Notch Stress Approach Research Articles

Finite element based fatigue assessment of corrugated steel web beams in highway bridges

This paper presents a finite element analysis and its related experimental test of corrugated steel web beams subjected to fatigue loading. A particular focus in this study was set on the fatigue failure arising from web-to-flange welded joints of corrugated steel web beams. Using ANSYS and FE-SAFE software packages, a detailed three-dimensional finite element model which explicitly includes the geometry of the web-to-flange welds along the corrugated web was developed to simulate the test corrugated web beams. The finite element model is verified by comparing with related fatigue experimental test results. The effective notch stress approach was also applied to analyse the web-to-flange welded joint replicating the local critical region in the corrugated web beam. The fatigue strength of the web-to-flange welded joint was evaluated and compared numerically by considering the stress distribution at potential fatigue crack initiation location. The fatigue life of the corrugated steel web beam was assessed numerically by incorporating material S-N relation and employing fracture mechanics approach. The comparison with fatigue test results show that it is possible to expect the fatigue crack failure arising at the weld root or weld toe corresponding to the sections with reference angle using the effective notch stress analysis. The range of these predictions was evaluated by comparing with fatigue test results with good accuracy and can be considered between AASHTO fatigue categories B and B’. The parametric notch stress analysis incorporating the influences of corrugation angle was performed and demonstrates it is possible to expect the fatigue crack failure arising at the weld root or weld toe. Finally, a practical solution for possible fatigue life enhancement of such structure is suggested by decreasing corrugation angle or smoothing the intersection geometry of the corrugated web together with moderate increase of the flange thickness.

Review of the fatigue strength of welded joints based on the notch stress intensity factor and SED approaches

Several approaches exist for the fatigue strength assessment of welded joints. In addition to the traditional nominal stress approach, various approaches were developed using a local stress as fatigue parameter. In recent times, the N-SIF based approaches using the notch stress intensity at the weld toe or root have been developed. Based on this, the more practical strain energy density (SED) and the Peak Stress approaches were proposed. This paper reviews the proposed design S–N curves of the N-SIF and SED approaches questioning in particular the consideration of misalignment effects, which should be included on the load side of local approaches in order to consider them individually in different types of welded joints. A re-analysis of fatigue tests evaluated for the effective notch stress approach leads to slight changes of the design S–N curves and of the radius of the control volume used for averaging the SED at the notches. Further, on purpose fatigue tests of artificially notched specimens show that the fatigue assessment using a single-point fatigue parameter might be problematic because the crack propagation phase, being part of the fatigue life, is strongly affected by the stress distribution along the crack path that may vary considerably between different geometries and loading cases.

Consideration of manufacturing effects on fatigue design for welded chassis components

The manufacturing process of chassis components determines variations in the formation of welded joints, which has a direct effect on the fatigue strength of chassis components in cyclic component tests. These influences are evaluated with the method “Design of Experiments” under fully reversed bending and axial loading at deduced T- and lap joints with a sheet thickness of 2.50 mm. Afterwards, the decisive geometry sizes are identified by measurements of the weld seam geometry and applied to FE models. By using the notch stress approach in combination with the stress-averaging method according to Neuber, the identified geometry sizes and their effects can be validated. For this purpose, the measured average radius is used for the weld toe and a radius of 0.05 mm for the weld root. Furthermore, the stress field at the notch is evaluated by stress values from a FE model to consider the stress gradient induced by the stress type. Thereby, the same design S-N line can be used for the fatigue strength assessment of weld toe and weld root failures.

Assessment of fatigue strength of steel butt-welded joints in as-welded condition – Alternative approaches for curve fitting and mean stress effect analysis

Experimental fatigue data for butt-welded joints in as-welded condition and under constant amplitude tensile loading (secondary bending included) were analyzed using the nominal stress system and the notch stress system. Two approaches were used; a standard fitting procedure and minimization of the sum of squared perpendicular distances from a line with a fixed and free slope. In all cases, the latter method gave better agreement between the experimental and predicted fatigue life and fatigue strength. The analyses showed both with all broken specimen data included and with reduced data that the FAT225 curve, as recommended by IIW, might be too optimistic for the notch stress approach in the case of butt-welded joints in as-welded condition. It was also found that use of the local stress ratio instead of the applied stress ratio might explain many issues concerning current observations and apparent inconsistencies in reported literature.

Multiaxial fatigue assessment of welded joints using the notch stress approach

This paper presents an evaluation of the safety involved when performing fatigue assessment of multiaxially loaded welded joints. The notch stress approach according to the IIW is used together with 8 different multiaxial criteria, including equivalent stress-, interaction equation- and critical plane approaches. The investigation is carried out by testing the criteria on a large amount of fatigue test results collected from the literature (351 specimens total). Subsequently, the probability of achieving a non-conservative fatigue assessment is calculated in order to evaluate the different criteria quantitatively. Large variation in safety is observed, especially for non-proportional loading.

Effect of post-weld heat treatment on the fatigue strength of HFMI-treated mild steel joints

Welding as a thermo-mechanical joining process generally induces residual stresses and distortion in welded components or structures. Mechanical post-treatment methods as the high-frequency mechanical impact treatment (HFMI) reduces the geometrical notch and introduces compressive stresses in the locally treated weld toe area, whereas post-weld heat treatment (PWHT) globally affects the whole structure. In this paper, the fatigue strength of HFMI-treated transverse non-load-carrying attachments and cruciform joints made of structural mild steel S355 before and after PWHT is investigated. Comprehensive tumescent fatigue tests and evaluation of notch topography, residual stress and distortion show the influence of the investigated post-treatment methods. To analyse the effect of distortion on the resulting stress condition during the fatigue tests, simulations and strain gauge measurements are carried out for different load cases. Finally, a local fatigue assessment based on the effective notch stress approach shows that an additional PWHT is not beneficial for fatigue strength. As an increase in distortion of the samples, and an influence on the base material properties, caused by the heat-treatment is not observable, the decrease in fatigue is mainly caused by the entire relieve of manufacturing induced (as-welded/HFMI-treated) prior compressive residual stresses to an almost zero stress value.

High cycle fatigue behaviour of impact treated welds under variable amplitude loading conditions

A comprehensive variable amplitude (VA) fatigue testing program and analysis was performed to address a number of concerns raised regarding the use of impact treatments for the fatigue enhancement of welds in the high and ultra-high cycle (up to 100 million cycles) domains. A total of 67 fatigue tests were conducted on two different welded joints representing load-carrying and non-load carrying welds in steel structures. Two different VA loading spectrums, generated using traffic data and influence lines for highway bridge girders, were used. The effects of load cycles with high stress ratios (R>0.4) and large tensile overloads (greater than the yield strength) were studied. The test results were then used to evaluate a number of previously proposed recommendations for the fatigue design of impact treated welds. The nominal, structural, and effective notch stress approaches were considered. Finite element (FE) analysis was performed to determine the structural and effective notch stresses. A statistical analysis of the fatigue test results was conducted and characteristic S–N curves with slope, m=5 are proposed for the fatigue design of treated welds under VA loading in the high cycle domain.

Fatigue tests and fatigue assessment approaches for rib-to-diaphragm in steel orthotropic decks

In orthotropic steel decks, a rib-to-diaphragm welded joint was one of the most vulnerable details prone to fatigue. Fatigue resistance and evaluation of the connection are challenges to be dealt with, due to a complicated local stress condition and fatigue cracking mechanism. Fatigue tests of full-scale orthotropic steel decks were carried out focusing on determining actual fatigue resistance and demonstrating the applicability of the fatigue assessment approaches for rib-to-diaphragm joints. Hot spot stress as well as the effective notch stress approaches were introduced to alleviate the difficulties of fatigue resistance assessment, and the evaluation results were compared with consistent nominal stress approach and model tests. Finite element analyses of the test specimen were performed to supplement the laboratory testing and provide additional information on the fatigue behavior of the targeted detail. The evaluation results of hot spot stress and notch stress approaches correlated with model tests better than consistent nominal stress approach. The results indicated that fatigue strength of the rib-to-diaphragm welded joints could be evaluated by hot spot stress or notch stress approaches with a single S–N curve corresponding to fatigue strength design class FAT90 (a stress range of 90MPa at 2×106cycles with a survival probability Ps=97.7%) or FAT225. The main reason lying behind the notable divergence in evaluation results was whether local stress condition could be accurately simulated or not. The investigations gave an insight into the fatigue strength of key fatigue details in orthotropic steel decks.

Fatigue assessment of welded joints using stress averaging and critical distance approaches

This paper comprises a brief methodical analysis and practical application of both the stress averaging approach according to Neuber and the critical distance approach according to Taylor for a fatigue assessment on welded steel structures under axial loading. The stress averaging approach, as established theoretically by Neuber, is discussed and compared to the effective notch stress approach with a reference radius r ref = 1 mm and also to the r ref = 0.05 mm concept which is usually applied for thin-walled structures. Furthermore, both approaches are applied for an assessment of 38 individual test series. As a result of the evaluations, a microstructural length ρ* = 0.40 mm is recommended. For the critical distance approach, a distance a = 0.10 mm was derived. Fatigue design (FAT) values are recommended for the practical application. Compared to the notch stress approach with a radius r ref = 0.05 mm, it was possible to improve the assessment quality significantly. The non-local approaches should be preferred to the r ref = 0.05 mm concept, especially for welded components using thin sheets for which the radius of 1 mm cannot be applied.

Fatigue assessment of full-scale welded crane runway girders*

Abstract This publication is focused on a critical plane approach to assess the influence of the multiaxial stress state on the lifetime of welded crane runway girders cyclically stressed by travelling wheel loads. Cyclic stress-strain curves of specimens both from the weld region and the base material of the girder are evaluated in strain-controlled reversible fatigue tests as a basis for the assessment of the lifetime. Finite element analyses are performed to estimate the multiaxial and non-proportional local stress condition acting on sharp notches of the girder, i. e. the weld toes and the weld root. The MPA AIM-life concept is used to evaluate the lifetime of crane runways on the basis of advanced fatigue parameters and the cyclic material properties. Furthermore, the notch stress approach according to IIW guideline [1] is applied to investigate the fatigue behavior. The estimated lifetimes are compared to experimental results of component fatigue tests conducted with full-scale welded crane runway girders.

Fatigue assessment of joints using the local stress field

This paper comprises a brief methodical analysis as well as a practical application of both, the stress averaging approach according to Neuber and the critical distance approach according to Taylor for a fatigue assessment on welded and adhesively bonded thin‐walled steel structures. The approaches are discussed and in case of the welded joints compared to the notch stress approach with a reference radius of rref = 0.05 mm, which is usually applied for thin‐walled structures. Furthermore, both approaches are applied for an assessment of welded joints and adhesively bonded joints. Using results from fatigue tests, a micro‐structural length of ρ* = 0.5 mm could be identified for welds, and ρ* = 2.75 mm for the examined adhesive bonds. The value for steel welds is close to the proposed value of ρ* = 0.4 mm, which was used by Radaj deriving the effective notch radius of rref = 1 mm. For the critical distance approach, a distance of a* = 0.1 mm was derived for welds and a* = 0.9 mm for the examined adhesive bonds. Using the stress averaging or critical distance approach and the identified material values, a reliable fatigue strength assessment can be performed for both welded and adhesively bonded joints.

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.

Calculations of notch stress factor of a thin-walled spreader bracket fillet weld with the use of a local stress approach

Presence of geometric notches in welded joints causes concentration of strains and stresses, therefore reducing fatigue strength of such joints. This article presents an analysis of stress concentrations in a fillet weld of a spreader mounting bracket on a small sailing yacht. The aim of this article is to direct the attention of designers, manufacturers and regulatory bodies to issues of fatigue cracks that form in brackets fastening spreaders to masts. The issue concerns mainly yachts with overall length of a hull below 10m. Despite frequent defects the solution is still being chosen by designers, mainly for technological reasons. Current design regulations of classification societies and other regulatory bodies for small recreational yachts do not specify a technical solution for spreader fastening. With the use of finite element analysis it was shown that for short term design loads structural strength of a bracket is sufficient. Then, with the use of the notch stress approach, it was proven that the spreader bracket welded of thin plates is a structural solution that runs a risk of fatigue cracks. The below presented calculations of the geometric concentration factor of weld section in the locations of fatigue cracks were performed for different geometric cases of joints. The analysis results indicate the need for greater attention that should be paid to the problem of fatigue cracks that form in welds of brackets fastening spreaders to masts. The determined values of effective stress concentration coefficients show the need for conducting fatigue calculations of brackets welded of thin plates, or choosing an alternative solution for fastening spreaders to masts. In both cases it seems appropriate to conduct fatigue calculations for a structural element which is “a weak link” of the main drive on a sailing yacht.

Fatigue strength assessment of Invar alloy weld joints using the notch stress approach

The aim of this study investigated the fatigue strength of Invar alloy weld joints. Invar steel (Fe-Ni 36%) is widely used in the primary and secondary barriers of membrane-type liquified natural gas (LNG) containment vessels. The fatigue test was carried out for two different types of welded joints with raised edge specimens and with overlap joint specimens based on the nominal and notch stress approaches. The thickness of the Invar plate is less than 1.5mm, so the notch stress approach with rref=0.05mm was applied. Our evaluation of the results in terms of the FAT value and the slopes of the design curves are compared with steel, aluminum, and magnesium weld joints in accordance with International Institute of Welding (IIW) recommendations.

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 Life Analysis of Crane K-Type Welded Joints Based on Non-Linear Cumulative Damage Theory

There are large numbers of welds on cranes’ lattice booms. Due to notably complicated force conditions, the fatigue life analysis of the lattice boom is difficult when designing the limit life. The fatigue cycle of the utilization level of a lattice boom crane is in the overlap zone of low-cycle fatigue and high-cycle fatigue. The poor accuracy of measurement of the fatigue life of lattice booms indicates that the calculation dispersion of the linear Palmgren-Miner (PM) rule was high. To obtain real crane stress spectra inexpensively and conveniently, a new stress spectra acquisition method based on the ‘measured & simulated & compared & statistics’ integrated strategy of crane K-type welded joints is proposed. The errors of the maximum stress amplitudes between the measured stress spectra and the simulated stress spectra were less than 10%. The fatigue test results also indicated that errors of the test fatigue life were less than 10% under both the simulated and the measured stress spectra. A new, simplified Huffman non-linear cumulative damage theory is proposed to calculate the fatigue life of crane K-type welded joints based on the notch stress and strain approaches. The calculation results indicated that the accuracy of the non-linear damage accumulation was higher than that of the PM rule, although the calculation result based on the non-linear damage accumulation method was slightly un-conservative when the initial damage was not considered in the calculation. By setting different initial damage conditions, various results were analysed, which revealed that the calculation errors of fatigue life based on the non-linear theory were less than 10% when the initial damage levels were set from 0.02 to 0.04. Such results are appropriate for engineering applications. When the fatigue life calculation needs to be more conservative, the initial damage levels may be set from 0.04 to 0.07; the resulting calculation errors could be less than 25%. As the Huffman non-linear cumulative damage theory requires data from only a few material properties, such as the cyclic stress-strain curve and the constant amplitude strain-life curve, it could therefore be more suitable for engineering applications with higher calculation accuracy and fewer costs.

Transferability of fatigue resistance data for welded joints

The original notch stress approach for fatigue design is limited to thick sheet structures and a fixed notch radius of 1 mm. Concepts utilizing the effective notch stress claim to account for the variation of the weld shape parameters, however, size effects are not taken into account. Therefore, an expansion of the notch stress approach has been developed, which includes consideration of the geometrical and statistical size effect in order to extend the application to thin and thick sheet structures. It is also capable of evaluating highly localized three-dimensional stress concentrations at weld start and end points. To validate the quality of this approach, experimentally determined fatigue data for thin steel sheet specimens from the Institute of Material Mechanics, including weld start and end points, and data from the literature regarding conventional thick sheet components without weld start and end points were reevaluated. The final result is a unified scatter band of fatigue lives of thin and thick sheet specimens, with and without weld start and end points, including also several modes of seam weld end point loading: transversal and longitudinal tension compression as well as shear loading at seam welds.