Effective Notch Stress Approach Research Articles

Fatigue Strength of HFMI-treated High-strength Steel Joints under Constant and Variable Amplitude Block Loading

Lightweight-design of welded high-strength steel structures in cyclic service necessitates the use of post-treatment methods like the high frequency mechanical impact treatment (HFMI). Service loads during operation mostly consist of variable amplitudes, whereat recommendations are only available for the as-welded condition. Therefore, this paper deals with the effect of variable amplitude block loading on the fatigue strength of HFMI-treated T-joints. An evaluation of the real damage sum exhibits characteristic distinctions to constant amplitude test results in regard to the base material strength. The application of an equivalent stress range method by nominal and effective notch stress approach is finally presented.

Fatigue Assessment of the Diaphragm-to-Rib Welded Connection in Orthotropic Steel Deck Using Effective Notch Stress Approach

The effective notch stress approach to fatigue assessment of existing large-size specimens for diaphragm-to-rib welded connection in orthotropic steel deck is investigated. Two kinds of specimens are designed. Specimen 1 has the form of circular arc transition; specimen 2 has the form of vertical transition. The first principal stress distribution at weld toe and weld root is calculated by the sub-model method of ANSYS software. By means of the maximum first principal stress analysis, it is possible to distinguish whether fatigue cracks will initiate at the weld toe or the weld root, which is agreed with the fatigue test results. Furthermore, the fatigue strength of specimen 1 and specimen 2 are evaluated by the effective notch stress approach according to fatigue tests. Finally, the factors affecting the stress intensity at the weld toe are studied.

Evaluation of Crack Initiation Points of Load-Carrying Cruciform Welded Joints Based on Effective Notch Stress Approach

For fatigue assessment, recently effective notch stress (ENS) approach is accepted as a valid alternative to fracture mechanics approaches in Japan. However, at least one study showed that the fatigue assessment at weld root of load-carrying cruciform welded joints based on ENS approach does not capture the influence of plate thickness, weld size and weld penetration. Consequently, there are still uncertainties about the fatigue assessment. This paper discusses the evaluation method of the joints based on ENS approach. Effects of weld size, weld penetration, and plate thickness on ENS are determined by using fine-meshed FEM analyses. Validity of this method is determined by comparing the evaluation results with fracture mechanics approach. It is shown that results from this method are comparable to those of the previous study and the results seem to be conservative when ratio of weld penetration to plate thickness becomes smaller and plate thickness becomes larger.

Effects of weaving technique on the fatigue strength of transverse loaded fillet welds made of ultra-high-strength steel

The effect of weaved weld toe geometry on the fatigue strength of S960 steel grade fillet welds has been studied by means of experimental testing and finite element analysis. The comparisons have been made between normal straight weld toe lines and weaved weld toe lines by using the weld toe radius and structural stress fatigue class (FAT) value as reference values. The effective notch stress approach was utilized to conduct finite element (FE) analyses on weaved weld toe geometry, and the results were compared with the experimental results. The FE analysis showed quite good agreement with the experimental test results, although residual stresses or heat-affected zone (HAZ) softening in welded joint were not included in the modeling. The experimental tests also indicated that it is possible to achieve high fatigue properties for ultra-high-strength steel (UHSS) fillet welded joints without any postweld treatments. By taking account all the essential factors—joint geometry, welding metallurgy, and residual stresses—the reached fatigue strength of UHSS weldments can be much higher than normal fatigue resistance values presented in standards and recommendations.

Low-cycle fatigue analysis of a web frame corner in ship structures

Ship structures are subjected to cyclic loads which may lead to fatigue failures particularly at welded joints. In addition to loads induced by waves and structural vibrations, which contribute to high-cycle fatigue, various structural details are also subjected to a limited number of high-stress cycles caused by changing loading conditions, which may lead to low-cycle fatigue. Thus, appropriate procedures are required for the shipbuilding industry, which are missing in the existing design codes. Experimental investigations and numerical analyses were performed to validate a procedure for the design of ship structures in the low-cycle fatigue regime. Three large-scale fatigue tests were carried out with high bending moments on test models of web frame corners in order to initiate fatigue cracks after few hundred cycles. Different cracks occurred; however, a crack at the cruciform joint has shown to determine the failure of the connection. An extension of the effective notch stress approach was applied to this joint to assess the low-cycle fatigue life, considering the elastic–plastic strain, evaluated by nonlinear finite element analyses, in the notch of the weld toe. This practical method offers promising possibilities to assess the fatigue life of welded joints in the low-cycle fatigue regime.

Fatigue strength behaviour of stud-arc welded joints in load-carrying ship structures

The application of stud-arc welding to cyclic loaded primary ship structures was investigated. Thereby M8- and M12 studs were welded on the bulb of Holland profiles using the drawn stud-arc welding with ceramic ferrules. Several fatigue tests were performed, varying the stiffener size, the stud diameter, its position on the bulb, the welding position, weld parameters, residual stress state and the static load in the stud. The obtained fatigue strength is above FAT 80 and only slightly affected by these factors. Hence, the investigated stud-arc welding can be applied at Holland profiles in primary ship structures, provided the quality criteria are met. In order to transfer the results to other geometries and load situations, the applicability of the effective notch stress approach was investigated. The corresponding finite element calculations using idealized weld shapes and a reference weld toe radius of 1 mm showed an insignificant influence of the weld geometry on the notch stress. The determined notch stresses for each test series was above the design S-N curve FAT 225 for the notch stress approach and make it also applicable to stud-arc welding. Further computations were performed using the real weld profile and the stress-averaging approach by Neuber, showing similar results.

Effective Notch Stress Method for Fatigue Evaluation of Welded Joints in a Steel Bridge Deck

Effective notch stress, as an approach to evaluate the local stress at a notch (weld toe or root), is defined as the total stress assuming linear-elastic material behavior. This method can be effectively used to evaluate the fatigue performance of welded joints. In this study, finite element analysis results using the effective notch stress method were correlated with fatigue test results of rib-to-deck welded joints in a steel orthotropic bridge deck. Effective notch stress approach provided a good correlation with the crack pattern observed in the full-scale fatigue test. A higher effective notch stress at the critical weld toe than at the weld root was consistent with the dominant crack pattern observed at the weld toe during testing. The effective notch stress at the toe on the deck plate was about 80% higher than that on the rib; no cracks at the weld toe on the rib in the testing were observed. Maximum effective notch stress at the weld root occurred on the upper side of the root notch, which indicates that cracks are more likely to propagate into the deck plate, not into the weld metal. This is also consistent with the observed crack pattern in which the crack from the root propagated upward into the deck plate. No such crack pattern, propagating into the weld metal, was observed in the testing.

Fatigue damage assessment of corroded oil tanker details based on global and local stress approaches

Fatigue damage assessment of double hull oil tanker structural details is performed, based on global and local structural finite element models. The wave-induced vertical and horizontal bending moments, as well as local pressure loads are accounted for in the fatigue damage calculations. Local stress analyses are considered based on the notch stress approach. Time-dependent stresses as a function of corrosion deterioration are analyzed based on nonlinear corrosion wastage during the design life of ship. The effective notch stress approach is applied for analyzing the stress distributions and the fatigue damage of the welded joint at two hotspots. The hotspots are located between the flat bar stiffener of a transverse web frame and the flange of a longitudinal stiffener at the side shell of a tanker ship hull. The details under consideration are modeled separately in a fine mesh employing the sub model techniques. Finally the fatigue damage assessment accounting for corrosion deterioration of the considered hotspots is analyzed.

Modelling and fatigue life assessment of orthotropic bridge deck details using FEM

The fatigue life estimation of orthotropic steel bridge decks using the finite element method is most frequently associated with the application of the structural hot spot stress approach or the effective notch stress approach, rather than the traditional nominal stress approach. The application of these approaches to a welded joint with cut-out holes in orthotropic bridge decks, where it is not easy to distinguish the non-linear stress caused by the notch at the weld toe from the stress concentration effect emanating from the hole in the detail, was investigated. The results of the finite element calculations were compared with the results of the fatigue tests which were carried out on full-scale specimens. The results of the finite element analyses revealed that the structural hot spot stresses obtained from the shell element models were unrealistically high when the welds were omitted. Moreover, the way in which the welds were represented had a substantial influence on the magnitude of the hot spot stress. The results of the analysis when using the effective notch stress approach showed that the agreement between the estimated fatigue life using this approach and the fatigue life obtained from the fatigue tests was good.

Estimate on fatigue welded units of orthotropic slabs of bridge mental spans in the local directions

The aim of this paper was to examine the application of the local stress method in the fatigue life assessment of rib-to-crossbeam joints of orthotropic decks. The hot spot stress approach and the effective notch stress approach were used. The fatigue life predicted by the local and nominal stress method was compared with the experimental results.

Modeling and fatigue assessment of weld start‐end locations based on the effective notch stress approach

AbstractThe present paper contains a methodology for modeling and life assessment of fatigue loaded welded components providing distinct weld start and end locations. The proposed methodology follows the IIW recommendation regarding modeling and finite element meshing of weld toe and root by means of an effective notch radius and uses the corresponding Wöhler curve (FAT class) to assess the durability. Geometrical singularities and, therewith, numerical discontinuities, can be overcome especially when 3D weld root problems are treated. The fatigue life assessment is performed on the basis of normal stresses acting at the failure‐critical weld toe and root locations. Comprehensive experimental database containing stress and fatigue life results derived from motor truck's hypoid rear axles providing complex 3D welds subject to vertical, longitudinal, and torsional loading is used to verify the calculation accuracy of the proposed methodology. The agreement between experimentally determined and calculated fatigue results is satisfactory.

Fatigue strength investigations of welded details of stiffened plate structures in steel ships

Ships are prone to fatigue due to high cyclic loads mainly caused by waves and changing loading conditions. Therefore, fatigue is an important criterion during design. Different approaches are applied to the fatigue strength assessment. However, the results are varying and the validity of results from small-scale fatigue tests for real structures is sometimes unclear. Therefore, deeper fatigue strength investigations were performed within a German industry-wide joint research project aiming at the harmonization of the approaches. Regarding ship structures, two types of structures were selected for the investigations. The first concerns web frame corners being typical for ro/ro ships from which three models were tested. The associated small-scale specimen is a cruciform joint. The second type is the intersection between longitudinals and transverse web frames, which recently showed fatigue failures in containerships. Five models were tested, three under constant and two under variable amplitude loading. The associated small-scale specimen is a longitudinal attachment on a stiffener top. All large-scale tests showed a relatively long crack propagation phase after first cracks had appeared, calling for a reasonable failure criterion. For the numerical analysis, the nominal, the structural hot-spot as well as the effective notch stress approach have been applied. The latter allows the consideration of the weld shape which could partly explain differences in the observed and calculated failure behaviour. The applicability of the different approaches is quite good if some specific aspects are observed. Insofar the investigations give a good insight into the strength behaviour of complex welded structures and into current problems and opportunities offered by numerical analyses.

Application of fatigue approaches on fillet welds of high strength steel

AbstractTo calculate the fatigue behaviour of welded structures local fatigue approaches like the effective notch stress approach are used. This paper examines the application of the notch stress approach for thin‐walled specimens made of high‐strength steel. Focus is laid on the end‐of‐seam fatigue behaviour. To highlight the differences between welding of transversal stiffener and longitudinal gusset a coupled weld simulation was set‐up. The comparative results and measurements showed that the intersecting start and stop areas of the longitudinal gusset define the regions with highest residual stresses. The thin‐walled fatigue investigations showed that the notch stress approach supports the use of a single master SN‐curve both for continuous fillet welds and end‐of‐seams possessing a slope of three. The influence of the clamping condition can be minimized by using small reference radii. The observed corresponding fatigue ratio is not thoroughly satisfying and will be examined in detail in further work.

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.

Full-scale fatigue tests of ship structures to validate the S–N approaches for fatigue strength assessment

Ships belong to those welded structures which are prone to fatigue due to high cyclic loads. Different approaches exist for the fatigue strength assessment which are varying between the industrial sectors. Therefore, deeper fatigue strength investigations were performed in Germany within an industry-wide joint research project aiming at the harmonization of the approaches. Regarding ship structures, two types were selected for full-scale tests. The first concerned web frame corners being typical for roll-on/roll-off ships (ro/ro) ships, from which three models were tested under constant amplitude loading. The second type was the intersection between longitudinals and transverse web frames, which recently showed fatigue failures in containerships. Five models were tested, three under constant and two under variable amplitude loading. All tests showed a relatively long crack propagation phase after first cracks had appeared, calling for a reasonable failure criterion. For the numerical analysis, the structural hot-spot stress as well as the effective notch stress approach have been applied. The latter allows the consideration of the weld shape which could partly explain differences in the observed and calculated failure behaviour. Another factor is the distribution of welding-induced residual stresses, which obviously affected the failure behaviour in the web frame corner as well. Insofar the investigations give a good insight into the strength behaviour of complex welded structures and into current problems and opportunities offered by numerical analyses.

Fatigue Tests and Numerical Analyses of a Connection of Steel Sandwich Plates

In shipbuilding, steel sandwich plates are applied in several areas. Particularly the connections between the sandwich plates may be prone to fatigue in the case of cyclic loads. Therefore, the fatigue strength of a fillet-welded connection of a steel sandwich plate under in-plane loading has been investigated experimentally and numerically. Numerical calculations are performed using the crack propagation and the effective notch stress approach. In the notch stress approach, the modelling of the weld root may create problems as the relatively thin face plates are affected by the notch rounding of 1 mm. Two different shapes are considered. Furthermore, the small size notch approach with a reference radius of 0.05 mm is applied. From the comparison of the results with the fatigue strength found in the tests, conclusions are drawn for appropriate modelling.

Full-size testing to determine stress concentration factors of dragline tubular joints

This paper describes the determination of stress concentration factors (SCFs) of dragline tubular joints through laboratory testing of four full-size dragline tubular joints. Strip gauges are installed both outside and inside the footprint of the joints to measure stress distribution at the weld toe and weld root. Measurements for SCF are carried out for two load cases, i.e. tension or compression forces in the main chord alone and tension or compression forces in the vertical bracings alone with reaction forces in the inclined braces. The relationship of the SCF between the corresponding weld toes and weld roots are presented in the paper. The weld profile and weld root gaps that have been measured using the silicon imprint technique and feeler gauges, provide valuable information for future FE simulation and weld root failure study using the effective notch stress approach.

Fatigue assessment of out-of-plane attachments with various angles by using local stress approaches

In order to investigate the effect of the angle between principal stress direction and weld attachment on the fatigue behavior, crack initiation points and fatigue strengths have been examined by using the fatigue test results of out-of-plane attachments with various angles under uniaxial loading. As fatigue assessment methods, the nominal stress approach, the structural hot spot stress approach and the effective notch stress approach are used. It is possible to find fatigue crack initiation points by using the effective notch stress approach, regardless of the angle between principal stress direction and weld attachment. The effect of the angle between principal stress direction and weld attachment on the fatigue crack initiation life cannot be considered when the nominal stress approach and the structural hot spot stress approach are used. However, by using the effective notch stress approach, it is possible to consider the effect of the angle on the crack initiation life.

Numerical and experimental investigation of weld root fatigue in fillet-welded structures

For fabrication reasons, a great part of the joints in ship structures are welded with non-penetrating fillet welds. This implies the risk of fatigue failure originating from the weld root under cyclic loading. A fatigue analysis may be performed using the nominal stress approach, which is rather coarse, and the crack propagation approach, which is less practical due to the high computational effort. Alternatives are offered by the effective notch stress approach, which assumes a fictitious radius at the tip of the non-fused root faces and by a newly developed structural stress approach for weld failures. The latter is outlined in the paper and applied to the fillet-welded end of a rectangular hollow section (RHS) and to a doubler plate under out-of-plane loads. These represent typical situations with one-sided welds subjected to local throat bending, where fatigue tests have generally shown weld root failure. From the fatigue lives, a design S-N curve based on the structural weld stress is derived. For further verification, the effective notch stress approach is applied to the RHS-joint showing similar results in relation to the design S-N curve as the structural stress approach.

Fatigue assessment of large-size welded joints based on the effective notch stress approach

In this paper the applicability of the effective notch stress approach to fatigue assessment of existing large-size specimens for cruciform joints, diaphragm joints and out-of-plane gusset joints has been investigated, by considering fatigue crack initiation points, stress distributions and fatigue strengths. By using the maximum principal stress, it is possible to distinguish whether fatigue cracks will initiate at the weld toe or the weld root. Fatigue strengths of the existing test results using the effective notch stress approach are above the SN curve of FAT300. For out-of-plane gusset joints, the effective notch stresses along the weld root line depend significantly on weld size and weld penetration size.