Improving fatigue life for aluminium cruciform joints by weld toe grinding

The increase of fatigue life in aluminium cruciform joints by weld toe grinding was the focus of the current study. The test data are presented by both a nominal stress range approach and by the more refined structural and notch stress range approaches. The influence of the weld toe angle, weld leg length and weld toe radius on the structural and notch stress concentration factor (SCF) was systematically studied by means of finite element analysis. Experimental data based on 18 pieces of as-welded and 13 pieces of weld toe-ground specimens made of 12 mm thick plates showed a significant improvement in fatigue life in aluminium by grinding the weld toe and confirmed the permitted improvement in fatigue life by design codes.     

Crack aspect development curves and fatigue life prediction for surface cracks at weld toes in the presence of residual stress

An engineering procedure is proposed for estimating the crack growth behaviour and fatigue lives of semi-elliptical surface cracks at weld toes, based on a database of stress intensity factors. Some examples of crack aspect development curves (CADC) are given for some typical cracked welded joints subjected to service loading and residual stress conditions. The significance for predicting fatigue life according to the natural crack growth path, namely along the CADC, is emphasized through examples.     

Fatigue failure transition analysis in load‐carrying cruciform welded joints based on strain energy density approach

This paper details a study of the application of notch stress intensity theory to the fatigue failure mode analysis of the transition in load‐carrying cruciform welded joints. The weldment fatigue crack initiation point is difficult to predict precisely because it usually occurs in the vicinity of the weld toe or weld root. To investigate the relationship between fatigue failure location and the geometry of the weldments, we analysed the weld toe and root asymptotic notch stress fields were analysed using the notch stress intensity factors on the basis of the Williams' solution in Linear Elastic Fracture Mechanics (LEFM). Numerous configurations of cruciform joints of various plate thicknesses, transverse plate thickness, weld sizes and incomplete penetration size were used to investigate the location of the fatigue failure. The strain energy density (SED) surrounding the notch tip was introduced to unify the scalar quantity and preclude the inconsistency of the dimensionality of the notch stress intensity factors for various notch opening angles. The results of the investigation showed that the SED approach can be used to determine the transition zone for a variety of joint geometries. The validity of the SED criteria was verified by comparing the experimental results of this study with the complied results for load‐carrying cruciform welded joints reported in literature.     

New weld toe magnification factors for semi‐elliptical cracks in plate‐to‐plate butt‐welded joints

British Standard 7910:2013 includes a single set of equations for estimating the weld toe magnification factor (M_k) of different types of welded joints with a semi‐elliptical surface crack. In this study, extensive finite element analyses are performed to determine the M_k of plate‐to‐plate butt‐welded joints subjected to axial and bending loading. The M_k values at the crack deepest points and the crack ends are determined. It is found that the M_k results of butt welds are different with those of single T‐butt joints with percentage difference in values being as high as 63.8% and 97.4% for axial and bending loading cases, respectively. Percentage differences of 60.4% and 358.2% for axial and bending loading cases are observed between butt welds and X‐joints. New M_k equations for butt welds are proposed using multiple regression analyses.     

Parametric equations for T-butt weld toe stress intensity factors

This paper describes the generation of parametric equations for weld toe stress intensity factors. The methodology employed used a two-dimensional finite element analysis to evaluate the ‘crack opening’ stress distribution in the uncracked plane of T-butt geometries. This was then used as input into a dedicated weight function solution for the determination of stress intensity factors. The final parametric equations describe the stress intensity factor distributions for tension and bending as a function of plate thickness, weld attachment width, weld angle, weld root radius, crack length and crack shape. The equations are compared and validated against a wide spectrum of published values and appear by comparison accurate and wide ranging. The validation exercise uncovered situations where present design guidance is unconservative.     

Stress intensity factor solutions for estimation of fatigue lives of laser welds in lap-shear specimens

Fatigue behavior of laser welds in lap-shear specimens of high strength low alloy (HSLA) steel is investigated based on experimental observations and two fatigue life estimation models. Fatigue experiments of laser welded lap-shear specimens are first reviewed. Analytical stress intensity factor solutions for laser welded lap-shear specimens based on the beam bending theory are derived and compared with the analytical solutions for two semi-infinite solids with connection. Finite element analyses of laser welded lap-shear specimens with different weld widths were also conducted to obtain the stress intensity factor solutions. Approximate closed-form stress intensity factor solutions based on the results of the finite element analyses in combination with the analytical solutions based on the beam bending theory and Westergaard stress function for a full range of the normalized weld widths are developed for future engineering applications. Next, finite element analyses for laser welded lap-shear specimens with three weld widths were conducted to obtain the local stress intensity factor solutions for kinked cracks as functions of the kink length. The computational results indicate that the kinked cracks are under dominant mode I loading conditions and the normalized local stress intensity factor solutions can be used in combination with the global stress intensity factor solutions to estimate fatigue lives of laser welds with the weld width as small as the sheet thickness. The global stress intensity factor solutions and the local stress intensity factor solutions for vanishing and finite kinked cracks are then adopted in a fatigue crack growth model to estimate the fatigue lives of the laser welds. Also, a structural stress model based on the beam bending theory is adopted to estimate the fatigue lives of the welds. The fatigue life estimations based on the kinked fatigue crack growth model agree well with the experimental results whereas the fatigue life estimations based on the structural stress model agree with the experimental results under larger load ranges but are higher than the experimental results under smaller load ranges.     

New weld toe magnification factors for semi-elliptical cracks in double-sided T-butt joints and cruciform X-joints

British Standard BS7910:2005 has a single set of equations for estimating the weld toe magnification factor (M_k) of different types of welded joints containing a surface crack. The equations are based on the finite element (FE) results of single sided T-butt joint. In this study, extensive FE analyses are carried out to determine the M_k of double-sided T-butt joint and cruciform X-joint. Both the joints are subjected to axial and bending loading, and M_k at crack ends and deepest points of the surface crack are determined. It is found that M_k results for cruciform X-joint differ with those of double-sided T-butt joint with percentage difference in values being as high as 136% and 76% for axial and bending loading cases, respectively. Finally, a new set of M_k equations for cracked double-sided T-butt joint and X-joint are proposed based on multiple regression analyses.     

The relative effects of residual stresses and weld toe geometry on fatigue life of weldments

The weld toe is one of the most probable fatigue crack initiation sites in welded components. In this paper, the relative influences of residual stresses and weld toe geometry on the fatigue life of cruciform welds was studied. Fatigue strength of cruciform welds produced using Low Transformation Temperature (LTT) filler material has been compared to that of welds produced with a conventional filler material. LTT welds had higher fatigue strength than conventional welds. A moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm. It was concluded that residual stress had a relatively larger influence than the weld toe geometry on fatigue strength.     

Fatigue crack growth analyses of offshore structural tubular joints

This study investigated various aspects of a fatigue crack growth analysis, ranging from the stress intensity factor solutions to the simulation of a fatigue crack coalescence process of a tubular joint weld toe surface flaw. Fracture mechanics fatigue crack growth analyses for offshore structural tubular joints are not simple, because of the difficulty to calculate the stress intensity factors due to their geometric complexity. The fully mixed-mode stress intensity factors of nine weld toe surface cracks of an X-shaped tubular joint under tension loading were calculated by detailed three-dimensional finite element analyses. Using these stress intensity factor solutions, a fatigue crack growth study was performed for the X-joint until (the crack surface length grew to two times the tube thickness. Through this study, the crack shape change during the fatigue crack propagation was investigated in detail. Fatigue life calculations were also performed for a range of crack geometries using the stress intensity factor solutions of the nine flaws. These calculations indicate that the natural fatigue crack growing path for a crack is its quickest growing path. The study demonstrated that detailed fracture mechanics fatigue analyses of tubular joints can be practical using the finite element method.     

Fatigue properties of arc-welded lap joints with weld start and end points

Fatigue properties of arc‐welded lap joints with weld start and end points were investigated through experiments with 2.3‐mm and 3.2‐mm thick 440 MPa‐class steel sheets. Macroscopic fatigue crack‐initiation sites depended on the length of the weld bead to the specimen width. In joints with shorter weld beads, cracks mainly initiated at the toe of the weld start points, while joints with longer beads had initial cracks at the toe of the bead centre. Crack‐propagation analyses, taking stress distribution around the weld toe and residual stress into account, suggested that residual stress distribution could move crack‐initiation sites from the weld start point to the bead centre, although the applied stress at the toe of the weld start point remains the highest.     

STRESS INTENSITY FACTOR SOLUTIONS FOR SEMI-ELLIPTICAL WELD-TOE CRACKS IN T-BUTT GEOMETRIES

Abstract— Weld toe magnification factors are widely used in the evaluation of stress intensity factors for cracks in welded structures. Traditionally, the weld magnification factor has been determined from 2-D plane strain models containing edge cracks. However, it has long been recognised that a semi-elliptical weld toe crack cannot be accurately represented by a 2-D approximation due to the 3-D nature of the geometry. As a consequence, some recent research has been carried out using 3-D numerical modelling, which highlights the limitations of the 2-D approach. Nevertheless, 3-D solutions are still scarce and are of limited validity due to the difficulties associated with creating the numerical models. This paper reports the most extensive 3-D numerical investigation of semi-elliptical cracks in T-butt geometries to date. Based on the numerical results, new and accurate equations for weld magnification factors were derived, which quantify the 3-D effects present and emphasise the importance of the attachment. The results obtained from these equations are then used in an assessment of existing solutions.     

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

The paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.     

A notch stress intensity approach applied to fatigue life predictions of welded joints with different local toe geometry

In the Notch Stress Intensity Factor (N‐SIF) approach the weld toe region is modelled as a sharp V‐shaped corner and local stress distributions in planar problems can be expressed in closed form on the basis of the relevant mode I and mode II N‐SIFs. Initially thought of as parameters suitable for quantifying only the crack initiation life, N‐SIFs were shown able to predict also the total fatigue life, at least when a large part of the life is spent as in the propagation of small cracks in the highly stressed region close to the notch tip. While the assumption of a welded toe radius equal to zero seems to be reasonable in many cases of practical interest, it is well known that some welding procedures are able to assure the presence of a mean value of the weld toe radius substantially different from zero. Under such conditions any N‐SIF‐based prediction is expected to underestimate the fatigue life. In order to investigate the degree of conservatism, a total of 128 fillet welded specimens are re‐analysed in the present work by using an energy‐based N‐SIF approach. The local weld toe geometry, characterised by its angle and radius, has been measured with accuracy for the actual test series. The aim of the work is to determine if the N‐SIF‐based model is capable of taking into account the large variability of the toe angle, and to quantify the inaccuracy in the predictions due to the simplification of setting the toe radius equal to zero.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of HT80 steel at several stress ratios

Fatigue crack propagation rates and threshold stress intensity factors were measured for welded joints and base metal by using 200 mm wide centre-cracked specimens. The fatigue crack propagation properties of welded joints were similar in spite of the different zones in which the cracks propagated (ie, in the heat-affected zone and in the weld metal) and the different welding process used (submerged arc welding and gas metal arc welding). They were, however, inferior to those of the base metal. It was revealed by observation of the crack closure that the fatigue cracks were fully open during the whole range of loading, due to the tensile residual stress distribution in the middle part of the welded joints. This observation also explains the lack of a stress ratio effect on the fatigue crack propagation properties of welded joints, and their inferiority to those of the base metal.     

A simplified fatigue assessment method for high quality welded cruciform joints

The primary goal of this study was to develop an equation relating the geometric parameters to fatigue strength which can be used is routine design assessment. To attain this, the influence of local geometrical weld variations on the fatigue strength of non-load-carrying cruciform fillet welded joints were systematically studied using plane strain linear elastic fracture mechanics (LEFM). The effects of weld toe radius, flank angle and weld size were considered. Both continuous weld toe cracks and semi-elliptical toe cracks with alternate pre-existing defect depths were considered. A previously developed experimental crack aspect ratio development curve was used for assessing the growth of the semi-elliptical cracks using 2D FE models. A total of 152 experimental fatigue data points from six published studies of welded cruciform joints were evaluated. Details of the actual weld toe radius, flank angle and weld size were available for these joints. For the high quality welds evaluated, an assumed initial crack depth of 0.05 mm was found to correlate best with the experimental data. Of all the geometric parameters considered analytically, weld toe radius was found to have the most dramatic influence on fatigue life. A simple equation is proposed which relates welded joint fatigue strength to the ratio weld toe radius/plate thickness for high quality welds.     

Fatigue analysis of non-load-carrying fillet welded cruciform joints

The goal of this investigation was to study the effect of local geometrical variations of the weld on the fatigue strength. Therefore the fatigue behaviour of non-load-carrying cruciform fillet welded joint under tensile loading has been studied parametrically. Several two-dimensional (2D) finite element models of the joint were analysed using plane strain linear elastic fracture mechanics (LEFM) calculations in order to get the magnification function M_k. A maximum tangential stress criterion was used to predict the crack growth direction under mixed mode K_I-K_II conditions. The derived M_k solution was then applied both for continuous weld toe cracks and also for semi-elliptical toe cracks at the deepest point of the crack front. An experimental crack aspect ratio development curve was used for propagating semi-elliptical cracks. The as-welded condition was assumed with the result that no crack initiation period was considered and stress ranges were fully effective. The Paris crack growth law was used to predict the growth rate. The effects of weld toe radius, flank angle and weld size on the fatigue strength were systematically studied. Finally, predicted fatigue strength values corresponding to different assumed crack sizes were compared with the available test results.     

Experimental and analytical investigation of fatigue crack propagation of T‐welded joints considering the effect of boundary condition

T‐welded joints are commonly employed in ship and ocean structures. The fatigue failure of structure components subjected to cyclic loading always occurs in T‐welded joints because of the metallurgical differences, tensile residual stress fields and stress concentrations. The former researches about T‐welded joints fatigue have in common that the boundary condition needs to be taken into account as an influencing parameter to predict the crack propagation during cyclic loading. In this paper, the crack growth behaviour in T‐welded joint processed with Q345D steel (Pingxiang Iron & Steel Co., Ltd, Hukou, Jiangxi Province in China) under the fatigue loading was analysed via analytical model and verified via experiment. The results show that the influence of boundary condition should be considered in T‐welded joint structure during crack propagation in weld toe area. The correction factor concerning the effect of boundary condition and modified Paris' equation was proposed according to the experimental results and verified by the following repeated experiments.     

Importance of toe irregularity for fatigue resistance of automatic welds

The early propagation stages of fatigue cracks along the toe of automatic bead-on-plate welds in a structural steel are studied. Three methods to introduce a controlled degree of waviness on the weld toe, to optimize the degree of crack interaction during fatigue growth, were tested: arc rotation, variable arc voltage and variable weld speed. The development of fatigue cracks in seven specimens was monitored using a strain-gauge method and beach marking. Crack mismatch and depth to coalescence were found to be much smaller in the case of straight welds, which also showed shorter propagation lives. The period of toe waves, as well as local toe geometry, strongly influence fatigue crack initiation and propagation lives. Best fatigue life improvements were obtained with arc rotation techniques.     

An analysis of fatigue cracks in fillet welded joints

In most of the lower fatigue strength welded joints failure occurs by the propagation of a semi-elliptical surface crack which initiates at the weld toe. In order to analyse the progress of these cracks using fracture mechanics techniques, the solution for the stress intensity factor, K, is required. Fatigue cracks in most welded joints adopt shapes which give low a/2c values (up to approximately 0.3) while solutions in the literature are more applicable to a/2c values close to 0.5. Therefore, results in the literature were used to estimate the stress intensity factor for cracks with low a/2c values. Furthermore, the effect of the weld stress concentration factor was incorporated in the solution. The accuracy of the resulting solution was confirmed by using it to determine ΔK values of weld toe cracks for which crack propagation data were available. The results agreed with the expected da/dN vs. ΔK scatterband obtained from centre-notched specimens.