Weld Toe Cracks Research Articles

Stress intensity factors for three‐dimensional weld toe cracks using weld toe magnification factors

ABSTRACTIn welded components, particularly those with complex geometrical shapes, evaluating stress intensity factors is a difficult task. To effectively calculate the stress intensity factors, a weld toe magnification factor is introduced that can be derived from data obtained in a parametric study performed by finite element method (FEM). Although solutions for the weld toe magnification factor have been presented, these are applicable only to non‐load‐carrying cruciform or T‐butt joints, due possibly to the requirement of very complicated calculations. In the majority of cases for various welded joints, the currently used weld toe magnification factors do not adequately describe the behaviour of weld toe cracks. In this study, the weld toe magnification factor solutions for the three types of welded joints such as cruciform, cover plate and longitudinal stiffener joints were provided through a parametric study using three‐dimensional finite elements. The solutions were formed with exponents and fractions that have polynomial functions in terms of a/c and a/t – that is, crack depths normalised by corresponding half crack lengths and specimen thickness. The proposed weld toe magnification factors were applied to evaluate the fatigue crack propagation life considering the propagation mechanisms of multiple‐surface cracks for all welded joints. It showed good agreement within a deviation factor of two between the experimental and calculated results for the fatigue crack propagation life.

Fatigue performance of tubular X-joints with PJP+ welds: II — Numerical investigation

This paper summarizes the numerical investigation on the fatigue crack propagation in CHS X-joints under constant amplitude brace in-plane bending, in parallel with the experimental investigation reported in the companion paper. The finite element procedure examines the effects of the crack propagation angle, the crack-front profile and the interaction between adjacent fatigue cracks on the fatigue driving force, measured by the stress-intensity factors. The Paris type estimation provides a close prediction on the propagation life of weld toe cracks observed in the experiment. The initiation of the root crack, based on the Paris prediction, occurs much later than that of the toe crack.

After-Fracture Reserve Strength of Two Four-Chord Aluminum Sign Trusses

AbstractMany departments of transportation use four-chord aluminum tubular truss structures to support dynamic message signs (DMS). Inspections have often revealed that these structures contain cracks that likely occurred during or shortly after fabrication or were caused by fatigue while in service. Nearly all of these cracks are located at welded tube-to-tube connections, such as the diagonal-to-main chord fillet welded connections. Both weld toe cracks and surface-breaking root cracks have been observed. Destructive testing was conducted to evaluate the after-fracture reserve strength (AFRS) of 21.3-m-span and 16.8-m-span aluminum four-chord trusses. The dead weight of a DMS was simulated by attaching steel beams of the same weight eccentric to one exterior side of the truss near midspan. Wind load was simulated by hydraulic jacks and a whiffle-tree loading configuration. Simple steel frames were fabricated to mimic the sign truss end supports. Realistic boundary conditions, similar to the truss-to-end...

Three-dimensional weld toe magnification factors for various welded joints

To effectively calculate the stress intensity factors in welded components, a weld toe magnification factor is introduced here that will allow for the influence of geometrical effects. This factor was derived from the data obtained in a parametric study performed by FEM. Several sets of weld toe magnification factor solutions have been presented, but these are applicable only to non-load-carrying cruciform or T-butt joints, due possibly to the requirement of very complicated calculations. In the majority of cases for various welded joints such as cruciform, cover plate and longitudinal stiffener joints, the currently used weld toe magnification factors do not adequately describe the behavior of weld toe cracks. In the present study, the weld toe magnification factor solutions with exponents and fractions that have polynomial functions in terms of a/c and a/t — i.e., crack depths normalized by corresponding crack lengths and specimen thickness — were developed through a parametric study using 3-dimensional finite elements for the above mentioned three types of welded joints. These weld toe magnification factor solutions showed a tendency to increase at a/t < 0.2. Meanwhile, for 0.2 < a/t < 0.7, the effect of the attachment on the weld toe magnification factor decreased asymptotically. When the a/t ratio exceeded this range, the weld toe magnification factor became almost one of unity. The fatigue crack propagation life was evaluated by using the proposed weld toe magnification factor and by considering the propagation mechanisms of multiple-surface cracks, and it showed good agreement-to within a deviation factor of 2-between the experimental and calculated results for the fatigue crack propagation life for all welded joint.

Welded Sleeve Connection Design of Cantilevered Steel Sign Structures

Cantilevered steel sign structures are widely used for on-premises freestanding signs. A welded sleeve connection detail is commonly used to splice pipes of different diameters. This type of structure can be flexible and the connections are vulnerable to fatigue cracking attributable to wind-induced vibration. Fatigue has not been considered in design, although the connection has an inherently low fatigue resistance. To investigate the cause of failure at the sleeve connection and to mitigate the problem, a study of 10 sample failure cases and a series of finite-element analyses were performed. The finite-element analyses showed that the actual load-transfer mechanism in the sleeve connection was not very consistent with that assumed in design. In addition, very high-stress concentration caused the weld-toe crack to propagate into the pipe section. The common practice of strengthening the connection by welded gusset plates did not alleviate the problem. A frequency analysis indicated that the second-mode resonance because of wind-induced vortex shedding may cause a large number of damage cycles. Two alternative connection details were proposed to mitigate weld cracking.

Determination of Some Parameters for Fatigue Life in Welded Joints Using Fracture Mechanics Method

In this work, the parameters stress intensity factor (SIF), initial and final crack lengths (a i and a f), crack growth parameters (C and m), and fatigue strength (FAT) are investigated. The determination of initial crack length seems to be the most serious factor in fatigue life and strength calculations for welded joints. A fracture mechanics approach was used in these calculations based on SIF which was calculated with the finite element method (FEM). The weld toe crack was determined to be equal to 0.1 mm, whereas the weld root crack’s length was varied depending on the degree of the weld penetration. These initial crack length values are applicable for all types of joints which have the same crack phenomenon. As based on the above calculated parameters, the new limits of FAT for new geometries which are not listed yet in recommendations can be calculated according to the current approach.

Crack paths in weld details under combined normal and shear loading

In welded plate structures, fatigue cracks may initiate at the intersections between the flanges of longitudinal stiffeners and the web-stiffener attached to a transverse girder. The weld-toe cracks may initiate and propagate in the flange of the longitudinal stiffener, while in certain cases they may initiate from the root of the fillet weld resulting in the separation of the welded attachment from the flange. In order to distinguish such a morphological change of crack growth behavior, series of fatigue tests have been carried out, and the morphological variation of these fatigue cracks are successfully identified in terms of the normal and shear stress distribution acting along the fillet weld between the flange and the attachment.

Damage Tolerance Analyses of Laser Welded “Skin-Clip” Joints for Aerospace Applications

Future metallic airframes may contain laser beam welded clips (to the skin) between two stringers to achieve lightweight integral airframes. In this study, an investigation involving stress analysis and fatigue testing of skin-clip laser welded T-joints in 6156 T4 aluminium alloy plates was carried out. The thickness of the base plate (skin) was locally reduced between the welded joints by machining to produce “pockets” to obtain weight reduction. The effects of pocket thickness (3.0 and 1.5 mm) and socket width (the width of the full-thickness part between pockets; 11 and 20 mm) as skin-clip joint parameters on stress distributions around the weld region were studied by FE analyses of 400 mm wide plates under tension. Several numerical analyses were performed considering the effect of different locations of the tips of weld toe cracks in the T-joints, and the results obtained were compared with the experimental observations from fatigue tests of such joints in order to describe the fatigue performance of the laser welded skin-clip joints. Pocketed skin-clip joints exhibited weld region protection as a result of a reduction in the stress in the part containing the welded joint. Among the joint configurations analyzed in the absence of weld toe cracking, 3.0 mm pocket thickness with 11 mm socket width (narrowly prepared pocket design) provided almost full protection of the weld toe with little increase in stress on the opposite side of the plate containing the weld detail. However, in the presence of a weld toe crack, a region of stress concentration arose in the pocket itself, in line with the crack tip. This encouraged deviation of the crack path away from the weld toe but, if the socket width was too small, as in the narrowly prepared pocket design, the two stress fields combined and led to an increase in fatigue crack growth rate. In this study, the best fatigue performance was obtained when crack deviation into the base material occurred but remained within the socket area.

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.

応力集中部に存在するき裂のCTOD簡易推定方法に関する研究

In this study, new evaluation method of local strain for the fracture assessment of the crack in the strain concentration was proposed. This method based on the relationship between the definition of local strain applied to the assessment method for brittle fracture of steel-frame structure WES2808-03 and liner fracture mechanics.2-dimensional FE-analyses of the cross weld joint models with weld toe crack were conducted. Local strains at the stress concentration were calculated by the proposed method and CTODs were measured. And the relationships between local strains and CTODs were discussed. These results were compared with the CTOD design curve employed in WES2805-97 and the Level 2 FAD employed in BS7910-99, and the accuracy of assessment curves was investigated. The results demonstrate that the CTOD design curve gives slightly conservative estimation but FAD gives unsafe estimation in the case of Lr > 1.The relationship among stress concentration factor and crack size and K-value was also investigated, and the suitability of estimation equation of strain concentration factor employed in WES2805 was discussed. The equation gives smaller strain concentration factor, but the influence of the results on the CTOD estimation by CTOD design curve is not serious.

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.

Prediction of fatigue crack growth rate in welded tubular joints using neural network

In the past, several methods have been proposed to predict fatigue crack growth rate in tubular joints of offshore structures, however reasonably accurate solution for this problem is still lacking. Dramatic increase in the use of neural neural networks (NN) in material science, specially fatigue area, inspired an investigation on the application of NN in estimating fatigue crack growth rates and stress intensity factors in tubular joints. In this research, four MLP networks are developed to predict weld magnification factor for weld toe cracks in T-butt joints under membrane and bending loading. The training data for these networks are obtained from results of finite element modeling. In addition, two types of neural networks, i.e. MLP and RBF are developed to predict stress intensity modification factors for deepest point of fatigue cracks in tubular T-joint, under axial loading. Experimental data are used to train these networks. The results of above mentioned networks are used to predict fatigue lives of tubular T-joints. The comparison between network results and fatigue lives reported in experiments shows that NN is a successful prediction technique if properly used in this area.

Mode mixity for tubular K-joints with weld toe cracks

This paper examines the mode-mixity of stress-intensity factors for surface cracks located near weld toes of the crown point at the toe of the tension brace in circular hollow section K-joints under different chord/brace end constraints. The remote loading applies a uniform stress at the end of the brace along its axis. The 3-D finite element models employ mesh-tieing between a topologically continuous, global mesh and a separate, local crack-front mesh. The linear-elastic analyses compute the mixed-mode stress-intensity factors along the crack front using an interaction-integral approach. The analyses for three different crack locations along the brace–chord intersection demonstrate that the critical location for the surface flaw lies at the crown point of the brace toe. The numerical investigation of six different boundary conditions indicate that the unbalanced loading conditions cause significant bending stresses in the chord and generate a considerably larger mode I stress-intensity factor than found for the balanced loading case. The mixed-mode stress-intensity factors indicate that the crack front experiences predominantly mode I loading, with K III → 0 near the deepest point on the front ( ϕ = π/2). The non-dimensional, mode I crack driving force, described by F I = K I / σ ¯ br π a , reaches a maximum value at the deepest point of the crack for the crack aspect ratio a/ c = 0.25 considered here. The mode-mixity angle, ψ = tan −1( K II/ K I), at ϕ = π/2 is compared for a range of practical K-joint configurations. The present study demonstrates that the mode-mixity angle ψ becomes significantly larger in the balanced loading conditions than in the unbalanced loading condition. Values of the non-dimensional stress-intensity factors ( F I = K I / σ ¯ br π a and F II = K II / σ ¯ br π a ), however, decrease for the balanced loading condition. Variations in the brace-to-chord diameter ratio ( β) and chord radius to wall thickness ratio ( γ) generate significant changes in the mode mixity. Thin-walled joints ( γ = 20) generally experience a larger crack driving force with a higher mode-mixity angle.

Thermal constraint considerations in design of a heat recovery boiler

Heat recovery boilers are typically designed to conform to the rules of the ASME Boiler and Pressure Vessel Code, Section I on Power Boilers. While the unfired pressure vessel design rules of Section VIII of the ASME Code have evolved to accommodate a wide range of vessels and loading conditions (including system thermal loads), Section I of the Code remains characteristically simple, specifying explicit rules only for primary loading (pressure mainly). By and large, the combination of boilermaker experience and Section I rules makes for robust boilers. However, departure from conventional design, although in conformance to the explicit rules of Section I, can result in premature failures. This paper describes one such design of a heat recovery boiler in a hardwood Kraft pulp mill, wherein system thermal stresses induced on superheater tube-to-tube tie welds were high enough to result in weld toe cracking and early tube leaks. A finite element stress analysis of the design showed that operational thermal stresses due to constraint imposed by the tie welds were in excess of the tube material yield strength and above a level that would be considered acceptable for design. The analysis highlights the need for considering effects of thermal constraint in designs that otherwise meet the basic Code requirements for boilers.

Stress Intensity Factor Solutions for Semi-Elliptical Weld-Toe Cracks in Tubular K-Joints

Stress intensity factors (SIFs) are generally used to predict the remaining life of cracked tubular K-joints used in the offshore structures. The accuracy of SIFs depends very much on accurate modelling of the surface crack. In this paper, a new mesh generation method is proposed for any cracked tubular K-joint where five types of elements are adopted in modelling the surface crack so as to avoid high elements aspect ratio around the crack front. The mesh of any K-joint is obtained by merging the mesh of several distinct regions, and each region is generated separately according to different requirements. To check the accuracy of the computed SIFs, two large scale K-joint specimens were tested to failure under fatigue loading conditions. The alternating current potential drop (ACPD) technique is used to monitor the rate of crack propagation of the surface crack located at the crown position. The experimental test results are then used to validate the numerical results. Generally, they agree well and the relative errors are acceptable.

Fatigue Life Prediction of Ship Welded Materials

Fatigue crack propagation life of weld toe crack through residual stress field was estimated using Elber's crack closure concept. Propagation of weld toe crack is heavily influenced by residual stresses caused by the welding process, so it is essential to take into account the effect of residual stresses on the propagation life of a weld toe crack. Fatigue cracks at transverse and longitudinal weld toe was studied, these two cases represent the typical weld joints in ship structures. Numerical and experimental studies are performed for both cases. Residual stresses near the welding area were estimated through a nonlinear thermo-elasto-plastic finite element method and the residual stress intensity factor with Glinka's weight function method. Effective stress intensity factor was calculated using the Newman-Forman-de Koning-Henriksen equation, which is based on the Dugdale strip yield model in estimating the crack closure level, U, at different stress ratios. Calculated crack propagation life coincided well with experimental results.

Mode Mixity for Circular Hollow Section X Joints With Weld Toe Cracks

This paper describes the mode mixity of stress-intensity factors for surface cracks at weld toes located at the saddle point in circular hollow section X joints. The remote loading applies a uniform tensile stress at the end of the brace along its axis. The three-dimensional finite element models employ mesh tieing between a topologically continuous, global mesh and a separate, local crack-front mesh. Analyses of a simple plate model that approximates key features of toe cracks at the brace-chord intersection verify the negligible effects of the recommended mesh-tieing scheme on stress intensity factors. The linear-elastic analyses compute the mixed-mode stress intensity factors along the crack front using an interaction-integral approach. The mixed-mode stress intensity factors indicate that the crack front experiences predominantly mode I loading, with KIII→0 near the deepest point on the front (ϕ=π∕2). The total crack driving force, described by the J integral, reaches a maximum value at the deepest point of the crack for the crack aspect ratio a∕c=0.25 considered here. The mode-mixity angle, ψ=tan−1(KII∕KI), at ϕ=π∕2 is compared for a range of practical X-joint configurations and crack-depth ratios. The present study demonstrates that the mode-mixity angle ψ increases with increasing brace-to-chord diameter ratio (β) and decreasing chord radius to wall thickness ratio (γ). Values of the nondimensional stress intensity factors (FI=KI∕σ¯brπa and FII=KII∕σ¯brπa), however, show an opposite trend, with higher crack driving forces for small β and large γ ratios. The variations in the brace-to-chord wall thickness ratio (τ) and the crack depth ratio (a∕t0) do not generate significant effects on the mode mixity.

Fatigue and static behaviour of aluminium box-stiffener lap joints

The fatigue and static behaviour of aluminium box-stiffener lap joints were investigated in this study. Finite element analysis showed that stress concentration factor (SCF) at the weld toe decreased greatly as the weld leg length increased, while the location of the greatest SCF deviated from the connecting point to the middle of the flange. It is also found that the SCF at the weld root is much less than that at the weld toe. Fatigue tests showed that weld toe cracking was the main fatigue failure mode. In addition, fatigue crack initiation site changed from the connecting point to the middle point of the flange. Weld root failure was only possible when the weld leg length was less than twice the main plate thickness (6.0 mm). The test data agree with those for toe failure given by Eurocode 9.

Estimation of stress intensity factor solutions for weld toe cracks in offshore tubular joints

This paper proposes an engineering methodology for estimating stress intensity factor ( K) solutions for semi-elliptical weld toe cracks in tubular joints. The methodology uses the T-butt solutions proposed previously by the authors in conjunction with the stress concentration factors and the degrees of bending in the uncracked tubular joints. The estimated K factors are then compared with those obtained from a well-validated finite element procedure and the validity of the proposed methodology is demonstrated. A more refined way of calculating the uncracked stress field parameters which leads to more accurate predictions is also proposed. Fatigue crack growth calculations on experimental joints indicate that the proposed methodology is conservative.

Corrosion fatigue strength of ship structural steel butt welded joints in synthetic sea-water

Summary This paper describes an investigation into the corrosion fatigue strength of a MAG-welded 50 MPa class TMCP steel in fatigue tests carried out under a constant-amplitude load at a stress ratio of 0.1 in air and synthetic sea-water. The tests took place under natural corrosion conditions at 25°C at a cycling rate in air and sea water of 3–12 and 0.17 Hz respectively. The fatigue corrosion initiation life Nc was evaluated from the crack length measured by the DC electric potential difference method. This paper describes the S/N relationships, the macroscopic stress concentration factors and the corrosion initiation behaviour at the toes of butt welded joints. The results obtained may be summarised as follows. 1. The ΔSn/Nc relationship between the nominal stress range and the corrosion initiation life as determined in the tests made in synthetic sea-water is represented by a straight line. In the high applied stress range, the corrosion initiation life as determined in the tests made in air is virtually equal to that determined in the tests made in synthetic sea water. The ΔSn/Nc relationship found in the tests made in synthetic sea-water can be applied as the corrosion initiation life for purposes of ensuring the greatest margin of safety in tests made in air. 2. Even when a weld toe crack is present in the specimens tested in air, there is an applied stress range leading to no fatigue failure after fatigue cycling through 1 × 107 cycles. As distinct from the crack arrested at the weld toe in the tests made in air, the crack found in the specimens tested in synthetic sea-water propagates through corrosion dissolution of the crack tip. 3. The stress concentration factor at the weld toe Kt2 determined from strain gauge data is smaller than the local stress concentration factor Kt1 calculated from the two-dimensional cross section at the corrosion initiation point. Its scatter is also smaller. The ΔSt/Nc relationship of the weld toe in which the bending stress of welding angular distortion is superposed on the tensile stress of the axial force is therefore determined as ΔSt = Kt* 2ΔSn. 4. The corrosion initiation life to failure life Nc/Nf has a value of around 0.84 and 0.91 in air and synthetic sea-water, respectively. This difference is due to the effect of the environment on the crack propagation rate. 5. Fatigue cracks are always initiated in the weld toe bond and propagate at right-angles to the loading direction.