Strength Of Fillet Welds Research Articles

Fillet weld strength analysis for cantilever loading: an investigation of single-sided fillet weld strength in bending applications

Theoretical calculations for assessing the strength of a welded connection in design rely on two parameters: the tensile strength of the weld filler metal and the effective area. It is important to note that the type of load applied can significantly affect the theoretical strength of the weld. According to the AWS Structural Welding Code D1.1, when the load is applied parallel to the weld in a welded member, a reduction of 70% is recommended. This remaining factor of 0.30 has been determined through well-accepted tests to provide factors of safety between 2.2 for shearing forces parallel to the longitudinal axis of the weld and 4.6 for forces normal to the axis under service loading. When a load is applied perpendicular to the weld in a welded member, the entire value of the tensile strength of the weld filler metal is used to calculate the strength. However, there are no similar considerations for a load applied in a bending configuration. While it is not recommended for structural design, fillet welded members can experience loading that causes material deflection, resulting in a bending scenario. This is particularly relevant in repairs. The configuration of a cantilevered beam creates a different loading scenario with additional stresses on the weld, which differ from those of a perpendicular or parallel load. This research experiment was conducted to initially understand and analyze the strength of a GMAW weld under cantilevered bending and to derive a mathematical equation that provides a factor of safety in the range of 2.2-4.6, similar to the previous findings.

Experimental evaluation and design of fillet welds under in-plane eccentric loads

The principal objective of this investigation is to furnish equations to compute the design strength of fillet welds under in-plane eccentric loads suitable for load and resistance factor design (LRFD) applications. Over the years, experimental and analytical techniques have been used to arrive at safe estimates for the strength of such welds in common practical applications. The conventional elastic design (ED) method is very conservative and is not compatible with the LRFD design philosophy. In addition, the ED method neglects the weld group's ductility and strain compatibility. On the other hand, the instantaneous center of rotation (IC) method considers the above parameters but requires complex calculations. In the present study, a method for designing fillet welds under eccentric loading is introduced called the plastic design (PD) method. This method considers the inelastic properties of the welds in a simple manner while providing an accurate prediction of the weld group's capacity. The performance of the proposed PD method was compared with the IC method for several cases. The ultimate loads obtained were mostly within 10% difference on the conservative side and only marginally different for a few. Additionally, the fillet weld group's resistance under in-plane eccentric loads was evaluated experimentally using 12C-shaped specimens with the load applied at different angles of 0, 45, and 75°. The test results also confirmed the efficacy of the proposed PD method in evaluating the design strength of such weld groups.

Fatigue Resistance of Fillet Welds of Traction Rod Brackets on a Locomotive Bogie Based on International Union of Railways Standards and Improvement Measures Adopted

To solve the problem of fatigue failure in fillet welds of traction rod brackets on locomotive bogies of a given model, the cause for failure and the improvement method were studied. The results show that when there is maximum clearance at weld roots, maximum incomplete fusion of sidewalls, maximum incomplete fusion at weld roots, and maximum pores allowable in the ISO 5817 standard, the stress amplitude separately increases by 70~97%, 53~55%, 40~46%, and 19~34%. Despite this, when various types of defects of the maximum size are present in the weld alone, the static and fatigue strengths of fillet welds with a throat depth of 6 mm on the traction rod bracket can still meet the requirements in the UIC615-4 standard. In practical fillet welds, defects including clearance at weld roots, incomplete fusion, and pores are very likely to occur at the same time, which may induce fatigue failure in fillet welds of traction rod brackets within the original design life. According to the size of the frame and the traction rod brackets, a strengthening scheme for increasing the throat depth of fillet welds of traction rod brackets to 8 mm was designed. Calculation results of the strengthening scheme show that for new structures subjected to overall post-weld stress-relief thermal treatments, the maximum stress amplitude decreases by 5~29% when increasing the throat depth of fillet welds from 6 to 8 mm. For structures in service with the throat depth of fillet welds increased from 6 to 8 mm through repair welding, peak residual stress at the weld root after repair welding can reach 383 MPa. Because overall stress-relief thermal treatments cannot be performed on repair-welded structures, the fatigue strength of repair-welded fillet welds cannot meet the requirements of UIC615-4; therefore, local stress-relief treatments have to be performed in the welded zone. The results are of guiding significance for the treatment of locomotives in service and performance improvement of new locomotives and suggest that the current standard is relatively conservative.

Design of fillet welds in RHS-to-RHS moment T-connections under branch in-plane bending

Non-linear finite element (FE) models were developed to assess the AISC 360-16 Chapter K design approach for fillet welds in rectangular hollow section (RHS) moment T-connections under branch in-plane bending. The FE models were validated by comparison of the weld fracture moments, load-deflection responses, and spot-strain measurements to results from six previous, large-scale, weld-critical experiments. Based on all available experimental and FE data, the AISC 360-16 design approach is shown to be over-conservative. A key reason for this is that it does not account for bearing between the branch and chord on the compression side of the connection. New design formulae that take bearing into account are hence proposed. These formulae are shown to provide more accurate predictions of fillet weld strength in RHS moment T-connections under branch in-plane bending, and yet still achieve a safety (reliability) index that meets the AISC's target value of 4.0 for connections. The scope of this paper covers connections with all-around fillet welds and branch-to-chord width ratios up to 0.85.

Strength of duplex stainless steel fillet welded connections

Standard uniaxial tests were conducted on round bar specimens made of stainless steel and the corresponding weld metal. The stress-strain curves of both base metal and the corresponding weld metal were obtained. The constitutive relationship for base metal and the corresponding weld metal were calibrated against the test results and were taken into consideration for further finite element analysis on stainless steel fillet welded connections. Six transverse fillet welded connections and five longitudinal fillet welded connections were tested to investigate the strength, deformation, and fracture angle of stainless steel fillet weld connections. According to the test results, the average angle of stainless steel transverse fillet weld was 29°; the average angle of stainless steel longitudinal fillet weld connection was 46°. The ultimate strength for a transverse fillet weld connection was 1.5 times than that of a longitudinal fillet weld. Relevant numerical models using ANSYS were calibrated against experimental results. Further parametric studies were conducted to investigate the effect of weld length and weld size on the strength of stainless steel fillet welded connections. Finally, both the experimental and numerical results were compared with the current design provisions. The conservatism of the European, American, and Chinese design codes was discussed. A revised prediction of fillet weld strength was proposed based on the strength of the longitudinal fillet weld strength.

Fillet weld effective lengths in CHS X-connections. II: Finite element modelling, parametric study and design

Finite element (FE) modelling was performed to extend the results of a recently completed experimental test program to evaluate the static strength of fillet welds in X-connections between circular hollow sections (CHS). Non-linear FE models with weld fracture were validated by comparison of spot strains, load-deformation response, and fracture load with 12 experimental tests. Two hundred and fifty-six FE weld-critical CHS-to-CHS X-connections, with varied branch-to-chord diameter ratio, chord wall slenderness, branch inclination angle, and branch-to-chord thickness ratio, were analysed under quasi-static branch tension. The effect of these parameters on fillet weld strength is illustrated, and the structural reliability (or safety index) of North American specification provisions for weld effective lengths is confirmed. An alternative method for estimating fillet weld strength, with specific weld effective lengths, is proposed. Recommendations for a new design approach that meets the minimum target safety index in North America are made.

Fillet weld effective lengths in CHS X-connections. I: Experimentation

For the first time, an experimental test program was conducted to assess the strength of fillet welds in circular hollow section (CHS) connections. Six large-scale, fillet-welded CHS-to-CHS X-connections were designed with varied key parameters that affect the weld strength: branch-to-chord diameter ratio, chord wall slenderness, and branch inclination angle. By means of quasi-static tension applied to the branches, fracture of 12 test welds (two per connection) was obtained. Strain distributions measured adjacent to the welds indicated a weld effective length less than the total weld length, under pure axial load. Branch loads at rupture were also measured and used to determine the structural reliability of existing AWS provisions for weld effective lengths in CHS-to-CHS X-connections. For the range of parameters studied, the provisions are found to be very conservative. Hence, methods are assessed to accurately quantify the weld effective length. A parametric modelling study is presented in a companion paper to develop more liberal recommendations.

Root fatigue strength assessment of fillet welded tube-to-plate joints subjected to multi-axial stress state using stress based local methods

In this study the fatigue strength of fillet welded tube-to-plate joints failing at the weld root and subjected to multi-axial stress states is investigated. The fatigue test data is collected from the literature and it is assessed together with the experimental data generated in this study. Finite element analysis is used to analyze the stress state at the weld root. The fatigue strength estimation capabilities of local stress based methods such as the Principal Stress Hypothesis (PSH), von Mises Stress Hypothesis (vMH), Modified Wöhler Curve Method (MWCM), and Effective Equivalent Stress Hypothesis (EESH) are compared. The applicability of modified Gough Pollard Equation (GPE) in local stress system is also assessed. It is observed that most of the proposed local stress assessment methods can estimate the fatigue strength of fillet welds subjected to multiaxial stress states with constant principal stress direction, e.g. proportional loading. In case of load histories which produce varying principal stress directions with respect to time, e.g. non-proportional loading, better estimation capability is shown by MWCM and EESH. In most of the cases of varying principal stress direction load histories, vMH and PSH fail to estimate the fatigue strength. The fatigue strength of specimens tested with combined loading is reduced in comparison to the fatigue strength of specimens tested with only internal pressure and only bending loading. Out-of-phase loading does not affect the fatigue strength significantly for the specimens in this study. However; a decrease in fatigue strength is observed for the test data for out-of-phase loading collected from the literature.

Strength analysis of fillet welds under longitudinal and transverse shear conditions

In support of the development of improved fillet weld sizing criteria for lightweight shipboard structures, a comprehensive static strength test program using longitudinal and transverse shear specimens according to AWS B4.0 Standards has been conducted. This test program covers base material with strength ranging from 71 ksi (490 MPa) to 96 ksi (660 MPa) and weld size ranging from 1/8″ (3 mm) to 3/8″ (10 mm). This paper focuses on a traction stress based analysis of the test data as an effort to establish a unified shear strength definition for load-carrying fillet weld specimens regardless of shear loading conditions. The proposed shear strength definition proves to be effective in correlating the fillet weld strength test data of the longitudinal and transverse shear specimens. The results of this investigation demonstrate that existing shear strength definitions used by various weld sizing criteria such as those given by Class Societies have two major limitations: (1) it cannot be related to a critical stress state on experimentally observed failure plane in transverse shear specimens; (2) it underestimates shear stress at failure due to severe stress concentration at weld end in typical longitudinal shear specimens. These two limitations have been shown to be the major cause for having two significantly different shear strength values: one is transverse shear strength obtained from transverse shear specimens and the other is longitudinal shear strength obtained from longitudinal shear specimens.

Influence of fatigue impact loading on radial carrier and rapping device fillet weld strength

The device used to shake off the ash of the steam boiler harp consists of a drive shaft which has radial carriers welded on. During the operation of the waste incinerator steam boiler in Riverside, a fracture of the radial carrier and drive shaft weld occurred. Such a welded T-joint is made as a double fillet weld. It is loaded with the impact loading which causes the fatigue of material because of its repetition. The analytical calculation, with which the load-bearing capacity of the welded joint is determined, is suggested in this paper. The suggested analytical calculation of the mean impact force is based on the law of momentum conservation during the collision of rapping device and elastically deformable heat exchanger harp. The mean impact force is applied for the weld strength calculation. The results showed that the use of double fillet weld loaded with the fatigue impact loading was a bad choice. The suggested analytical calculation model is applicable for the rapping device design.

Fatigue strength of fillet welds subjected to multi-axial stresses

Fatigue endurance assessments of welded details are normally carried out by calculating the relevant stress acting on the detail and identifying a relevant fatigue class (or detail category) with its associated S–N curve. The fatigue strength of most structural details incorporated in design codes has been obtained from fatigue tests conducted under uni-axial loading conditions, which normally result in a uni-axial stress state in the detail. Many of the structural details that exist in fatigue-loaded structures experience some kind of multi-axial loading condition. The subject of the fatigue strength of welded details under multi-axial loads has been the topic of numerous research projects in recent years. The vast majority of these projects were, however, devoted to cracking in the base metal (i.e. toe cracking). Very little has been done with reference to the cracking of fillet welds in combined loading situations (i.e. root cracking). This paper presents new test results from cruciform specimens, in which weld failure initiated at the root in a multi-axial stress state. The tests have been performed at two different load levels and on three different specimen configurations giving different τ/σ ratios. This permitted an examination of the effect of the shear to normal stress ratio on the fatigue strength of fillet welds. The results of these tests, together with other relevant tests reported in the literature, are then evaluated in relation to the design models proposed in three design standards: Eurocode, IIW and DNV. No obvious dependence on the τ/σ ratios could be found. The evaluated models all appear to be able to predict the fatigue life of a cruciform weld failing from the root under combined shear and normal stress.

Fatigue Tests and Numerical Analyses of Partial-Load and Full-Load Carrying Fillet Welds at Cover Plates and Lap Joints

The fatigue strength of fillet welds is dependent on the amount of loads transferred. This is considered in the approaches for fatigue assessment in different ways. Particularly in the structural stress approaches, modified stress distributions in thickness direction of the parent plate have recently been proposed to consider the amount of load carried by the fillet weld in the fatigue assessment. In order to validate the approaches, fatigue tests have been performed with cover plates having partial-load carrying fillet welds and lap joints having full-load carrying fillet welds. The thickness of all plates was 12 mm, while two throat thicknesses with 3 and 7 mm were realized. The approaches for fatigue strength assessment include different structural stress approaches and the effective notch stress approach. Conclusions are drawn from a comparison of the approaches and from the fatigue lives obtained with respect to the consideration of the amount of load carried by fillet welds.

Strength and ductility of fillet welds with transverse root notch

Results from twenty-four cruciform weld experiments and complementary finite element simulations are presented to study the effect of the weld root notch on strength and ductility of fillet welds. All specimens are loaded in tension such that the weld root notch is perpendicular to the direction of loading, producing a crack-like effect. Two weld classifications (toughness rated E70T7-K2 and non-toughness rated E70T7; both Grade 480 MPa), two weld sizes (12 mm and 8 mm) and two root notch lengths (32 mm and 64 mm) are tested. Results of supporting ancillary tests, including all-weld tension coupons, Charpy V Notch impact tests and spectrochemical analysis are presented. The cruciform tests indicate that the strength of fillet welds is not affected significantly by the presence of the root notch. The E70T7-K2 welds show approximately twice the ductility of the E70T7 welds, whereas the E70T7 welds show a slightly reduced ductility as compared to previous published data for lap-welded joints. However, for the range of parameters tested, the ductility for all the welds is relatively insensitive to root notch length. Complementary fracture mechanics based finite element simulations are conducted to examine and generalize experimental findings. The simulations indicate that larger notch lengths do not further reduce the strength and ductility of the welds.

Behavior of Transverse Fillet Welds: Parametric and Reliability Analyses

Transverse fillet weld specimens were tested to assess the influence of various parameters on fillet weld strength and ductility and to evaluate the level of safety being provided currently by North American design standards. Details of the test procedures and results are reported in the companion paper by Ng, Deng, Grondin, and Driver, also published in the 2nd Quarter 2004 issue of the Engineering Journal. The variables included in the study were the filler metal classification (including classifications both with and without a toughness requirement), fillet weld size and number of passes, electrode manufacturer, steel fabricator, root notch orientation, and test temperature.

Errata: Rational Basis for Increased Fillet Weld Strength

Errata

The static strength of end and T fillet weld connections

The ultimate strength of fillet welded joints is of vital importance when analysing the collapse behaviour of a welded structure. However, both the small size and difficulties in determining accurately, for the different regions of the weld, relevant material properties to input in a finite element analysis complicates modelling. Hence the strength of fillet welds forming lap (cover plate) and cruciform (T) joints has been analysed by reviewing the various empirical relationships, based on both experimental and theoretical treatments, proposed in the literature. These results are compared with the strength of fillet welds predicted by different design codes, and are used to establish the correct materials parameters for input to a finite element model of a welded T joint analysed by the finite element program DYNA 3D.

Errata: Rational Basis for Increased Fillet Weld Strength

Errata

Rational Basis for Increased Fillet Weld Strength

The fillet weld strength increase occurs because of sensitivity to the direction of loading. At a loading perpendicular to the longitudinal axis of the weld, the weld has been shown to be on the order of 50 percent stronger than the lower bound for parallel loading. A full range of strength variation exists for intermediate force angles of 0-90 degrees. In the past, this effect had been conservatively and conveniently ignored for design but has now been recognized. While the detailed experimental justification reported in the literature is convincing by itself, the objective of this paper is to also demonstrate this increased strength with a simple rational model derived from first principles.

Examination of Fillet Weld Strength

The strength of a fillet weld is influenced by many different factors. For example, the welding process, electrode type and strength, weld-metal chemistry, welder skill, weld profile, joint penetration, joint fit-up and restraint, and preheat and interpass temperatures all contribute, in some part, to the resultant weld strength. Many of these factors, such as skill of the welder, joint fit-up, and preheat and interpass temperatures, are dealt with through particular requirements and provisions of the AWS Structural Welding Code. Weld qualification tests, for instance, are required to demonstrate whether an individual has adequate skill and training to produce a particular weld with a desirable level of quality. The design methodology used to compute the strength of fillet welds does not directly reflect all of the factors that influence fillet weld strength. The design method utilized for many years to compute the strength of a fillet weld involves multiplying the effective fillet weld area—the product of the effective throat size and the weld length—times an allowable or ultimate stress, depending upon whether an allowable stress or limit states design approach is being used. The effective throat dimension of the weld is taken as the shortest distance from the root of the weld to a line that connects the top and bottom weld toes; this procedure gives an effective throat that is 71 percent of the weld leg size for welds with equal leg sizes. It should be noted that this procedure ignore the influence of the weld root penetration and the weld profile. These factors will result in a true weld throat size that is different than the theoretical effective throat utilized for design purposes. The purpose of the study reported herein was to study the influence of geometrical factors that influence the true effective throat of fillet welds. Specifically, a series of tests were conducted to evaluate the influence of weld leg size and fabrication gaps on the strength of fillet welds.

Influeuce of Bead Toe Shapes on Fatigue Strength of Fillet Welds (2nd Report)

Experimental studies were carried out to examine the influence of undercut depth at the weld toe on the fatigue strength. Non-load carrying cruciform and T fillet welded joints were tested and quantitative life reductions due to undercut were obtained. Test results and life estimation procedure were reported in the previous paper.In this report, the fatigue strengths for 2 × 106 cycles were studied and were compared with each other. Adding to 16 test series joints concerning to undercuts, 40 test joints having various fillet shapes were also analysed. The equivalent fatigue strengths for 2 × 106 cycles were calculated from each failure life of test data, referring to the estimated S·N relations of these joints. The joint strengths were evaluated through the mean and the standard deviation of these equivalent strengths. The influence of the undercut depth and the bead toe shapes could quantitatively be discussed considering the undercut dependence of estimated strength as well as the scatter of equivalent strength data. These results led to the draft of the acceptance level that the allowable maximum undercut depth is 0.3 mm for the special members of offshore structures.