High Frequency Mechanical Impact Research Articles

Strain-based critical plane approach to predict the fatigue life of high frequency mechanical impact (HFMI)-treated welded joints depending on the material condition

After post-treatment with high frequency impact treatment (HFMI) of weld toes, a significant increase of fatigue strength or fatigue life can be achieved. Due to the lack of methods for the quantitative prediction of the HFMI benefits depending on the process and treated material, a widespread use of this process has not yet happened. One reason for that issue is that at the moment, no methods exist to predict the influence of the HFMI-treatment on welded components and structures at the moment in advance depending on the HFMI-treatment parameters. For this case, a strain-based approach based on the finite element (FE) simulation of the HFMI-process and the subsequent evaluation by critical plane approaches of the FE-post-processing was developed. This approach principally considers the beneficial effects of compressive residual stresses, notch effect reduction, and hardening of the HFMI-treated surface layer. Furthermore, residual stress relaxation during fatigue loading was taken into account. For numerical prediction of the influence of HFMI-treatment of welds, process parameters at the real device were measured and a suitable material hardening model was used from previous work to describe the conditions in the surface layer. After this, the process chain of welding, HFMI-treatment, and fatigue loading of butt joint specimen were simulated. The estimated fatigue life values were compared with the values of the fatigue tests for two different treatment intensities and also a good agreement was reached.

A Practical Method for Measuring Residual Stress Distribution by XRD after Needle or Hammer Peening: Application to a Simple Case

Ultrasonic needle peening (UNP) is a post-weld high frequency mechanical impact (HFMI) treatment process. This study describes a practical method for measuring residual stress (RS) distribution after UNP and its application to a simple case. Laboratory sin²ψ XRD RS analysis is performed on an UNP single-track on a flat structural steel specimen. This situation is studied as a reference case to characterize RS creation during UNP. It is demonstrated that successive mechanical and electrochemical polishing stages combined with relatively high resolution sin²ψ XRD measurement permits to get an effective surface and in-depth characterization of longitudinal and transverse RS around the UNP groove. High magnitude longitudinal compressive RS are found in the middle of the groove whereas transverse RS compression peaks are located on both sides of the groove. Discussion is conducted about sources of error associated with the experimental method and outlook for UNP process fundamental characterization.

Numerical assessment of fatigue design curve of welded T-joint improved by high-frequency mechanical impact (HFMI) treatment

In the paper, the fatigue performances of as-welded T-joint and T-joint improved by high frequency mechanical impact (HFMI) were numerically investigated using structural hot spot stress approaches: linear surface extrapolation (LSE) and through thickness at the weld toe (TTWT). The effects of main plate thickness and material strength for HFMI-treated joints were investigated. The results showed that the TTWT method was more effective to study the effect of thickness on T-joints improved by HFMI treatment than LSE method. For as-welded T-joints, the thickness correction exponent n = 0.04 was obtained when the attachment plate thickness was set as constant. For HFMI-treated T-joints, a reverse thickness effect was observed with negative thickness correction exponents, and the thickness correction exponents increased with material strength. In addition, the adoption of S–N slope varying with yield strength was proven to be more proper for HFMI improvement assessment.

Stability of high frequency mechanical impact (HFMI) post-treatment induced residual stress states under cyclic loading of welded steel joints

This paper investigates the effect of cyclic loading on the stability of compressive residual stress fields induced by high frequency mechanical impact (HFMI) post-weld treatment. First, the effectiveness of the post-treatment technique is shown by fatigue tests incorporating mild steel S355 and high-strength steel S960 longitudinal stiffener specimens. Extensive X-ray residual stress measurements support the beneficial impact on the compressive residual stress state for mild and high-strength steel structures. They also illustrate that cyclic loading leads to a significant local relaxation of this condition. Second, a numerical simulation chain incorporating a structural weld simulation, numerical analysis of the HFMI-treatment, and a final cyclic loading step for the investigated mild steel specimen is set-up. The results show that the residual stresses at the surface of the weld toe are in agreement to the X-ray measurements for both the as-welded and HFMI-treated condition, which basically proofs the applicability of the manufacturing simulation. The numerical computation including the first five load-cycles demonstrates that the simulated residual stress relaxation again exhibits consistent results with the measurements. An additional utilization of an analytical relaxation model from literature reveals that the estimation of the residual stress state in the high-cycle fatigue region is well employable. Therefore, the scientific results in this paper proof the applicability of the presented consecutive numerical-analytical procedure to assess the local compressive residual stress stability of HFMI-treated welded steel joints in both the low- and high-cycle fatigue region.

Improving fatigue strength of welded 1300 MPa yield strength steel using HFMI treatment or LTT fillers

Abstract Fatigue improvement techniques are widely used to increase fatigue strength of welded high strength steels. In this paper high frequency mechanical impact (HFMI) and a Low Transformation Temperature (LTT) filler material were employed to investigate the effect on fatigue strength of welded 1300 MPa yield strength steel. Fatigue testing was done under fully reversed, constant amplitude bending load on T-joint samples. Fatigue strength of LTT welds was the same as for welds produced using a conventional filler material. However, HFMI treatment increased the mean fatigue strength of conventional welds about 26% and of LTT welds about 13%. Similar distributions of residual stresses and almost the same weld toe radii were observed for welds produced using LTT and conventional consumables. HFMI increased the weld toe radius slightly and produced a more uniform geometry along the treated weld toes. Relatively large compressive residual stresses, adjacent to the weld toe were produced and the surface hardness was increased in the treated region for conventional welds after HFMI. For this specific combination of weld geometry, steel strength and loading conditions HFMI treatment gave higher fatigue strength than LTT consumables.

HFMI: understanding the mechanisms for fatigue life improvement and repair of welded structures

Fatigue resistance of high-frequency mechanical impact (HFMI) treatment is investigated on high-strength welded steel S690. Cyclic four-point bending tests are performed on butt weld samples and HFMI samples with different conditions of treatment with or without stress-relieving heat treatment. Microstructural characterisations with electron backscattering diffraction observations and residual stress analyses are presented. The main objective of this paper is to reveal and identify the parameters (residual stresses, geometry, strain hardening, etc.) which have an effect during the HFMI operation and to understand the phenomena involved during fatigue stressing. The optimisation of the geometry of the fatigue specimen allowed to repetitively obtain failures in the treated areas, thereby showing the relationship between the geometry, the strain hardening, the residual stresses and the fatigue strength. The main conclusions of this paper showed that HFMI treatment improves the fatigue performance of welded joints (in keeping with the International Institute of Welding results). The residual compressive stresses are prominent. The microstructure does not seem to have any effect, despite the nanostructured layer. The geometry is of secondary importance despite a significant change in the local geometry. HFMI treatment is also presented as retrofitting treatment.

Influence of effective stress ratio on the fatigue strength of welded and HFMI-treated high-strength steel joints

This paper contributes to quantify the influence of the local mean stress condition and the local weld geometry on the high-cycle fatigue strength of welded and high frequency mechanical impact (HFMI) treated S960 high-strength steel joints. A calculation procedure is presented, which bases on the effective stress ratio at the weld toe considering the local residual stress condition as well as the specimen clamping-induced and load-dependent notch mean stress state to assess a final influence fatigue factor. Fatigue tests incorporating butt joint, T-joint, and longitudinal stiffener specimens present the effect of different welding parameters and the HFMI technique as post-treatment method. The results show that firstly, by an application of optimized welding parameters, such as filler material and shielding gas, an increase in fatigue strength of 7% for the butt joint and of 30% for the T-joint specimens is experimentally observed. Secondly, it is shown that all investigated specimen types reveal a significant increase in fatigue strength due to the HFMI-treatment up to an enhancement of factor 2.46 in case of the longitudinal stiffener specimens. A study on the effect of stress-relief annealing as post-weld heat treatment (PWHT) process highlights a decrease of about 25% in the high-cycle fatigue strength in case of HFMI-treated T-joint specimens. Applying the introduced calculation method, an effective stress ratio of Reff=0.05 for the butt joint and T-joint specimens, and Reff=−0.81 for the longitudinal stiffener specimens in the as-welded condition is calculated. Comparing the computed fatigue factors with the statistically evaluated nominal fatigue strengths it is observed that the effect by the effective stress ratio is dominant, but not fully covers the fatigue enhancement. Furthermore, it reveals that the influence by the effective stress ratio may be related to the fundamental change in the local residual stress state. Especially the HFMI-treatment leads to a reduction of the local residual stress condition up to a difference of 650MPa for the S960 high-strength steel, which significantly contributes to this influence.

The thickness effect of welded details improved by high-frequency mechanical impact treatment

High-frequency mechanical impact (HFMI) treatment can enable resource-efficient structural design by improving the fatigue strength of welded joints. While the thickness effect with reference to the fatigue of welded details is well known and covered in design codes, this effect has not been investigated systematically when the welds are improved by HFMI. In this study, experimental data of 582 small-scale fatigue tests on welded details with HFMI treatment has been collected from the literature and evaluated with respect to the thickness effect. In order to separate the effects of yield strength and thickness on the fatigue strength, a new approach was developed to adjust the data to a reference yield strength of 355MPa prior to thickness evaluation. The test data covered transverse butt welds, details with non-load-carrying transverse attachments and details with non-load-carrying longitudinal attachments. The thickness effect of details with transverse attachments corresponds well with the IIW recommendation of n=0.2, whereas transverse butt welds have a much weaker thickness effect. Details with longitudinal attachments show a “reverse” thickness effect.

Fatigue assessment of welded and high frequency mechanical impact (HFMI) treated joints by master notch stress approach

High frequency mechanical impact (HFMI) treatment is a reliable and utmost effective method for post-weld fatigue strength improvement of steel joints, especially in case of high-strength steel applications. In 2014, the HFMI master notch stress approach was firstly introduced as an alternative design concept. It features an engineering-feasible method to assess the notch fatigue strength of HFMI-treated joints based on weld toe notch stress concentration, base material yield strength and load stress ratio.This paper presents an essential update of the HFMI master notch stress approach by facilitating the notch factor KW, obtained as the ratio of effective notch stress σk to structural stress σs. This enables a thorough fatigue assessment of welded structures without the need of a nominal cross-section definition.To proof the applicability, a comprehensive validation of the HFMI master notch stress approach incorporating over 230 additional steel joint specimen results covering both constant and variable amplitude tests is conducted. The constant amplitude study reveals that the fatigue strength of the HFMI master notch stress approach, utilizing the notch factor KW, is well applicable for material strengths up to ultra high-strength steels with a nominal yield limit of 1300MPa. In addition, the notch stress based service strength evaluation of variable amplitude loaded HFMI-treated high-strength steel joints considering a specified damage sum of D=0.3 according to Sonsino can be also well utilized for HFMI-treated joints.

Residual stress state induced by high frequency mechanical impact treatment in different steel grades – Numerical and experimental study

High frequency mechanical impact treatment is observed to increase the fatigue strength of welded joints. This technique induces compressive residual stresses, increases the local hardness, and reduces the stress concentration by modifying the weld toe radius. The goal of this study was to investigate residual stresses induced by ultrasonic impact treatment in S355, S700MC, and S960 grades steel experimentally and numerically. Plate specimens were manufactured and treated with different treatment intensities i.e. vibration amplitudes of the Sonotrode. The indentation depths were measured by the aid of a laser scanner and residual stresses using X-ray diffraction technique. The effect of steel grade and treatment intensity on the induced compressive residual stress state was firstly studied experimentally. In addition, displacement controlled simulations were carried out to estimate the local residual stress condition considering the effect of different material models. Both the numerically estimated and experimentally measured residual stresses were qualitatively in good agreement. Residual stress state in S355 and S700MC can be estimated well using combined strain rate dependent material model. No significant effect of the treatment intensity is observed on the indentation depth and residual stress state for S355 grade steel. The indentation depth decreases with the increase in the yield strength of the steel.

Life Extension of Welded Structures Using HFMI Techniques - Potential Application to Offshore Structures

Fatigue damage development in welded structures is a local phenomenon and if one need to achieve an extension of the life for the structure local post weld improvements need to be use in order to reduce/remove local features which contribute to the fatigue damage. In order to enhance the life time of load carrying welded structures without large amount of cost investments, e.g. redesign and replacement of existing structures, post weld improvement techniques need to be more applied. New High Frequency Mechanical Impact (HFMI) technologies have been developed in the last 10 years which enables cost-effective life extension and reparation of welded structures. The use improvement techniques for technical life enhancement upgrade and repair of welded structures within various industries, e.g. oil and gas, have been an accepted practice. HFMI treatment techniques are based on localized peening process of the welded joints and the devices are portable. The impacting results in a local cold plastic deformation which remove weld defects reduce stress concentration and induce compressive residual stresses which eventually will enhance the fatigue life of the welded structure. An overview of existing improvement techniques for welded structures is given followed by description of new technologies (HFMI). A brief description of the new international guideline and design recommendations within the International Institute of Welding (IIW) is given. Several validation studies on the fatigue performance of HFMI techniques are presented and onsite potential applications of the techniques for joints in fixed offshore structures are outlined.

Allowable stresses in high-frequency mechanical impact (HFMI)-treated joints subjected to variable amplitude loading

The effectiveness of high-frequency mechanical impact (HFMI) is considered to rely on the existence of compressive residual stresses. To determine when residual stress relaxation occurs, and what the resulting influence on fatigue improvement is, local stress-strain response in as-welded and HFMI-treated weld toes was modelled under different peak stress conditions. Then, effective notch stress analysis was used to correlate these results with available experimental observations. The simulations showed that high stress ratios and compressive peak stresses were critical with respect to residual stress relaxation, as expected. A compressive peak stress of 0.6f y (nominal yield strength) resulted in full residual stress relaxation. The relative fatigue damage calculations and the notch stress analysis indicated, however, that fatigue improvement could be expected even after significant residual stress relaxation. Based on this and previously observed benefit for high stress ratios, an increase in maximum allowable stresses for HFMI-treated welded steel joints is suggested. The maximum stress ratio is proposed to be increased from R = 0.52 to R = 0.7, and the maximum stress range to limit compressive stresses is proposed to be increased from ΔS max = 0.9f y to ΔS max = 1.2f y , which corresponds to S min = −0.6f y for stress ratio R = −1.

Structural life enhancement on friction stir welded AA6061 with optimized process and HFMI/PIT parameters

This novel study presents an unconventional approach to find the best governing process parameters of high frequency mechanical impact technique based on multi-objective optimization method. In this investigation, the post-weld mechanical treatment is aimed to enhance fatigue resistance of structural friction-stirred weld subjected to fluctuating loads by obtaining nominal sub-surface hardness. The experimental study was conducted on aluminium alloy AA 6061 with thickness of 6 mm under varied parameters centred on indenter diameter, air pressure and impact frequency. The investigation began with obtaining optimum parameters for single response by using conventional Taguchi method with L9 orthogonal array. Next, advanced optimization approach by means of multi-objective Taguchi method attempts to consider the multiple responses simultaneously which are sub-surface hardness and structural life. As the final results, the optimum value was acquired by calculating the total normalized quality loss and multiple signal-to-noise ratios based on unequal desirability. The significant level of the parameters was evaluated by using analysis of variance. Furthermore, the second-order model for predicting the objectives was derived by applying response surface methodology. It can be summarized that, first, the affecting parameters to obtain superior structural life can be ordered at significant level of ca. 65, 25 and 10% for air pressure, impact frequency and indenter diameter, respectively. Secondly, using subsequent post-weld mechanical treatment, the life cycle number can be extended up to 12 times on friction-stirred weld. Finally, based on the experimental confirmation test, the proposed method can effectively estimate the structural life and surface hardness within the acceptable range of relative error.

Recent results on fatigue strength improvement of high-strength steel welded joints

Economic considerations push technology towards new lightweight structural solutions. This results in great interest in using high-strength steels (HSS) for structural applications like in railway vehicles, bridges, offshore structure and high-speed ships. In all these applications, welding is the main joining method and fatigue of welds is the major design criterion despite the existence of locally deteriorated microstructure, increased stress concentration and unfavourable tensile residual stresses after welding. Good weld design is a must to ensure the structural durability and performance, however this does not guarantee lightweight design alone. Special applications, such as post-weld treatment methods, can be performed following the welding. Therefore, this study concerns about the recent developments on such improvement techniques by considering two most-commonly used fatigue strength improvement methods; high frequency mechanical impact (HFMI) treatment and tungsten inert gas (TIG) dressing. Evaluations based on more than 1500 fatigue data points extracted from the literature. Investigations include presentation of the individual data analyses and fatigue strength assessment of all the data points by the effective notch stress approach with the reference radius rref=1.00mm. The influence of material strength, residual stress state, weld toe profile and loading conditions on the fatigue strength improvement are all discussed.

Plastic strain characteristics of butt-welded ultra-high strength steel (UHSS)

This paper investigates the mechanical properties of direct quenched (DQ) ultra-high strength steel (UHSS) joints in butt-welded configuration. Experimental and numerical investigation of the monotonic behavior of heat-treated and welded UHSS is undertaken. The tensile tests in the study investigate the effects of three different parameters on the plastic strain capacity of UHSS, namely: size of the heat affected zone (HAZ), matching between the filler material and the BM, and welding and post welding treatments such as TIG-dressing and high frequency mechanical impact (HMFI) treatment. Laser welding with a narrow HAZ was also performed without filler material and with zero air gap. A digital image correlation technique was utilized to monitor the plastic behavior of the specimens under tensile loading. The results indicate that the strain capacity of the joint is highly sensitive to heat input (HI); even a small HAZ leads to a considerable drop in the strain capacity. A strong constraint at the boundaries of the HAZ with the surrounding material generates a hydrostatic stress component, and thus an increase in both the ductility and strength of the joint can be expected.

A finite element study on residual stress stability and fatigue damage in high-frequency mechanical impact (HFMI)-treated welded joint

Fatigue improvement from high-frequency mechanical impact (HFMI) is considered to rely on compressive residual stresses, improved weld toe geometry and localized strain hardening at the weld toe. Of these, the existence of compressive residual stresses is generally assumed critical for the effectiveness of the method. In this study, the influence of stress ratio and peak loads on residual stress relaxation and fatigue damage in as-welded and HFMI-treated S700 transverse attachments was investigated. Elastic-plastic stress-strain response for as-welded and HFMI-treated conditions was simulated considering the effects of initial residual stress distribution, local geometry and local material properties. Relative fatigue damage was estimated using the Smith-Watson-Topper parameter. Full residual stress relaxation was observed for a stress ratio of 0.5 and a compressive overload of 0.6 times the nominal yield strength. The fatigue damage assessment showed benefit from HFMI with respect to the as-welded state for all simulated load conditions. The results are consistent with experimentally observed behaviour. The stepwise analysis indicated that the remaining benefit after residual stress relaxation was due to geometry improvement and strain hardening.

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

<p class="TRANSAffiliation">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.</p>

Numerical implementation for fatigue assessment of butt joint improved by high frequency mechanical impact treatment: A structural hot spot stress approach

In the study, three-dimensional finite element modelling of high frequency mechanical impact (HFMI) treatment is presented for butt joints of four strength grade materials (Q235B, AISI 1006, 45 Steel and AISI 2205). The fatigue assessment of butt joints by HFMI is implemented by structural hot spot stress approaches, i.e., linear surface extrapolation (LSE) and through thickness at the weld toe (TTWT). The effective structural hot spot stress concentration factors for HFMI-treated joints are determined, and the dependence of HFMI improvement on material strength and external stress is well captured by TTWT method. Following structural hot spot stress S-N curves for as-welded joints recommended by IIW, the structural hot spot stress S-N curves for HFMI-treated joints are suggested if the slope m of S-N curves for HFMI-treated joints is designated to vary from 5 to 10, inversely proportional to material strength. At final, the characteristic fatigue strength (FAT) has been determined and verified with the available experimental data.

Erratum to: Testing and fracture mechanics analysis of strength effects on the fatigue behavior of HFMI-treated welds

In this paper, recent fatigue tests conducted on welded specimens subjected to high frequency mechanical impact (HFMI) treatment are described, geometry measurements and metallurgical analyses of the tested specimens are presented, and efforts to estimate the test results using a nonlinear fracture mechanics model are discussed. The specimens were fabricated from 9.5-mm-thick (3/8 in.) aluminum (5083-H321) and high-strength steel (ASTM A514) plate. The specimen geometry and preparation followed procedures used in previous studies on mild steel (CSA 350W). Fatigue tests were performed on the as-welded and impact-treated specimens under two loading histories (constant amplitude with and without periodic under-loads) at several equivalent stress ranges. Residual stress distributions were determined by x-ray diffraction. In addition, weld toe geometry measurements were obtained using silicon impressions and micro-hardness distributions were obtained on polished weld samples for each material type. This information was used to establish parameter values for a nonlinear fracture mechanics analysis. The employed fracture mechanics model is reviewed in this paper, and its benefits as a tool for modelling the fatigue behavior of impact-treated welds are discussed. Following this, the effectiveness of the model in estimating the test results for the three materials is assessed.

Effect of high frequency mechanical impact treatment on fatigue strength of welded 1300 MPa yield strength steel

High frequency mechanical impact (HFMI) is a recent post weld treatment method which can be employed to increase the fatigue strength of welded components. In this paper the fatigue strength of as-welded and HFMI treated fillet welds in a 1300MPa yield strength steel was compared. Fatigue testing was done under fully reversed, constant amplitude bending load. Finite element analysis was used to calculate the stress distribution in the weld toe region to permit evaluation of the fatigue data with the effective notch stress approach. As-welded samples showed a mean fatigue strength of 353MPa and a characteristic fatigue strength of 306MPa. HFMI treatment increased the mean fatigue strength by 26% and the characteristic fatigue strengths by 3%. The weld toe radii in as-welded condition were large. HFMI only increased the weld toe radii slightly but resulted in a more uniform weld toe geometry along the weld. A depth of indentation in the base metal in the range of 0.15–0.19mm and a width of indentation in the range of 2.5–3mm, were achieved. Maximum compressive residual stresses of about 800MPa in the longitudinal and 250MPa in the transverse direction were introduced by HFMI treatment, adjacent to the weld toe. The surface hardness was increased in the entire HFMI treated region. It is concluded that the increase in fatigue strength is due to the combined effects of the weld toe geometry modification, increase in surface hardness and creation of compressive residual stresses in the treated region.