Effective Notch Stress Method Research Articles

Fatigue life analysis based on the groove angle reduction in the butt welding of excavator booms

This study analyzes the effect of changes in groove angle on the fatigue strength of the V-groove butt weld joint made of SM490A steel used in excavators. Butt-type specimens with different groove angles of 40° and 30° were manufactured by CO2 welding at a speed of 300 mm/min on a 13.5 mm thick steel plate. Four-point bending fatigue tests were conducted at room temperature with a frequency of 5 Hz and stress ratio of R = 0.1, and fatigue limits were set to 2 × 106 cycles. The high-cycle fatigue life of VG40 was approximately 7% higher than that of VG30. The stress concentration coefficient calculated using the effective notch stress method was 1.62 for the VG40, and 1.78 for the VG30, with an approximate difference of 9%. This decrease in fatigue life was caused by stress concentration due to the notch effect at the weld root, which was attributed to the adjacent backplate. Although the fatigue life decreased slightly owing to the improvement angle of the welded part, no difference was observed in microstructure and hardness. Therefore, it was confirmed that a sound weld was formed. The results of this study can be used as important basic data for determining the V-groove angle.

Specimen design for gigacycle fatigue testing of structural steel welded joints

Structures and components across a range of industries exhibit service lives above 107 cycles, but there is limited fatigue data in this domain. Conducting fatigue tests in the gigacycle regime is only feasible by using ultrasonic fatigue testing machines in which specimens are typically excited at 20 kHz. This research focuses on the design of a novel specimen which captures the geometric stress concentration at the weld toe, as an alternative to typically used cylindrical ‘hourglass’ shaped specimens. The gas-shielded flux-core arc welding method was used to manufacture butt joints of 080A15 structural steel. Specimens were tested in fully reversed axial loading at room temperature using a Shimadzu USF-2000A ultrasonic fatigue testing machine. Specimens failed in the range of 104-107 stress cycles with all fatigue cracks initiating at the weld toe. One specimen was classed as a run-out after being subjected to 3 × 109 stress cycles without failure. The fatigue data obtained was assessed using the effective notch stress method and showed similar fatigue performance to the IIW guidelines.

Considerations regarding the fatigue design of the steel bridges

The fatigue damage of steel bridges is prone to occur under a large amount of heavy traffic and be a worldwide problem leading to serious disasters. This communication will discuss the importance of the correct evaluation of the fatigue category of resistance for the safety and durability of bridges. Although much attention is usually rightly paid by engineers to the effects of traffic, on the other hand the effect of stress concentrations which modify the category of detail of the tables of the Eurocode is still very poorly known.We will deal with multiple examples to illustrate the question: the replacement of the cylinder head of the François I bridge in Le Havre at the very beginning of the 80s is an excellent example of an approach to the calculation of stress concentration factors by finite elements, which made it possible to solve the fatigue problems of this movable bridge.On the other hand, the cracking of the bridges of Chamousset where a then forbidden constructive arrangement was nevertheless built shows that the question is not a view of the mind: If the problem had not been identified and the bridge put under annual surveillance for the hot spots concerned, the two bridges could have collapsed.Today, Clause 7.1(5) of the French National Annex to EC3 1-9 has put manufacturers under the obligation to take into account stress concentration factors since 2011. Several features will be reviewed, such as for example the thickness transition zones with and without cope hole, the anchorages of hangers, the studs under tension and bending, the bottom cuts of the vertical stiffeners that actually do not help regarding fatigue, and the interruptions of guide rails.Finally, the effects of corrosion, in particular the case of the weathering steels when used in the transition zones will be discussed in detail using models using the “Effective Notch Stress Method” based on tests carried out on samples for the Academy of Sciences of the Czech Republic by Ludwig Kunz. Of course, the aging of weathering steels in case of birds droppings is not considered in this study. Even without birds droppings, the fatigue resistance of some weathering steels details has to be reduced considering the Clause 7.1(5) of the French National Annex to EC3 1-9 when compared to the categories given by the tables of the Eurocodes for painted details.

Life-cycle Cost assessment of Post Weld treatments: Effect of local weld geometries

The fatigue life of welded structures is dependent on the local geometry and imperfections of the welds. Therefore, optimizing the manufacturing processes to improve the local weld geometry and remove possible imperfections can considerably affect the fatigue life of the structure. Post-weld treatments such as burr grinding, and High Frequency Mechanical Impact (HFMI) treatment are commonly used techniques to extend the fatigue life of a weld by modifying the local weld toe geometry. However, employing additional manufacturing processes can have an adverse effect on the Life-cycle cost of a welded structure by increasing production costs. On the other hand, extending the fatigue life of a structure can result in lower maintenance and replacement costs. Therefore, a thorough yet predictive life-cycle cost assessment is required to assess the viability of such treatments and design economically efficient weld structures.This study assesses the life-cycle cost of welded joints. The fatigue life of the weld in this study is analyzed using the effective Notch Stress method (NS). Furthermore, the weld is post-treated with an automated HFMI treatment to prolong its fatigue life. Moreover, the effect of weld quality on the production cost is analyzed. The results show that every phase of the weld's life-cycle has a significant contribution to the life-cycle cost with the use-phase being the more dominant. The results also depict the impact of the changes in weld quality in the overall life-cycle cost.

Modelling of Size Effect in Fatigue Strength for Welded Joints using Effective Notch Stress and Probabilistic Methods

Modelling of Size Effect in Fatigue Strength for Welded Joints using Effective Notch Stress and Probabilistic Methods

New developments on fatigue in Eurocode on steel structures

The design rules for the verification of civil engineering steel structures against fatigue are compiled in the European harmonized design standard Eurocode EN 1993-1-9 released in 2005. This design standard is currently under revision, together with all other Eurocode design standards. Besides the inclusion of new developments and clarification, this revision aims for an improvement of user-friendliness and an extended degree of harmonization by reducing the number of Nationally Determined Parameters. The structure of the revised standard has also been modified to facilitate the application of the standard. In addition, the following major changes have been implemented: (i) Improvement of S-N curves in terms of the nominal stress method: clarification of stress computation, and re-organization of detail category tables. (ii) Improvement and extension of design rules for the hot-spot-stress and inclusion of design rules for the effective notch stress methods, in separate annexes. (iii) New annexes to determine the most common magnification and stress concentration factors for the modified nominal stress method as well as the stress ranges in preloaded bolts. Meanwhile, the final document of the project of the European design standard Eurocode prEN 1993-1-9 has been submitted to the “CEN Enquiry”. Thereafter it will go through the “Formal Vote”, planned for October 2024, before replacing the 2005 version throughout Europe.

A simplified fracture mechanics method for fatigue life analysis of weld roots

Volvo CE currently uses the effective notch stress method for assessing fatigue life in welded structures. For large structures such as frames, a sub modelling technique with a very fine mesh is necessary using millions of DOF. Stress tensors from unit load cases, estimated with FEA, are combined with time history loading from Multi Body Simulations of the complete machine operating on customer representative digitized test tracks. A critical plane approach (the Modified Wöhler Curve Method) is used to estimate the fatigue life. The usage of the effective notch method implies a very time-consuming design process. An LEFM approach for weld roots would be even more time-consuming but would be a more natural approach since the weld root consists of crack like defects. To overcome the drawbacks with the current design process that involves usage of the effective notch method, a hybrid method is defined using pre-calculated geometry factors for different crack sizes and weld geometries. The geometry factors as a function of crack size, and the weld geometry, are stored in a database. From the unit load cases the structural stress using a coarse mesh in the weld is extracted. Together with the geometry factors, the stress intensity factors may be estimated and used with Paris law to calculate the fatigue life of welds. A comparison between the effective notch method, LEFM and the new hybrid method is carried out for typical fillet welds is presented in this paper.

Fatigue performance assessment of thick TIG-Dressing cruciform welded joints made by Q355D structural steel

TIG-Dressing treatment is often used for steel welded joints in construction machinery manufacturing as an effective and reliable method for post-weld fatigue strength improvement. This paper investigates the fatigue performance of thick Q355D cruciform joints in heavy load-carrying steel structures under different TIG-Dressing treatments. Two TIG-Dressing treatments for the full-penetration welded joints were used for fatigue tests considering filling metal during this post-welded processing. Experimental tests studied the fatigue strength of TIG-Dressing cruciform welded joints of Q355D structural steel. The geometric parameters and relevant statistical analyses were performed by actual 3D optical measurement. Moreover, the stress concentration factors are calculated according to the actual geometrics of the welded joint. On the other hand, the effective notch stress and hot spot stress methods were employed to evaluate the fatigue data of Load-Carrying Welded joints (LCWJs) under different notch radius. The collected fatigue data from the literature was compared with the tested data by the above methods. The results show that the fatigue strength of LCWJs under TIG-Dressing processing without filler metal is higher than those under the same TIG-Dressing treatment with filler metal. The effective notch stress method with a fictitious notch radius r = rtrue + 1 mm (rtrue is the actual notch radius at weld toe) could obtain smaller scatter bands for the tested fatigue data.

Misalignment factors to affect the fatigue of welded load-carrying joints

To assess the effects of misalignment on the weld fatigue, we present experimental fatigue test results of load- carrying cruciform joints which are subjected to axial variable amplitude loading. The welds were produced from steel AH36 grade and fatigue strength improved by high-frequently mechanical impact treatment. We identify several misalignment factors affecting the fatigue by considering the real and reference welded joint geometries. Our methodology includes proposals for the empirical calibration functions and local stress analyses by using the Structural Hot Spot Stress, Effective Notch Stress and Peak Stress Method. Additionally, we apply our methodology to other sixteen data sets which are extracted from the literature for load-carrying cruciform and butt welded joints subjected to constant amplitude loading. Finally, we present the resultant calibration functions with respect to corresponding fatigue test data.

Different fatigue design stress methods for common constructional details in cylindrical shells under axial loading – Influence of the r/t‐ratio and the shell thickness

AbstractCylindrical shells are frequently used in industrial constructions, for example in pipes, towers and masts and are often subjected to fatigue loading. Two common welded constructional details were studied: i) detail 1, the wall thickness transition with a circumferential butt weld and ii) detail 2, the welded ring stiffener. Both details are loaded with axial stresses due to N and/or M. In the new Eurocode prEN 1993‐1‐9:2021 three different fatigue design stress methods are provided in general. For these two constructional details, beside the modified nominal stress method, the hot spot stresses and effective notch stresses are determined with Finite‐Element‐Analyses, with variation of geometric parameters, like shell thickness t, radius‐over‐thickness‐ratio, t2/t1‐ratio at the thickness transition and thickness ts of the ring stiffener. Stress concentration factors for the different fatigue design stress methods are presented and the influence of the geometric parameters are investigated. Especially the difference between a plate with an infinite radius‐over‐thickness‐ratio and the pipe with a finite radius‐over‐thickness‐ratio is shown. Also, the influence of the shell thickness is investigated. For comparison of the three different methods the utilization factors of the hotspot stress method and the effective notch stress method are compared with those of the modified nominal stress method. A recommendation is given, which design stress method is appropriate for the two studied details, depending on the geometric parameters.

Low-cycle fatigue in ultra-high-strength steel welded joints in the as-welded and post-weld-treated conditions

This study investigates four different joints under fluctuating tension cycles in the low-cycle fatigue (LCF) regime. The selected material was S960 ultra-high-strength steel, manufactured via the direct-quenching manufacturing route. Based on typical engineering solutions, the following joints were chosen to be investigated in the experimental tests: metal active gas and laser welded butt joints, fillet-welded load-carrying cruciform joints, and non-load-carrying double-sided transverse attachment joints. To evaluate the improvement gained by the post-weld treatment, both as-welded condition and high-frequency mechanical impact treated joints from all joint types were tested. In addition to the experimental tests, some of the joints were analyzed using finite element (FE) models, with the aim of comparing the accuracy of the different fatigue assessment methods and the effect of the toe radius on the computational fatigue lives. The results obtained from the experimental tests and FE analyses were compared with each other and with IIW recommendations using the nominal stress, structural hot-spot (HS) stress, effective notch stress (ENS) and 4R methods. Based on the results, the structural HS and 4R methods are very suitable for the fatigue strength assessment in the LCF regime. With the nominal stress method, the computational fatigue lives remained uniformly conservative, while with the ENS method, the results varied depending on the calculation method. Using the value r = 1 mm as the weld toe rounding is justified in the FE analysis.

Fatigue strength assessment of cover plate joints subjected to axial and bending loading

AbstractThis study investigates stress‐based fatigue assessment methods to determine their applicability to welded cover plate joints subject to axial and bending loading. The nominal stress (NS), hot spot stress (HSS), 1 mm stress (OM), effective notch stress (ENS), theory of critical distances (TCD), and stress averaging (SA) methods are covered, and their accuracy and reliability are evaluated. To the best of the authors' knowledge, there is limited fatigue test data available for cover plate joints subjected to bending loading. In this study, fatigue tests are performed with cover plate joints under axial and bending loading. Evaluation of the fatigue assessment methods is based on the test results. It is observed that for axial loading, the ENS and OM method have the highest accuracy. For bending, the OM method is non‐conservative, and the other methods are overly conservative. Using design curves recommended for thin‐walled welded joints subjected to bending highly improves accuracy.

Fatigue failure investigation of thick plate butt‐welded joints with incomplete penetration effect

AbstractThe objective of this paper was to investigate the fatigue performance of the thick plate structural steel butt‐welded joints influenced by its geometrical factors, such as the notch angles, incomplete penetration length, and plate thickness. Utilizing experimental tests, finite element calculations, and analytical models, the fatigue behavior of various weld joint shapes and dimensions was analyzed, and the discrepancy of fatigue strength for different incomplete penetration lengths was evaluated using the nominal stress method. Meanwhile, the local approaches, including notch stress intensity factor (NSIF), averaged strain energy density (SED), effective notch stress (ENS) method, peak stress method (PSM), and implicit gradient effective stress (IGES) method, are employed to assess the fatigue life of thick plate welded joints. According to the results, incomplete penetration ratios in butt‐welded joints led to a decrease in fatigue strength. The analytical solutions of butt‐welded joints in high cycle regime based on SED results were obtained to estimate the fatigue characteristic values for different plate thickness and incomplete penetration ratios. The proposed equations could illustrate the relationship between the nominal stress and local SED dealing with the variations of incomplete penetration length and geometrical dimensions. Moreover, the results obtained by the analysis of ENS, PSM, and IGES could be deduced by the analytical models to determine the fatigue life of welded joints.

Fatigue assessment of U-rib full penetration welded joints based on local methods

Experimental and numerical methods were used to carry out fatigue assessment of double-sided full penetration U-rib welded joints with different deck thicknesses in Orthotropic Bridge Decks (OBD). The local stress field analysis was conducted by the finite element method. S-N curves for different thicknesses can be obtained based on fatigue test results. The Nominal Stress (NS) method, the Effective Notch Stress (ENS) method, and the Theory of Critical Distances (TCD) were applied to estimate the fatigue life. The feasibility of these three approaches was validated by fatigue test results. The results show that two types of fatigue crack initiation sites (inner and outer weld toes) and typical smooth and rough regions are found. The maximum principal stress decreases by about 30% when the deck thickness increasing from 14 mm to 18 mm, while the fatigue life increases by 6–11 times for different stress ranges. The stress concentration factors (SCFs) for U ribs with 14 mm, 16 mm, and 18 mm deck thicknesses are 2.71, 3.16, and 3.15, respectively. Double-sided U ribs welded joints with thicker decks have better fatigue performance. All predicted results using the NS method and the ENS method are greater than the FAT71 and FAT225 fatigue curves, respectively. The Point Method (PM) and Line Method (LM) in the form of TCD are found to be successful in predicting the fatigue life and recommended to fatigue assessment for double-sided welded U-ribs.

Evaluating the fatigue property of S355J2W steel butt-welded joint: Multiple notch effects

The fatigue fracture mechanisms and the fatigue property evaluations of S355J2W steel butt-welded joints relieved welding residual stresses with and without welding reinforcement were systematically investigated by microstructure characterization and fatigue tests. Experimental results show that the complex evolution of microstructure at welded joint results in the notable degradation of mechanical properties. The fatigue performance of the as-welded specimen with welding reinforcement is significantly poorer than that of the smooth-welded specimen without welding reinforcement. Stress concentration under the fish-scale pattern at the weld toe caused by geometric dimension change is the major reason for the fatigue fracture of the as-welded specimen. By comparison, the cementite segregation at grain boundary in the partial recrystallization zone of the heat affected zone caused by the sharp change of microstructure should be responsible for the fatigue fracture of the smooth-welded specimen. The fatigue notch coefficient (Kf=1.89) calculated by the effective notch stress method is consistent with that (Kf=1.9) measured by the fatigue tests, which shows that the effective notch stress method can well evaluate the multiple notch effect under the coupling of the weld toe and the fish-scale pattern. Furthermore, the fatigue risk factor (Rf) model was employed to evaluate the fatigue performance of welded joint, and the intuitive Rf curves show that the fatigue fracture occurs at the fatigue life exceeding 2 × 106 cycles, i.e., Rfmax exceeds 0.7.

Fatigue performance of transverse welded attachment of high-strength structural steel series

This study presents an experimental investigation of the fatigue performance of non-load-carrying transverse fillet-welded attachments made of high-strength steel (HSS). Four typical HSSs were selected, with steel yield strength of 460 MPa, 550 MPa, 690 MPa, and 960 MPa. A total of 88 high-cycle fatigue tests were conducted. Based on the analysis of the morphological details of the fracture surfaces and the failure modes, three different methods for fatigue assessment, such as the nominal stress method, the effective notch stress method and the strain energy density method, were applied to directly compare the fatigue performance of HSS and conventional strength steel (CSS, with a yield strength of approximately 235–355 MPa). HSS transverse fillet-welded attachments have demonstrated excellent fatigue resistance compared with specifications (FAT 80) in the design codes by using the nominal stress method, the natural slope of the S–N curves is nearly 4.0, rather than 3.0 prescribed in the current design codes for fatigue assessment of welded structures. For the notch stress method, HSS transverse fillet-welded attachments show a greater increase in fatigue life for low notch stress ranges (approximately Δσk≤ 400 MPa). However, for high notch stress ranges (approximately Δσk > 400 MPa), the increase in fatigue life is limited. The results with the strain energy density method are similar to those with the notch stress method.

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.

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.