1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-06a71 Center Point of Contact: MSFCa71 Submitted by: Wilson HarkinsSubject: Controlling Stress Corrosion Cracking in Aerospace Applications Practice: This practice presents considerations that should be evaluated and appli
2、ed concerning stress corrosion and subsequent crack propagation in mechanical devices, structural devices, and related components used in aerospace applications. Material selection, heat treat methods, fabrication methodology, testing regimes, and loading path assessments are presented as methods to
3、 reduce the potential for stress corrosion cracking in a materials operational environment.Programs that Certify Usage: This practice has been used on Saturn IB, Saturn V, Lunar Roving Vehicle, Space Shuttle Solid Rocket Booster, Space Shuttle External Tank, Space Shuttle Solid Rocket Motor, Materia
4、l Experiments Assembly, Inertial Upper Stage, Skylab, High Energy Astronomy Observatory, Hubble Space Telescope.Center to Contact for Information: MSFCImplementation Method: This Lesson Learned is based on Reliability Practice No. PD-ED-1227; from NASA Technical Memorandum 4322A, NASA Reliability Pr
5、eferred Practices for Design and Test.Selection of materials, heat treating methods, fabrication methodologies, testing regimes, and loading paths that are not susceptible to stress corrosion cracking will promote fewer failures due to Stress Corrosion Cracking (SCC) and will eliminate downtime due
6、to the change-out of components.Implementation:Numerous materials have been tested for susceptibility to SCC in a 3.5 percent NaCl alternate immersion Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-bath, a 5 percent NaCl salt fog cabinet and a 90-10
7、0 percent relative humidity cabinet. These tests resulted in the development of a specification on Design Criteria for Controlling Stress Corrosion (reference 3). The information contained in the specification is based upon laboratory tests in which specimens were either sprayed with salt water or p
8、eriodically immersed and withdrawn; by exposure of the specimen to simulated seacoast or mild industrial environments; and by service experience with fabricated hardware. This specification also lists materials that have a high resistance, a moderate resistance, or a low resistance to SCC. MSFCs Mat
9、erial Selection List for Space Hardware (reference 4) also lists materials that have a high resistance to SCC.refer to D descriptionDrefer to D descriptionDrefer to D descriptionD To avoid failure, the tensile stress in service must be maintained at a safe level. Since stresses are additive, all sou
10、rces of stress (see Table 1) must be considered to ensure that the threshold stress (the stress level which will result in a failure if stress corrosion is present) is not exceeded. There is not an absolute threshold stress for Provided by IHSNot for ResaleNo reproduction or networking permitted wit
11、hout license from IHS-,-,-stress corrosion as there is with other material properties. Therefore, estimates of the stress corrosion threshold for a specific service application must be determined for each alloy and heat treatment by using a test piece, a stressing procedure, and a corrosive environm
12、ent that are appropriate for the materials intended application. A simplified stress corrosion test fixture with a round tensile specimen installed is illustrated on Figure 1 in a simulated corrosion environment. A masking material is applied to the test fixture to ensure that the specimen alone is
13、exposed to the corrosive environment. The tensile specimen is stressed to a desired level (typically 25, 50, 75 or 90 percent yield strength). The specimen is then submerged in a 3.5 percent NaCl alternate immersion bath, in a 5 percent NaCl salt spray (fog), or in a 90-95 percent relative humidity
14、test. Test duration is typically three months for low alloy steels and aluminum alloys, and six months for stainless steel.The most common processing methods for production of wrought metal are rolling, forging and extruding. These processing methods produce a granular structure which is parallel to
15、 the flow of metal. As shown on Figure 2, grain orientation is parallel to the longitudinal direction of rolling, extrusion or drawing. When thin shapes are rolled or extruded, grains are oriented in a short transverse or long transverse direction as shown on Figure 2. The resistance of metals to SC
16、C is always less when tension is applied in a transverse direction. It is least for the short transverse direction. Stress corrosion is aggravated when tensile stresses due to assembly have been applied in the short transverse direction. Table 2 lists typical materials and environments that may caus
17、e stress corrosion.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Technical Rationale:SCC is caused by the combined action of sustained tensile stress and corrosion which result in premature failure of materials. Certain mate
18、rials are more susceptible than others. If a susceptible material is placed in service in a corrosive environment under a tension of sufficient magnitude, and the duration of service is sufficient to permit the initiation and growth of cracks, failure will occur at a stress lower than the material w
19、ill normally be expected to withstand.References1. Hall, A. and Hongola, M: “Stress Corrosion Test Procedure Applicable to Lot Acceptance Testing and Qualification Testing for Forward Separation Bolt Assembly.“ Hi-shear Corporation, Ordnance Group, Torrance, CA, June 1, 1988.2. Fontana, Mars G.: “Co
20、rrosion Engineering.“ Third Edition, McGraw-Hill Book Company, New York, 1986.3. “Design Criteria for Controlling Stress Corrosion Cracking.“ MSFC-SPEC-522B, NASA/Marshall Space Flight Center, AL, September 30, 1988.4. “Material Selection List for Space Hardware.“ MSFC-HDBK-527F, NASA/Marshall Space
21、 Flight Center, AL, September 30, 1988.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-5. Crain, Bruce D.: “Handbook of Corrosion Data.“ ASM International, Metals Park, OH, August 1989.6. McEvily, A. J. Jr.: “Atlas of Stress Corrosion and Corrosion F
22、atigue Curves.“ ASM International, Metals Park, OH, 1990.7. Scully, J. C.: “The Fundamentals of Corrosion.“ Third Edition, Pergamon Press, Inc., Elmsford, NY, 1990.8. Torres, P.D.: “MSFC Corrosion Test Procedure Currently Followed for Testing Round Tensile Specimens.“ Memo No. EH24(92-24), NASA/Mars
23、hall Space Flight Center, AL, October 22, 1992.9. “Standard Practice for Evaluating Stress Corrosion Cracking Resistance of Metals and Alloys by Alternate Immersion in 3.5% Sodium Chloride Solution.“ ASTM G44-88, American Society for Testing and Materials, Philadelphia, PA, June 1988.10. “Standard P
24、ractice for Preparation of Stress Corrosion Test Specimens for Weldments.“ ASTM G58-85, American Society for Testing and Materials, Philadelphia, PA, November 1985, (reapproved 1990).11. “Standard Test Method for Salt Spray Testing.“ ASTM B117-90, American Society for Testing and Materials, Philadel
25、phia, PA, May 1990.Impact of Non-Practice: Failure to adhere to proven criteria for controlling SCC could result in hardware failure, which could result in schedule slippages, excessive resource expenditures, shortened mission life, mission failure, and, in extreme cases, loss of life.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-06a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
