High Frequency Mechanical Impact Research Articles

Sequence effect of p(1/3) spectrum loading on service fatigue strength of as-welded and high-frequency mechanical impact (HFMI)-treated transverse stiffeners of mild steel

In the meantime, it’s well known that post-weld fatigue strength improvement techniques for welded structures like high-frequency mechanical impact (HFMI) treatment increase the fatigue live of welded joints. Although the current design recommendations for HFMI-treated welded joints give first design proposals for the HFMI-treated welds, in practice the application of HFMI treatment and the associated increase in fatigue resistance are still being discussed. There are, for example, reservations regarding the efficiency of HFMI-treated welded joints under variable amplitude loading (VAL). This paper analyses first results for the sequence effect of VAL of a p (1/3) spectrum on the service fatigue strength of HFMI-treated transverse stiffeners (TS) of mild steel (S355). Fatigue test results with random and high-low loading for the two states as-welded (AW) and HFMI-treated joints will be presented. The modified linear damage accumulation and the failure locations will be discussed. The experimental results show a clear change in the slope of the S-N curve from the as-welded (AW) state to the HFMI state and additionally in the HFMI state from constant amplitude loading (CAL) to variable amplitude loading (VAL). It was particularly noticeable in the experimental results of all tested HFMI series that the specimens failed exclusively in the base material 2–4mm before the HFMI-treated welds. The presented results of the investigations show that with application of the nominal stress concept, no sequence effect was recognizable.

Assessment of in-service stresses in steel bridges for high-frequency mechanical impact applications

The application of high-frequency mechanical impact (HFMI) treatment to improve the fatigue performance of composite steel and concrete road bridges was studied through a state-of-the-art review in conjunction with simulations of variable amplitude in-service stresses in four case-study bridges in Sweden. Empirical stress range spectra with associated mean stresses were characterised for HFMI-treated bridges. It was shown 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. Calculations also showed a considerably better fatigue performance if HFMI treatment is performed on site, after the application of self-weight stresses.

Investigation of corrosive influence on the fatigue behaviour of HFMI-treated and as-welded transverse non-load-carrying attachments made of mild steel S355

This study reports on the corrosion fatigue behaviour of fillet welds in the as-welded and the high frequency mechanical impact (HFMI) treated condition. For this purpose, transverse attachment specimens made of steel S355J2+N were investigated. Seawater corrosion was simulated by immersing the specimens in artificial seawater and by the salt spray chamber test. Sequential and simultaneous corrosion fatigue tests were carried out on the corroded specimens under axial and bending loading. FEM simulations show that the stress concentration factors are reduced due to the corrosion-related change in HFMI-treated weld geometry. HFMI significantly improves fatigue life in all investigated cases.

Influence of the accelerated corrosion exposure on the fatigue behaviour of welded joints treated by high frequency mechanical impact

This study investigates the effectiveness of the HFMI treatment to improve the fatigue of welded joints before and after the exposure of neutral salt spray fog. Microstructural analysis of surface layer of weld toe after corrosion testing has been performed. HFMI-treated joints after the corrosion has resulted 3 times increase in the fatigue life. Following the preloading and corrosion, the further application of HFMI has let to 4 times increase in the fatigue life. The corrosion rate of the base metal before and after HFMI has changed non-monotonically over the time. Plastic deformation of the HFMI-treated surface layer after corrosion test has decreased partially.

2D fracture mechanics analysis of HFMI treatment effects on the fatigue behaviour of structural steel welds

The purpose of this paper is to present the extension of a previously developed strain-based fracture mechanics (SBFM) model from one to two dimensions for simulating fatigue crack growth in steel arc welds. The one-dimensional (1D) SBFM model is first briefly reviewed. Steps for extending it to simulate 2D growth of a semi-elliptical surface crack in a weld are then described. An application of the new model is lastly presented, which consists of analysing the behaviour of structural steel weld specimens from a previously published fatigue test program, in which the fatigue performance of weld specimens was enhanced by high frequency mechanical impact (HFMI) treatment. In this application, the effect of the treatment-induced compressive residual stress on the crack shape is simulated, and it is shown how the extended 2D SBFM model can be used to establish probabilistic design stress-life (S-N) curves for various loading conditions.

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 of the 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 damage of welded high-strength steel details improved by post-weld treatment subjected to critical cyclic loading conditions

High-frequency mechanical impact (HFMI) treatment has proven its potential in many aspects, however the improvement under the conditions of high stress ratios, stress ranges and variable amplitude loading (VAL) has not been sufficiently addressed. In that respect, the aim of this work is to verify the indications and trends in the recently published literature and to understand the local weld toe mechanisms related to several cyclic loading conditions. The investigations were performed for three types of welds subjected to high stress ratios, ranges as well as VAL in order to represent the real-life scenarios, where the aim was to include the state of compressive residual stresses, cyclic behaviour of material and eventually estimate the fatigue life.The induced beneficial stresses were relaxed after applying compressive overloads (OL). However, a tensile OL indicated a potential benefit, by reducing the extent of residual stress relaxation. With the increasing stress ratio (R) and maximum stress (σmax), the improve- ment due to the HFMI decreased 59% as compared with the cases for OL = 0.8fy. The estimated damage decreased rapidly from R = −1 to R = 0.2, however it decreased slowly from R = 0.2 to R = 0.5, indicating that the effect of HFMI was reduced for high stress ratios due to the stress relaxation. The remaining improvement was then governed by the modification of weld geometry and the strain hardening effect. The fatigue damage was decreased from as-welded (AW) to HFMI states. For R = −1, longitudinal attachments were the most improved joint with 86%, while transverse attachments and butt joints were improved by 73% and 61%, respectively. The fatigue damage was increased when relatively larger block loads applied. Hence, the HFMI improvement was reduced 20% for transverse and longitudinal attachments and 14% for butt joints. The damage for σmax = 0.6fy in HFMI was similar to the damage for σmax = 0.3fy in AW, despite a doubling of the maximum CAL, which indicated promising potential of the HFMI improvement.

The efficiency of HFMI treatment and TIG remelting for extending the fatigue life of existing welded structures

AbstractDifferent post‐weld treatment methods have been developed to enhance the fatigue strength of welded steel structures and extend the service lives of their components. High‐frequency mechanical impact (HFMI) treatment and tungsten inert gas (TIG) remelting are two methods that have attracted considerable interest in recent decades. This paper presents the results of a study of fatigue life extension for pre‐fatigued welded steel details which can be achieved using HFMI treatment and TIG remelting. More than 250 fatigue test results were collected – including different details such as butt welds, longitudinal attachments, transverse attachments and cover plate attachments. HFMI treatment was found to extend the life considerably when the specimens treated were free from cracks or when existing cracks were < 2.25 mm deep. TIG remelting could extend fatigue lives even with cracks > 4 mm deep. In comparison to TIG remelting, HFMI treatment results in a longer fatigue life extension for pre‐fatigued details, provided existing cracks are < 2.25 mm deep. Regarding TIG remelting, the depth of possible remaining cracks was found to be a substantial parameter when assessing the degree of life extension.

Investigating the fatigue behaviour of small scale and real size HFMI-treated components of high strength steels

The present study reports on results of a completed research project, which investigated amongst others the fatigue strength of weldments made of structural steels S355J2+N, S690QL and S960QL and treated with the post-weld treatment process High Frequency Mechanical Impact (HFMI). Reference tests on as-welded (AW) specimens, which were manufactured from the same material batches, were carried out in order to enable a direct comparison. Main goal of the present tests series was, apart from the revalidation of the higher efficiency of HFMI on high strength steels, to investigate possible scale effects, when the post-weld treatment method is applied on real size structural components. Therefore, usual fatigue test specimens and real size structural beams HEA 260 were tested. The welded structural detail of transverse stiffeners was investigated in all cases. All results are evaluated with the nominal stress approach and they are presented in S-N curves along with the respective FAT classes, which are proposed by the design guidelines of Eurocode and the recommendations of International Institute of Welding (IIW). Beach mark tests were carried out on HFMI-treated small scale fatigue test specimens, in order to investigate the influence of HFMI on the fatigue crack growth behaviour (FCG). In all cases, a significant increase in fatigue strength was achieved by the application of HFMI with the fatigue life extension being more significant for the high strength steels. No scaling effect was documented regarding the application of the method on the structural beams having similar plate thicknesses as the small scale specimens but more complex geometries and significantly longer weldments. Finally, the evaluation of the beach mark tests and their comparison with reference curves found in literature did not allow a clear assessment of the HFMI influence on the fatigue crack propagation (FCP).

A reanalysis of fatigue test data for longitudinal welded gusset joints in as-welded and high frequency mechanical impact (HFMI)-treated state

This study aims at clarifying the combined effect of different geometric parameters on the fatigue strength of longitudinal welded gusset joints. Statistical reanalyses with an up-to-date database for this detail were thus carried out on larger parameter ranges, first on as-welded joints, including also loading, weld detailing, steel grade, and R-ratio. The results showed that the combined effect of gusset length, main plate width and thickness significantly affects the characteristic values and a relationship is proposed. With this basis, analyses are then performed on high frequency mechanical impact (HFMI)-treated joints and an accurate assessment of the improvement effect is proposed.

Paper Review of “A reanalysis of fatigue test data for longitudinal welded gusset joints in as-welded and high frequency mechanical impact (HFMI)-treated state”

Paper Review of “A reanalysis of fatigue test data for longitudinal welded gusset joints in as-welded and high frequency mechanical impact (HFMI)-treated state”

応力比が負の条件下におけるHFMI処理の疲労強度改善効果及び板厚効果

Fatigue tests under the condition of stress ratio ＝ -0.3 are conducted for three types of welded joints (as-weld, grinding treatment, and HFMI (High Frequency Mechanical Impact) treatment.), and the fatigue strength of these welded joints are obtained experimentally. By comparing the present results with the results by Iwata et al., the fatigue strength improvement effect of HFMI treatment under the condition of negative stress ratio is discussed. The stress concentration factor of the HFMI-treated welded joint increases as the plate thickness increases, and the weld toe may be yielded in compression when the plate thickness exceeds 40 mm. The fatigue strength of the HFMI-treated welded joint decreases when the plate thickness exceeds 40 mm. However, even when the weld toe of the HFMItreated welded joints is compressively yielded, a significant decrease in fatigue strength is not observed. In addition, the modified MILHDBK- 5 method is applied to all experimental data, which includes differences in stress ratio. By the equivalent stress based on the modified MIL-HDBK-5 method, a good correlation between the fatigue test results is achieved regardless of joint type, plate thickness, and stress ratio.

Corrosion fatigue behaviour of HFMI-treated butt welds

The present study reports on the results of a completed research project, which investigated the corrosion fatigue behaviour of weldments made of structural steel S355J2+N treated with the High Frequency Mechanical Impact (HFMI). Main goal has been the estimation of short corrosion on the surface layer (<1 year) on the fatigue strength of uncoated HFMI-treated specimens. Axial and 4-point bending fatigue tests were carried out on uncorroded and corroded butt welds both for the as-welded and HFMI-treated state. Marine corrosion is simulated in the laboratory with exposure in a Salt Spray Chamber (SSC) tests and deposition in Artificial Seawater (ASW). SSC was applied as pre-corrosion means while corrosion in ASW took place simultaneously during fatigue testing of specimens pre-corroded in the same corrosive medium. Extrapolation of the laboratory results to real marine corrosion (RC) is carried out based on findings from an earlier study of the authors, where specimens corroded in the laboratory were compared with reference specimens exposed to the real marine splash zone in the Baltic Sea. Material removal rate, increase of surface roughness and reduction of fatigue life are considered for this comparison. Significant conclusions regarding corrosion fatigue resistance of both as-welded and HFMI-treated specimens are drawn. Assumptions and initial expectations regarding the influence of corrosion on both fatigue crack initiation and crack growth regimes are either confirmed or overthrown. In all investigated cases for thickness reduction up to 0.1 mm, significant increase in fatigue life is achieved through the application of HFMI. Finally, recommendations for future work regarding the clarification of open questions from the present study are presented.

Enhancing fatigue strength of welded joints made of SBHS700 by hammer peening with ICR apparatus and HFMI treatment

This paper investigates the fatigue strength enhancement of two types of peening techniques: hammer peening with impact crack-closure retrofit apparatus and high-frequency mechanical impact (HFMI) treatment on out-of-plane gusset-welded joints made of SBHS700. In this work, fatigue tests were continuously carried out using plate specimens that were removed from a previously tested girder specimen. The results showed that the fatigue strength enhancement of these peening techniques on the welded joints was expected to be at least three classes higher compared with as-welded details with similar dimensions. Unexpectedly, the improvement effect of the hammer peening on one specimen was slightly lower than that of the other specimens. According to SEM observation, the cause of the slight reduction was that due to its weld shape, the distance between the crack initiation point and the treatment edge became greater than 2 mm. Finally, evaluations were performed on available data, including the test results from this study, based on an extended Modified Goodman model to estimate the fatigue limits of HFMI-treated joints. Consequently, the estimation results by the model indicated a good agreement with the fatigue strength enhancement of HFMI treatment and a possibility to be able to assess the enhancement of the hammer peening.

Fatigue crack repair in welded structures via tungsten inert gas remelting and high frequency mechanical impact

Rehabilitation of welded structures has gained increasing attention lately. This paper aims at investigating the efficiency of Tungsten Inert Gas (TIG)-remelting and TIG-remelting followed by High Frequency Mechanical Impact treatment (TIG-HFMI) in fatigue life extension. Fatigue tests were carried out on as-welded and cracked specimens after treatment. The lives of the treated specimens increased remarkably by the two methods (TIG and TIG-HFMI). Many of the treated specimens ran-out after 10 million cycles of loading and failed at the clamping location when tested at a higher stress range. The improvement in compressive residual stresses, hardness values and weld toe radii were the reasons behind the life extension. These factors were used for fatigue life estimation in as-welded and TIG-treated specimens using the base metal SN curve. Moreover, the test results together with results from previous tests in the literature demonstrated that these methods can be useful for crack retrofitting as for new structures.

Comparison of the fatigue performance of ferrite–pearlite and ferrite–bainite dual-phase steels

Recent demands for large vessels and deep water exploration have increased the requirements for thicker plates to ensure the sufficient strength of such structures. The use of thicker plates, however, has an adverse influence on the fatigue performance. To enhance the fatigue life, post-weld treatments, such as weld-toe grinding, TIG (tungsten inert gas) dressing, hammer peening, and high-frequency mechanical impact (HFMI) treatments are commonly employed. On the other hand, these techniques require additional fabrication time and production cost. Therefore, there is keen interest with respect to thick steel plates with improved fatigue strength. This study examined the fatigue characteristics of conventional steel (Ferrite–Pearlite, F–P steel) and Ferrite–Bainite dual-phase steel (F–B steel). The fatigue behavior of both steels was investigated by a series of fatigue tests with three different types of welded joints. In case of gusset and cruciform-welded joints, fatigue strength of F–B steel is higher than that of F–P steel. F–B steel is known to exhibit improved fatigue performance associated with the existence of bainite microstructure. In addition, the fatigue data in this study are validated against the existing literatures. Based on the fatigue test results, a modified fatigue design curve is suggested from this study. In order to discuss the effects of fatigue crack path, the fatigue crack growth rate (FCGR) of both steels was investigated in terms of Paris’ law. Finally, microstructural analysis was conducted to discuss the fatigue performance.

Obrada zavarenih priključaka metodom mehaničkog udara visokom frekvencijom

Obrada zavarenih priključaka metodom mehaničkog udara visokom frekvencijom

Effect of increased yield strength, R-ratio, and plate thickness on the fatigue resistance of high-frequency mechanical impact (HFMI)\u2013treated steel joints

In 2016, the International Institute of Welding (IIW) published a recommendation for high-frequency mechanical impact (HFMI) treatment for improving the fatigue strength of welded joints. Since the publication of the HFMI recommendations, numerous of studies have been published with a considerable amount of new fatigue test data focusing on various aspects of the improvement; influence of base material yield strength, and loading and thickness effects. Since the data was scarce covering some of the aspects when the recommendation was published, re-evaluating the recommendation with these new test data, presented within this work, will further validate and extend the recommendations. The analysis reveals that the recommended improvement of fatigue classes based on the base material yield strength is well applicable. In addition, the reduction of fatigue classes for higher R-ratios up to the defined value of R = 0.52 is well considered. Finally, the practicability of the thickness correction factor is also confirmed by leading to a conservative fatigue assessment. The ratio of the statistically evaluated FAT class to the recommended value is shown to be conservative with a value of above one for almost every data set; however, even the ratio is below one in some minor cases, every single test data point in this study is assessed conservatively validating the applicability of the recommendation.

Comparison of effect of shot-peening with HFMI treatment or use of LTT consumables on fatigue strength of 1300\xa0MPa yield strength steel weldments

In this paper, T-joint samples in a 1300 MPa yield strength steel were produced using conventional or low transformation temperature (LTT) type consumables. The welded samples were either subjected to high-frequency mechanical impact (HFMI) treatment or to shot-peening. Fatigue testing was performed under fully reversed, constant amplitude bending load. Shot-peening gave a significant increase in fatigue strength for more than around 50,000 cycles. Shot-peened LTT welds had the highest fatigue strength, with conventional welds being shot-peened having slightly lower fatigue strength. HFMI treatment of conventional and LTT welds improved the fatigue strength also, but to a lesser extent, keeping the slope in the SN diagram close to three, while shot-peened samples had a slope of 5–7. Significant differences in compressive residual stress were seen between the different welds, with the most compressive stress found in the shot-peened samples. This was probably one of the main reasons for the improved fatigue life of shot-peened samples.

Calibration of High-Frequency Mechanical Impact Simulation Based on Drop Tests

A series of drop tests was implemented in the present study in order to allow the reproduction of a single impact identical to the high frequency mechanical impact (HFMI) under monitored conditions in the laboratory. Therewith, characterization of the investigated material’s mechanical behavior by explicitly considering possible irregularities concerning the present deformation modes would be enabled. Main goal was the determination of the investigated material’s dynamic yield stress for various strain rates inside the spectrum of interest, so that the Cowper–Symonds viscous material model would be calibrated for the subsequent HFMI simulation. The values of the dynamic yield stress extracted by the present drop tests show good agreement with other experimental methods regarding the investigated material S355. The introduction of the calibrated material behavior on the present drop tests in the finite element (FE) analysis of HFMI led to reduced preciseness though, in comparison with the FE analysis, which considered high strain rate tensile tests found in literature. A series of conclusions was drawn from both the experimental and numerical investigations, confirming most of the initial expectations. Further work is proposed, in order to clarify an incompatibility met during the numerical investigations.