Nominal Stress Approach Research Articles

Fatigue assessment of CFST joints using the effective notch stress approach

Compared to tubular joints formed by hollow steel sections, concrete-filled steel tubular (CFST) joints exhibit more complicated failure modes and mechanism under fatigue load due to the nonlinear material confinement and the associated stress redistribution. For a reliable fatigue design of such composite tubular joints, these complexities need to be fully addressed and reasonably quantified, which necessitates an accurate approach for assessing their fatigue performance. The effective notch stress approach is widely recognized in fatigue design for its applicability and efficiency in evaluating the fatigue strength since it can rationally account for the effects of geometric configurations and welding defects on the fatigue performance. However, it has not been widely adopted in the assessment of CFST joints due to the high costs involved in modelling and computing. In this paper, a refined nonlinear finite element model is established, which facilitates the acquisition of reliable notch stress values. Subsequently, a statistical analysis is conducted on the available fatigue data of CFST joints, employing the computed notch stress to yield an S–N curve. A comparative evaluation on the results obtained from the nominal stress approach, the hot-spot stress approach and the effective notch stress approach testifies the superior accuracy of the last in assessing the fatigue strength of CFST joints.

Influence of shear cut holes on the fatigue performance of hot-rolled 800 MPa automotive steel grades

Experimental fatigue tests were carried out for two t = 3 mm 800 MPa hot-rolled (HR) high-strength steel grades. These steels have good formability, and it is possible to use shear cutting to enable high productivity. The test campaign included unnotched base metal specimens to evaluate the fatigue performance of HR surface and notched specimens with shear-punched and electrical discharge-machined circular holes to evaluate the effect of cutting process and geometrical shape on the fatigue strength capacity. In this context, the properties of the shear-cut affected zone (SAZ) and resulting high tensile residual stresses, and their effects on the fatigue performance, were evaluated. The fatigue test results were first evaluated based on the nominal stress approach, followed by the applications of local approaches. The mean fatigue strength of the specimens with shear cut holes in the nominal stress system was 125 MPa (with a slope parameter of m = 3, stress range at the net cross section), which is at the comparable level to laser cut holes. Fractography analysis, conducted with scanning electron microscopy (SEM), showed local crack-like defects at the SAZ, and the characterization of local defect feature was utilized in the fatigue strength assessment using a multiparametric notch stress method applying effective stresses obtained by the critical distance approach.

Analytische Methode zur Berechnung der Nennspannungen im Zahnfuß von Schneckenwellen

AbstractAccording to the state of the art, worm gears are designed due to the softer bronze-material of the worm wheel primarily against damage of the wheel because of wear, pitting or breakage of the teeth. The aim of research in the last decades is to optimize the load capacity on the worm wheel by using higher-strength bronze alloys, cast iron or steel to increase the transmission capacity of worm gears. This development may lead to an increased number of damage cases on the worm shaft. In literature, documented cases of tooth segment fractures of the worm shaft can already be found. Since the worm is only designed against deflection according to the state of the art, there is a need for a method to calculate the material stress in worm shafts. This paper presents an analytical method based on the nominal stress approach for calculating bending, compression, and shear stresses in the tooth root of worm shafts to close this gap. The stresses resulting from different load distributions in the tooth contact due to assembly deviations are calculated with the presented method and compared with results from the Finite Element Method.

Implementation of a nominal stress approach for the fatigue assessment of aluminium naval ships

Fatigue analysis is an important part of the structural design of weight-optimised naval ships, because they are often constructed from aluminium alloys or high tensile steel. Due to the low fatigue strength of these metals, the welded joints are vulnerable to cracking. Furthermore, naval ships can be required to operate in demanding environments, and remain in service longer than assumed during design. Thus, ship designers and in-service managers require efficient fatigue evaluation approaches. One industry-accepted and relatively quick method, documented in design codes, to assess the fatigue strength of welded joints is the nominal stress or S-N curve approach. However, there are uncertainties associated with using a design code; if all of the conditions for its applicability are not met, the analyst must reliably interpret the code. Further, different specifications for the use of S-N curves leads to different fatigue life estimates. In this paper, a refinement of the nominal stress approach for joints, typical of aluminium welded ship details, is proposed. The goal is to inject rigour and practicality into the fatigue analysis of naval ships. The refinement process leverages both in-service maintenance information and long-term strain measurements of a 56 m aluminium patrol boat. The sensitivity of the predicted fatigue life of welded details to the choice of S-N curve, available from a structural design code, and stress parameter extraction is investigated. Finally, recommendations for future work are provided.

Influence of the geometry on the fatigue strength of welded joints using the effective notch stress approach

Using the example of a cruciform joint, this study enquires into the relationship between the geometry of weld (the wall thickness of base metal plates, thickness of weld and weld penetration depth) and the fatigue strength of weld joint using the finite element method. It is assumed that the effective notch stress approach can be used to determine a FAT class for dimensioning the connection with the nominal stress approach. Filled throat thickness and weld penetration depth or the geometry of weld root were found to have a strong influence on the fatigue strength of a welded joint. When using the effective notch stress approach, the exact geometry of the calculated weld seam must be known and should be modelled accordingly. It was also found that with thin-walled welds, the effect of wall thickness produces a significant deviation between the load capacity calculated with the effective notch stress approach (effect of wall thickness up to 100%), and with the nominal stress approach (effect of wall thickness max. 10%). At the end, a proposal will be made on how thin-walled welds can be dimensioned to facilitate the use of the same thickness factor for both the notch and nominal stress approaches.

Laser welding of 16MnCr5 butt welds with gap: resulting weld quality and fatigue strength assessment

For laser welding, gaps should normally be avoided to ensure stable weld processes and high weld quality. Nonetheless, sometimes, gaps are resulting from non-optimal weld preparation in industrial applications. Within this investigation, the effects of a defocussed beam and laser beam oscillation on gap bridging abilities at reduced ambient pressure were investigated. For reference purpose, conventional laser welding with zero gap at ambient pressure was performed, too. The resulting weld geometry was investigated and correlated to gap bridging strategies and weld quality groups according to ISO 13919–1. The welds were characterized regarding their hardness and weld microstructure. Residual stress was determined by means of X-ray diffraction, and tensile tests as well as fatigue tests were conducted. The fatigue tests were evaluated by the nominal stress approach, the critical distance approach, and the stress averaging approach and correlated to weld quality measures. Resulting from this, fatigue resistance of laser welded butt welds with gap can be estimated by the FAT160 design S–N curve. The stress evaluation parameters for the determination of keff-values (ρ* = 0.4 mm, a = 0.1 mm) were confirmed.

Comparison between PSM and IBESS approaches for the fatigue life estimation of weldments

In the framework of the fatigue assessment of welded components, several methods are available in design standards such as the nominal stress, hot-spot stress, notch stress and linear elastic fracture mechanics approaches. The present paper aims at comparing two advanced local approaches for the fatigue strength assessment of different welded joints made of steel. The first one is IBESS which is based on short crack fracture mechanics. The second one is the PSM which instead involves the strain energy density approach. Both methods will be briefly presented, and the fatigue life prediction results discussed. The results obtained for the joint geometries considered in this work show advantages and drawbacks of the approaches, which are thoroughly analysed as well.

Analysis of the scatter in fatigue life testing of thick thermal cut plate edges

ABSTRACT Structural elements of ships are subjected to a high number of cyclic loads, which are critical to fatigue life. Especially for ultra-large container ships the cutting of thick steel plates with high finishing quality and good fatigue life is important, e.g. located at openings in walls and decks of ships. This study presents a series of fatigue tests with 50 mm thick S355 specimens that are thermally gas cut on different shipyards. The obtained stress-life (S-N) curve contains significant horizontal scatter which often occurs for specimens that spend a high number of cycles on crack initiation. However, it is found that both the local stress concentrations and the origin of failure affect the number of load cycles significantly. Accounting for local stress concentrations provides an explanation for the initially encountered scatter in the S-N curves and indicates possible limitations of the nominal stress approach for thermal cut edges.

Fatigue and Ultimate Strength Assessment of Post Weld Treated Strenx® 1100 Plus Butt Welds

The demand of stronger and lighter structures is rapidly growing. Today’s applications vary over a large range of industries where the requirements are different for different products which requires application driven research of the 3rd generation Advance High-Strength Steels (AHSS). These are high-strength steels that are tailor-made to get desired mechanical properties. One AHSS manufactured by SSAB is their Strenx® 1100Plus steel that is specially designed to get excellent welding properties. In the current study the fatigue life improvement was investigated when using post weld treatments, TIG dressing and HFMI (High Frequency Mechanical Impact), for butt welded joints in Strenx® 1100Plus. Fatigue testing and assessment was carried out, using nominal stress and effective notch stress approaches, and the results were compared with fatigue strength recommendations for similar joints and steel grades given by the International institute of Welding (IIW). It is concluded that one additional FAT class increase could be observed in comparison to what is recommended by IIW for both types of post weld treatment methods.

Probabilistic fatigue strength modelling based on various statistical approaches for a double-side welded connection

S355 steel is currently used in the fabrication of most wind turbine monopile support structures and offshore structures. In these types of structures, most fatigue failures occur in the welded connections where cyclic loading is the main responsible for this phenomenon. For the high-cycle fatigue analysis, the stress life method utilizing S-N curves can be used to determine the strength of a welded joint under fatigue loading. In this research, the evaluation of design S-N curves for a double-side welded connection made of S355 steel is proposed. This study concludes with a comparison between the experimental fatigue curves obtained and the design S-N curves at 5% and 2.3% probability of failure proposed in design codes (EN1993-1-9, DNVGL-RP-C203 standards and IIW recommendations) for offshore structures and general steel structures. For the specimens under investigation, the hot-spot and nominal stress approaches are taken into consideration. The characteristic fatigue curves of the double-side welded connection are obtained using statistical analyses based on the ASTM E739, ISO 12107, Bayesian inference, and Weibull distribution. The hot-spot and nominal stress approaches yield very similar S-N results considering the specimens under study.

Fatigue design of mild and high-strength steel cruciform joints in as-welded and HFMI-treated condition by nominal and effective notch stress approach

According to the Recommendations for Fatigue Design of Welded Joints and Components by the International Institute of Welding (IIW), the fatigue strength of welded steel joints is in general independent of the base material strength. Post-treatment methods, such as the High Frequency Mechanical Impact (HFMI) treatment, can significantly increase the fatigue performance of welded joints especially in case of high-strength steel applications, which is already considered within the IIW Recommendations for the HFMI Treatment. This paper firstly investigates the effect of the base material strength on the fatigue resistance of welded and HFMI-treated steel joints. Therefore, mild steel S355 and high-strength steel S700 cruciform joints are cyclically tested in both conditions and the statistically evaluated S/N-curves are compared. The results reveal an increase of the high-cycle fatigue strength by the HFMI-treatment by a factor of 1.35 in case of the S355, and of 1.59 for the S700 specimens. Secondly, the test results are assessed by the fatigue design curves of the corresponding structural detail within the IIW-recommendations using the nominal stress concept. Moreover, the applicability of the procedure applying the effective notch approach is analyzed. In order to numerically evaluate the effective notch stress, the geometry of the cruciform joint is modelled according to the given guidelines within the recommendations applying a reference radius of 1 mm at the weld toe for both conditions. On the basis of the numerically computed effective notch stress, a local fatigue design is performed for all test series, which reveals sound accordance between the recommended design curves and the statistically evaluated fatigue test S/N-curves.

Comparison of local fatigue assessment methods for high-quality butt-welded joints made of high-strength steel

Fatigue strength of welded joints is typically independent of the parent material static strength; however, there are two exceptions from this rule—post-weld treated joints and high-quality joints. The reason for both is the absence of sharp weld transitions. Thus, fatigue life is not fully governed by fatigue crack propagation but also by fatigue crack initiation. This is intensified by the fact that crack initiation behaviour is proportional to static strength properties. Commonly, this effect is not accounted for by design guidelines and typical fatigue assessment methods for welded joints. This study hence investigates the applicability and accuracy of different local fatigue assessment methods for high-quality welded joints made of high-strength steel. For this purpose, fatigue test results of S500 structural steel joints with weld toe and weld root failure are assessed using a variety of local approaches and then compared to the nominal stress approach. Finally, the transferability of results to large-scale structures are discussed and recommendations are given for practical applications.

Application of high frequency mechanical impact treatment to improve the fatigue strength of corroding welded joints

This study deals with the fatigue strength of high-frequency mechanical impact (HFMI)-treated unprotected structural details made of mild steel S355 considering the influence of corrosive environmental conditions. The investigations are carried out on butt welded specimens with sheet thickness t = 15 mm and on transverse non-load-carrying attachment specimens with sheet thickness t = 25 mm. Two different methods were applied for the simulation of marine corrosive environment in the laboratory. Specimens first were deposited in a salt spray chamber and then tested subsequently dry at laboratory-air conditions considering the influence of corrosion on the crack initiation. Alternatively, and to cover the effects of corrosion on the crack growth, artificial seawater was used for pre-corrosion, and after a defined timespan, fatigue tests were performed simultaneously with the specimen resting in the corrosive medium. The corrosion fatigue tests were performed in as-welded and HFMI-treated conditions at a stress ratio R = 0.1 under axial tensile and 4-point bending cyclic loading. The test results are evaluated to determine the characteristic fatigue strengths for fixed slopes m = 3 and m = 5 according to IIW recommendations for the as-welded and for the HFMI-treated condition respectively. The results of the experimental investigations based on the nominal stress approach show that the fatigue strength of both specimen types could be significantly increased by the application of HFMI treatment compared to the corresponding specimens in the as-welded condition even if exposed to the investigated corrosive conditions. The comparison with the design proposals of IIW shows that for HFMI-treated butt welds, no reduction of the FAT class due to corrosion is required and the recommended FAT class is still valid. The results for the HFMI-treated transverse attachments are slightly below the design curve recommended by IIW and a proposal to consider corrosion is derived for this case. Additional numerical investigations by applying the effective notch stress (ENS) approach are performed to determine notch stress curves. It was found that for the corroded specimens in the as-welded condition, the FAT class according to IIW could not be reached and adjustments of the existing rules are necessary to consider corrosion effects. However, it can be concluded that the effective notch fatigue resistance recommended by IIW is still applicable in the case of corroded HFMI-treated structural details.

Stress Analysis of Rib-to-Deck Joints in Orthotropic Steel Deck Based on Nominal and Effective Notch Stress Approaches

Stress Analysis of Rib-to-Deck Joints in Orthotropic Steel Deck Based on Nominal and Effective Notch Stress Approaches

Simulation study on fatigue behavior of wrap-around weld at rib-to-floorbeam joint in a steel-UHPC composite orthotropic bridge deck

As an innovative cement-based material, the ultra-high performance concrete (UHPC) has been recently utilized to reinforce the orthotropic steel bridges. Based on an in-situ steel-UHPC composite orthotropic deck bridge, both the field testing and multi-scale finite element analysis modelling were conducted to investigate the fatigue behavior of wrap-around weld at rib-to-floorbeam (RF) joint, and factors influencing stress were further studied to achieve an infinite fatigue life. It is found that a primary tensile stress cycle is jointly produced by the axle group of truck, while a small reversal compressive cycle will be generated when the axle is directly located on the floorbeam. The stress behavior of fatigue detail is extremely sensitive to the localized effect of axle loading instead of entire truck configuration. Since significant stress concentration and high stress gradient exist in the wrap-around weld, it is essential to utilize the hot-spot stress approach rather than nominal stress approach. The wrap-around weld presents an obvious out-of-plane bending deformation, which results from the torsion effect and Poisson’s effect. The Dong’s structural stress reveals that the surface stress is dominated by bending stress. The application of bulkhead can greatly improve the fatigue performance of wrap-around weld. Both the large-scale rib and U-shaped rib are preferable, which are available to achieve an infinite fatigue life without extra fabrication procedure.

Investigating the fatigue behaviour of small scale and real size HFMI-treated components of high strength steels

The present study reports on results of a completed research project, which investigated amongst others the fatigue strength of weldments made of structural steels S355J2+N, S690QL and S960QL and treated with the post-weld treatment process High Frequency Mechanical Impact (HFMI). Reference tests on as-welded (AW) specimens, which were manufactured from the same material batches, were carried out in order to enable a direct comparison. Main goal of the present tests series was, apart from the revalidation of the higher efficiency of HFMI on high strength steels, to investigate possible scale effects, when the post-weld treatment method is applied on real size structural components. Therefore, usual fatigue test specimens and real size structural beams HEA 260 were tested. The welded structural detail of transverse stiffeners was investigated in all cases. All results are evaluated with the nominal stress approach and they are presented in S-N curves along with the respective FAT classes, which are proposed by the design guidelines of Eurocode and the recommendations of International Institute of Welding (IIW). Beach mark tests were carried out on HFMI-treated small scale fatigue test specimens, in order to investigate the influence of HFMI on the fatigue crack growth behaviour (FCG). In all cases, a significant increase in fatigue strength was achieved by the application of HFMI with the fatigue life extension being more significant for the high strength steels. No scaling effect was documented regarding the application of the method on the structural beams having similar plate thicknesses as the small scale specimens but more complex geometries and significantly longer weldments. Finally, the evaluation of the beach mark tests and their comparison with reference curves found in literature did not allow a clear assessment of the HFMI influence on the fatigue crack propagation (FCP).

Fatigue assessment of laser beam and friction stir welded joints made of aluminium

Available fatigue recommendations are typically developed for arc-welded joints. Some of them may need to be adjusted to fit fatigue life of joints manufactured with non-conventional welding techniques since they show different mechanical properties. Laser beam and friction stir welded butt and overlap joints made from aluminium are addressed in this paper focused on a comparison between different fatigue assessment methods. For an evaluation, S-N data from published papers was collected. Additionally, fatigue tests on laser beam and friction stir welded overlap joints were carried out. For a fatigue assessment, the nominal stress, notch stress and effective stress approach were applied. The comparison of the endurable nominal stresses showed a comparatively high fatigue strength of the tested friction stir welded overlap joints in comparison to literature data. The notches at the interface of the overlap joints were observed to have an asymmetric geometry compared to the common symmetric one, frequently found in literature, leading to a lower stress concentration. A comparison between endurable nominal stresses and the classes FAT 28 for butt welded and FAT 12 for overlap welded joints resulted in a conservative design for the majority of specimens. An evaluation based on notch stresses lead to partly non-conservative results that could be explained by the mild notches present at the laser-beam welded butt joints. For the effective stress approach, an evaluation of the micro-structural length was performed and a FAT-value is proposed.

Imperfections and Modelling of the Weld Bead Profile of Laser Butt Joints in HSLA Steel Thin Plate

In many applications that use high strength steels, structural integrity depends greatly on weld quality. Imperfections and the weld bead geometry are influencing factors on mechanical properties of the welded joints but, especially in the fatigue strength, they cause a great decrease. The proper knowledge of these two factors is important from the nominal stress approach to the fracture mechanics approaches. Studies concerning the profile and imperfections of the weld bead in laser welding for thin plates of high strength steels are scarce. In this work, these two aspects are covered for five series single and double-welded joints, butt joints in a 3 mm thick HSLA steel, welded in a small range of welding parameters. The actual profiles captured with profilometer were modeled with proposed geometric parameters achieving an adequate fit with values of the coefficient of determination ℜ2 greater than 0.9000. Description of imperfections includes the distributions of porosity and undercuts. The evaluation of the weld quality, taking as guide the ISO 13919-1 standard determined B and D levels for the welded series while based on the stress-concentrating effect, showed a greater detriment in those series with undercuts and excessive penetration. The analysis of variance validated the results of the different combinations of laser welding parameters and showed, for the factorial experimental design, a more significant effect of the welding speed.

Неопределенности в оценке усталости конструкций при проектировании и эксплуатации

Неопределенности в оценке усталости конструкций при проектировании и эксплуатации

Finite element shape optimization of weld orientation in simple plate structure considering different fatigue estimation methods

Dynamically loaded structures are generally in risk of fatigue damage. The fatigue lives of such structures can be improved using traditional methods like producing a better weld or using post-weld treatment. Another method is to improve the overall design of the structure by using the finite element method in combination with shape optimization. In this paper, a framework for finite element shape optimization has been developed, considering the fatigue life of welded joints. Simplicity and the possibility of including different fatigue estimation methods have been in focus for the framework. The framework has been used on a simple plate structure, where the weld orientation has been optimized based on four different fatigue criteria. The results show that the criteria from governing guidelines predict the same optimized weld orientation. However, the critical plane approaches cannot produce optimized weld orientations, when using the nominal stress approach as it does not account for increased stresses from the weld.