REG NASA-CR-242-1965 Some Considerations in the Fatigue Design of Launch and Spacecraft Structures.pdf

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1、i NASA CONTRACTOR NASA CR-242 REPORT 174 NfyJ Lb L O k (I ALES) COJE) 4 3/9- 2 INiSA CR OR TMX OR AD NUMBER) (CATEGORY) Hard copy (HC) Microfiche (MF) ;/ I ;y3 SOME CONSIDERATIONS IN THE FATIGUE DESIGN OF LAUNCH AND SPACECRAFT STRUCTURES by R. H, Christensen und R. J. Bellinfdnte Prepared under Cont

2、ract No. NAS 7-298 by DOUGLAS AIRCRAFT COMPANY, INC. Santa Monica, Calif. for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. JUNE 1965 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NASA CR-242 * SOME CONSIDERATIONS IN THE FATIGUE D

3、ESIGN OF LAUNCH AND SPACECRAFT STRUCTURES By R. H. Christensen and R. J. Bellinfante Distribution of this report is provided in the interest of information exchange. resides in the author or organization that prepared it. Responsibility for the contents Prepared under Contract No. NAS 7-298 by DOUGL

4、AS AIRCRAFT COMPANY, INC. Santa Monica, Calif. for NATIONAL AERONAUT ICs AND SPACE ADMINISTRATION For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - Price 84.00 Provided by IHSNot for ResaleNo reproduction or networking permitted without lice

5、nse from IHS-,-,-NOTICES When U. S. Government drawings, specifications, or other data are used for any purpose other than a definitely related Govern- ment procurement operation, the Government thereby incurs no responsibility nor any obligation whatsoever; and the fact that the Government may have

6、 formulated, furnished, or in any other way supplied the said drawings, specifications, or other data is not to be regarded, by implication or otherwise, as in any manner licensing the holder or any other person or corporation, or con- veying any rights or permission to manufacture, use, or sell any

7、 patented invention that may in any way be related thereto. This document may not be reproduced or published in any form in whole or in part without prior approval of the Government. iii Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-FORE WORD This

8、NASA contract, “Some Considerations in the Fatigue Design of Launch and Spacecraft Structures, I was initiated under Project No. NAS 7-298. The work was administered under the direction of T. V. Gooney, OAR for Spectrum Loadings Based on Henrys Equation Modified 58 Effect of Prior Preload 59 Growth

9、of Fatigue Cracks as a Function of Tempera- ture (Schematic) 63 Growth of Crack under Steady Load and Temperature 63 Fatigue Life as a Function of Test Temperature 64 Effect of Temperature 64 Effect of Frequency at Elevated Temperature 65 C-B Fatigue Diagram 66 Master Fatigue Diagram S-816 Alloy 67

10、Fatigue- Crack Growth under Programmed Loads 68 Delayed Fracture (Static Fatigue) of AIS1 4340 Steel in Water Environment (PH = . 5 to 1. 0) 70 Crack Growth in Inert Gases. Gas Pressures Noted (MM. HG. ) 70 Crack Growth of Titanium in Ozone 71 Fatigue of Ferromagnetic Material 71 Effect of Prior Irr

11、adiation on Unnotched 7075-T6 Aluminum Alloy (Rotating Beam) 73 Effect of Outgassing Time 73 Short Time Vacuum Tests (20 Hrs. ) Influence of Atmosphere on Fatigue Life 74 75 X Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-FIGURES 1. 2. 3. 4. 5. 6.

12、7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. Trends in Fatigue/Vibration Testing of Flight Vehicle Structure 9 Aerospace Structural Designs (Aluminum Alloy Construction) 16 Experience in Bracket Design Vibration Testing S-N Curve for Component X Extrapolated from Notched Coupon Data 19 16 Fatigue Diagra

13、m 20 Vibration Qualification Test for SIV Thrust Structure and Fuel Lines 22 Wave Shapes- -Qualification Specs 26 Vibration Levels Measured at Gimbal Point in SIV-5 Flight 27 External Sound Pressure Levels Measured on SI/SIV Interstage Flight 28 Example Histogram of Vibration Input to “Component XJ4

14、 during Expected Life 29 Flow Chart for Fatigue Life Prediction 31 Sinusoidal Vibration Qualification Specification 33 Random Vibration Qualification Specification 33 Steady State Transfer Functions 34 Response Load Levels for Sine Sweep and Sine Dwell 37 Random Response Load Levels 37 Parabolic Res

15、ponse Curve for Sine Sweep Damage Si mu la t ion 44 IX Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-c1. Biaxial SN Curves for 24ST Extruded Tubes (NACA 1889) 78 78 80 c2. c3. c4. Biaxial SN Curves for 14ST-4 Tubing Flaw Growth under Uniaxial and B

16、iaxial Loading Fracture Strength as a Function of Flaw Orientation 80 c5. Prediction of Fracture Strength of Flawed Structure under Biaxial Loading 81 83 C6. c7. S-IVB Hydrostatic Burst Test Specimen Notch Resistance as a Function of Material Ductility (For Fatigue- Cracked Structure) 84 86 D1. D2.

17、Sinusoidal and Random SN Data Fatigue Crack Growth under Discrete and Random Cyclic Loading 90 El. Boundary Between Catastrophic and Fracture- Safe Design 95 E2. Safe-Fracture as a Function of Reinforcement Area 95 E3. Optimum Weight vs. Structural Reinforcement for Fracture -Saf e Design 96 E4. Str

18、uctural Reinforcements Providing Fracture Arrest 97 100 100 F1. F2. F3. Life Variability in Fatigue Test Results Scatter of Fatigue Crack Growth Characteristics Scatter of Crack Growth Behavior in 3 Test Panels Beyond the Detectable Crack Stage 101 104 105 G1. G2. Strength of Structure as a Function

19、 of Life Pressure Vessel Structure xi Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SUMMARY Five years ago, metal fatigue was considered by many to be an unimportant problem in the design of space vehicle systems. Within this period, potential prob

20、lems were reviewed period- ically as experience in the operation of these vehicles increased. No fatigue design criteria have yet been formally documented for this new class of vehicles. Now, trends in future space system design, in addition to some current experience, dictate that the problem can n

21、o longer be neglected. This report is intended to supply background information useful in the design of space vehicle system structures. The report presents a definition of the fatigue problem as it relates to the strength of structure. knowledge in designing to prevent the occurrence of this unde-

22、sirable phenomenon. for use in design, test, and analysis are also reviewed. Several appendixes appear at the end of the report. pose is to serve as a checklist for the designer on those aspects A review of the appendixes will also reveal those areas in which of the problem that are often neglected

23、or are not well known. additional research is required. It briefly reviews present Current evaluation methods and guides Their pur- Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. strength of structure will occur if no concerted effort is made to a

24、pply test loads and environments in a realistic manner. Further, the fatigue strength evaluation of a vehicle, whether by analysis or by test, should be made early in its design and fabrication stage. approach, it is practical either to redesign or to reinforce marginal and sus- pect regions of a st

25、ructure before reaching the production stage of the vehicle. Careful attention to design details, coupled with the use of the most advanced structure stress -analysis techniques, will reduce the probability and the frequency of occurrence of underdesigned and fatigue-critical areas. With this When k

26、nowledge of the magnitude of loads, frequencies of loads, and effect of environments is uncertain, there is but one course to pursue. rated vehicles and for vehicles in which the retrieval of equipment and data is mandatory, the “fail-safe“ design philosophy should be used. philosophy, the necessary

27、 structural reinforcements and redundant members are incorporated into the design so that, should accidental rupture or fatigue cracking in structure take place, a safe rather than catastrophic fracture would occur. For man- In this An alternate design and analysis procedure for the evaluation of fa

28、tigue re- sistance of structure is termed the tlsafe-lifelt method. reliability of the fatigue resistance of structure is assured through knowledge that the vehicle either will be retired or will have accomplished its mission hiig Sefore the fatigue life =f its parts has heen reached. that the use o

29、f this design approach should be limited to very few cases and then only with extreme caution. carrying structure, the safe -life method is not recommended for use. In this concept, It is believed In the fatigue design of primary load- 4 Provided by IHSNot for ResaleNo reproduction or networking per

30、mitted without license from IHS-,-,-STATEMENT OF THE PROBLEM The whole subject of fatigue of structures is concerned with the fact that during the operation of a vehicle, fatigue cracks can be formed within its stressed members; once initiated, these cracks may propagate to critical dimensions. stru

31、cture are the cause of this damaging phenomenon and the eventual gener- ation of flaws. of highly stressed components of a vehicle structure with their time in serv- ice. This undesirable situation can result in catastrophic consequences Designing to prevent the occurrence of this phenomenon is ther

32、efore important if vehicles are to be economically built and yet possess adequate safety and construction that requires minimal repair. Repetitive loads experienced throughout the useful life of a The mechanism is accumulative and decreases the strength In order to assure integrity of structure and

33、a satisfactory life for vehicles, the spectra of loads and environments which the structure will encounter must be defined. definition for the majority of vehicles, are those induced by wind, shock, vibration, engine exhaust noise, cycling, pressurization, kinetic heating, operational heating, and a

34、tmospheric corrosion. (The above conditions are applicable to space vehicle systems. ) The major load and environmental parameters requiring Knowledge of the response of structure and of the behavior of materials of construction under these anticipated loads should then enable the designer to design

35、 safe vehicles. The normal practice has been to accomplish this task by accepted stress analysis methods in conjunction with laboratory fatigue tests on component parts. evaluate fatigue resistance of structure is to fatigue-test the entire vehicle. Regardless of the method chosen, it will be possib

36、le to make valid assess- ments of fatigue life only through accurate simulation of the environment the vehicle will encounter. structure of combined environments. experienced during the operation of vehicles is time dependent and this factor cannot be overlooked. Large errors in the prediction of fa

37、tigue life and fatigue An alternate but far more costly procedure to It often will be necessary to consider the effect on The effect of the many environments 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Experience in Space Vehicle System Designs

38、 Conversely, current service experience with launch vehicle and spacecraft structure is rather limited. Fortunately, this lack of experience has not affected the majority of static firings and launches made, since they have been successful. However, this past performance alone does not assure contin

39、ued success for all subsequent flights, structural testing and analysis techniques are required to maintain a high degree of structural integrity for future designs. position for those who have conjectured that fatigue of launch vehicle struc- ture is no problem because of its relatively short opera

40、tional life. conclusion erroneously may be deduced for long -time operational spacecraft with its infrequently and low stressed structure. rated space vehicle systems, coupled with the realization of the vastly dif- ferent and unexplored fatigue regimes, this problem becomes important; since the occ

41、urrence of structural fatigue in flight is certainly probable. Table I identifies various structural categories related to the newer fatigue modes that must now be considered. These are high load-high cycle and low load-low cycle regions for both extremely short- and long-time exposure s . Concerted

42、 efforts to develop It may be an unfortunate The same With the advent of man- At present, it is known that fatigue is dependent on time, environment, and load cycle; for these reasons alone, the probability of occurrence of fatigue in metals and structures should not be overlooked. for the present c

43、lass of space vehicle designs. any other philosophy. This is particularly true It would be suicidal to adopt Current Evaluation Methods The usual practice has been to define the fatigue characteristics of metals or of structural components by subjecting test specimens of the structural elements to r

44、epeated or alternating loads. By this procedure, design data are obtained to evaluate the effect of these alternating loads on structures. However, such fatigue programs have not always been wholly satisfactory for obtaining estimates of probable structure life or for defining rates of non-linear ac

45、cumulative damage to structures. One of the principal reasons 6 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-STATE OF THE ART General Before the mid-1930ts, the design of flight vehicles was based solely on the maximum load a structure could exper

46、ience once in its lifetime. guides used during this period fortunately and unknowingly proved to be reasonable fatigue design criteria. Few, if any, catastrophic failures of structure could be attributed directly to metal fatigue. However, progress in mans demand for vehicles possessing increased pe

47、rformance character- istics advanced rapidly during the 1940ts, and in the majority of cases, the design concepts required material allowables not yet defined and stress analysis techniques not yet developed. lagged the requirements of the designer, new materials eventually were pro- duced and more

48、exacting structural analysis methods were developed which, by degrees, evolved into designs possessing increased efficiency. The design Although development of materials However, a forced trend toward higher and higher working stresses soon occurred which resulted in decreased margins of safety. ris

49、e in the number of structural fatigue problems was experienced. the past 20 years there has been a gradual recognition of the fatigue problem and, today, the design of structural members, and especially structural joints, is no longer based on one maximum load. designed and analyzed for a satisfactory life