High Frequency Mechanical Impact Treatment Research Articles

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.

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

AbstractDue 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).

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.

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.

Corrosion Effect on the Efficiency of High-Frequency Mechanical Impact Treatment in Enhancing Fatigue Strength of Welded Steel Structures

Fatigue and corrosion are two material degradation phenomena that occur in welded steel structures. High-frequency mechanical impact (HFMI) treatment is a post-weld treatment method that aims to increase the fatigue strength of welded details. This paper investigates the effect of steel corrosion on the efficiency of this method in enhancing fatigue resistance. More than 150 fatigue test results on corroded and HFMI-treated welded details are collected from several research articles and analyzed for both transverse welded attachment and butt-welded details. The efficiency of HFMI treatment decreases in corroded details as the corrosion level increases. However, HFMI treatment is found to have a high potential in prolonging the fatigue life, even in circumstances of an extremely corrosive environment.

Fatigue performance of the welded details of an old, demolished steel railway bridge

This study investigates the fatigue performance of the welded details of an old disused steel railway bridge. The Saimaa canal railway bridge was demolished after 60 years of service, and as a part of the demolition process, a fillet-welded vertical web stiffener joint of an I-profile girder was taken for experimental testing. Fatigue tests were carried out on the joints tested in both as-welded and post-weld-treated conditions, using burr grinding, high-frequency mechanical impact (HFMI) treatment, and TIG dressing as improvement techniques. To examine the reduction in stress concentration due to the post-weld treatments, numerical analyses were performed as per the effective notch stress (ENS) approach and the theory of critical distance (TCD) methods, and different local approaches were used to assess the fatigue strength of the welded details. According to the obtained results, the fatigue strength of the joints in the as-welded condition was higher than that of recommended by fatigue design standards, indicating good welding quality and suggesting that the tested welded details had not experienced severe fatigue loads during service. HFMI treatment and burr grinding improved the fatigue strength by 30%, meanwhile no failures in the joint area were observed with the TIG dressing. Of the applied local approaches, the structural hot-spot stress method, 4R method, and ENS concept showed the highest accuracy in the fatigue assessments, and had lower scatter than the TCD methods.

Damage-Based Assessment of the Fatigue Crack Initiation Site in High-Strength Steel Welded Joints Treated by HFMI

This study aimed to identify the fatigue crack initiation site of high-frequency mechanical impact (HFMI)-treated high-strength steel welded joints subjected to high peak stresses; the impact of HFMI treatment residual stress relaxation being of particular interest. First, the compressive residual stresses induced by HFMI treatment and their changes due to applied high peak stresses were quantified using advanced measurement techniques. Then, several features of crack initiation sites according to levels of applied peak stresses were identified through fracture surface observation of failed specimens. The relaxation behavior was simulated with finite element (FE) analyses incorporating the experimentally characterized residual stress field, load cycles including high peak load, improved weld geometry and non-linear material behavior. With local strain and local mean stress after relaxation, fatigue damage assessments along the surface of the HFMI groove were performed using the Smith–Watson–Topper (SWT) parameter to identify the critical location and compared with actual crack initiation sites. The obtained results demonstrate the shift of the crack initiation most prone position along the surface of the HFMI groove, resulting from a combination of stress concentration and residual stress relaxation effect.

Fatigue design of mild and high-strength steel cruciform joints in as-welded and HFMI-treated condition by nominal and effective notch stress approach

According to the Recommendations for Fatigue Design of Welded Joints and Components by the International Institute of Welding (IIW), the fatigue strength of welded steel joints is in general independent of the base material strength. Post-treatment methods, such as the High Frequency Mechanical Impact (HFMI) treatment, can significantly increase the fatigue performance of welded joints especially in case of high-strength steel applications, which is already considered within the IIW Recommendations for the HFMI Treatment. This paper firstly investigates the effect of the base material strength on the fatigue resistance of welded and HFMI-treated steel joints. Therefore, mild steel S355 and high-strength steel S700 cruciform joints are cyclically tested in both conditions and the statistically evaluated S/N-curves are compared. The results reveal an increase of the high-cycle fatigue strength by the HFMI-treatment by a factor of 1.35 in case of the S355, and of 1.59 for the S700 specimens. Secondly, the test results are assessed by the fatigue design curves of the corresponding structural detail within the IIW-recommendations using the nominal stress concept. Moreover, the applicability of the procedure applying the effective notch approach is analyzed. In order to numerically evaluate the effective notch stress, the geometry of the cruciform joint is modelled according to the given guidelines within the recommendations applying a reference radius of 1 mm at the weld toe for both conditions. On the basis of the numerically computed effective notch stress, a local fatigue design is performed for all test series, which reveals sound accordance between the recommended design curves and the statistically evaluated fatigue test S/N-curves.

Opportunities and partial problems of HFMI implementation in design of welded structures of rail vehicles

This paper highlights the main impacts of HFMI (High Frequency Mechanical Impact) technology on increasing the fatigue resistance and extending the fatigue life of welded joints. In the case study there are SN curves of T-welded joints made of two different steels (conventional steel S235JR and high strength steel S460MC). For both materials there is a comparison of SN curves with and without HFMI treatment. Both samples are loaded by bending for cycle asymmetry R = 0. The failure criterion is a decrease in stiffness caused by the formation of a macrocrack. The results of fatigue testing are then applied in a parametric study, which assumes a random loading of a selected structural detail in a railway application. Opportunities and barriers of the implementation of HFMI in practice are mentioned and documented according to results of laboratory experiments, which take place in cooperation with the university research center and the manufacturer of railway vehicles.

VHCF Behavior of Welded Joints with HFMI Treatment under Moisture Conditions

Fatigue tests of cruciform welded joints made of Q355B steel at very-high-cycle fatigue (VHCF) regimes were carried out on as-welded specimens using high-frequency mechanical impact (HFMI) treatment in dry air and water-spray environments, respectively. The influence of the environment on fatigue life was more obvious in the VHCF regime. It was found that S-N curves became flat over the range of 106–108 cycles for as-welded specimens, while a continuously decreasing S-N curve existed for HFMI-treated specimens. Fatigue cracks initiated from the weld toe of the as-welded specimens in dry air and water-spray environments. Due to residual stress, the crack initiation site transition of HFMI-treated specimens from the weld toe to the weld root and base metal was observed at lower stress levels. Moreover, hydrogen-assisted quasi-cleavage and intergranular fracture were captured using a scanning electron microscope and a hydrogen permeation test.

Material and residual stress improvement in S355 welded structural steel using mechanical and thermal post‐weld treatment methods

AbstractWelding is by far the most widely used metal joining method. High‐Frequency Mechanical Impact (HFMI) treatment and Tungsten Inert Gas (TIG) remelting are two post‐weld treatment methods that aim to enhance the strength of the steel. In this paper, the improvement in residual stress and material characteristic obtained with these methods are studied by conducting several experimental investigations such as hole drilling, hardness testing and microscopy. Hole drilling shows that HFMI treatment improves the status of residual stress at the weld toe in the first 1 mm from the surface. Furthermore, Vickers testing shows a remarkable improvement in the hardness values at the weld toe in the first 2 mm. This can be attributed to the reduction in grain size after treatment. Moreover, acicular ferrite and tempered bainite are found to be the main constituents in the fusion and heat‐affected zones after TIG‐remelting.

Computational crack propagation modeling of welded structures under as‐welded and high frequency mechanical impact (HFMI) treatment conditions

AbstractThe fatigue damage assessment of welded structures is of critical importance in the design of many engineering structures. The thermomechanical nature of the welding process leads to undesired tensile residual stresses, which results in fatigue performance degradation of welded structures. High‐frequency mechanical impact (HFMI) technique offers significant potentials to induce compressive residual stresses into the welded joints, thus improving fatigue life of welded parts. In this paper, a new crack propagation modeling approach considering effects of residual stress fields and the crack closure based on a modification of the Forman model is proposed to assess fatigue crack propagation performance of welded joints under as‐welded and HFMI treatment conditions. Experimental residual stress and fatigue data sets of 5083‐H321 aluminum, CSA 350W, ASTM A514, S355, and S960 steel welded joints are used to validate the proposed approach. Both predicted and experimental results show that the fatigue life improvement by the HFMI treatment depends on the applied stress level and material type. The fatigue life improvement is greater at the lower stress levels with a factor of more than 10–30 times, and the fatigue life increase becomes less at the higher stress levels with a factor of two to three times depending on the material type of welded joints. The fatigue life improvement by the HFMI is more significant with a higher material yield strength. Compared results showed that the proposed modeling approach provides efficient and accurate life predictions of the welded joints of five different materials under both as‐weld and HFMI conditions. The proposed modeling framework can be used as an effective analysis technique for fatigue crack propagation life of welded structures under pre‐ and post‐weld treatment conditions to account for residual stress fields.

Application of critical distance approach for fatigue assessment of welded and HFMI-treated steel joints

High Frequency Mechanical Impact (HFMI) treatment offers a great opportunity to enhance the lightweight potential of welded structures using high strength steel materials. Focusing on the IIW Recommendations for the HFMI Treatment, a fatigue assessment based on the nominal, structural as well as notch stress approach is considered. Due to its sound applicability and proper assessment of the fatigue strength, the critical distance approach acts as a valuable concept in fatigue design, which has not been broadly studied in terms of HFMI-treated weld joints. Therefore, this paper deals with the application of the critical distance approach for welded mild and high-strength steel joints in both as-welded and HFMI-treated condition. Critical distance values were determined as a = 0.1 mm for as-welded specimens and a = 0.2 mm for HFMI treated specimens but for the sake of reliability, a = 0.15 mm have been proposed for HFMI treated welded joints.

Application of high frequency mechanical impact treatment to improve the fatigue strength of corroding welded joints

This study deals with the fatigue strength of high-frequency mechanical impact (HFMI)-treated unprotected structural details made of mild steel S355 considering the influence of corrosive environmental conditions. The investigations are carried out on butt welded specimens with sheet thickness t = 15 mm and on transverse non-load-carrying attachment specimens with sheet thickness t = 25 mm. Two different methods were applied for the simulation of marine corrosive environment in the laboratory. Specimens first were deposited in a salt spray chamber and then tested subsequently dry at laboratory-air conditions considering the influence of corrosion on the crack initiation. Alternatively, and to cover the effects of corrosion on the crack growth, artificial seawater was used for pre-corrosion, and after a defined timespan, fatigue tests were performed simultaneously with the specimen resting in the corrosive medium. The corrosion fatigue tests were performed in as-welded and HFMI-treated conditions at a stress ratio R = 0.1 under axial tensile and 4-point bending cyclic loading. The test results are evaluated to determine the characteristic fatigue strengths for fixed slopes m = 3 and m = 5 according to IIW recommendations for the as-welded and for the HFMI-treated condition respectively. The results of the experimental investigations based on the nominal stress approach show that the fatigue strength of both specimen types could be significantly increased by the application of HFMI treatment compared to the corresponding specimens in the as-welded condition even if exposed to the investigated corrosive conditions. The comparison with the design proposals of IIW shows that for HFMI-treated butt welds, no reduction of the FAT class due to corrosion is required and the recommended FAT class is still valid. The results for the HFMI-treated transverse attachments are slightly below the design curve recommended by IIW and a proposal to consider corrosion is derived for this case. Additional numerical investigations by applying the effective notch stress (ENS) approach are performed to determine notch stress curves. It was found that for the corroded specimens in the as-welded condition, the FAT class according to IIW could not be reached and adjustments of the existing rules are necessary to consider corrosion effects. However, it can be concluded that the effective notch fatigue resistance recommended by IIW is still applicable in the case of corroded HFMI-treated structural details.

Fatigue-performance of PWHT welded joints: As-welded vs. high-frequency mechanical impact treatment

In the study, fatigue tests were conducted to discuss the detrimental effect of local softening induced by post-weld heat treatment (PWHT) on the fatigue behavior of welded joints and subsequent high frequency mechanical impact treatment (HFMI) improvement. According to the distribution of stress and strain by numerical simulation, the difference between fatigue behavior of welded joints with and without PWHT becomes significant with the increase of stress range since the dominant mechanism of fatigue behavior changes from ‘stress-controlled’ to ‘strain-controlled’. The fatigue life of as-welded joints treated by HFMI is longer than that with PWHT+HFMI treatment. In particular, the difference is more obvious with the decrease of stress range due to the difference among the release amounts of compressive residual stress induced by HFMI. The fatigue crack initiation sites are dependent on stress range, local softening effect by PWHT and HFMI treatment.

Fatigue strength of normal and high strength steel joints improved by weld profiling

While some post-weld treatment techniques increase the fatigue life mainly due to induced compressive residual stress others are trying to create a smooth transition at weld toes. One of the latter is weld profiling. This study investigates the effect of weld profiling for four different steel types from S355 to S900 including one duplex stainless-steel and performs a comparison with high frequency mechanical impact (HFMI) treatment. The observed fatigue strength improvement is significantly higher than typically assumed for such techniques and comparable to increases observed for HFMI treatment. The fatigue strength further increases with parent material strength and is only slightly below the estimated fatigue strength of the parent material. Finally, fatigue design curves are proposed for weld profiling that include an increasing fatigue strength improvement for higher strength materials, but that are still conservative for low weld quality.