Weld Toe Magnification Factors Research Articles

Multi-crack propagation analysis of double-side welded rib-to-deck joint in orthotropic steel decks

Rib-to-deck welded joints are among the most susceptible to fatigue in orthotropic bridge decks (OSDs). Prior research has primarily examined the stress intensity factor (SIF) and propagation morphology of a solitary fracture at these joints, allocating limited attention to the investigation of the propagation of multiple cracks. This paper presents a methodology for simulating and computing the weld toe magnification factor and the SIF of semi-elliptical surface cracks in innovative double-sided welded rib-to-deck (DWRTD) joints. The intention was to simplify the calculation of the SIF for this fatigue detail. The SIF obtained from the calculation model in combination with a multiple crack coalescence approach resulted in a series of fatigue life prediction models of multiple co-planar semi-elliptical cracks. Compared with the results of fatigue tests on the DWRTD joints, the calculated fatigue life and propagation morphology of multiple cracks from the model showed good agreement. Finally, the parameters were analysed to investigate the effects of crack number, bridge deck thickness, residual stress, and penetration rate. Compared to a single crack, multiple cracks reduced the service life of the bridge deck by approximately 30%. When the bridge deck thickness increases from 12 mm to 20 mm, the nominal stress amplitude near the weld ∆σ decreases by 2.2 times, resulting in a 7-fold increase in crack propagation time. Compared to penetration rate and residual stress, ∆σ is the most crucial factor affecting fatigue propagation life. Therefore, it is suggested to increase the thickness of the bridge deck during design.

Fatigue Life Estimation for HFMI-Treated Weldments Considering Weld Toe Magnification Factors

AbstractHigh-frequency mechanical impact (HFMI) post-treatment is a proven method to improve the fatigue life of welded structures. The positive effects of HFMI treatment are influenced by the weld toe geometries and residual stresses. This study investigates the effect of geometric and mechanical improvements by HFMI treatment on fatigue strength with explicit consideration of the weld toe magnification factor. General fatigue life estimation method is based on experiment data, and residual stress may be considered in addition. In terms of HFMI-treated structures, geometric improvement also affects the fatigue life. Thus, a more efficient method is suggested by considering the weld toe magnification factor to assess the effects of HFMI treatment. First, the weld toe magnification factor in HFMI-treated conditions is calculated to consider the geometrical effect of HFMI treatment at the weld toe region. Second, a stress ratio model is introduced to consider the compressive residual stress by HFMI treatment based on the Paris equation. The results were validated by comparing them with the estimated fatigue life from previous studies on HFMI-treated welded specimens.

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

AbstractBritish Standard 7910:2013 includes a single set of equations for estimating the weld toe magnification factor (Mk) of different types of welded joints with a semi‐elliptical surface crack. In this study, extensive finite element analyses are performed to determine the Mk of plate‐to‐plate butt‐welded joints subjected to axial and bending loading. The Mk values at the crack deepest points and the crack ends are determined. It is found that the Mk 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 Mk equations for butt welds are proposed using multiple regression analyses.

3D stress intensity factors for weld toe semi-elliptical surface cracks using XFEM

Stress intensity factor and weld toe magnification factor solutions have been developed for T-butt joint geometries using the three-dimensional extended finite element method to model a semi-elliptical weld toe crack in a fillet-welded T-butt joint plate. The stress intensity factors of the semi-elliptical surface crack in a plain plate under tension were in good agreement with the reference solution by Newman and Raju cited in BS 7910 standard. The effect of the fillet welded T-butt joint over the stress intensity factor solution for a semi-elliptical surface crack in a plain plate are presented as a series of weld-toe magnification factors. The new magnification factor solutions were curve-fitted to upper bound, mean and lower bound solutions for practical use in a fatigue life prediction model. The predictions were verified by comparison to available fatigue life data, as well as the commonly used weight function technique. Fatigue crack growth modelling of weld toe semi-elliptical surface cracks were investigated for initial surface cracks with crack depth to plate thickness ratio (a/T) of 0.01 and three different crack aspect ratio (a/c) of 0.25, 0.5 and 1.0.

Stress intensity factors for fatigue analysis of weld toe cracks in a girth-welded pipe

This paper addresses the lack of three-dimensional stress intensity factor solutions for small to large surface cracks located at the weld toe region of welded pipes. A methodology was developed to model and calculate the stress intensity factors and weld toe magnification factors for semi-elliptical surface cracks in a circumferentially welded pipe. Three-dimensional finite element models were developed for a wide range of surface crack geometries. The stress intensity factors for surface cracks in a plain pipe were benchmarked with reported results and showed good agreement to within 2.6%. Stress intensity factors and weld toe magnification factors solutions were determined for weld toe surface cracks located at a circumferentially welded pipe. The weld toe semi-elliptical surface cracks geometries vary from small crack depth-to-thickness (a/t) ratios of 0.05 to large crack depth-to-thickness (a/t) ratio of 0.5. Three semi-elliptical surface crack shape aspect ratios (a/c) were modelled for values of 1.0, 0.5 and 0.25. The stress intensity factor and weld toe magnification factors solutions are reported in this paper for a wide range of semi-elliptical surface crack geometries with crack depth-to-thickness ratios (a/t) from 0.05 to 0.5 and crack shape aspect ratios (a/c) from 0.25 to 1.0 subject to pipe wall tension and bending loading. The stress intensity factor and weld toe magnification factors solutions are used for fatigue crack growth prediction assessment of weld toe surface cracks propagating in a circumferentially welded pipe.

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 (Mk) 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 Mk of double-sided T-butt joint and cruciform X-joint. Both the joints are subjected to axial and bending loading, and Mk at crack ends and deepest points of the surface crack are determined. It is found that Mk 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 Mk equations for cracked double-sided T-butt joint and X-joint are proposed based on multiple regression analyses.

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.

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.

A study of fatigue crack propagation at a web stiffener on a longitudinal stiffener

Predicting fatigue crack growth after its detection during in-service inspection is necessary to prevent a loss of serviceability, such as the oil and/or water tightness of critical compartments. This paper focuses on the most typical fatigue cracks that start at the weld joint between a flat bar stiffener on a transverse web frame and the flange of a longitudinal stiffener on a bottom plate or inner bottom plate. An experiment is carried out to observe the fatigue crack propagation for two kinds of flat bars at the abovementioned connection. The experimental results, especially the surface crack growth on the flange (which dominates during the total fatigue life of the longitudinal stiffener), are compared with crack growth curves predicted using a few existing formulas. Based on the comparative study, a formula that shows the best agreement with the experiment results is selected. Weld toe magnification factors for the web stiffener are computed from the crack propagation rates measured in the experiment, and two equations for the magnification factors versus crack depth are developed for two types of web stiffeners. The selected existing formula and the proposed equations are applied to two connections at the inner bottom and side longitudinal bulkhead of an LNG carrier. The equivalent stress approach based on a long-term distribution is employed to avoid the complexity involved in dealing with the actual stress history. Using this prediction, the remaining service life until an oil or water leakage occurs at a tank boundary can be estimated when a fatigue crack at the connection is detected.

Weld toe magnification factors for semi-elliptical cracks in T–butt joints — comparison with existing solutions

A set of comprehensive estimation equations for the weld toe magnification factor (Mk) for semi-elliptical cracks in T–butt geometries was developed and reported in a companion paper. Here the solutions obtained from these equations are compared with existing Mk and stress intensity factor solutions for cracks in plain plates, T–butts and tubular joints to demonstrate their applicability to practical situations. These comparisons show that the new solutions, in general, agree well with the existing solutions; where the agreement is poor, deficiencies in the existing solutions are identified. The Mk estimation equations proposed in this work have been included in the new British Standard BS7910.

Prediction of weld toe magnification factors for semi-elliptical cracks in T–butt joints

This paper reports the background development of a set of equations for estimating weld toe magnification factors (Mk) for semi-elliptical cracks in T–butt joints. This set of equations has been included in the new British Standard BS 7910. The extensive finite element parametric study from which the database of Mk factors was generated is first reported. The parameters studied — crack depth and aspect ratios, attachment footprint, weld angle and weld toe radius — and the ranges covered are those likely to be found in real structures. The procedures for numerical modelling, analysis and stress intensity factor evaluation are described, and the effects of the various influencing parameters on the Mk factors are discussed. Estimation equations that were developed from multiple regression analyses of the database of Mk factors are detailed. The equations are shown, through statistical analyses, to be a very good fit to the database from which they were derived.

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.

Fatigue performance of repaired aluminium panels

Stress intensity factor and weld toe magnification factor solutions have been developed for T-butt joint geometries using the three-dimensional extended finite element method to model a semi-elliptical weld toe crack in a fillet-welded T-butt joint plate. The stress intensity factors of the semi-elliptical surface crack in a plain plate under tension were in good agreement with the reference solution by Newman and Raju cited in BS 7910 standard. The effect of the fillet welded T-butt joint over the stress intensity factor solution for a semi-elliptical surface crack in a plain plate are presented as a series of weld-toe magnification factors. The new magnification factor solutions were curve-fitted to upper bound, mean and lower bound solutions for practical use in a fatigue life prediction model. The predictions were verified by comparison to available fatigue life data, as well as the commonly used weight function technique. Fatigue crack growth modelling of weld toe semi-elliptical surface cracks were investigated for initial surface cracks with crack depth to plate thickness ratio (a/T) of 0.01 and three different crack aspect ratio (a/c) of 0.25, 0.5 and 1.0.