1、Lessons Learned Entry: 2038Lesson Info:a71 Lesson Number: 2038a71 Lesson Date: 2008-11-25a71 Submitting Organization: JPLa71 Submitted by: David Oberhettingera71 POC Name: Paul Willis; Gustav Forsberga71 POC Email: Paul.B.Willisjpl.nasa.gov; Gustaf.A.Forsbergjpl.nasa.gova71 POC Phone: 818-354-6998 (
2、Willis); 818-354-1191 (Forsberg)Subject: To Bond or to Bolt, That is the Question Export Version Abstract: Following vibration test and bakeout of the OSTM Advanced Microwave Radiometer (AMR) Reflector Structure Assembly (RSA), it was discovered that thermal stress had caused delamination in the joi
3、nt between metallic fittings and the graphite composite skin of the RSA. The joints were subsequently reinforced with bolts. Although adhesive bonding provides an elegant design solution for joining spacecraft structures and other components, alternative joining methods such as mechanical fasteners
4、should be considered where environmental stress analysis suggests uncertainty in the safety margins. Evaluate thermal stress of bonded joints early in the design phase, and perform coupon tests that accurately represent the joint application.Description of Driving Event: The Ocean Surface Topography
5、 Mission features an Advanced Microwave Radiometer (AMR) instrument, built by the NASA/Caltech Jet Propulsion Laboratory (JPL), which measures water vapor in the Earths atmosphere. Following vibration test of the AMR at a contractor facility, inspection revealed a small region of de-bonding between
6、a titanium fitting and the graphite composite skin of the radar Reflector Structure Assembly (RSA) (Reference (1). After a bakeout of the AMR RSA that occurred 3 weeks later, inspection found a similar delamination between a pair of RSA pallet fittings and the graphite composite skin of the pallet (
7、Figure 1). Thermal stress analysis traced the failures to thermal stresses induced during thermal cycling, which then propagated during subsequent vibration testing and thermal vacuum testing (Reference (2). Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IH
8、S-,-,-Figure 1 is a color photo of flight equipment in a laboratory environment (JPL high bay). A complex composite structure that is gray in color and shaped somewhat like an open clamshell is supported by 6 tubes that look like shock absorbers. A metallic fitting or bracket joins each of these cyl
9、indrical devices similar to shock absorbers to the side of the pallet-shaped base of the clamshell-like composite structure. A red circle has been added to highlight the joint. No de-bonding or damage is readily evident from the photo.Figure 2 is a color photo that provides a close-up of Figure 1, r
10、evealing the details of the juncture of the top of one of the cylindrical tubes to (approximately the midsection of) the pallet. The metal bracket fills most of the image, except unlike Figure 1 two metal shims are visible. These shims are very thin sheets of film or sheet metal inserted between a p
11、ortion of the bracket and the pallet, illustrating a delamination that itself is not visible.Figure 1.Inspection at JPL of RSA de-bond/crack area. (On right side fixture, disbond/crack observed that moved under slight load; on left side fixture, disbond/crack observed that moved under load.)Figure 2
12、.Close-up of right side fixture (circled in Figure 1) with 1 mil shims inserted into micro-crack area. (No shim would fit into left side fixture.)The failure mechanism was a mismatch in the coefficients of thermal expansion (CTE) for the bonded joints between the titanium fittings and the graphite s
13、tructure at low temperature. The remainder of this paragraph describes the inadequate inspection methods performed after the first delamination and the results of the 80x bondline inspection, ultrasonic imaging, and finite element method (FEM) thermal stress analysis used to characterize the damage
14、after the second incident. It has been redacted for International Traffic in Arms Regulations (ITAR) compliance. “U.S. Persons“ may obtain a copy of the complete document by contacting the JPL Office of the Chief Engineer (David Oberhettinger at davidonasa.gov). The RSA flight hardware was reworked,
15、 with titanium bolts added to secure all metallic-to-composite bonded joints. In addition, JPL has added institutional design rules (References (3) and (4) that specifically require projects to design flight structures with specified safety factors for thermally-induced loading over the allowable qu
16、alification/flight temperature ranges. These structural design rules are more specific about the required safety factors to use for structural design when it comes to analyzing different CTEs of the structure. References: 1. “OSTM AMR RSA S/N 001 Local De-Bond Around ESS Fitting,“ Problem/Failure Re
17、port No. 4481, July/July Jet Propulsion Laboratory, May 3, 2006.2. “OSTM AMR RSA S/N 001 Local Dis-Bond Around Pallet Fitting,“ Problem/Failure Report No. 4579, July/July Jet Propulsion Laboratory, May 26, 2006.3. “Design, Verification/Validation and Operations Principles for Flight Systems (Design
18、Principles),“ JPL Document D-17868, Rev. 3, December 11, 2006, Paragraph 4.2.5.7.4. “Design, Verification/Validation and Operations Principles for Flight Systems (Design Principles),“ JPL Document D-17868, Rev. 3, December 11, 2006, Paragraph 4.2.5.5.Lesson(s) Learned: The full text of the 5 lessons
19、 that were learned (below) has also been redacted for ITAR compliance. “U.S. Persons“ may obtain a copy of the complete document by contacting the JPL Office of the Chief Engineer (David Oberhettinger at davidonasa.gov). 1. Use of adhesive bonding for joining spacecraft fittings to structural compon
20、ents provides an opportunity for elegant design solutions, but such bonds are subject to thermal stress and may be difficult to properly characterize and inspect. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2. This redacted lesson learned provide
21、s cautions regarding alternative methods of joining, such as fasteners, that may provide an additional margin of safety. 3. This redacted lesson learned discusses the benefits of alternative methods of thermal stress analysis for bonded joints. 4. This 4th redacted lesson learned provides guidelines
22、 for design and test of test coupons. 5. This redacted lesson learned provides a material properties criterion for selecting a method for joining mechanical components that does not overstress the hardware.Recommendation(s): 1. Evaluate thermal stresses and stress relief early in the design phase of
23、 hardware, but particularly where adhesive bonding will be used for joining spacecraft fittings. Approaches should not be limited to thermal cycle testing but could include other options for strain distribution in a bonded joint. 2. Consider alternative joining methods for attaching fittings to spac
24、ecraft structures. For example, mechanical fasteners may serve as a substitute or reinforcement for bonding where there is uncertainty in analyses of the integrity of the bonded joint design. 3. Conduct thermal stress analysis using an FEM whenever composite structures are to be joined with material
25、s having a significantly different CTE. 4. Perform coupon tests that accurately represent the joint application in configuration, stiffness and environmental conditions. 5. Evaluate the materials for both their spaceflight and ground test environments.Evidence of Recurrence Control Effectiveness: JP
26、L has referenced this lesson learned as additional rationale and guidance supporting Paragraph 4.2.5.5 (“Mechanical Configuration/Systems Design: Structural Design - Thermally-Induced Loading Factor of Safety“) and Paragraph 4.2.5.7 (“Mechanical Configuration/Systems Design: Structural Design - Desi
27、gn Temperature Range for Spacecraft Structure“) in the JPL standard “Design, Verification/Validation and Operations Principles for Flight Systems (Design Principles),“ JPL Document D-17868, Rev. 3, December 11, 2006.Documents Related to Lesson: N/AMission Directorate(s): a71 Exploration Systemsa71 S
28、cienceAdditional Key Phrase(s): a71 0.a71 0.a71 0.a71 1.Spacecraft and Spacecraft Instrumentsa71 0a71 0a71 0.a71 1.QualityProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 1.Reliabilitya71 1.Review systems and boardsa71 1.Flight Equipmenta71 1.Hard
29、warea71 1.Parts, Materials, & Processesa71 1.Payloadsa71 1.Spacecrafta71 1.Test & VerificationAdditional Info: a71 Project: OSTM/Jason 2Approval Info: a71 Approval Date: 2009-07-09a71 Approval Name: mbella71 Approval Organization: HQProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
