Notch Stress Method Research Articles

Modified effective notch stress method for fatigue evaluation of rib-deck welds integrating the critical distance approach

To evaluate the reliability of the IIW-recommended effective notch stress (ENS) method and propose proper parameters for modification, fatigue testing data of 109 rib-deck welded specimens covering four cracking types were reviewed, and corresponding finite element models were established for parametric analysis. The critical distance approach was also integrated for improving the evaluation reliability. The effects of notch radius (r) and critical distance (a) on fatigue strength, and the effects of different methods for mean stress correction on the scatter ratio (Tσ) were investigated. Results show that the FAT71 S-N curve could be directly applied to the fatigue evaluation of rib-deck welds without mean stress correction; however, referring to the FAT225 S-N curve in notch stress system provides unconservative results. Instead, employing the FAT198 S-N curve combined with the Gerber method for mean stress correction or using the FAT200 S-N curve combined with the Walker method is recommended. Additionally, the FAT22 S-N curve is applicable while the notch radius (r) and critical distance (a) are valued 0.5 and 0.01 mm, respectively. The findings are of importance for improving the reliability of applying the ENS method to the fatigue evaluation of rib-deck welds.

Evaluation of local stress‐based fatigue strength assessment methods for cover plates and T‐joints subjected to axial and bending loading

AbstractThis study aims to find suitable fatigue assessment methods for welded structures (cover plates and T‐joints) subjected to axial and bending loading. The Hot Spot Stress (HSS), 1‐mm stress (OM), Theory of Critical Distances (TCD), Stress Averaging (SA), and Effective Notch Stress (ENS) methods are evaluated in terms of accuracy and reliability. The evaluation is based on fatigue test data extracted from the literature and carried out in this study. It is found that the SA method can be used to assess the fatigue strength of cover plate joints under axial loading with relatively good accuracy and low scatter, followed by the ENS method. The HSS, TCD, SA, and ENS methods are conservative estimation methods for T‐joints under bending, while the accuracy is low. Furthermore, fatigue design curves applicable for T‐joints under bending are discussed, which can be used in the TCD method and SA method.

Misalignment effect on the fatigue failure behavior of load-carrying cruciform welded joints

This paper presents an analytical model based on the Strain Energy Density (SED) method to predict the high cycle fatigue failure behavior of load-carrying cruciform welded joints (LCWJ) weakened by axial and angular misalignments. Compared with the Hot Spot Stress (HSS) and Effective Notch Stress (ENS) methods recommended in standard codes, the SED values at weld toe and weld root were carried out systematically considering the welded details and the types of misalignments. Moreover, the analytical model was applied to estimate the magnitudes of fatigue indicators and illustrated the effects of misalignment on the fatigue failure transition region in LCWJ. It is validated effectively as a supplementary analysis method for fatigue life prediction and fatigue design of welded joints compared with the experimental data and published reports.

Numerical study of the influence of weld geometry variations on fatigue life using the notch stress analysis

Idealized geometry is typically used in standards for the fatigue life assessment of welded joints. In the presence of stochastic geometrical variations along the weld, the choice of the idealized geometry is however ambiguous. In the notch stress (NS) method with a fictitious notch radius rref = 1 mm, the FAT 225 curve is derived for welds with relatively good quality in toe profiles. In the NS method with rref = ractual + 1 mm, a lower FAT 200 curve is recommended. Both approaches neglect the stochastic variability in toe radius, toe angle and leg length along the weld. The aim of this paper is two-fold. First, a numerical comparison between both approaches in terms of their predicted fatigue life is performed for a non-load carrying fillet cruciform joints. The results show that the NS method with rref = 1 mm and FAT 225 is substantially more conservative. Second, these methods are enhanced by replacing the deterministic stress concentration factor by a probability distribution computed using Monte Carlo simulation. It is shown that NS with rref = 1 mm and FAT 225 does not predict any substantial influence of the stochastic variability in process parameters since the actual toe radius is not included in the analysis. However, the NS method with rref = ractual + 1 mm and FAT 200 predicts a decrease in fatigue life when uncertainties in geometrical parameters is included. This numerical study paves the way for an experimental validation of the predicted influence of stochastic variability of geometrical parameters based on the stochastic notch stress analysis.

Fatigue strength of laser-dressed non-load-carrying fillet weld joints made of ultra-high-strength steel

Laser dressing or remelting is one of the post-weld treatment methods that is based on the modification of the local weld toe geometry, and generally, it is similar to TIG or plasma dressing. However, the laser dressing is not covered by common fatigue design guidelines and recommendations of welded structures at the moment. In this research, the effect of laser dressing on the fatigue performance of non-load-carrying fillet-welded cruciform joints made of direct quenched S960 grade steel was studied by means of experimental fatigue testing and geometry and residual stress measurements together with finite element analyses and different statistical calculation processes, such as nominal, structural and effective notch stress methods. In addition, the 4R method was also employed and its applicability for fatigue assessment of laser-dressed weld joints in question was investigated. The experimental results showed the enhancement of the fatigue strength for laser-dressed fillet weld joints compared to as-welded condition and the recommended FAT class for similar TIG dressing case was exceeded. However, the fatigue strength improvement of laser dressing was less compared to the corresponding TIG-dressed joints when applied stress ratio was low. In proportion, the disparity between the improvement effect of laser and TIG dressing methods was reduced with higher applied stress ratio. This can be explained by different residual stress state in the critical weld toe treatment area of laser- and TIG-dressed joints that can be taken into account when using the 4R fatigue assessment method.

Structural performance of laser welded edge joints made of high-strength steel

Laser welding is a widely-used fusion welding process in industry. However, laser welding is not a common welding process in the manufacture of industrial crane structures. There has been a remarkable increase in the available strength classes of steel grades over the last 10 years, such that strengths of up to 1200 MPa are now commercially available. This enables the use of thinner materials in welded products and at the same time, has opened up new possibilities for using laser welding more widely in the manufacture of steel structures. This study focuses on the static and fatigue strength of the laser-welded joints. Details investigated are edge joint with flange preparation between two rectangular tubes and edge joint between flat bar and rectangular tube. A novel fatigue strength assessment concept, the FATmod method, is applied to assess the theoretical fatigue performance of the joint in comparison with the effective notch stress method with a FAT630 design curve. The FATmod method is based on the local stress ratio at a fatigue-critical point of the joint and the analysis considers the strength of the material, surface quality and applied stress ratio in the assessment of fatigue. The study shows that the samples failed from the base material side in static tests and the FATmod method developed was found to agree well with the test results.

Fatigue reliability analysis of metro bogie frame based on effective notch stress method

Fatigue cracks may occur at the weld toe or weld root in the fatigue control locations of the metro bogie frame, an effective fatigue assessment method is needed to analyze the bogie’s fatigue reliability thoroughly. In this paper, the fatigue property tests of typical welded joints were completed, combined with the effective notch stress concentration factor, the effective notch stress S-N curve was determined. The effects of joint dimensions were studied to get a safer S-N curve which could be applied to the fatigue reliability analysis. This paper provides a theoretical and practical basis for the fatigue assessment of the metro bogie frame under service conditions.

Fatigue limit prediction of notched plates using the zero-point effective notch stress method

A zero-point effective notch stress method is proposed to predict the fatigue limit of notched plates. The effective stress is the sum of the structural hot-spot stress and the notch hot-spot stress, which are obtained by stress linearization of the overall stress and the non-linear peak stress at a characteristic length d0, respectively. A good agreement with experimental data in the literature can be achieved by the zero-point effective notch stress. The geometry dependence and mesh sensitivity of the proposed method are discussed. Comparing with the TCD, the proposed method can achieve identical prediction accuracy and is only geometry-dependent.

Comparison of Findley and MWCM multiaxial fatigue methods using the notch stress method on welded joints

Fatigue assessment of welded joints is vital for accurately designing and predicting lifetime of many engineering structures. Assessing weld fatigue is challenging due to the high variability in weld quality and geometry because of the volatile nature of the joining method and further challenges arise when assessing multiaxial fatigue. In this paper, two approaches, the Findley criterion and Modified Wöhler Curve Method, on several multiaxial experimental data sets of welded joints are compared to predict the fatigue life of the welded joints. Six experimental sets from literature are tested under a proportional torsion/bending multiaxial load. The notch stress method is used for calculating stresses at the toes of a weld.

A semi‐analytical formula for calculating the notch stress field of cruciform welded joint under bending loading

AbstractA fine finite element model should be established in fatigue assessment of cruciform welded specimens in notch stress method, and there is no analytical or empirical notch stress formula suitable for engineering application. Based on linear elastic fracture mechanics (LEFM) and singular strength theory, this paper put forward a simple semi‐analytical formula of notch stress for commonly used cruciform joints. Considering the influence of incision types and different opening angles, the proposed formula is further extended to U‐shaped notch and other common opening angles. After a series of verification, it is found that the proposed semi‐analytical stress formula based on singular strength values can simply and accurately predict the notch stress under bending loading, which provides a simple assessment approach for fatigue assessment of cruciform welded joints in engineering.

유효노치응력법을 이용한 T형 필렛 용접부의 피로수명 분석

This study aims to investigate the fatigue life of T-Type fillet welded joints for excavators subjected to bending loads, and also to verify the predicted fatigue life of the welded part using the effective notch stress method. Moreover, this study aims to determine an optimal toe angle of the T-Type fillet welded structure. In this context, the fatigue lives of T-Type fillet welded specimens (SM490A) were measured and the effective notch stress method for predicting the fatigue life of the T-Type fillet welded structure was verified by comparing with the FAT-225 curve of IIW (International Institute of Welding) as was suggested for the current types of welded structures. Considering simultaneously the scattering factor of the welded structure, the stress condition at the toe part higher than the root part, and the stress minimization condition of the toe part, the optimum toe angle at the T-Type fillet welding was identified at 30°. Likewise, the maximum stress (310.5 MPa) when the toe angle was 30° was about 14% less than the maximum stress (354.0 MPa) at 45°, and the fatigue life was improved by about 30%.

Fracture mechanics and fatigue life assessment of box-shaped welded structures: FEM analysis and parametric design

In this study, Linear Elastic Fracture Mechanics (LEFM) approach is used to evaluate the fatigue strength of a box-shaped welded structure. A parametric study is also undertaken to study the effect of various weld parameters on the fatigue strength, such as lack of weld metal penetration, load position, and plate thicknesses. FRANC3D software was adopted to obtain the stress intensity factor values for two types of full-length and intermediate crack sizes, located at the critical region of the weld of the box-shaped structure. It was concluded that the LEFM approach could capture the crack propagation from the weld root reasonably well under the given conditions and estimate residual fatigue life of the welded structures conservatively. Compared to fatigue life estimations by nominal stress method (1,714,564 cycles) or effective notch stress method (63,385 cycles), the LEFM approach can estimate the residual life more accurately. Especially for intermediate (4 mm) lack of penetration (LOP) of weld metal case (589,198 cycles) in comparison to the experiments (1,216,595 cycles). The parametric study showed that the fatigue life increases with increase in the thickness of flanges, lesser LOP in the weld root, and when load is applied more toward the center of the plate.

Fatigue strength assessment of TIG-dressed ultra-high-strength steel fillet weld joints using the 4R method

Post-weld treatments can enhance the fatigue strength of weldments by means of improving their geometry or residual stresses, or both. In this study, the fatigue strength of TIG-dressed non-load-carrying cruciform joints made of ultra-high-strength steel were analyzed using stress-based approaches, i.e. nominal, structural, and effective notch stress (ENS) methods, and a multi-parametric approach, entitled the 4R method. Comparisons were made between the methods, along with the applied parameters, and the results showed the 4R and ENS approaches to have the best correspondence with the experimental test data, whereas the nominal and structural stress methods resulted in conservative fatigue strength estimations.

Evaluation of magnesium weldment fatigue data using traction and notch stress methods

Recent magnesium weldment fatigue test data (Karakas et al., 2008), which involve partial- and full-penetration butt welds, as well as non-load carrying fillet welds, have been analyzed using both traction structural stress and equivalent notch stress methods. Both methods were compared for their effectiveness in correlating the test data by incorporating fracture mechanics-based load ratio parameters and was successfully used in conjunction with the load ratio parameter. Furthermore, the equivalent traction stress parameter, once converted into an equivalent structural strain parameter, enables comparison between magnesium weldment test data with the master S-N curve scatter band adopted in ASME Div. 2.

Assessment of grinding weld treatment methods using effective notch stresses

An experimental and numerical research program is executed to investigate the fatigue lifetime increase due to grinding-type post-weld treatment method. Plate edge gusset, longitudinal attachment and T-type joint specimens are investigated using fatigue test results, numerical models and mathematical statistical tools. The favourable impact of grinding on fatigue lifetime of the analysed joints is commonly known for researchers. However, considering the lifetime increasing effect is mainly based on experimental results. In the current research program, numerical models for as-welded and post-weld-treated specimens are developed and the lifetime increasing effect of weld treatment is investigated using effective notch stress (ENS) method. Fatigue tests are executed on as-welded and weld-treated specimens, and their results are comparatively evaluated based on numerical calculations and measured lifetimes. The results prove that the fatigue lifetime of weld treated (ground) details can be predicted with high accuracy by applying modified weld geometry in the numerical model and ENS method. Based on the current investigation, it is proved that the advantage of weld geometry treatment method comes mainly from the modification of the weld geometry at the fatigue critical point. Results also show that the same S-N curve can be applied for both as-welded and ground specimens, if the modified weld geometry is implemented in the numerical model.

Effective notch stress method for fatigue assessment of sheet alloy material and bi-material welded joints

Abstract As notch stress can effectively reflect the influence of welding geometry on local stress concentration, it can be used for fatigue strength evaluation of complex welded joints. In order to avoid stress singularity effect caused by the corner, virtual radius is introduced into fatigue evaluation of welded structures. However, the virtual radius recommended by the current specifications is only for steel and aluminum alloy materials, and is not applicable for thin-plate structures, especially for welded structures of special alloy materials and dissimilar materials. Considering the influence of alloy materials and size effect on hypothetical virtual notch stress, based on effective notch stress theory and finite element method, the optimal virtual radius values suitable for thin-plate alloy welded structures are proposed and verified respectively. After optimizing the virtual radius, the scatter band index of the notch stress S–N curve is significantly lower than the result of nominal stress.

Fatigue life prediction of spot-welded joints with a notch stress approach

Spot-welded joints are widely used in the construction of vehicle structures and frequently become critical locations for fatigue failure. Hence, it is essential to have reliable fatigue life prediction method for the spot-welded joints during vehicle structure design. In this paper, a new notch stress approach is developed for fatigue life prediction of the spot-welded joints. Currently, structural stress methods are widely used in automotive industry for fatigue life prediction of spot-welded joints. However, these methods are not well considering local geometry information. This paper introduces a notch stress based method to overcome the limitation of the structural stress methods. In the notch stress method, stress concentration factors for spot-welded joints are calculated from stress intensity factor equations. Then, the notch stress method is validated with fatigue test results of lap-shear and coach peel specimens.

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

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

Summary of research on fatigue life assessment of welded joints

In order to deepen the understanding of the fatigue problem of welded joints, this paper compares the research principles and limitations of four commonly used welding fatigue life prediction methods, including nominal stress method, hot spot stress method, notched stress method and fracture mechanics method. The nominal stress method is the most widely used in the four fatigue prediction methods, but it is too conservative. The hot spot stress method has high calculation accuracy, but it cannot fully reflect the fatigue behavior of the joint details. The notch stress method includes all sources of stress concentration, but the S-N curve recommended by IIW does not apply to the fatigue evaluation of butt joints. The fracture mechanics method is applicable to the crack propagation stage of joints. Through the analysis and research on the fatigue life assessment methods of four welded joints, it lays a theoretical foundation for the fatigue life prediction of welded joints.

Evaluation of crack propagation and fatigue strength of rib-to-deck welds based on effective notch stress method

The fatigue data of tested specimens was referred to, and an additional three specimens were tested to obtain the crack section. The stress distribution, crack initiation position, and crack propagation path were compared with the effective notch-based numerical results. A simplified simulation of the fatigue test, considering the effect of welding residual stress, was also performed. The fatigue lives of tested specimens, as well as numerical results, were investigated in the effective notch stress system to investigate the feasibility of the effective notch stress approach. The results show that the effective notch stress approach is precise enough to determine the crack propagation path. The proposed simulation method for fatigue testing is demonstrated to be feasible when predicting the fatigue life, and the SWT parameter is found with higher accuracy by taking into consideration the welding residual stress. It is reasonable to assess the fatigue strength of rib-to-deck welds by the classification of FAT-225 recommended by IIW using an effective notch stress approach.