Fatigue Behavior Of Welded Joints Research Articles

Very High Cycle Fatigue of Welds: A Review

The design life of welded structures and components extends into the very high cycle fatigue (VHCF) regime across various applications. However, the availability of data on the fatigue behaviour of welded joints in the VHCF regime is limited, particularly when compared to the low and high cycle fatigue regimes. The development of ultrasonic fatigue testing equipment has accelerated fatigue testing and allowed for the VHCF properties of welds to be investigated in a feasible timeframe. In the present review, the emerging research concerning the VHCF behaviour of welds of various steels and non-ferrous alloys are individually explored. Overall, it is observed that welded joints have significantly lower fatigue strength than the base metal in the VHCF regime and that welding defects have a considerable influence on fatigue strength. Through the discussion of the relevant literature, important findings concerning the effects of specimen geometry and fatigue improvement methods are underlined. Furthermore, the guidance provided within design standards is compared, and some examples of VHCF failures of in-service components are highlighted. Finally, perspectives on future directions of investigation are put forward with the aim of encouraging further research in the field of VHCF of welds.

A novel elastic strain energy density approach for fatigue evaluation of welded components

The averaged elastic strain energy density (SED) within a characteristic volume radius R has been investigated and applied for characterizing the fatigue strength of welded joints for decades. However, engineering applications are still relatively rare. The main reason is that the calculation process is cumbersome due to fine mesh size and circular pattern requirements. This study proposes a novel approach of SED without requiring special finite element size and special layout requirements. First, a novel approach of the notch stress solution is presented. As a result, the present solution differs from the conventional approach which has been focused on notch tip singular stress field. Then, an averaged strain energy density can be obtained analytically by taking advantage of the new notch stress solutions presented in this study. Both the closed-form notch stress solutions and resulting expression for average strain energy density have been validated by FEA results. Subsequently, a large number of fatigue tests published in the literature are selected to examine the effectiveness of the evaluation approach of SED on interpreting fatigue behaviors of welded joints. Finally, a procedure for practical engineering application is established and verified by a group of fatigue tests of 3D gusset welded components.

Fatigue & Fracture of Engineering Materials & Structures Virtual Special Issue: Data science and machine learning for fatigue and fracture assessment

<scp>Fatigue &amp; Fracture of Engineering Materials &amp; Structures Virtual Special Issue</scp>: Data science and machine learning for fatigue and fracture assessment

Actual weld profile fatigue performance by digital prototyping of defected and un‐defected joints

AbstractThis paper considers the effects of the actual weld profile on the fatigue behavior of welded joints by means of the implicit gradient approach. Different types of geometrical approximation are considered in the analysis. In order to define the fatigue strength reduction factor of the actual weld profile, FE analyses have been considered based on a three‐dimensional scan of the welded joints. Furthermore, the fatigue behavior of butt‐welded joints with a discontinuous undercut is considered, and a comparison has been made with experimental data taken from the literature.

On the Compression Instability during Static and Low-Cycle Fatigue Loadings of AA 5083 Welded Joints: Full-Field and Numerical Analyses

The aim of this scientific work was to evaluate the compression instability effects during static and low-cycle fatigue loadings of AA 5083 welded joints, commonly used in marine structures. Low-cycle fatigue assessment in marine structures is of utmost importance since high levels of plastic deformation can arise in the proximity of high-stress concentration areas. Displacement ratios equal to minus one and zero were used to perform experimental low-cycle fatigue tests. The tests were monitored by means of the Digital Image Correlation technique in order to detect the strain patterns, with particular attention paid to stress concentration areas, indicating that a specimen tends to buckle during high compression loads, for tests with a displacement ratio of minus one. The tests at displacement ratios equal to −1 showed a lowering of the strain–life curve revealing a considerable effect on compression instability. A nonlinear finite element modelling procedure, depending only on hardness measurements, was developed. The hardness measurements were used in order to assess the distinct mechanical properties of the different zones that were included in the finite element model. The finite element model results were compared to the data achieved by means of the digital image correlation technique, demonstrating that hardness measurements can help predict the low-cycle fatigue behaviour of welded joints and consider compression instability phenomena.

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.

Infrared thermographic method to rapidly evaluate high-cycle fatigue behavior of welded joints

A thermography-based energetic damage model is put forward to rapidly evaluate the high-cycle fatigue behavior of welded joints subjected to fatigue loadings. The energy dissipation is calculated using the thermal signal, and then is used to create fatigue damage model. The model is extended to predict the damage accumulation of welded specimens. The predicted fatigue damage evolution is shown to be in good agreement with the results given by the existing models. Experiments are conducted to verify the applicability of the present model for residual life prediction. It is shown that the residual life under multistage loading is well predicted in comparison with the experimental life.

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.

Influence of porosity on the fatigue behaviour of welded joints

In this paper, the influence of a single pore on the fatigue behaviour of welded joints has been examined by means of the implicit gradient approach. The pore has been considered a stress raiser that interacts with the weld toe or weld root. As an example, in the case of a cruciform joint with a load-carrying fillet weld, the influence of a single spherical pore is examined. Furthermore, the fatigue behaviour of an aluminium laser welded joint with ellipsoid equivalent pores is considered and a comparison is made with the fatigue strength of an aluminium arc welded joint.

Evaluation of the Effect of the TIG-Dressing Technique on Welded Joints through the Strain Energy Density Method

Among the various techniques employed to increase the fatigue strength of a welded joint, the TIG dressing technique acts on the weld geometry re-melting the welding toe to eliminate the presence of sharp V-notches. The main aim of this work is to investigate the effect of the TIG dressing technique on the fatigue behavior of welded joints summarizing the results of experimental tests taken from the literature through the Strain Energy Density method. The experimental tests considered were performed both on as-welded and TIG-dressed joints in order to better understand the improvement that such a technique leads to the welded joints. Finally, the data were summarized through the Strain Energy Density technique to show that the same fatigue curve, defined in terms of Strain Energy Density versus number of cycles, obtained for the as-welded joints could be used also for the TIG-dressed joints being the method not dependent on the geometry considered.

FEM simulation of welded joint geometry influence on fatigue crack growth resistance

Fatigue behaviour of welded joints is investigated, in terms of welded joint geometry and the fatigue crack position. It is based on previous work which involved numerical simulation of fatigue crack growth in a welded joint made of micro-alloyed, low-carbon pressure vessel steel P460NL1, with the main focus on fatigue crack growth rate through different welded joint regions. The goal here was to change the size of the heat affected zone, as the region in which the fatigue crack initiated, and to compare the results obtained for new crack length values with the original ones, obtained by creating numerical models based on experimental data. A number of models were created, some of which simulated the case with a bigger heat affected zone (and, consequently, a smaller crack length in the weld metal), and other which simulated the case with a smaller heat affected zone. Due to the micro-structural differences between these two welded joint regions, noticeable differences appeared in the numbers of cycles obtained for each zone with varying fatigue crack lengths, as well as in the total number of cycles for both zones through which the crack propagated.

Mechanical behavior and fatigue life of micro welded joints obtained by TIG spots in NiTi wires

This work evaluates the effect of welding energy and cycling strain amplitude on fatigue behavior of welded joints obtained by micro Tungsten Inert Gas spots in NiTi SMA superelastic wires. For this purpose, a Dynamic Mechanical Analyzer operating in single cantilever bending mode is employed. The equipment monitors the evolution of the force level to maintain constant the strain amplitude as cycling proceeds. Fatigue lives of as-received and welded NiTi wires are compared for different strain amplitudes applied during mechanical cycling. In the evaluated range of strain amplitude, as-received NiTi wires showed fatigue lives between 103 and 104 cycles. The fatigue life of the welded joints is lower, although it remains in the same range observed for the as-received wires.

Analysis of fatigue behavior of welded joints in orthotropic bridge deck using traction structural stress

In this study, the fatigue behavior of welded joints in an orthotropic steel bridge is simulated and analyzed. The traction structural stress method is proven to be more accurate and effective, and the predicted results agree well with the test results compared to traditional assessment methods, including the nominal stress, hot-spot stress, and effective notch stress methods. The traction structural stress concentration factor curves of welded joints under cyclic tensile and bending moment loading are obtained. The accuracy and validity of finite-element simulation methods for welded joints in orthotropic steel bridge are verified by comparing simulation results with full-scale tests results. Equivalent nominal stresses predicted by the master S–N curve method based on traction structural stress results agree well with the fatigue test results with small standard deviation. The fatigue behavior of orthotropic steel bridge analysis specifications Eurocode3 and GB50017 is combined with the traction structural stress method using unified traction structural stress concentration factor curves. This approach provides practical guidance for the fatigue behavior design of orthotropic steel bridges.

Effect of dilution on fatigue behaviour of welded joints produced by low-transformation-temperature fillers

ABSTRACTLow-transformation-temperature (LTT) fillers with various martensitic transformation start (Ms) temperatures were used to produce fillet welds. In comparison with conventional welds, the fatigue strength of the LTT fillet welds offers a significant improvement, with a minimum increase of 145%. Owing to the substantial dilution of base metal, the Cr and Ni alloying elements in the LTT weld metals decrease, resulting in an increase of the Ms temperatures. Therefore, the fillet welds produced using the LTT filler material with the lower Ms temperature (92°C) exhibit a larger compressive residual stress and higher fatigue strength.

Francis-99: Evaluation of the strain energy density value for welded joints typical of turbine runner blades

The main aim of this work is to investigate the fatigue behaviour of welded joints through an energetic approach based on the Strain Energy Density failure criteria. The geometries, taken from the literature, are typical of turbine runner blades. The results of the fatigue tests on these details were summarised through the Strain Energy Density approach. The application of this method to these geometries is the first step of a wider research with the aim to provide a suitable tool in FEM code for the lifetime estimation of components characterised by complex geometries.

Influence of fillet end geometry on fatigue behaviour of welded joints

This paper presents a fatigue analysis of a type of fillet welded joint representative of one main joint of the steel box girder of the Alcácer do Sal railway bridge. From previous studies, it was found that the welded joint between the box girder diagonal and the central hanger gusset is one of the most stressed details of the bridge. This welded joint was not fully manufactured according to current construction procedures, as regards the fillet weld end configuration. In order to assess the fatigue behaviour of such welded joint, the present study combines an experimental campaign and numerical analysis. A total of four welded joint series were produced in order to allow the comparison of the fatigue performance of similar type of welded joint of the Alcácer do Sal bridge with welded joints produced according to existing recommendations, such as EC3. Since scale-down specimens were considered, two different thicknesses were included in this study for each joint configuration, to allow the verification of any thickness effect. Concerning the numerical analyses, two main numerical tools were used: the standard Finite Element Method (FEM) with ANSYS and the eXtended Finite Element Method (XFEM) with ABAQUS. Fatigue life predictions were performed including both fatigue crack initiation and fatigue crack propagation phases. The number of cycles to initiate a fatigue crack was computed using local notch strain-life approaches, and the number of cycles for fatigue crack propagation was computed by integrating the Paris fatigue crack growth law with stress intensity factors computed with ANSYS (virtual crack closure technique) and ABAQUS (contour integral method, 3D XFEM model). Experimental tests demonstrated little influence of fillet weld end geometry on fatigue behaviour of welded joints and plate thickness effects were also reduced as also confirmed by the similar fatigue crack propagation rates. Both numerical simulations provided very accurate predictions of the experimental S-N curves, however the XFEM modelling opens new possibilities for mix-mode fatigue crack propagation simulations.

Experimental determining of fracture behaviour of P460NL1 steel welded joint specimens

This paper is focused on the experimental measuring of impact energy as a part of the doctoral dissertation involving the fatigue behaviour of welded joints, made of fine grain normalised micro-alloyed low carbon high strength steel P460NL1 [1,2], typically used for pressure vessels working at subzero temperatures. VAC 65 [3] was used as filler material, and two plates were welded using the MAG welding procedure (with 82% Ar + 18% CO2 shielding gas).The specimens were taken from two opposite ends of the welded plate, taking into account the measured values of groove edge temperature in these locations, wherein the temperatures in Location 2 (near the end of the weld) were higher than those in Location 1 (near the weld’s beginning). Based on their specific position, in terms of temperature and notch position (weld root or face), the specimens were divided into four groups of three. This testing was performed using a SCHENCK-TREBEL instrumented Charpy pendulum. The total impact energy was determined, along with its components, crack initiation and crack propagation energy.The obtained results have confirmed that the test specimens were of high toughness, with total impact energy ranging from 165-200 J, at room temperature. The crack propagation energy was the dominant component, being several times greater than the crack initiation energy, as was expected.

Study of the effect of microstructural variation on the low cycle fatigue behavior of laser welded DP600 steel: Simulation and experimental validation

Non-isothermal heat treatment during welding results in a wide variety of microstructures which strongly affect the fatigue properties of the weldment. To improve the durability of weldments, the effect of microstructural heterogeneity on the fatigue behavior of welded joints should be thoroughly understood. In this paper, the role of microstructure features on the low cycle fatigue behavior of laser welded dual-phase 600 steel was studied by both experimental analysis and simulation. In-situ cyclic tests combined with scanning electron microscopy (SEM) imaging and electron backscattered diffraction (EBSD) were performed to identify microstructural deformations in different laser welded regions during cyclic loading. Microstructural characterization revealed that the microcracks caused by fatigue mostly occurred in the ferrite phase and close to the interface boundaries of ferrite/martensite. In the numerical part, the hierarchical micromechanical modeling using a representative volume element (RVE) was employed to study the regional mechanical properties of welded joints. 2D RVE calculations for different parts of the welded joints were based on the metallographic morphologies. The cyclic behavior of constituent phases (ferrite, martensite) in the microstructure of different regions was quantified based on the dislocation-density-based model and Ramberg-Osgood model. The laser weld joint model containing the RVEs of different weld regions could accurately predict the fatigue weak area of the laser welded joint. The hysteresis stress-strain loop of the welded joint with a heterogeneous multiphase microstructure was simulated and was in line with the experimental data.

Numerical simulation of low-cycle fatigue behavior of welded joints for naval applications: influence of residual stresses

A methodology is described and evaluated for the numerical prediction of the low-cycle fatigue strength of welded assemblies taking into account the effect of local residual stresses. The methodology involves (i) modeling the cyclic behavior of base metal and heat affected zone materials of welded joints, (ii) establishing a simplified method to analytically predict the local notch fatigue behavior, (iii) defining a crack initiation criterion including mean stress effect. Notched specimens are first designed to produce a first configuration without residual stresses, a second one with tensile residual stresses, and a third one with compressive residual stresses. Cyclic tests are performed under repeated tensile loading. These tests allowed describing the influence of the initial residual stress states on the fatigue life quantified and analyzed on the basis of experiments. The application of the simplified method managed to capture the impact of residual stresses on the local cyclic behavior and the crack initiation resistance. Finally, the fatigue life prediction method is applied on thick welded T-joints. The deep hole drilling and the contour method are used to determine residual stresses in the weld toe. The proposed modeling approach is finally evaluated and discussed by correlating numerical predictions with experimental results obtained on T-joints loaded in four-point bending fatigue.

Probabilistic S-N curves for constant and variable amplitude

Welded details in steel bridges are subjected to fatigue lives up to 200·106 cycles due to traffic loads. While the majority of these stress ranges are below the Constant Amplitude Fatigue Limit (CAFL), some few high stress cycles can trigger the start of fatigue damage and lead the remaining load spectra to become damaging. The behaviour under spectra loading is thus of major importance for the fatigue design of steel bridges. This paper focuses on the fatigue behaviour of welded joints under variable amplitude loads. Fatigue tests have been conducted under constant and variable amplitudes on a typical bridge detail. Experimental crack growth curves were obtained using the Alternative Current Potential Drop method (ACPD), which showed the detrimental effect of stress ranges below the conventional CAFL. A two-step model with initiation-propagation was established to estimate the experimental fatigue lives, using a local strain approach for the initiation life and fracture mechanics for crack propagation. The model was implemented in a probabilistic Monte Carlo framework to include variability on the main parameters and establish S-N curves for Constant and Variable amplitude. The results of the simulations show that load spectra shape can be correlated with the S-N curves, namely the 2nd slope value below the CAFL.