Fatigue Design Curves Research Articles

Austempered ductile iron-to-steel dissimilar arc-welded joints: fatigue strength assessment according to local approaches

The design of mechanical components increasingly requires the adoption of different materials in the same structure with the aim of improving its performance. One of the available solutions is to adopt dissimilar arc-welded joints, which typically must withstand high fatigue loadings during in-service life. In a recent paper, the fatigue strength of EN-JS-1050 austempered ductile iron-to-S355J2 steel dissimilar arc-welded joints was experimentally investigated by testing some typical welded details in the as-welded conditions, under either axial or four-point bending loadings. In the present work, previous experimental campaign has been extended by fatigue testing other ADI-to-steel joint details. The fracture surfaces of all tested joints have been analysed to identify the fatigue crack initiation locations. Then, experimental fatigue results have been re-analysed to determine the fatigue strength category of each welded detail, which have been compared with those provided by International Standards for corresponding homogeneous steel joints. Afterwards, the local approach based on the Peak Stress Method (PSM) combined with the averaged Strain Energy Density (SED) fatigue criterion, has been used for the first time for fatigue strength assessment of ADI-to-steel dissimilar arc-welded joints. First, the structural volume size R0 has been calibrated for joints exhibiting fatigue crack initiation and propagation at the ADI side; then, a PSM-based fatigue design curve has been determined by fitting the available experimental results.

Multiaxial fatigue assessment of S355 steel in the high-cycle region by using Susmel’s criterion

Multiaxial fatigue is frequently present on engineering structures and is the cause of many mechanical failures. However, multiaxial fatigue analysis is full of questions and different points of view. Thus, throughout this study, Susmel’s criterion, a recent multiaxial fatigue damage model also known as the Modified Wöhler Curve Method, is presented, explained and assessed. Experimental data of axial, torsional and proportional (axial+torsional) fatigue tests conducted on S355 structural steel and under different stress ratios were analysed and evaluated according to this model. Mean fatigue design curves for each loading condition were obtained and plotted in the high cycle fatigue region. Finally, the ability of Susmel’s criterion to assess the multiaxial fatigue behaviour of S355 steel in the high cycle region was evaluated by determining the error index between the theoretically estimated and the experimental fatigue damage. Susmel’s model was found to be adequate to describe the fatigue behaviour of the steel under study in high cycle region.

Automated fatigue strength assessment of arc-welded structures according to the Peak Stress Method

The Peak Stress Method (PSM) is an approximate, FE-oriented application of the notch stress intensity factor (NSIF) approach to fatigue design of welded joints. This approach is based on the singular linear elastic peak stresses calculated from coarse FE models. The PSM was calibrated by using 2D, four-node plane quadrilateral elements; 3D, eight-node brick elements and 3D, four-node or ten-node tetra elements. By adopting the averaged Strain Energy Density (SED) as a fatigue strength criterion, a design stress, the so-called equivalent peak stress, has been defined and adopted in conjunction with a reference design fatigue curve to estimate the fatigue life of welded structures. The equivalent peak stress has successfully been adopted to assess the fatigue strength of arc-welded joints made of either structural steels or aluminum alloys and subjected to axial, torsion or multiaxial fatigue loadings. In the present work, a subroutine, named ANSYS-PSM, has been developed in the post-processing environment of ANSYS® FE code to automate the PSM for 2D or 3D FE models. After having briefly recalled the theoretical background of the PSM and the procedure to apply it step by step, the paper presents the adopted programming languages and the workflow of the developed subroutine.

Fatigue strength of fillet‐welded joints at subzero temperatures

AbstractShips and offshore structures may be operated in areas with seasonal freezing temperatures and extreme environmental conditions. While current standards state that attention should be given to the validity of fatigue design curves at subzero temperatures, studies on fatigue strength of structural steel at subzero temperatures are scarce. This study addresses the issue by analysing the fatigue strength of welded steel joints under subzero temperatures. Although critical weld details in large welded structures are mostly fillet‐welded joints, most published data are based on fatigue crack growth rate specimens cut out of butt‐welded joints. This study analyses fillet‐welded specimens at −20°C and −50°C against controls at room temperature. Significantly higher fatigue strength was measured in comparison to estimates based on international standards and data from design codes even at temperatures far below the allowed service temperature based on fracture toughness results.

Modification and Extension of Screening Criteria for Fatigue Analysis

Abstract ASME Code VIII-2-2019 and previous versions provided three screening criteria for fatigue analysis. From edition 2004 to 2019, the design factor for material allowable stress decreased and the considered range of permissible cyclic number for design fatigue curve extended. However, screening criteria are almost unchanged except one restriction: If the specified number of cycles is greater than 106, then the screening criteria are not applicable and a fatigue analysis is required. In this paper, percentage limit of the design pressure in method A is modified and the specified number of cycles is extended. Some revision suggestions are also proposed to broaden the applicable range of the screening criterion.

The influence of pulsed gas metal arc welding on the fatigue and corrosion-fatigue properties of wrought aluminium 6061-T651

Purpose This paper aims to characterise the hardness, tensile properties, corrosion behaviour and fatigue properties (in air and in a 3.5 per cent NaCl solution) of aluminium 6061-T651 in the as-received and as-welded conditions. Design/methodology/approach Aluminium 6061-T651 plate material, prepared with double-V or square butt joint preparations was welded using semi-mechanised or mechanised pulsed gas metal arc welding. Magnesium-alloyed ER5356 or ER5183 filler material or silicon-alloyed ER4043 filler wire was used. The material was characterised in the as-supplied and as-welded conditions, and fatigue tests were performed in air and in a 3.5 per cent NaCl solution. The fatigue results were compared to the reference fatigue design curves for aluminium published in Eurocode 9 – Part 1-3. Findings Significant softening, attributed to the partial dissolution and coarsening of precipitates, grain growth and recrystallisation during welding, was observed in the heat-affected zone (HAZ) of the 6061-T651 welds. During tensile testing, failure occurred in the HAZ of all 6061 welds tested. Welding reduced the room temperature fatigue life of all specimens evaluated. In 6061 welds, failure occurred preferentially in the softened HAZ of the welds. The presence of a corrosive environment (a 3.5 per cent NaCl solution in this investigation) during fatigue testing reduced the fatigue properties of all the samples tested. Corrosion pits formed preferentially at second phase particles and reduced the overall fatigue life by accelerating fatigue crack initiation. Originality/value The fatigue properties of welded aluminium structures under dynamic loading conditions have been studied extensively. Welding is known to create tensile residual stresses, to promote grain growth, recrystallisation and softening in the HAZ, and to introduce weld defects that act as stress concentrations and preferential fatigue crack initiation sites. Several fatigue studies of aluminium welds emphasised the role of precipitates, second phase particles and inclusions in initiating fatigue cracks. When simultaneously subjected to a corrosive environment and dynamic loading, the fatigue properties are often adversely affected and even alloys with good corrosion resistance may fail prematurely under conditions promoting fatigue failure. The corrosion-fatigue performance of aluminium welds has not been systematically examined to date.

Multiaxial Fatigue Experimental Analysis of 6063-T66 Aluminum Alloy of the Base Material and the Welded Material

Abstract This article deals with determining of fatigue lifetime of aluminum alloy 6063-T66 during by multiaxial cyclic loading. The experiments deal with the testing of specimens for identification of the strain-life behavior of material, the modeling of combined loading and determining the number of cycles to fracture in the region of low-cycle fatigue. Fatigue tests under constant amplitude loading were performed in a standard electromechanical machine with a suitable gripping system. Based on the experimental results the fatigue design curves are compared to the fatigue data from the base material and the welded material and also multiaxial fatigue models, which are able to predict fatigue life at different loads.

Fatigue strength of cruciform joints and longitudinal joints with laser-arc hybrid welding

This study examined the fatigue strength of cruciform joints and longitudinal joints fabricated with laser-arc hybrid welding. Load-carrying and non-load-carrying cruciform welded joints with plate thicknesses of 8, 16, and 25 mm were tested under tensile cyclic loads and out-of-plane bending cyclic loads. In longitudinal joints, four-point bending fatigue tests were performed on plate girders with flanges and a web connected by laser-arc hybrid welding. When assembling the plate girder, tack welds were installed at the flange-to-web connection to clarify their effect on the fatigue strength of the longitudinal joints. Measurements of weld toe profiles and residual stress were also conducted. The residual stress was measured using X-ray diffraction. The test results revealed that the fatigue strengths of hybrid welded joints are equivalent to those of arc welded joints and satisfy the fatigue design curves specified by the Japanese Society of Steel Construction. In addition, the effect of plate thickness on the fatigue strength was observed in hybrid welded joints as well as in arc welded joints.

Fatigue of Preloaded Bolted Connections with Injection Bolts

Injection bolts have been used to replace faulty rivets on strengthening operations of old riveted steel bridges. This technology has been essentially tested for its quasi-static or creep behaviour and there are very few studies concerned the assessment of its fatigue resistance. This paper presents an experimental program developed to evaluate the fatigue behaviour of single shear preloaded bolted connections with injection and standard bolts. Specimens combine S355 steel and old metallic material obtained from structural elements of the centenary Eiffel bridge in Portugal. Fatigue failure was found either at the net cross-section or at a gross cross-section of the plates. Statistical analysis was conducted on the experimental data to compare with the fatigue design curves proposed in Eurocode 3, part 1-9. It is concluded that the detail category suggested on the reference code is not able to represent the fatigue life of old steel materials. An alternative S-N curve is proposed which is able to represent the fatigue behaviour of the studied detail. Furthermore, it was concluded that the influence of the adhesive on the detail’s fatigue strength is not significant.

Fatigue strength of laser-arc hybrid welded joints with tack welds

ABSTRACT This study examined the fatigue strength of cruciform joints and longitudinal joints fabricated with laser-arc hybrid welding, in which tack welds were installed at the connection. The cruciform joints were tested under tensile cyclic loads and out-of-plane bending cyclic loads. In the longitudinal joints, four-point bending fatigue tests were performed on plate girders fabricated with the hybrid welding. In addition, finite element analysis was performed on the cruciform joint to investigate the effect of tack welds on stress concentration at weld toes. The test results revealed that the fatigue strengths of the hybrid welded joints are equivalent to those of arc welded joints with or without tack welds and satisfy the fatigue design curves specified in JSSC recommendation. Moreover, the analysis results indicated that tack welds have little influence on stress concentration at weld toes in cruciform joints fabricated with laser-arc hybrid welding.

Fatigue design curve under LCF as well as combined LCF and HCF regime at 923 K in a type 316LN stainless steel

AbstractThe paper presents a novel approach towards developing fatigue design curve under combined loading involving low cycle fatigue (LCF) and high cycle fatigue (HCF), in a type 316LN austenitic stainless steel. The total strain life curve used for fatigue design is modified taking into account the effect of varying load history. The methodology relies on the test data obtained to previous studies by authors pertaining to LCF‐HCF interaction using a sequential pattern at 923 K. Modified design curves are generated at 923 K where the effect of varying degree of prior LCF exposure at strain range of 0.12% is accounted for, on HCF.

Low cycle fatigue and creep-fatigue response of the 316Ti stainless steel

SS 316Ti is widely used in bellows industry and is a good candidate material for high temperature bellows in sodium cooled fast reactor (SFR) systems. Typical operating temperature experienced by SFR systems is around 823K. Design of bellows for nuclear applications need to be in compliance with the standard design codes such as ASME section-III and RCC-MR. The fatigue data and cyclic stress strain curve of SS316Ti are not available in design codes such as RCC-MR or ASME section-III/NH. Hence, the material data required for high temperature design of bellows are generated experimentally. Initially, the basic tensile data such as yield strength, ultimate tensile strength and % elongation of the material were obtained from tensile testing at 823K. Low cycle fatigue tests were carried out in strain controlled mode on SS316Ti at 823K different strain ranges in air and variation of number of cycles with strain range was obtained. Creep-fatigue interaction (CFI) experiment was also conducted at 823K and strain amplitude of ± 0.4% with 1 minute hold time in peak tensile strain. The stress response (peak stress variation with number of cycles) of the material showed continuous hardening up to saturation followed by crack nucleation and final failure. The fatigue life was found to decrease with increase in strain range. The fatigue life decreased in presence of hold period in tension. The design fatigue curve for SS316Ti at 823K has been generated using the LCF data by incorporating factors of safety on strain and number of cycles. Cyclic stress strain curve was generated for the material at 823K. The tensile, LCF and CFI data generated will be useful in design of SS316Ti bellows for SFR systems.

A finite element based approach for fatigue life prediction of headed shear studs

Steel shear studs in bridges are subjected to rapidly fluctuating stresses causing fatigue failure. Research on fatigue of shear studs mainly focused on tests. Both AASHTO and Canadian design curve for fatigue resistance of shear studs are based on the tests conducted in the mid1960s by Slutter and Fisher. This paper presents a finite element based approach using push-out specimen for fatigue life estimation of headed shear stud connectors. Both crack initiation and crack propagation life are estimated and an excellent correlation is found when compared against test results. In addition, since a significant amount of push-out tests data on headed shear studs are now available, this paper evaluates the fatigue design curves of different standards, with special focus given to evaluation of the value of constant amplitude fatigue limit (CAFL) given in the current AASHTO and Canadian code (CSA S6-14). The regression analysis also shows that the current fatigue curves in different codes can be used for shear studs as large as 31.8 mm. Thus, restriction of use of studs larger than 25 mm (1 in.) in different bridge codes (CSA S6, Eurocode 4, and AASHTO) can be waived.

Discussion of fracture surface using beach marks on fatigue test data with large scale piping

Design Fatigue Curve (DFC) Phase 1 subcommittee and Phase 2 subcommittee were established in the Atomic Energy Research Committee in the Japan Welding Engineering Society. DFC 1 and DFC 2 subcommittees were proposed new design fatigue curves for carbon steels & low alloy steels and austenitic stainless steels and a new design fatigue evaluation method. And a Japanese utility project performed large scale fatigue tests using austenitic stainless steel piping to obtain not only basic data but also fatigue data of mean stress effect.We reported that the results of the large-scale fatigue piping tests showed good agreement with the best-fit curve developed by the DFC 1 and DFC 2 subcommittees and the size effect could be considered as negligible. It was confirmed that the effect of mean stress can be accurately evaluated by the Smith-Watson-Topper approach.In this paper, crack growth analysis was carried out for the beach marks introduced during the large-scale fatigue piping tests using JSME S NA1 crack propagation equation of austenitic stainless steel and the analysis results were compared with the crack depths obtained from the beach marks.

A review of fatigue performance of bolted connections in offshore wind turbines

Flanged bolted connections are used in offshore wind turbines to connect the monopile to the transition piece, and the transition piece to the tower. Forces and moments generated by the effects of winds, waves and currents subject these connections to variable amplitude cyclic loads. In the harsh marine offshore environment areas such as bolt threads are vulnerable to fatigue cracking and failure. Their structural integrity and life are influenced by coatings, lubricants, bolt size, design and tension, manufacturing tolerances and flange contact.This paper presents a review of the main challenges for fatigue life assessment of M72 bolted connections used in offshore wind turbines to connect the monopile to the transition piece, and the factors affecting the fatigue performance. Existing standards and guidelines along with different fatigue assessment methods presented in the literature are discussed in terms of their suitability for fatigue life assessment of M72 bolted connections for offshore wind turbines. One of the key challenges in fatigue life assessment of large scale bolts (e.g. M72) is the lack of experimental data points from which fatigue design curves can be derived. Recommendations are made for how to improve the current best practice for fatigue assessment of large scale bolted connections in the offshore wind industry and potential areas for further investigation are proposed for future research.

Discussion on crack growth behavior in large-scale fatigue tests of carbon and low-alloy steel plates based on fracture surface observation

In Japan, the Design Fatigue Curve (DFC) Phase 1 and Phase 2 subcommittees, organized under the Atomic Energy Research Committee of the Japan Welding Engineering Society, have proposed new best-fit fatigue curves and design fatigue curves for carbon, low-alloy, and austenitic stainless steels. To investigate the correlation between the fatigue life by the proposed best-fit curves and that of the actual components, the authors carried out reversed four-point bending fatigue tests for large-scale specimens of carbon steel and low-alloy steel plates with 90 mm thickness and 150 mm width. As a result, the fatigue lives given by the best-fit curve were found to correspond to approximately 3-4-mm-deep crack initiation lives in large-scale specimens. The result implies that only several-millimeter crack initiation defines a large structure. It would be reasonable to consider the crack growth behavior after reaching the design fatigue life. During the fatigue tests, the crack growth behavior was monitored with a plastic replica and a beach mark method. Since the main crack grew with the coalescing of micro cracks, the applicability of fracture mechanics method is uncertain. In this study, a crack growth analysis was performed on the cracks observed in large-scale specimens using a crack growth law specified in a fitness-for-service (FFS) code or literature. With respect to the surface crack length, some experimental data surpassed the analysis results, which might be attributed to crack coalescence. The crack depth data that are not easily affected by the coalescence was plotted within the upper limit of data scatter. The analysis results enabled the prediction of the total fatigue life of the large-scale specimens as the sum of the crack initiation life by the best-fit curve and the growth life from a 3-mm-deep crack.

Design and analysis of formed bellows for nuclear applications- Case study

Bellows are highly engineered components which find large number of applications in nuclear industry. Formed bellows are the most commonly used type of bellows especially when stoke is less than one thirds of bellows free length. Conventionally, design and manufacturing of the formed bellows are extensively addressed in standards of Expansion Joint Manufacturers Association (EJMA). In nuclear applications, design of the bellows shall comply with standard nuclear design codes such as ASME section-III, RCC-MR etc, which follow design by analysis. Hence, the design of the bellows for nuclear systems needs detailed Finite Element Analysis (FEA) and fatigue life estimation as per the design curves given in the codes. In this work, a case study of design of formed bellows, made of SS316LN, operating below creep range for nuclear application is presented. The bellows are preliminarily designed by EJMA and the geometry arrived based on preliminary design is analyzed using FEA. The stress/ strain range in the bellows is estimated by both linear elastic and inelastic FEA. Non-linear isotropic-kinematic hardening constitutive model is employed for inelastic analysis of the bellows. The fatigue life of the bellows is estimated as per the guidelines given in RCC-MR. It is found that the fatigue life estimated by linear elastic analysis is more conservative as compared to inelastic analysis. In the design fatigue curves given in the nuclear design codes, factor of safety (FOS) of 2 on stress range is considered. By considering similar factor of safety on the equivalent stress given in EJMA, the cycle life of the bellows is estimated and compared with the design life obtained by detailed stress analysis. It is found that after including FOS of 2 on equivalent stress, the life predicted by EJMA design curve is conservative. Hence, FOS modified EJMA equation can be used for design of the nuclear bellows when operating temperature is less than creep range of the bellows material. Use of FOS modified EJMA design equation is easy and eliminates the need for detailed stress analysis by the bellows suppliers. However, other aspects such as manufacturing tolerances, inspection, testing and qualification of the bellows shall comply with standard nuclear design codes.

Multiaxial fatigue assessment of welded steel details according to the peak stress method based on tetra elements

The Peak Stress Method (PSM) is an engineering, FE-oriented application of the notch stress intensity factor (NSIF) approach to fatigue design of welded joints, which takes advantage of the singular linear elastic peak stresses from FE analyses with coarse meshes. Originally, the PSM was calibrated by using 3D, eight-node brick elements, taking advantage of the submodeling technique. Recently, the PSM has been calibrated by using ten-node tetra elements, which are able to directly discretize complex 3D geometries without the need for submodels. The PSM was validated for pure axial or bending loadings as well as pure torsion loadings; recently it has been extended to multiaxial loadings by adopting a design stress, the so-called equivalent peak stress, in conjunction with a reference design fatigue curve. After having briefly recalled the calibration of the PSM with tetra elements, the paper presents some applications of the PSM relevant to steel plate-to-tube welded details of industrial interest under in-phase bending-torsion fatigue loadings. Experimental data have been re-analysed using the PSM based on tetra elements. Eventually a good agreement between experimental and theoretical results has been obtained in terms of fatigue crack initiation location as well as total fatigue life.

Low Cycle Fatigue Behavior of Steam Generator Tubes under Axial Loading.

Compared with the fatigue properties of the material (Inconel Alloy 690), the real fatigue lives of tubes are more meaningful in the fatigue design and assessment of steam generator (SG) tube bundles. However, it is almost impossible to get a satisfactory result by conducting fatigue tests on the tube directly. A tube with a uniform and thin wall easily fails near the clamping ends under cyclic loading due to the stress concentration. In this research, a set-up for fatigue tests of real tubes is proposed to overcome the stress concentration. With the set-up, low cycle fatigue tests were conducted in accordance with an existing fatigue design curve for Alloy 690. Strain control mode was applied with fully reversed push–pull loading under different strain amplitudes (0.15%, 0.2%, 0.3%, and 0.4%). A favourable result was obtained, and the low cycle fatigue behavior was investigated. The results showed that the fatigue life tested by the real tube was below the strain–life curve of Alloy 690 which was fitted by conventional solid specimens. A cyclic hardening behavior was found by the cyclic stress–strain curve when compared with the monotonic stress–strain curve.

High cycle fatigue and fatigue crack propagation design curves for 5754-H22 and 6082-T6 aluminium alloys and their friction stir welded joints

Friction stir welding (FSW) is a dynamically developing version of pressure welding processes. Nowadays, learning the properties and behaviour of the welded joints is an important direction of investigation, especially under cyclic loading. The research work aimed (i) to demonstrate the behaviour of FS welded joints under cyclic loading conditions and (ii) to determine fatigue design curves for two aluminium alloys and their welded joints made by the FSW process. Experiments were performed on 5754-H22 and 6082-T6 aluminium alloys and their welded joints; high cycle fatigue (HCF) and fatigue crack propagation (FCG) tests were performed. Statistical approaches were applied during the experiments and the evaluation process. HCF design curves were determined based on the staircase method and FCG design curves were determined by our own previously developed six-step method. The results of the HCF and FCG tests on FS welded joints confirm the applicability of the applied FSW technology and show good correspondence with results of the comparative base materials and welded joints that can be found in the literature. Based on our experimental results and the evaluation methods used, statistically established parameters of HCF and FCG design curves were calculated.