1、Lessons Learned Entry: 1010Lesson Info:a71 Lesson Number: 1010a71 Lesson Date: 1997-02-01a71 Submitting Organization: HQa71 Submitted by: David M. LengyelSubject: Space Shuttle Program/Super Light Weight External Tank/Materials & Test and Verification Changes Description of Driving Event: Lower than
2、 Anticipated Fracture Toughness of the 2195 Aluminum-Lithium Alloy Used in the SLWT at Cryogenic Temperatures Causing Changes to Test & Verification ProceduresLesson(s) Learned: The 2195 aluminum-lithium alloy used in the tank walls and domes of the new Super Light Weight Tank (SLWT) has a lower fra
3、cture toughness at cryogenic temperatures than was anticipated in the design. To compensate for this potentially critical short-coming, NASA has limited the pressure used in the full tank proof test and has recognized that acceptance of each SLWT for flight is highly dependent on far more stringent
4、quality control of the materials and processes used to manufacture the SLWT than is required for the current external tanks.Recommendation(s): 16a. Assure that the acceptance tests of the 2195 material and the quality control procedures used in the manufacture of each SLWT continue to be sufficientl
5、y stringent, clearly specified, conscientiously adhered to, and their use unambiguously documented.16b. The criticality of these quality control operations makes it mandatory for NASA to retain buyoff of the results of those fabrication operations and tests that are essential in determining SLWT saf
6、ety.16c. As quality control data on the size of flaws detected in 2195 aluminum-lithium material are collected, they should be used in an updated analysis of the SLWT structure, because it may permit the verifiable spread between flight limit stress and proof stress to be raised above that presently
7、 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-reported.Evidence of Recurrence Control Effectiveness: 16a. The SSP and Marshall Space Flight Center (MSFC) will continue to ensure that material acceptance testing and the quality control procedures u
8、sed in the manufacturing of SLWTs are of a sufficient quality to validate that each tank is fully in compliance with all program requirements and is safe to fly.16b. The SSP and MSFC will retain NASA approval of the quality control program and changes to that baseline.16c. The simulated service data
9、base has been developed from data collected on fracture specimens with flaws that are 0.175 inch long. The data verify a 2.9% positive spread between the flight and proof-test conditions. Using the demonstrated flaw detectability level for our nondestructive evaluation dye penetrant process (0.086 i
10、nch long) would increase the spread to approximately 14%. Because of uncertainties, it is NASAs standard policy to use a factor of two on our flaw detectability limit. This methodology provides the proper risk allocation between the nondestructive evaluation capability and proof-test levels. The use
11、 of a flaw size of 0.175 inch for the simulated service tests is conservative for the SLWT.Documents Related to Lesson: N/AMission Directorate(s): a71 Space Operationsa71 Exploration SystemsAdditional Key Phrase(s): a71 Aerospace Safety Advisory Panela71 Cryogenic Systemsa71 Energetic Materials - Ex
12、plosive/Propellant/Pyrotechnica71 Hardwarea71 Parts Materials & Processesa71 Policy & Planninga71 Risk Management/Assessmenta71 Spacecrafta71 Standarda71 Test & VerificationProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Additional Info: Approval Info: a71 Approval Date: 2001-11-15a71 Approval Name: Bill Loewya71 Approval Organization: QSa71 Approval Phone Number: 202-358-0528Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
