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structures have used the concept of local "hot-spot" stress or

strain near the weld region of interest to quantify the magnitude

of the fatigue loading. Previous guidelines for bridges and other

structures incorporated a series of parallel fatigue life design S-N

curves, where each curve is a function of weld detail severity. The

local approach was implemented to overcome the difficulty of selecting

the weld severity type in complex geometry structures.

In the ground vehicle industry much of the local stress analysis is

done using elastic finite element (or simple stress*Kt) type calculations.

These elastic stresses are then transformed into local stresses and strains

using some form of Plasticity correction tool which requires a definition

of the cyclic stress-strain curve along with the fatigue life curve.

The analysis method proposed here allows this type of correction to be

made for aluminum weld data sets, and thus will conform to the standard

methods presently applied in the ground vehicle industry.

**Base Metal Data:**Aluminum AA6061 was selected as the base metal starting point for a

Leever[22] type model of fatigue of aluminum welds. Although there is quite

a bit of information available for base metal AA6061 fatigue curves, not

a great deal of it has been made public. After the creation of the

SAE J1099 database, an event which in my opinion gave a major impetus to

fatigue analysis in the ground vehicle industry, most of the original

contributing corporations "clammed up" and kept further testing to themselves.

In my opinion this lack of openness is generally harmful to all

concerned. My commendations to NRIM (National Res. Inst. for Metals) in

Tokyo, Japan for continuing to "do the right thing" for engineering.

Thus far I have only found two public sets of AA_6061 stress-strain-life

fatigue results:[1] R.F.Brodrick, G.A.Spiering, "Low Cycle Fatigue of Aluminum Alloys",

ASTM J.of Matls, V7 N4 Dec 1972, pp.515-526

[2] Y.S. Chung, A.Abel, "Low Cycle Fatigue of Some Aluminum Alloys,"

ASTM STP 942 1988, pg. 94-106. also available in book by

A.BĂ¤umel Jr and T.Seeger, "Materials Data for Cyclic Loading,"

Supplement 1, Elsevier, 1990.

The data sets are also available in the following location:

http://fde.uwaterloo.ca/Fde/Materials/Alum/AA6xxx/aa6xxx.html

Unfortunately they do not have any results beyond about 50000 reversals to

failure. In order to obtain stress-strain and fatigue curves for longer

lives it was necessary to use the Neuber plot from the paper:

[3] F.A.Conle and J.J.F.Bonnen, "Using the Neuber Plot to Account for the

Effects of Scatter, Corrosion and Welding in Strain-Life Fatigue Test Data,"

2008 ISOPE Conf. Vancouver BC, July 10.

Neuber Stress = SQRT(StressRange * StrainRange* Modulus ) is depicted

versus fatigue life in a combined plot of data from [1],[2], and [3].

By using the elastic behavior of aluminum at longer fatigue lives it was

possible to add the long life information to a merged "Fitted" file

available here:

See: AA 6061- Merged Room Temp data files __ | _**_ Fitted**__ | __ Calculator

The fitted curve points and the data from the three references are

depicted in the Neuber plot:

*Click image to enlarge*The fitted curve, or one like it, will form the material "base line"

curve for the Leever method applied to welded aluminum components.

**Weld Results to date:**After scanning a typical aluminum weld fatigue curve figure and digitizing

the data one must compensate somehow for the effects of tests run under

different load ratios, in order to plot the data on a combined Neuber type

plot. Thus far I have found that using:

SequivAmpl = SQRT( Smax * Sa )

where SequivAmpl is an equivalent constant R=-1 stress amplitude, Smax

is the maximum stress in the test cycle, and Sa is the stress amplitude of

the test cycle. It is a form of the Smith-Watson-Topper mean stress

correction factor and should work reasonably well under primarily elastic

conditions, which these aluminum weldment tests appear to have.

In some of the graphs below I have not yet deleted the regions of the test

results where one would expect substantial plasticity; typically lives shorter

than 10**4 cycles. I have also left in Sanders' AA3003 data curve but have

not yet obtained the original reference work. Also still missing are

2000 series alloy results. The first three graphs are provided to show the

evolution of the results as more curves are added to the plot.

Finally, all of above data accumulated to date are plotted here:

Click image to enlarge

Figure: Neuber plot of aluminum weld data along with un-notched AA_6061

base line data. Also included is a stress-life plot of the IIW1823 weld curve.

**A Comparison to Cast 319 Aluminum:**Given the porosity found in many weldments, it was thought to be of

interest to compare the fatigue behavior of a typical cast aluminum,

in this case a 319 engine material. The baseline data for both simple

constant amplitude strain control tests and periodic overload tests

can be found here.

A summary figure for cast 319 aluminum is shown in the following graph.

Figure: Strain life comparison plot of Constant Amplitude and Periodic

Overstrained fatigue test results of 319 Cast Aluminum.

With the added consideration of half life cyclic stresses and elastic modulus

the test data of the above plot can be added to the aluminum weld Neuber Stress

plots shown previously.

(Click image to enlarge)

Figure: Neuber plot of aluminum weld data, AA_6061 constant amplitude

data, IIW1823 weld curve, and Cast 319 aluminum.

In the above figure the filled brown circles are for 319 constant amplitude

tests and filled square points are 319 periodic overstrain test results.

One can conclude that this particular cast aluminum is very similar to weld

aluminum fatigue data and that the IIW1823 variable amplitude design curve

is similar to the periodically overstrained cast 319 fatigue life curve.

**Tentative Design Procedure:**The data from the previous figure was replotted with the inclusion of

the base material "Fitted" Neuber Product reference curve and the same reference curve

with Yaxis terms divided by a factor 3; which amounts to a Kt=3 effect.

(Click image to enlarge)

Figure: Neuber plot of all found aluminum weld data along with un-notched

AA_6061 base material data and the Fitted base and Kt=3 tentative design curves.Note that the "enlarged" version of the above figure shows the data after

removal of any load control test points that appear to have nominal plasticity.

**Tentative Aluminum Weld Life Calculator:**With the assumption that the Leever method described above is suitable

for aluminum weldment fatigue life representations, one can create the

proposed Calculator

It consists of the Fitted base metal calculator with the setting

of the elastic magnification factor to a value of**3**to reflect

a Kt=3. This magnification can be changed at the discretion of the user.

**Summary:**- It appears that the Leever method can be applied to fatigue life

calculations for aluminum weldments. With base material curve for a

merged file of AA_6061 files and a stress concentration factor of**Kt=3**most of the

existing sample test results are encompassed. The exception of Sanders'

AA_3003F data set remains a question for further study.

- The data provided by Köbler used strain gages close to the fusion weld

toe and appears to be closer to the base material curve. It is not

clear, however, if this closer proximity is due to the use of gages

or because of a higher strength material effect. It would be useful for

FD+E members to perhaps try the strain gage method with Köbler type

specimens on one of the lower strength materials, such as AA_3003 perhaps.

- Constant amplitude cast 319 aluminum fatigue life test data falls into the

same band of life results as the welded aluminum tests.

- The IIW1823-7 curve appears to also be a good lower bound for much of

the welded results, but is perhaps too low at longer life for the

constant amplitude weld sample tests. A part of this effect

is attributed to test results with mixtures of small and

large cycles, which in other literature has been termed "periodic

overload effect." It appears that there is a correlation between the

cast 319 aluminum periodic overstrain curve and the IIW1823-7 design curve.

- Some confirmation tests using variable amplitude histories are also

necessary to determine the validity of this approach to service load

fatigue design.

- It appears that the Leever method can be applied to fatigue life

http://sourceforge.net/projects/digitizer/ "engauge" works fairly well. Suggestion: Only digitize one "curve" at a time. Then restart the program for the next curve.

- W. Schuetz and K.Winkler, "Betriebsfestigkeit geschweisster Schiffsaufbauten
aus AlMg4,5Mn," (Fatigue strength of welded AlMg4.5Mn ship structures)
Schiff und Hafen, v.21, N.9, Sept. 1969, pp.804-813.

- W. Sanders, "Fatigue Behavior of Aluminum Alloy Weldments,"
*Fatigue of Aluminum*Weld Res. Council Bulletin 171, 1972, pp.1-30.

- H.G. Köbler, "Beurteilung der Schwingfestigkeit von Schweissverbindungen
aus AlZnMg1 auf dem Weg einer Oertlichen Dehnungsmessung (Teil 2),
Aluminium, V.50, N.7, 1974, pp.445-449.

- W. Schuetz, "Zur Dimensionierung schwingbeanspruchter Aluminium-Schweissverbindungen,"
(On dimensioning welded aluminum joints subjected to cyclic loading)
ZEV-Glas. Ann. 100, Nr.2/3 Feb/Mar, 1976, pp.41-45.

- D. Kosteas, I.Kirou, W.W.Sanders, "Fatigue Behaviour of Aluminum Alloy
Weldments," Parts 1,2,3,4, Iowa State Univ. Rpes ISU-ERI-Ames-87028, Aug. 1986.

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welded plate T-joints based on the local stress-strain approach,"
Int.J.Fatigue V.11 N.3, 1989, pp.153-159.

- NRIM Fatigue Data Sheet No.64, "Data Sheeets on Fatigue Properties for Butt
Welded Joints of A5083P-O (Al-4.5Mg-0.6Mn) Aluminium Alloy Plates,"
Nat.Res.Inst.for Metals, Tokyo, 1990.

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of Welded Components: Fatigue Test Data fro Steel Plate Thicknesses up to 10mm,"
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- J.L. Fayard, A.Bignonnet, K.Dang Van, "Fatigue Design Criterion for Welded
Structures," Fat.Frac.Eng.Mat.Struc., V.19 N.6, 1996, pp.723-729.

- F.V. Lawrence, S.D.Dimitrakis, W.H.Munse, "The Variables Influencing the Fatigue Behavior of Structural Weldments," Submitted to ASM for inclusion in a Handbook. Jan. 1996. FCP Rep. No. 173/ UILU ENG-96-4001.
- M. Fermer, M.Andreasson and B.Frodin, "Fatigue Life Prediction of MAG-
Welded Thin-Sheet Structures," SAE Paper 982311, SAE-P331, 1998.

- M. Backstrom and G.Marquis, "A review of multiaxial fatigue of weldments:
experimental results, design code and critical plane approaches,"
Fat.Frac.Eng.Matl.Struc., V24, 2001, pp.279-291.

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method for structural stresses at welded joints," Int.J.Fatigue, v25 2003, pp.359-369.

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structures," Int.J.Fat. V25, 2003, pp.1359-1378.

- D. Kosteas and C.Radlbeck, "Static and Faigue Design of Aluminium Structures
according to the new Eurocode 9," June 2004.

- BS7910:2005 British Standard for Welded Structures

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- P. Darcis, T.Lassen and N.Recho, "Fatigue Behavior of Welded Joints Part 2:
Physical Modeling of the Fatigue Process," Welding Journal, Jan.2006, p.19s-26s.

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Fatigue of Weldments," Proc. of NAFEMS Vancouver, May22-25, 2007.

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*Behavior and Design of Aluminum Structures*, McGraw Hill, 1972. - R.C.Leever, "Application of Life Prediction Methods to As-Welded Steel Structures," ASME Int. Conf. on Advances in Life Prediction Methods, The Matls. Conf, Albany, NY, Apr. 18-20, 1983. Lib. of Congress 83-70330.