive stresses of 315 MPa and 100 MPa were respectively registered at distances of 3 mm and 13 mm from the HFMI-formed groove using the hole-drilling method and electron speckle interferometry. Transmission electron microscopy of the weld-heating affected zone reveals that the bimodal microstructure, containing the areas with ultrafine and larger grains, observed after sequentially applied preload and HFMI remains stable during further fatigue cycling and facilitates the enhanced fatigue strength/life.

Life-cycle Cost assessment of Post Weld treatments: Effect of local weld geometries

The fatigue life of welded structures is dependent on the local geometry and imperfections of the welds. Therefore, optimizing the manufacturing processes to improve the local weld geometry and remove possible imperfections can considerably affect the fatigue life of the structure. Post-weld treatments such as burr grinding, and High Frequency Mechanical Impact (HFMI) treatment are commonly used techniques to extend the fatigue life of a weld by modifying the local weld toe geometry. However, employing additional manufacturing processes can have an adverse effect on the Life-cycle cost of a welded structure by increasing production costs. On the other hand, extending the fatigue life of a structure can result in lower maintenance and replacement costs. Therefore, a thorough yet predictive life-cycle cost assessment is required to assess the viability of such treatments and design economically efficient weld structures.This study assesses the life-cycle cost of welded joints. The fatigue life of the weld in this study is analyzed using the effective Notch Stress method (NS). Furthermore, the weld is post-treated with an automated HFMI treatment to prolong its fatigue life. Moreover, the effect of weld quality on the production cost is analyzed. The results show that every phase of the weld's life-cycle has a significant contribution to the life-cycle cost with the use-phase being the more dominant. The results also depict the impact of the changes in weld quality in the overall life-cycle cost.

Ultimate strength and fatigue of stiffened welded tubular joints in floating energy production structures

Monotonic and cyclic loading experiments are performed on 1/6-scale stiffened steel joints representing the welded connection between the tubular braces and the vertical cylinders of a prototype offshore floating structure for combined wind-wave energy production. The structural behavior of the welded joints is investigated to determine their ultimate strength under monotonic loading and their fatigue life. The brace-to-cylinder welded connection is full penetration and semi-automatic, whereas the stiffeners are welded to the inner surface of the cylinder with fillet welds. High-frequency mechanical impact post-weld treatment is applied to five specimens. Experimental observations indicate two critical locations for fatigue crack initiation: the crown of the brace-to-cylinder weld and the fillet weld of the central stiffener. These locations are also verified by finite element simulations and fractographic examination of failed specimens. Post-weld treatment improves the fatigue resistance of the welded joints in the high-cycle fatigue domain but has negligible influence on the low-cycle fatigue response of the connection and its monotonic ultimate strength. Finally, one fatigue-cracked specimen is tested under monotonic loading exhibiting structural performance similar to the intact specimens.

Combining electric discharge surface alloying and high-frequency mechanic impact post-processing for increased corrosion fatigue life of as-welded transverse non-load-carrying attachments of the S355 steel

The combined effects of severe plastic deformation induced by high-frequency mechanical impact (HFMI) treatment and the electric discharge surface alloying (EDSA) with chromium on the corrosion fatigue life of low-carbon S355 steel transverse non-load-carrying specimens are studied in this paper. First, for various alternations of the combined HFMI + EDSA process, the surface alloyed layer integrity and depths were analyzed using scanning electron microscopy (SEM) accompanied with energy dispersive spectroscopy (EDS) and hardness measurements. The alloying and hardening depths were respectively observed to be 10–30 μm and 0.15–0.75 mm and the surface hardness increased up to 3.6–3.9 GPa. Polarization tests demonstrated higher corrosion resistance of the EDSA-processed samples in 3%NaCl. The near-surface layers of the HFMI + EDSA(Cr) + HFMI processed sample characterized by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) contain the dislocation-cell structure formation and partially dissolved cementite. The fatigue tests in 3%NaCl confirmed that the HFMI + EDSA(Cr) + HFMI post-weld process increased corrosion fatigue life (1.77 × 106 cycles) of the S355 steel welded joint at the stress level of 260 MPa by 2 and 12 times in comparison with the HFMI-treated (0.9 × 106) and as-welded (0.14 × 106) samples, respectively.

Mean stress effect in high‐frequency mechanical impact (HFMI)‐treated welded steel railway bridges

AbstractThe need for new railway bridges is driven by the growing volume of transportation demands for both passenger and freight traffic on railway networks. In the design of these bridges, the fatigue limit state is a criterion that usually limits the allowable applied load level and thus also the utilization of the high strength of the steel material. Therefore, improving the fatigue performance of welded details by high‐frequency mechanical impact (HFMI) treatment leads to a more efficient design. However, the fatigue performance of HFMI‐treated welds is known to be affected by the mean stress and this needs to be considered in the design of treated welded details in steel bridges. This is rather straightforward if the bridge is subjected to cycles from one type of train but becomes cumbersome when several different sets of trains (e. g. axle loads, axle distances) cross the bridge. In this article, a factor to take the mean stress effect (including self‐weight and traffic load variations) into account is derived from traffic data measured in Sweden. Moreover, the mean stress effect is also predicted using the different fatigue load models in the Eurocode. These models either consist of one‐load patterns such as LM71, SW/0, and SW/2 or are composed of different trains with different combinations. It was found that the mean stress effect is underestimated by the first group of models. On the other hand, the mean stress predicted by the light traffic mix is found to be close to that calculated using real traffic data, while other mixes (standard and heavy) underestimate the mean stress effect. Therefore, a correction factor to account for the mean stress effects in real traffic is derived (called here λHFMI). This factor can be used to correct the design stress range for fatigue verification of HFMI‐treated welded details in railway bridges.

Fatigue design and assessment guidelines for high-frequency mechanical impact treatment applied on steel bridges

Structural design of bridges in Europe should be carried out in accordance with Eurocode regulations. However, there is no guideline demonstrating how the fatigue design is to be conducted when high-frequency mechanical impact (HFMI) is used to enhance welded joints in steel bridges. The aim of this paper is to present the different design rules and equations and apply them to some example bridges enhanced by HFMI treatment. Fatigue verification of some welded details in these bridges is carried out via either “damage accumulation” or “λ-coefficients” methods in the Eurocode. Four fatigue load models are used in the fatigue verification (FLM3 and FLM4 for road bridge assessment, and LM71 and traffic mix, for railway bridge assessment). The effect of steel grade, mean stresses, self-weight, variation in stress ratios, and maximum stress on the treatment efficiency is considered in both examples. It is found that HFMI treatment causes a significant increase in fatigue lives in all studied cases.

On the Fatigue Strength of Welded High-Strength Steel Joints in the As-Welded, Post-Weld-Treated and Repaired Conditions in a Typical Ship Structural Detail

Weld quality and life extension methods of welded details in ship structures made of high-strength and ultra-high-strength steels are of high importance to overcome the issues related to the fatigue characteristics of welded high-strength steels. The current work experimentally and numerically investigated the fatigue strength of a longitudinal stiffener detail, typically present in the bulkhead connections of ship hull. Two high-strength steel grades, namely EQ47TM and EQ70QT steels, were studied in welded plate connections using gas metal arc welding with rutile-cored wires. Fatigue tests were carried out on both small-scale specimens under axial and large-scale beam specimens under four-point bending loading. In addition to the joints tested in the as-welded condition, the high-frequency mechanical impact (HFMI) treatment was considered as a post-weld treatment technique in the fatigue test series. Furthermore, the large-scale beam specimens were pre-fatigued until substantial fatigue cracks were observed, after which they were re-tested after weld repairing and post-weld treatments to investigate the potential to rehabilitate fatigue-cracked ship details. The joints in the as-welded condition were performed in accordance with the current design recommendations. Due to the severe transition from the base material to the weld reinforcement in the joints welded with the rutile-cored wire, a successful HFMI treatment required geometrical modification of weld toe using a rotary burr to avoid any detrimental sub-cracks at the HFMI-treated region. Alternatively, the use of solid filler wires could potentially overcome these issues related to the welding quality. Repaired and post-weld-treated welds performed well in the re-tests, and the fatigue strength was almost twice higher than that of tests in the as-welded condition.

Numerical study on local residual stresses induced by high frequency mechanical impact post-weld treatment using the optimized displacement-controlled simulation method

The High Frequency Mechanical Impact (HFMI) treatment is a reliable, effective, and user-friendly post-weld technique for fatigue life enhancement of welded structures. The working principle for HFMI can be characterized by impacts from cylindrical indenters with a high frequency which introduces the local work hardening and high compressive residual stresses (RS) and reduces the stress concentration at the weld toe. These beneficial effects are needed to be considered in the fatigue assessment and design of those HFMI-treated structures. This study proposes a multi-process numerical simulation model to estimate local RS and improved shape profiles induced by HFMI treatment in welded joints. Firstly, the welding simulation is performed by thermal-elastic-plastic finite element (TEPFE) analysis using a coarse finite element (FE) mesh model. The zooming technique is then applied to transfer simulated as-welded RS to the ultra-fine mesh used in HFMI analysis as initial stresses. Finally, the numerical analysis of HFMI-induced RS is carried out using the optimized displacement-controlled numerical simulation method. An HFMI-treated out-of-plate gusset welded joint specimen was fabricated using SM490 steel. The surface topography for the treated zone, RS before and after the HFMI process, and thermal cycles during the welding process were experimentally investigated to calibrate and validate the proposed simulation approach. Both simulated local deformed shape profiles and RS were in good agreement with the measurements. In this paper, the recommendations for FE modeling and the procedures for determining HFMI analysis parameters were discussed in detail.

Verification of the Maximum Stresses in Enhanced Welded Details via High-Frequency Mechanical Impact in Road Bridges

High-frequency mechanical impact (HFMI) is an efficient post-weld treatment technique that enhances fatigue strength in metallic welded structures. Steel or steel-concrete composite road bridges, where the fatigue limit state often governs the design, compose one category of structures that can benefit from the application of this technology. To assert an improvement in fatigue strength using HFMI, the induced compressive residual stresses must be stable. Therefore, the maximum service stresses that can be allowed on HFMI-treated joints should be controlled to avoid the relaxation of the induced beneficial compressive stresses by HFMI treatment. Using statistical analysis of recorded traffic, this paper compares the measured maximum traffic loads to those generated by a load model. More than 870,000 and 470,000 recorded vehicles from traffic measurements in Sweden and the Netherlands are used in this analysis. To capture the characteristic bending moment, the daily maxima of the resulting measured load effect are combined with the extreme value distribution of the bending moment. In addition, it is found that the characteristic load combination is the best-studied option to assess the maximum stress in HFMI-treated weldments in road bridges.