23590627 fatigue design

7/31/2019 23590627 Fatigue Design

1/18

\PERGAMON Engineering Failure Analysis 5 "0888# 002029

S02495296:88:, ! see front matter 0888 Published by Elsevier Science Ltd[ All rights reserved

P I I ] S 0 2 4 9 5 2 9 6 " 8 7 # 9 9 9 1 4 8

Fatigue design of welded aluminum rectangular hollowsection joints

K[A[ Macdonald\ P[J[ Haagensen

Norwegian University of Science and Technology\ Department of Structural Engineering\ Rich[ Birkelandsvei 0a \

N!6923 Trondheim\ Norway

Received 6 August 0887^ accepted 7 September 0887

Abstract

Fatigue design methods for welded aluminum joints are reviewed\ including various approaches to fatiguelife estimation currently adopted in design codes across a range of industrial applications[ The applicabilityof these established methodologies to the fatigue design of automotive space frame structures is criticallyassessed[ The hot spot stress method is identi_ed as the most promising in terms of providing a coherentand comprehensive approach to design[ Particular problems related to implementation are considered suchas failure sites and determination of appropriate stress concentration factors from physical models\ _nite

element calculations or parametric equations[ Preliminary results from _nite element stress analyses andfatigue tests are also presented for rectangular hollow sections welded in a T!joint con_guration[ Recom!mendations are made for a design methodology for welded rectangular hollow!section joints in aluminumspace frames\ including use of a single hot spot SN curve[ 0888 Published by Elsevier Science Ltd[ Allrights reserved[

Keywords] Automotive design^ Fatigue design^ SNcurves^ Space frames^ Weld fatigue

0[ Introduction

Aluminium welded hollow section "RHS# joints are _nding increased use in crane and bridge

structures\ transport vehicles and in automotive structures[ Tubular structures are occasionallyfabricated using forged or cast nodes but these are more expensive to produce than welded inter!tube connections which are consequently more common[ However\ the fatigue design basis forwelded RHS joints in aluminium is limited and no design recommendations currently exist forsuch joints[ An appropriate starting platform from which to establish a fatigue design methodologycould be the experience recently gained with similar structures fabricated in steel[ In particular\fatigue assessment based on the geometric hot spot stress range "or hot spot stress# concept could

Corresponding author[

7/31/2019 23590627 Fatigue Design

2/18

K[A[ Macdonald\ P[J[ Haa`ensen:En`ineerin` Failure Analysis 5 "0887# 002029003

be adopted for aluminium structures[ However\ in automotive design the material thicknesses tendto be smaller than those used in civil engineering design and this di}erence might limit thepossibilities for transfer of data from the steel industry[

This paper gives a brief overview of fatigue assessment procedures used in the design of steeltubular structures[ The limited SN data available for aluminium RHS joints are presented andthe need for further experimental research is outlined[

Trends in the development of design codes are also discussed in light of the fact that internationalcodes are rapidly harmonising through co!operation between organisations such as ISO "Inter!national Standards Organisation#\ CEN "Comite Europeen de Normalisation# responsible forEurocodes\ IIW "International Institute of Welding# and API "American Petroleum Institute#[

1[ Fatigue life assessment methods for welded tubular joints

In the majority of current fatigue design codes there are two generic types of SN curve used[In conventional welded structures involving plates and beams\ the nominal stress approach isnormally employed where the di}erent fatigue behaviour of various structural elements or detailsis described by assigning to them di}erent SN curves\ termed design categories or classes\ andcombining these with nominal stresses remote from the weld[ Since fatigue failure in weldedconstructions is not only related to geometry\ the direction of loading and failure site also in~uences

joint categorisation[ Examples of design categories of such basic connections are joints with buttor _llet weld\ having di}erent design categories dependent on the stress direction[ The main

advantage of the nominal stress approach is that the SNcurves for each weld category "or weldclass# include the notch e}ect of the weld as well as the e}ect of the component geometry[ Themain sources of scatter due to fabrication variables are thus included in the test data used to createthe curve[ Nominal stresses are calculated from a structural analysis and comprise the membranestress range\ Sm and the bending stress range Sb in general S SmSb[ The complexities of andinteractions between geometry and loading found in some structures\ e[g[\ in circular sectiontubular joints\ give rise to a plethora of possible failure sites and here the hot spot stress approachis used to reduce the design to a common basis[ Additionally\ the di.culties encountered in de_ninga nominal stress in complex structures also make the hot spot approach the only practicablemethod[ Fatigue failure will occur at sites of high peak stress in such joints and it is assumed thatfatigue life is related to the magnitude of these peaks[ Fatigue design is accomplished by combining

knowledge of these local stress peaks*usually in the form of a stress concentration factor " SCF#for a particular load con_guration*with an SN curve representing a simple weld without anystructural SCF[

1[0[ The geometric hot spot stress approach*circular hollow!section "CHS# joints

The hot spot approach has been used extensively in the o}shore industry in the analysis ofresults from tests on steel tubular joints 0[ Stresses in circular hollow!section "CHS# joints arisefrom three main sources]

"i# The basic stress response due to the global action of the remote applied load\ i[e[\ the stress

7/31/2019 23590627 Fatigue Design

3/18

K[A[ Macdonald\ P[J[ Haa`ensen:En`ineerin` Failure Analysis 5 "0887# 002029 004

that can be calculated from a frame analysis disregarding the stress concentrating e}ects ofboth the joint and the weld[

"ii# The geometric stresses resulting from local bending of the tube walls to maintain compatibilitybetween the members[

"iii# The highly local stress near the intersection lines between members[ These local stresses arestrongly in~uenced by the weld shape[

The maximum stress "or notch stress#\ located at the weld toe is the sum of the nominal stress\ thegeometric stress and the local stress components[ The geometric hot spot stress range or hot spotstress concept has evolved as the most practical basis for fatigue design of welded joints[ It capturesthe _ctitious local stress that characterises the fatigue performance of the joint\ but excludes the

very local stress perturbations caused by changes in the weld toe geometry[ Also excluded are thee}ects of any undetectable defects[ These e}ects are included in the SNcurve[ The hot spot stressconcept places di}erent structural geometries on a common basis\ enabling the use of a singleSNcurve[ The hot spot stress Sh is related to the global loads in the structure through]

Sh SCFSnom "0#

where Snom is the nominal stress range and the stress concentration factor "SCF# is normallyobtained from either _nite element analyses or from strain gauge measurements[ It is importantthat consistency with the SN curve is maintained by using the same method for estimating thehot spot in the fatigue test as used in obtaining SCFs[ The hot spot stress method for steel CHS

joints has been validated by SNdata for di}erent types of joints and loading conditions that plot

into a single scatter band 0[There is general agreement that the hot spot is located at the weld toe but there are many

opinions as to the proper method of determining the hot spot from strain gauge measurements[ Inthe early US practice for o}shore structures\ the API and AWS codes de_ned the hot spot stressrange as the total stress range measured by a strain gauge placed adjacent to the weld toe\perpendicular to the weld[ Therefore an attempt was made to measure the maximum stress at theweld toe\ including the notch e}ect of the weld[ Typically hot spot gauges were placed within5 mm to 9[0zrt of the weld toe with a gauge length of 2 mm\ r and t referring to the outside radiusand thickness of the instrumented member[

In the European Coal and Steel Community "ECSC# method 1\ also for o}shore steel structures\an extrapolation is made from two strain gauges placed just outside the weld notch zone and inthe region of stress linearity to determine the geometric hot spot stress range[ This method is also

used for non!linear stress distributions[ In the Det Norske Veritas "DNV# method 2 for o}shorestructures\ many strain gauges are placed near the weld to allow for a more accurate determinationof the region of hot spot stress[ The ECSC and the DNV recommended locations for the straingauges are shown in Fig[ 0[ The ECSC de_nition is based on the maximum principal stress\ i[e[\the stress components are extrapolated to the weld toe and the maximum principal stress calculated[The stress normal to the weld used in the US de_nition is somewhat lower than this but in theregions of highest stress\ the crown and saddle location the two are almost identical[

In the IIW design recommendations 3\ a non!linear "quadratic# extrapolation procedure isrecommended for cases of high local shell bending stresses caused for example by eccentricattachments in large diameter tubes or plane plates[ The quadratic extrapolation requires a

7/31/2019 23590627 Fatigue Design

4/18


Fig[ 0[ Recommended location of strain gauges to measure hot spot stress in tubular joints\ ECSC "top# and DNV

"bottom# 1\ 2[

minimum of three strain gauges and is based on points on a curve\ _tted to the strain gaugemeasurements by regression analysis[

The SCFis determined by several methods]

"i# Physical models using strain gauges\ as outlined above["ii# Finite element methods using appropriate meshes which model the overall joint geometry

without actually modelling the weld["iii# Parametric formulae based on either strain gauge measurements or FEM[

In the _nite element analysis\ care must be taken to obtain stresses at positions for extrapolationthat are consistent with the de_nition of the hot spot stress used[ In the IIW recommendations 3detailed instructions are given for the FEM analysis[

The value of the SCF is critical for the accuracy of the predicted life\ and large e}orts havetherefore been made to assess available data to de_ne sets of parametric equations for use in designcodes[ In the latest Health and Safety Executive revision of the guidance for o}shore structures

7/31/2019 23590627 Fatigue Design

5/18


4\ two sets of parametric equations are recommended for design of tubular CHS joints 5\ 6[ Acomprehensive survey of the changes in the HSE guidance has recently been presented 7[ Changessimilar to those in the HSE guidance are likely to be implemented in the API code 8 for o}shorestructures which is currently undergoing extensive revisions which are aimed at producing a newISO code for o}shore structures[ The new ISO code is in turn intended to become the Eurocode 2"steel structures# fatigue clauses for o}shore structures 09[ Since there is a trend towards har!monisation of European design rules\ the new ISO provisions in Eurocode 2 for hot spot basedfatigue assessment of tubular structures are expected to in~uence the hot spot assessment pro!cedures in future versions of Eurocode 8 for aluminium structures 00[

The SNcurve to be used with the hot spot concept is usually identical to or very similar to theSN curve for a simple butt weld[ In the early "0873# Department of Energy guidance on which

NS 2361 01 for steel structures are based\ the T!curve for tubular joints is almost identical to theD!curve which applies to transverse butt welds in plate structures[ This follows from the de_nitionof the hot spot stress[ For a transverse butt weld in a plate or pipe there is no geometric stressconcentration and the SCF 0[ Therefore the associated hot spot stress SN curve should onlyinclude the e}ect of the weld\ i[e[\ the SNcurve for a transverse butt weld should be used[

The physical size of the joint under consideration is important because the size of the straingauge limits the distance from the weld to the locations of the gauge[ In thin!walled structures thegauge has therefore to be placed farther away from the weld than has been possible in o}shoresteel joints\ and special guidance has therefore evolved for thin section welded joints[

The e}ect of plate thickness is usually handled the same way in design recommendations for thehot spot stress method as for the nominal stress method[ A thickness penalty factor is imposed for

plate thicknesses greater than the reference thickness\ t9[ The penalty factor\ when applied to thestress range\ is usually expressed as]

S

S9 0t9t 1

q

"1#

where Sis the stress range at thickness t[ In most of the older rules the value of thickness exponentq is 9[14[ However\ assessment of recent research data has indicated stronger in~uence of thicknessand in the latest HSE and API:ISO revision for o}shore structures a higher penalty factor ofq 9[29 is given[

1[1[ Rectangular and square hollow!section joints

The considerable knowledge gained regarding the static response of circular hollow!sectionjoints is unfortunately not directly transferable to rectangular hollow!section joints because ofmarked di}erences in the behaviour of the latter 02[ The sti}ness distribution in the ~at sidewallsis di}erent from that of cylindrical shells and the brace corners near to the chord walls have astrong in~uence on the maximum stress[ Interest in joints of this type has been con_ned to thosemade in steel 03 and recommendations have been made on the selection of hot spot stressde_nition\ parametric equations for SCFs and on appropriate SNcurves for RHS joints 04[ Incontrast\ static stress and fatigue guidance for RHS joints in aluminium are absent in the literatureand little data exists[ Some fatigue test results have been reviewed 05\ but the data are not analysed

7/31/2019 23590627 Fatigue Design

6/18


Fig[ 1[ Notch hot spot stress 06[

in terms of local or hot spot stress\ but these data nonetheless indicate that a design hot spot stressrange "FAT value# of 39 MPa would be applicable to welded square hollow!section joints[

1[2[ The hot spot stress concept applied to plate joints

Recent fatigue design guidance for welded details in steel ship structures 06\ and similarguidance for aluminium ships 07\ have in both cases adopted a hot spot stress approach[ This isdue mainly to the geometric complexity of the internal hull structure of modern ships that makesit di.cult to de_ne nominal stresses[ The guidance favours the maximum local stress at the weldtoe "called the notch stress# over the extrapolated geometric stress described earlier^ see Fig[ 1[ Thee}ect of the local weld geometry is therefore included in the notch stress\ dictating that the samee}ects are excluded from the SNcurve\ which is consequently elevated above that for a butt weldby a factor of approximately 0[4[

The notch stress concept is attractive in that it is an actual stress\ in contrast to the _ctitiousextrapolated hot spot stress already described[ In both practical and numerical applications\however\ the notch stress approach is problematic[ The two most important parameters in~uencing

the stress concentration factor for the weld are highly variable along the weld length\ the weld toeradius\ r\ and the weld angle a\ and these parameters can only be readily described in terms ofstatistical distribution[ Therefore\ in a deterministic _nite element analysis the variations in r anda cannot be modelled\ and some descriptive values "average or maximum# are employed as defaultvalues[ In the DNV rules for welded steel ship structures 07 these default values are r:t 9[04and a 34> for butt welds\ where t plate thickness[ Similar problems are encountered when thenotch stress is to be determined experimentally from strain gauge measurements since it cannot bemeasured directly at the weld because the strain gauge would have to straddle the weld toe[ Insteadan extrapolated hot spot stress is obtained from strain gauges placed at distances of 9[4 t and 0[4tfrom the weld[ To account for the stress concentration e}ect of the weld the hot spot stress is

7/31/2019 23590627 Fatigue Design

7/18


Fig[ 2[ The DNV design SNcurves for aluminium ships 07[

multiplied by a default SCF of Kw 0[4 for the weld[ Thus the DNV notch stress approach ise}ectively the same as the hot spot approach since a factor of 0[4 is _rst applied to the SNcurve\then it is accounted for by multiplying the hot spot stress by the same factor of 0[4[

1[3[ Hot spot SN curves for aluminium joints

In the DNV guidance for welded aluminium ship structures 07 two SN curves are given forwelded joints "three curves including one for welded joints in a corrosive environment#\ as shownin Fig[ 2[

The applied stress range is the notch stress ran`e as de_ned in Fig[ 1[ Curve I applies to basematerial[ The highest curve for welds "Curve II# represents butt welds while the lower curve\ CurveIII\ which also has a steeper slope "m 2[26#\ represents _llet welds as low strength "high severity#

joints[ Curve IV is for welds in a corrosive environment[ Since the slope of Curves II and III isclose to the value ofm 2 in the IIW guidance\ corresponding SNcurves\ characterised by theirFAT value "FAT stress range at N 1095# can be obtained from the DNV curves[ The stressrange of curve III at 1095 cycles is 34 MPa but this includes the Kw factor of 0[4 so thecorresponding FAT value becomes 34:0[4 29 MPa[ Similarly\ the FAT value of Curve II is44:0[4 26 MPa[ These curves can now be compared with other curves for aluminium welded

joints based on hot spot stresses[SNdata from a large number of welded aluminium plate specimens with a variety of geometries

and in thicknesses up to 5 mm have recently been collected 08 and analysed in terms of hot spotstresses obtained either from strain gauge measurements or from _nite element analyses[ The

7/31/2019 23590627 Fatigue Design

8/18


Fig[ 3[ SNdata for small MIG welded aluminium plate specimens 08[

results plotted as hot spot stresses are summarised in Fig[ 3[ It was concluded that a weld categorycurve corresponding to FAT 39 would constitute a conservative hot spot based design curve forwelded aluminium structures[

In the US the Category B curve in the Aluminum Association Design Manual has recently beenproposed 19 for use as a general design hot spot stress based design curve for welded aluminium

joints[ This is the SN curve for longitudinal butt welds[ In Eurocode 8 00 and in BS 7007 10\the hot spot stress method is mentioned but no advice is given on the choice of SNcurve[

The DNV classi_cation note for ships 07 is the only known code that uses the hot spotstress concept for welded aluminium structures[ However\ by analogy with design codes for steelstructures where the hot spot stress SNcurves are nearly identical to the curves for two sided buttwelds\ SNcurves for hot spot stress fatigue design can be obtained from other codes[ SNcurves

obtained in this way are compared in Table 0[

2[ Finite element analyses

The determination of the state of stress experienced by weldments is critical in design\ butespecially so where the hot spot stress method is used[ Appropriate SCFs are pivotal in analysisexperimental fatigue data for establishing hot spot SN curves and are also critical in terms ofrelating structural or nominal stresses to hot spot stresses for use in fatigue design[ The workreported here is primarily of an exploratory nature and was _rstly aimed at determining the SCF

7/31/2019 23590627 Fatigue Design

9/18


Table 0

Possible hot spot stress SNcurves for fatigue design of welded aluminium structures

Source Stress range Design Slope\ Comments

at two million category m

cycles\ S1 mill"MPa#

Eurocode 8 00 24 Class 24 3[9 Curve for two!sided butt welds

BS 7007 19 24 Class 24 2[1 Curve for two!sided butt welds

IIW 3 21 or 39 Fat 21 or 39 2[9 Curves for two!sided butt welds

Sharp et al[ 19 35 AA Cat[ B 3[64 Various types of welds

Partanen and Niemi 08 35 * 2[9 Various types of welds

Det Norse Varitas 07 29 or 26 Curve III or II 2[26 or 3[21 Fillet or butt welds\ respectivelyKosteas and Gietl\ 0884 05 39 * 2[4 Based on data for hollow section

joints

for welded aluminium RHS T!joints\ Fig[ 4\ under simple membrane and bending loading of thechord member\ and secondly to establish an SNcurve for this joint[

2[0[ Determination ofSCFs

Finite element analyses were performed using the I!Deas and Abaqus commecial codes[ Three!dimensional solid models were used representing a quarter of the test specimen geometry byemploying symmetry planes[ Geometrically linear elastostatic analysis routines were used[ Twosets of models were used] I!Deas based modelling of parametric variations in local weld geometry^Abaqus based analyses of the nominal weld geometry to independently verify the SCFs determined[The two series of analysis separately employed 09!node tetrahedral and 19!node brick elements[Meshing rules suggested by IIW 11 were employed and the stresses for extrapolation werealso extracted in accordance with these guidelines in order to maintain consistency between FEpredictions and experimental stress analyses[

The following nominal parameters were used for the model] weld toe radius\ r 9[7 mm^ weldangle\ a 34> weld throat size\ a 2 mm^ tube wall thickness\ t 2 mm[ Subjected to pure

bending of the chord member\ a weld toe SCFvalue of 0[82 was calculated based on extrapolation[The site of maximum stress along the weld toe is located near to the corner of the brace membertowards the sidewall of the chord[ An entirely separate FE analysis of the same geometry using adi}erent program and di}erent solid elements produced SCF values of 0[75 in bending and 0[40under membrane loading[

2[1[ Parametric study

In an investigation of the sensitivity of the calculated SCF to parametric variations in the weldand section geometry\ the four parameters r\ a\ a and t were systematically varied[ Identical tube

7/31/2019 23590627 Fatigue Design

10/18


Fig[ 4[ Specimen dimensions[

geometries were used on brace and chord members in a given analysis and during variation of tthe external dimensions remained constant[ The toe radius\ r\ was varied between 9[22[9 mm^ awas varied from 2444>^ a ranged between 13 mm^ and t was varied between 12[4 mm[ Statisticalanalysis of the resulting distribution ofSCFz "in this case de_ning the maximum stress at the weldtoe#\ based on both maximum principal stress and axial stress component\ revealed that r and thave the strongest in~uence\ while the SCFis only a weak function of both a and a[ Examples aregiven in Fig[ 5[ Models _tted to the data produced the eqns]

SCFz 0[689[221a9[62te1r9[22ta\ "2#

SCFs0 0[069[648a9[787te

1r9[05a\ "3#

7/31/2019 23590627 Fatigue Design

11/18


Fig[ 5[ Examples of parametric variation ofSCF"radius and toe angle#[

with a in radians and t\ r and a in mm[ Equation "2# refers to the axial "Z!direction# stresscomponent and eqn "3# refers to maximum principal stress at the weld toe[ The bounds investigated"stated above# strictly apply as limits to these equations[ These models behave in a similar way toestablished treatments for transverse _llet welds in plate 12 but\ as the joint under considerations

here is between hollow!sections\ there is strictly no valid basis on which to make a direct com!parison[ The variation ofSCFwith both r and t is plotted in Fig[ 6 where it is clear that\ as couldbe foreseen\ combinations of small weld toe radius and large weld angle\ weld throat and wallthickness lead to high SCFs[ In particular\ combinations of r and t have most in~uence[

3[ Fatigue tests

Fatigue testing of T!joint specimens was performed on a series of four specimen groups coveringtwo weld metals "3932 and 4072#\ and as!welded and improved "toe!ground# weldments[ Thespecimen geometry\ Fig[ 4\ is identical to that studied earlier using FE^ however\the unloaded

7/31/2019 23590627 Fatigue Design

12/18


Fig[ 6[ SCFas a function of r and t[

brace member was shortened in order to facilitate testing[ The parent material extrusions were5971[15!T4 aluminium alloy that has speci_ed minimum tensile properties of 189 MPa yieldstrength and 239 MPa ultimate tensile strength[ The specimens were loaded in 3!point bending ofthe chord member[ All tests were carried out at R 9[0 and under constant amplitude conditionsusing a 4 Hz sinusoidal waveform[ The nominal stresses were calculated from simple elasticbending theory for the loaded cross!section[ The hot!spot stresses were determined from a simplemultiplication of the nominal stresses by an SCF of 0[72\ derived from the FE work describedearlier[ A limited number of specimens were instrumented with strain gauges on the chord\ locatedclose to the anticipated failure site using the IIW guidelines 3 in order to measure the hot spot

strain\ Fig[ 7[ A polynomial curve _tted to the strain data was linearly extrapolated to the weldtoe[ Good agreement was found between the hot spot stresses based on experimental " SCF 0[72#and numerical "SCF 0[82# stress analyses[ Endurance data generated in the test programme aregiven in Tables 14\ presented in terms of both nominal and hot spot stress ranges\ using an SCFof 0[72[

The data exhibited no clear dependence on the _ller metal so these data were combined[ Linearregression analysis of the test data produced the design lines "mean minus two standard deviations#plotted in Fig[ 8] the corresponding SNconstants are noted in Table 5[

Some data are available in the literature 13 for welded T!joints in aluminium hollow pro_les[Directly relevant data reviewed by Kosteas and Gietl 05 were already expressed as local stress

7/31/2019 23590627 Fatigue Design

13/18


Fig[ 7[ Strain gauge locations[

Table 1

Results from fatigue testing of as!welded 4072 _ller metal specimens

Nominal stress Hot spot stress Endurancerange "MPa# range "MPa# "cycles#

091[8 077[2 83[819

74[6 045[7 101[793

74[6 045[7 116[698

74[6 045[7 089[166

57[5 014[4 593[533

57[5 014[4 090[664

57[5 014[4 143[022

57[5 014[4 188[567

57[5 014[4 232[999

40[3 40[3 0068[214

ranges but the Hagstro m and Sandstro m data had to be reevaluated in terms of hot spot stress[This involved performing an FE analysis on the test geometry used "axial loading of the brace# toget an SCF value of 3[3[ These data are all plotted in Fig[ 09 based on hot!spot stress range withthe results of the present study for as!welded joints where it is clear that the hot spot stress approachappears to be performing well in terms of reducing data from di}erent loading con_gurations toa common basis[

7/31/2019 23590627 Fatigue Design

14/18


Table 2

Results from fatigue testing of toe!ground 4072 _ller metal specimens

Nominal stress Hot spot stress Endurance

range "MPa# range "MPa# "cycles#

019[9 108[5 092[150

019[9 108[5 098[075

019[9 108[5 71[953

74[6 045[7 218[867

74[6 045[7 328[130

74[6 045[7 304[113

74[6 045[7 306[681

57[5 014[4 0897[528

Table 3

Results from fatigue testing of as!welded 3932 _ller metal specimens



096[0 085[9 52[969

096[0 085[9 88[069

74[6 045[7 086[93974[6 045[7 066[359

53[2 045[7 052[039

53[2 006[6 307[329

53[2 006[6 571[119

53[2 006[6 374[599

53[2 006[6 498[779

53[2 006[6 0601[529

Table 4

Results from fatigue testing of toe!ground 3932 _ller metal specimens



026[9 140[1 51[837

017[9 123[1 023[455

019[9 108[5 053[748

019[9 108[5 031[661

74[6 045[7 614[241

74[6 045[7 791[853

74[6 045[7 386[485

7/31/2019 23590627 Fatigue Design

15/18


Fig[ 8[ Fatigue test data from welded RHS T!joints[

Table 5Fatigue test results\ SNconstants for mean and design lines

Test series SNcurves

Log C S1 mill "MPa# m

As!welded\ mean line 02[68 2[76

As!welded\ design line 02[31 58[0 2[76

Toe ground\ mean line 03[64 3[01

Toe ground\ design line 03[35 84[4 3[01

7/31/2019 23590627 Fatigue Design

16/18


Fig[ 09[ Comparison of test data with published results\ all are design "mean1 SD# curves[

4[ Proposed design methodology

Based on an evaluation of the published literature\ the following recommendations are madefor the _rst version of a design methodology for welded aluminium space frames made of rec!tangular hollow section joints]

4[0[ De_nition of hot spot stress

Use the IIW 11 de_nition[

4[1[ Determination of hot spot stress by strain gauge measurements

Use the IIW 11 extrapolation procedure

4[2[ Finite element analysis to determine SCFs

Determine stress distribution by three!dimensional analysis\ determine SCFs by linear extra!polation from two points on the curve in accordance with the IIW procedure[ Use IIW 11guidance for FEM analysis[

7/31/2019 23590627 Fatigue Design

17/18


4[3[ Parametric formulae for SCFs

Use equations proposed by van Wingerde et al[ 03 for RHS T! and K!joints as preliminaryguidance[

4[4[ Hot spot stress design SN curve

Use the Partanen and Niemi curve with a stress range at 109 5 cycles of 39 MPa and a slope ofone!third[ This curve is slightly higher than the proposed DNV curves for example when correctedfor the di}erence between hot spot and notch stresses[

5[ Conclusions

Many di}erent de_nitions of hot spot stress range exist and are used in the fatigue design ofwelded structures[ The guidance recently published by IIW for derivation of the hot spot stressfrom strain gauge measurements and FEM analyses are recommended for use for preliminaryfatigue design of thin walled aluminium structures[

The hot spot stress SN curves for welded aluminium structures in design recommendationsthat give speci_c advice are remarkably similar\ apparently converging on a design fatigue stressrange at two million cycles in the region of 2939 MPa[

The hot spot method appears well suited to welded aluminium RHS joints\ however\ the design

database needs to be expanded[ Speci_c parametric equations for SCFs need to be developed "oradapted from existing methodologies for steel structures#[Parametric FE analysis of a T!joint con_guration under in!plane bending of the chord showed

that combinations of small weld toe radius and large weld angle\ weld throat and wall thicknesslead to high SCFs[ In particular\ combinations of r and t have most in~uence[

Stress concentration factors determined from strain gauge measurements and FE analysis werein good agreement[

Acknowledgements

The authors wish to acknowledge the assistance of R[ M[ Edvardsen and R[ Trandum both of

whom made contributions to the work reported here as part of their M[Sc[ thesis work[

References

0 Marshall PW[ Design of welded tubular connections[ Basis and use of AWS code provisions[ Amsterdam^ Elsevier\

0881[

1 Radenkovic D[ Stress analysis in tubular joints[ Int[ Conf[ on Steels in Marine Structures\ Paris\ 0870[

2 Gibstein MB\ Moe ET[ Fatigue of tubular joints[ In] Almar!N%ss T\ editor[ Fatigue handbook[ 0874[

3 Hobbacher A[ Fatigue design of welded joints and components[ Recommendations of IIW Joint Working Group

XIII!XV\ IIW Doc[ XIII!0428:XV!734!85[ Cambridge] Abington Publishing\ 0885[

7/31/2019 23590627 Fatigue Design

18/18


4 HSE\ Guidance on design\ construction and certi_cation[ 3th ed[ "including Feb[ 0884 amendments#[ London\

U[K[] HMSO\ 0884[

5 Efthymiou M[ Development of SCF formulae and generalised in~uence functions for use in fatigue analysis[

Proceedings of the O}shore Tubular Joint Conference[ Surrey\ U[K[\ 0877[

6 HSE\ Stress concentration factors for simple tubular joints] assessment of existing and developing of new parametric

formulae[ HSE Report OTH 80 246[ London] HMSO\ 0889[

7 Stacey A\ Sharp JV[ The revised HSE fatigue guidance[ Proceedings of the 03th International Conference on

O}shore Mechanics and Arctic Engineering[ Copenhagen\ Denmark] June 0711\ 0884[

8 API RP1A\ Recommended practice for planning\ designing and constructing _xed o}shore platforms[ 19th ed[

American Petroleum Institute\ 0882[

09 Baerheim\ M[ et al[ Proposed fatigue provisions in the new ISO code for o}shore structures[ Proceedings of the

03th International Conference on O}shore Mechanics and Arctic Engineering[ Copenhagen\ Denmark] June 07

11\ 0884[

00 CEN\ Eurocode 8] Design of aluminium structures[ Part 1] structures susceptible to fatigue[ prENV 080!1\ CEN:TC149SC 8\ 0885[

01 Norsk standard\ NS 2361\ Prosjektering av stalkonstruksjoner[ Beregning og dimensjonering[ 1[ Utg[\ Norges

Standardiseringsforbund\ 0873[

02 van Wingerde AM et al[ Proposed revisions for fatigue design of planar welded connections made of hollow

structural sections[ Proceedings of the 4th International Symposium on Tubular Structures\ Nottingham\ 0882[

03 van Wingerde AM et al[ Criteria for fatigue assessment of hollow structural section connections[ J Constr Steel

Research 0884^24]601004[

04 van Wingerde AM et al[ The fatigue behaviour of K!joints made of square hollow sections[ IIW Doc[ XIII!0515!

85\ 0885[

05 Kosteas D\ Gietl B[ A comparative analysis of existing test data on welded aluminium tubular joints[ Fatigue and

Fracture of Engineering Materials and Structures 0885^08]620627[

06 Det Norse Varitas\ Class Note] Fatigue assessment of aluminium structures[ Report No[ LIB!J!99909\ Dec[ 0884^

revised Jan[\ 0885[07 Det Norse Varitas\ Class Note] Fatigue assessment of ship structures[ Report No[ 82!9321\ revised Sept[\ 0885[

08 Partanen T\ Niemi E[ Hot spot S!N curves based on fatigue tests of small MIG welded aluminium specimens[ IIW

Doc[ 0525!85\ 0885[

19 Sharp ML et al[ Fatigue design of aluminium components and structures[ McGraw!Hill\ 0885[

10 BS 7007\ Structural use of aluminium[ Part 0[ Code of practice for design[ London] British Standards Institution\

0880[

11 Niemi E[ Stress determination for fatigue analysis of welded components[ Cambridge\ U[K] Abington Publishing\

0884[

12 Niu X\ Glinka G[ The weld pro_le e}ect on stress intensity factors in weldments[ Int J Fracture 0876^24]219[

13 Hagstro m J\ Sandstro m R[ Fatigue properties of welded T!joints in thin!walled aluminium pro_les[ Proceedings of

the International Conference 4th Int[ Conf[ on Aluminium Alloys\ ICAA!4\ Grenoble\ 04 July\ 0885[ Materials

Science Forum 0885^106111"2#]061621[

23590627 fatigue design

Documents