6061-T6鋁合金疲勞曲線

1、FATIGUE DESIGN CURVES FOR 6061-T6 ALUMINUM*G. T. YahrEngineering Technology DivisionOak Ridge National LaboratoryOak Ridge, Tennessee 37831-8051ABSTRACTA request has been made to the ASME Boiler and Pressure Vessel Committee that 6061-T6 aluminum be approved for use in the construction of Class 1 we

2、lded nuclear vessels so it can be used for the pressure vessel of the Advanced Neutron Source research reactor. Fatigue design curves with and without mean stress effects have been proposed. A knock-down factor of two is applied to the design curve for evaluation of welds. The basis of the curves is

3、 explained. The fatigue design curves are compared to fatigue data from base metal and weldments.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, m

4、akes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific c

5、ommercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessari

6、ly state or reflect those of the United States Government or any agency thereof.RECEO 二©APR 211993OSTI* Prepared by the OAK RIDGE NATIONAL LABORATORY, Oak Ridge, Tennessee 37831; managed by MARTIN MARIETTA ENERGY SYSTEMS, INC., for the U.S. DEPARTMENT OF ENERGY underContract No. DE-AC05-84ORDIS

7、TRIBUTION OF THIS DOCUMENT IS UNLIMITEDF» submitted manuscript has boon author” by s contractor of the U.S. Government under contract No. DE AC05-840R21400. AccordbgJy, the US Gownment retains a nonexdusivo. royahy-froe ficarue to pub&sh or reproduJ tho pub&shod form of this contnbution

8、. or others to do so. for U.S. Government purposes. DISCLAIMERPortions of this document may be illegible in electronic image products Images are produced from the best available original document 2NOMENCLATUREA=constant in Eq.1B=constant in Eq.1E=elastic modulusN=fatigue lifeR = ratio of minimum str

9、ess to maximum stress in a fatigue testRA = percent reduction of areaS = strain amplitude times elastic modulusSa = allowable amplitude of alternating stress intensitySe = endurance limitSf = fatigue strength under fully reversed loadingSf* = fatigue strength in presence of mean stressSm = mean stre

10、ssSu = tensile strengthSy = cyclic yield strength81. INTRODUCTIONOak Ridge National Laboratory (ORNL) has proposed a new facility for neutron research, centered around a new research reactor, called the Advanced Neutron Source (ANS) (West, 1988; Harrington, 1991). The reactor will require extensive

11、use of 6061-T6 aluminum in the primary pressure boundary. The ANS is to be a user experimental facility for many scientific fields, including neutron scattering with cold, thermal, and hot neutrons (the most important scientific justification for the project); engineering materials irradiation; isot

12、ope production (including transuranium isotopes); materials analysis; and nuclear science. It is anticipated that the ANS will lead to important technological developments that will enhance our global competitiveness.The user requirements, particularly the need for a large, accessible volume of a ve

13、ry high thermal neutron flux, determine the main features high power -300 MW(t) and small size) of the reactor core design. The basic concept is conventional: a heavy-water-cooled and reflected reactor. The coolant inlet temperature is expected to be 45°C (113°F) and the bulk coolant outle

14、t temperature is expected to be 85°C (185°F)> The inlet pressure will be 3.5 MPa (508 psi). The primary pressure boundary is a long 0.5 m (20 in.) diameter 6061-T6 aluminum vessel with flanges on both ends so that it can be periodically replaced as required due to embrittlement caused b

15、y neutron irradiation. Outside the primary pressure boundary vessel, there will also be a 3.5 m (11.5 ft) diameter 6061-T6 aluminum reflector tank with numerous penetrations and attachments that will contain heavy water at a pressure of about 0.5 MPa (75 psig). The heavy water in the reflector tank

16、will be at a temperature below 46°C (115°F).Aluminum has been employed as a structural material in most research reactors. The aluminum alloy 6061-T6 has been successfully used in structural applications in several research reactors including the High Flux Isotope Reactor (HFIR) at Oak Rid

17、ge and the High Flux Beam Reactor (HFBR) at Brookhaven National Laboratory. The HFBR reactor vessel is 6061-T6 aluminum. It was designed in accordance with the ASME Boiler and Pressure Vessel Code for Unfired Pressure Vessels, Section VIII (1959), including all revisions, addenda, and applicable cod

18、e case rulings in effect at the date of award of the construction contract. In addition, critical areas of the vessel where discontinuity stresses could be appreciable were examined and analyzed in accordance with the document 'Tentative Structural Design Basis for Reactor Pressure Vessels and D

19、irectly Associated Components5' (1958) along with formulae from reports, documents and publications by Bijlaard Horvay, Galletly, et al.Previous Department of Energy (DOE) research reactors have not been required to be built according to the ASME Nuclear Code. However, DOE Order 5480.4 Environme

20、ntal Protection, Safety, and Health Protection Standards (1984) specifically requires all DOE and DOE contractor operations to apply the ASME Boiler and Pressure Vessel Code to DOE-owned reactors. Therefore, it will be necessary that the materials of construction for the ANS, including 6061-T6 alumi

21、num, be acceptable Code materials.A proposed Section JU Code Case has been prepared for the ASME Boiler and Pressure Vessel Committee's consideration. The basis for the design fatigue curves included in the proposed Code Case is provided in this paper.2. BASIC FATIGUE CURVEFatigue data on 6061-T

22、6 aluminum were collected from the literature Structural Alloys Handbook 1989; Alcoa Structural Handbook, 1960; MIL-HDBK-5D, 1984; AFML-TDR- 64-280, 1970; Brodrick and Spiering, 1972) to construct a design fatigue curve. The data are plotted in Fig. L Data for test temperatures from room temperature

23、 up to 149°C (300°F) are included. The dau are for fully reversed loading. The low cycle data (<103 cycles) are all from axial strain controlled tests. Note that the data from the strain controlled tests fall right in with the rest of the data at greater than 103 cycles- Some of the hig

24、h cycle data are from rotating beam tests. A fit to the 6061-T6 fatigue data was developed using the equation given in the Code Section m criteria document (1969).where E = elastic modulusN = number of cycles to failureS = strain amplitude times elastic modulusand A and B are constants that are sele

25、cted to make the equation fit the data. Note that S is treated like a stress based on the assumption of elastic behavior. However, it does not represent a real stress when the elastic range is exceeded- As a first approximation, the fatigue properties can be estimated by taking A as the percentage r

26、eduction of area in a tensile test, RA, and B as the endurance limit Se- It should be noted that aluminum does not exhibit an endurance limit The fit to the data is14,479+ 96.5 MPaor s = 24000+ 14 qoo psjeThe curve is shown with the available data from tests without any mean stress in Fig. 1. It was

27、 used to construct the design curve by reducing the stress by a factor of two or the number of cycles to failure by twenty as shown in Fig. 2. The design curve is truncated at twice the yield strength to prevent significant strain accumulation.3. MEAN STRESS EFFECTSData from fatigue tests (Structura

28、l Alloys Handbook, 1989; MIL-HDBK-5D, 1984) with an applied mean stress fall below the fit to the fully reversed fatigue data as shown in Fig. 3. The mean stress data include test series with constant R ratios as high as +0.5 and test series with applied mean stress of up to 207 MPa (30 ksi). Mean s

29、tress effects are generally accounted for by use of the modified Goodman diagram (ASME Criteria Document, 1969) when the fatigue curves in the ASME Boiler and Pressure Vessel Code are developed. Experimental studies of aluminum (Fuchs and Stephens, 1980; Woodward, et al., 1956) show that the mean st

30、ress effect is better described by the Gerber equation than by the linear Goodman equationwhere Sf* :Sf :Sm :Sq=fatigue strength in presence of mean stress=fatigue strength under fully reversed cycling=mean stress=tensile strength.The modified Goodman diagram is generally used because it is simpler

31、to apply tlian the Gerber equation and is more conservative- However, for materials that have a high yieldstrength to tensile strength ratio, such as 6061-T6 aluminum or high strength steel bolts, the modified Goodman approach tends to become overly conservative. The mean stress effect in high stren

32、gth bolts was accounted for using the Peterson cubic equation when the Code fatigue curves were developed (Snow and Langer, 1967).The Gerber equation and the linear Goodman equation are represented in Fig. 4. The alternating stress is represented on the ordinate and the mean stress is represented on

33、 the abscissa* The straight solid line drawn from the fatigue strength for fully reversed loading for 200,000 cycles 129 MPa (1&7 ksi) to the tensile strength 290 MPa (42 ksi) is a graphical representation of the Goodman prediction of the effect of mean stress. The Gerber prediction is the curve

34、d line. The dashed line drawn between the cyclic yield stress 262 MPa (38 ksi) on both axes represents the boundary of sustained cyclic conditions; yielding will occur if the combination of alternating stress and mean stress is above the dashed line. Therefore, the intersection of the dashed line wi

35、th the Goodman or Gerber line gives the maximum effect of mean stress because yielding would reduce any higher mean stress. For the case of 200,000 cycles shown in Fig. 4, the Goodman approach would predict a maximum reduction in fatigue strength to 22 MPa (3.2 ksi). Using the Gerber approach, the m

36、aximum reduction is to 75 MPa (10.94 ksi). For this case, the allowable alternating stress is increased by three times because the Gerber approach was used instead of the Goodman approach.The difference between the maximum mean stress effect predicted by the Gerber and Goodman approaches becomes muc

37、h more significant when the yield strength approaches the tensile strength, especially for low fatigue strengths.Studies by Brodrick and Stephens (1972); and Chung and Abel, (1988) have shown that 6061-T6 cyclically hardens a small amount at room temperature typically 241 MPa (35 ksi) to 262 MPa (38

38、 ksi). At 149°C (300°F) it cyclically softens at total strain ranges below about two percent and cyclically hardens at higher total strain ranges.Because 6061-T6 aluminum cyclically hardens, a value of 262 MPa (38 ksi) was used for the yield strength when determining the maximum effect of

39、mean stress. This does result in lower values of allowable alternating stress than would have been obtained if the monotomic yield strength of 241 MPa (35 ksi) had been used. For example, the maximum mean stress reduction in the 200,000 cycle fatigue strength would have been to 93 MPa (13.5 ksi) ins

40、tead of to 75 MPa (10.94 ksi).Yield and ultimate strengths of 262 MPa (38 ksi) and 290 MPa (42 ksi), respectively, were used in making the mean stress corrections to the best fit to the fatigue data. The resulting curve shown in Fig. 5 was then reduced by a factor of two on stress or twenty on numbe

41、rof cycles to failure to arrive at a design curve that considers the effects of the maximum amount of mean stress that could be sustainedThe design curves with zero mean stress and with maximum mean stress are shown in Fig. 6 The design curves are both truncated at twice the minimum yield strength t

42、o help guard against excessive strain. Tabular values of allowable alternating stress are given in Table 1.4. FATIGUE OF WELDED 6061-T6 ALUMINUMThe fatigue behavior of aluminum alloy weldments has been surveyed by Sanders (1971), Sanders and Lawrence (1978), and Sanders and Day (1983). Weldment fati

43、gue data for 6061-T6 aluminum from Sanders and Day (1983) are compared with the design fatigue curves with no mean stress effect and with maximum mean stress effect in Fig. 1. This comparison seems to indicate that the design curve with maximum mean stress effect would be adequate for weldments.Cons

44、iderable testing on transverse butt joints in 9.5 mm (3/8 in.) thick 6061-T6 aluminum plate welded by the inert-gas tungsten-arc process using 4043 electrodes was reported by Hartmann, et al< (1954). The gage section was 139.7 mm (5.0 in.) wide and 393.7 mm (15.5 in.) long. The tests were conduct

45、ed in Alcoa's Aluminum Research Laboratories' Structural Fatigue Testing Machines which loaded the specimens axially with a tensile stress range from zero to a maximum stress (R=0). In general, the fatigue specimens with the weld bead left in the as-welded condition failed at the edge of the

46、 bead; however, a few specimens with very smooth weld bead failed in the heat-affected zone. The majority of the fatigue failures in specimens in which the weld bead was removed occurred in the heat- affected zone with about 20 percent of the failures occurring through the weld. Nordmark and Clark (

47、1964) extended those tests to include a number of different stress ratios. Comparison of data from Nordmark and Clark (1964) with the design curves in Fig- 8 shows that the design curve with maximum mean stress effects is not adequate for weldments subjected to cycles with high mean stress.It should

48、 be noted that fatigue allowables for welded 6061-T6 aluminum are included in specifications suggested by the American Society of Civil Engineers (ASCE) Task Committee on Lightweight Alloys (1962). The fatigue allowables in the ASCE specification were based on the data from Nordmark and Clark (1964)

49、. The ASCE design curves are compared with the proposed design fatigue curves in Fig- 9. Note that the ASCE fatigue design curve for welded joints with an R-ratio of 0.5 is in good agreement with the design curve with maximum mean stress effects and a weld reduction factor of two. Therefore, a weld

50、reduction factor of two must be applied to the fatigue design curves when they are applied to welds.The fatigue design curve reduced by a factor of two on stress is compared with the data from Sanders and Day (1983) and Nordmark and Clark (1964) for weldments in Fig. 10. The open point symbols are f

51、rom tests with no mean stress. The plus and x symbols are from tests with applied mean stress. As can be seen in Fig. 10, a weld reduction factor of two, i.e. dividing the allowable stress values by two, results in design curves that are adequately conservative.5. SUMMARYFatigue design curves have b

52、een developed for 6061-T6 aluminum and are given in Fig. 6 and Table 1. Mean stress effects were accounted for using the Gerber equation. A weld reduction factor of two is required when the fatigue design curves are applied to welds.6. ACKNOWLEDGMENTSC. R. Luttrelfs assistance in assembling the data

53、base is appreciated. Members of the ASME Boiler and Pressure Vessel Committee's Subgroup on Fatigue Strength chaired by W. J Oonnell provided many helpful suggestions and valuable guidance. Reviews by C. R. Brinkman, R. C. Gwaltney, and D. P. Jones are gratefully acknowledged. I thank Sherry Bye

54、rly for typing the manuscript. The support of this work by J. M. Corum, D. L. Selby, and C. D. West is gratefully acknowledged.10REFERENCESWes匚 C. D.t “Overview of the ANS Project/' Transactions American Nuclear Society. 57»1988, pp. 288-289.Harrington, R. M. /'Designing for Safety in t

55、he Conceptual Design of the Advanced Neutron Source,” Nuclear Sctfety, VoL 32, No. 2, April-June 1991."ASME Boiler and Pressure Vessel Code Section VIII, Rules for Construction of Unfired Pressure Vessels,n 1959 Edition, ASME, 1959."Tentative Structural Design Basis for Reactor Pressure Ve

56、ssels and Directly Associated Components (Pressurized, Water Cooled Systems), 1 December 1958 Revision, U.S. Department of Commerce, OTS Document No. PB151987.DOE Order 5480.4, Environmental Protection, Safety, and Health Protection Standards, May 15, 1984."6061 Aluminum,n Structural Alloys Han

57、dbook, Volume 3、Battelle, Columbus, Ohio,1989.Alcoa Structural Handbook, Aluminum Company of America, Pittsburgh, 1960.Military Standardization Handbook, Metallic Materials and Elements for Aerospace Vehicle Structures, Vol. 1, MIL-HDBK-5D, January 1,1984Cyrogenic Materials Data Handbook, Volume 1、A

58、FML-TDR-64-280, Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio, 1970.Brodrick, R. F. and Spiering, G. A., uLow-Cycle Fatigue of Aluminum Alloys J Journal of Materials VoL 7, No. 4, December 1972, pp. 515-526."'Criteria of the ASME Boiler and Pressure Vessel Code for Design by Analysis in Sections m and vm, Division 2 J ASME, 1969-Fuchs, H. 0. and Stephens, R. I., Metal Fatigue in Engineering, Wiley, 1980, p. 74.Woodward, A. R,Gunn, K. WM and Forrest, G'The Effect of Mean Stress on the Fatigue of Aluminum Alloys/1 Proceedings of the International Conferenc