1、Lessons Learned Entry: 1038Lesson Info:a71 Lesson Number: 1038a71 Lesson Date: 2001-08-22a71 Submitting Organization: WSTFa71 Submitted by: Jess WallerSubject: Optimized pre-forming of thermoplastic polymer parts Description of Driving Event: The Primary Reaction Control System (PRCS) pilot-operated
2、 valve (POV) controls the flow of hypergolic liquid propellants, nitrogen tetroxide (oxidizer) and monomethylhydrazine (fuel), to the Space Shuttle Orbiter attitude control thrusters. The POV has experienced numerous in-flight and ground turnaround failures, including leakage and failure to open upo
3、n command. Corrosion in the oxidizer valves and seal extrusion in the fuel valves have been linked to most of these failures. The desire to mitigate corrosion and seal extrusion led to an effort to develop a redesigned POV (RPOV) pilot seat assembly.Preform Size. Semitrapped PTFE seals, such as the
4、PRCS POV pilot seal, will undergo coldflow during service 1. This phenomenon can help to replenish lost material at a sealing interface or heal seal imperfections 2. However, if the preform used during fabrication is too small, gaps may form in the seal cavity during fabrication that will cause the
5、seal to recede into the seal cavity during qualification testing or service, causing loss of seal-to-poppet contact and ultimately leakage. Conversely, if the preform is too large, the compressive yield point of the PTFE can be exceeded during interference fitting of adjacent metal and plastic parts
6、, resulting in irreversible plastic deformation and associated property loss. In general, the compressive yield strain of thermoplastic polymers such as PTFE is of the order of several percent. The interference fit used during initial attempts to fabricate a RPOV pilot seal (17 to 26 percent) exceed
7、ed the yield point of PTFE by a factor of about 5 to 7, resulting in the formation of translucent PTFE in regions of highest internal stress (Figure 1).Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDFigure 1. Sectioned initial
8、 attempt RPOV seal made with Algoflon E-2 polytetrafluoroethylene showing translucent (plastically deformed) and opaque regions (normal appearance).By comparison, the interference fit used in POV and final RPOV pilot seal fabrication (7 percent) exceeded the yield point of PTFE by a factor of about
9、2.The use of an overly large seal preform can have another adverse consequence. Excessively oversized preforms give rise to higher levels internal stress within semitrapped parts. This frozen-in-stress will relieve over time or during thermal cycling, and could be manifested as unacceptable extrusio
10、n during service.Preform Shape. If the preform is shaped incorrectly relative to the metal cavity dimensions (producing non-uniform squeeze within the seal), an internal stress gradient within the semitrapped PTFE seal can result. Depending on the magnitude and direction of the gradient, either rece
11、ssion or extrusion of the seal could result. The use of correctly shaped seal preforms that minimize the squeeze gradient within the seal is recommended. This could be accomplished in the RPOV by maintaining equivalent radial and axial seal squeezes during fabrication.Exit Area/Volume Ratio. A lower
12、 seal exit area/entrapped seal volume (A/V) ratio was present in the initial and final RPOV designs (A/V = 0.22 mm-1) compared to original POV design (A/V = 0.35 mm-1). A differential expansion model for an annular, semitrapped seal like that in a POV configuration showed that twofold reduction in t
13、he A/V ratio will result in double the extrusion during heating 3. Therefore, the highest practical A/V ratio should be used in semitrapped seal design.Unoptimized Design Consequences. Evidence of undesirable structural and morphological changes was obtained during initial RPOV seal fabrication. Mac
14、hined PTFE seals had a mottled, splotchy appearance in areas where the greatest amount of seal squeeze had occurred. This material was especially prone to fracturing and fibrillation during trimming. Subsequent sectioning of the seat assembly revealed PTFE material with a translucent appearance in t
15、he region of highest interference Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-fit, while the remaining material in the seal retained a normal opaque appearance (Figure 1). One factor thought to be responsible for the formation of the translucent
16、material was excessive shearing accompanied by irreversible property loss. Evidence of irreversible property loss was supported by decreasing Shore durometer hardness of PTFE test coupons with increasing squeeze (percent strain) (Figure 2). Hardness reductions in semicrystalline thermoplastics such
17、as PTFE are generally regarded as indicative of irreversible deformation modes within crystalline lamellae involving chain slip and tilt at lower strains, followed by lamellar shear and crystal destruction at larger strains 4.refer to D descriptionDFigure 2. Effect of increasing percent squeeze on t
18、he Shore A durometer hardness of polytetrafluoroethyleneReferences:1. Singleton, K. B., Cold Facts about Cold Flow, The Journal of Teflon Reprint No. 39, DuPont and Nemours and Company, Plastics Department, Wilmington, DE.2. Wichmann, H., TF E Seat Evolution Report. ISC Report 90201, PO T-92270, NAS
19、A Johnson Space Center White Sands Test Facility, Las Cruces, NM, February 1999.3. Waller, J. M., Factors Contributing to the Extrusion of Monomethylhydrazine Pilot Operated Valve Seals, TR-WSTF-960-001, NASA-JSC White Sands Test Facility, June 1, 2000.4. Balta-Calleja, F., Martinez-Salazar, J., Rue
20、da, D. R., “Hardness,“ Polymers, An Encyclopaedic Sourcebook of Engineering Properties, J. Kroschwitz, Ed., Wiley & Sons, 415, 1987.Lesson(s) Learned: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Pre-forming of piece parts made from thermoplastic
21、polymers requires special care. Geometrical design criteria must not take precedence over polymer property limitations. For example, never exceed the elastic limit of a polymer unless some property degradation can be tolerated. Also, preclude fabrication of nonviable parts by anticipating adverse co
22、nsequences of poorly optimized preform size and shape.Recommendation(s): When designing with PTFE, the tendency of PTFE to undergo creep and cold flow must be considered. While a positive interference fit is necessary to avoid gap formation and seal recession in semitrapped seals, the interference f
23、it should remain close to the elastic limit of PTFE so that key mechanical properties are not changed irreversibly. The use of smaller, properly proportioned preforms may benefit long-term seal performance by reducing the magnitude and gradient of internal stress, and thus minimizing the tendency of
24、 seals to extrude. Internal stress could be further minimized by a slower strain rate when tightening down on the retainer via the hot-forming fixture during seat assembly, or by post-assembly annealing.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Director
25、ate(s): a71 Exploration Systemsa71 Aeronautics ResearchAdditional Key Phrase(s): a71 Hardwarea71 Parts Materials & Processesa71 Research & Developmenta71 Test & VerificationAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2001-12-11a71 Approval Name: Jan Jungewaeltera71 Approval Organization: WSTFa71 Approval Phone Number: 505-524-5161Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
