SAE AIR 1412C-2016 Designing for Long Life with Elastomers.pdf

1、_SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising theref

2、rom, is the sole responsibility of the user.”SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions.Copyright 2016 SAE InternationalAll rights reserved. No part of this publi

3、cation may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE.TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada)Tel: +1 724-776-4970 (out

4、side USA)Fax: 724-776-0790Email: CustomerServicesae.orgSAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provide feedbackon this Technical Report, please visithttp:/www.sae.org/technical/standards/AIR1412CAEROSPACEINFORMATION REPORTAIR1412 REV. CIssued 1976-11Revised 2016-08Superseding AIR

5、1412BDesigning for Long Life with ElastomersRATIONALEThis document has been revised to update the template, update contact information within Applicable Documents, and to resolve a technical comment received during 28-day ballot in order to remove certain inaccurate / misleading language, prior to s

6、tabilization.FOREWORDThe properties of elastomers change with time and temperature; in some cases, these changes are substantial. As a result of long-term storage stability problems with some early elastomeric materials, the aerospace industry to a large extent has become accustomed to the applicati

7、on of age controls on O-rings, hoses, and certain other rubber products. This has proven to be very costly, time-consuming, and unwieldy. Additionally, elastomeric materials qualified for service based on the results of short-term simulation tests conducted only at service temperature extremes have

8、not always performed adequately in the field. Accelerated tests, when required, should be performed significantly above the continuous service temperature to provide a meaningful estimate of life at reduced temperatures.Replacement and reassembly of parts have been found to lower reliability. Mainte

9、nance on very complex aerospace products is difficult to carry out because of compactness of these products and disassembly required to gain access to seals or other rubber goods. The reliability and cost requirements of aerospace components are very high, hence short life, unreliable elastomeric pa

10、rts cannot be tolerated. Long life elastomers are available for use in aerospace designs. It, therefore, follows that designing for long life is a much more viable approach.The designer must convey a specific requirement to all concerned that he is building critical aerospace equipment intended for

11、long life and high reliability. This can be as straightforward as a detailed drawing note citing the life requirement in years and the expected environments. This overall requirement has to be backed up by specific elastomer material performance and mechanical property specification requirements. Mo

12、reover, the designer cannot assume that published specifications or proprietary callouts will automatically provide elastomeric performance to meet his specific needs.SAE INTERNATIONAL AIR1412C Page 2 of 71. SCOPEThis document lists those guidelines recognized as being essential for consideration by

13、 the designer who is preparing to select an elastomer as part of an aerospace design.1.1 PurposeTo provide guidelines to the aerospace designer in the testing and selection of elastomers so that long life service will be realized in critical components.2. APPLICABLE DOCUMENTSThe following publicatio

14、ns form a part of this document to the extent specified herein. The latest issue of SAE publications shall apply. The applicable issue of other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references cited

15、 herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained.2.1 ASTM PublicationsAvailable from ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2

16、959, Tel: 610-832-9585, www.astm.org.ASTM D2990 Tensile, Compressive, and Flexural Creep and Creep Rupture of PlasticsASTM D3045 Heat Aging of Plastics Without LoadASTM D6147 Vulcanized Rubber and Thermoplastic Elastomer Determination of Force Decay (Stress Relaxation) in Compression2.2 ISO Publicat

17、ionsCopies of these documents are available online at http:/webstore.ansi.org/.ISO 3384 Rubber, vulcanized Determination of stress relaxation in compression at ambient and at elevated temperaturesISO 6056 Rubber, vulcanized or thermoplastic Determination of compression stress relaxation (rings)2.3 U

18、L PublicationsAvailable from UL, 333 Pfingsten Road, Northbrook, IL 60062-2096, Tel: 847-272-8800, .UL 746B Polymeric Materials: Long Term Property EvaluationsSAE INTERNATIONAL AIR1412C Page 3 of 72.4 Related PublicationsThe following publications are provided for information purposes only and are n

19、ot a required part of this SAE Aerospace Information Report.F. R. Eirich, ed., “Science and Technology of Rubber“, Academic Press, New York, 1978.A. V. Tobolsky and H. F. Mark, ed., “Polymer Science and Materials“, John Wiley and Sons, New York, 1980.H. Liebowitz, ed., “Fracture - An Advanced Treati

20、se“, Vol. VII, Academic Press, New York, 1972.H. F. Mark and N. G. Garylor, N. J. Bikales, ed., “Encyclopedia of Polymer Science and Technology“, Vol. 8, p. 419 complete tight cures give the best set resistance. However, for economic reasons, production cure cycles are kept as short as possible. The

21、 designer must insist on a thoroughly cured seal, controlled by invoking a specific accelerated compression set test and by running lot acceptance tests on actual production parts. Utilizing test parameters appropriate to the generic polymer base for the compound, the compression set for critical pa

22、rts as a general guide should preferably be less than 25% but most generally be less than 35%. With a few specific types of elastomers, such as the fluorocarbons, room temperature compression set tests must be run in addition to accelerated temperature tests. Accelerated tests using Arrhenius aging,

23、 may be run using ASTM D3045 or ASTM D2990, or alternately, UL 746B as guidelines. In addition, ISO 3384 and ISO 6056 reference stress relaxation in compression.4.5 High Initial Tensile and Elongation PropertiesThe higher the dilution of the elastomer with fillers and the poorer the strengthening ac

24、tion of the specific class of reinforcing fillers, the lower the strength and elongation of the resulting compound. Similarly, high dilution with plasticizers will reduce mechanical properties. Hence, tensile strength and elongation are primarily an index of quality rather than properties for use in

25、 stress analysis. Low strength (low quality) elastomer compounds generally age poorly. Further, high quality (high strength) compounds have better abrasion and tear resistance and can degrade to a greater extent and for much longer periods of time and still remain functional. Extensive studies have

26、shown that with many elastomers the tensile stress at 100% elongation will approximately double in about three years of aging at room temperature and will remain fairly constant from three years to ten years and beyond. Hardness does not measure this effect. Elongation drops continuously with aging

27、time. Many concepts consider 100% elongation as the end point because the material becomes too rigid and inelastic to be useful in most elastomeric applications.4.6 Cut, Tear, and Abrasion ResistanceA surprising number of elastomer items fail through cutting, tearing, or abrasive action. This is par

28、ticularly true of room temperature vulcanization (RTV) silicone materials, which are also deficient in high initial properties and compression set resistance. Certain non-RTV, high temperature press cured silicones also tend to be deficient in cut, tear, and abrasion resistance. The newest products

29、based on advanced heat cured silicone rubber technology overcome the cut and tear problem, provided specifications requiring them are invoked by design. They are still inferior in abrasion resistance. Hence, silicone and fluorosilicone elastomers should not be used in dynamic seals. Low quality seal

30、s based on other polymers may also be deficient in cut, tear, or abrasion resistance.4.7 Special EnvironmentsThe elastomer compound must be compatible with the appropriate special environments in addition to those noted. The resistance of elastomers to fungus growth depends upon additives, such as e

31、xtender oils and plasticizers, in addition to the base polymer. Therefore, only tests on specific compounds have any meaning. The resistance of elastomers to water, humidity, and nuclear radiation varies with the base polymer and the specific formulation. Antirads, similar to the antiozonants, are a

32、vailable for specific types of elastomers as a means of improving radiation resistance. Materials meant for low outgassing in space must be formulated without use of high vapor pressure ingredients.Removal of low molecular weight polymers is also helpful. This may be done by molecular distillation o

33、f the polymer, by heating the uncured polymer, by severe postcures of cured elastomer compounds, and by vacuum stripping at high temperature. The last is most employed but is the least cost-effective and is “after the fact.“4.8 Functional TestingIt is possible to artificially age specified hardware

34、containing elastomeric material by using time-temperature heating cycles appropriate to the generic class of rubber and the design limits of the hardware involved. This provides for significant deterioration of the elastomer resulting in high set and increased tensile stress in the seals, diaphragms

35、, etc. in a reduced time frame. Degradation of the functional properties of the component or system can then be measured and decisions made as to acceptability of such changes. Functional tests as described are separate from accelerated aging tests conducted on the rubber alone but can follow simila

36、r protocols to estimate lifetime at lower temperatures. Functional tests are the only means to simulate the complex stresses imposed on elastomeric components. These tests would be in addition to normal hardware qualification tests.SAE INTERNATIONAL AIR1412C Page 7 of 74.9 Quality and Product Contro

37、lInadequate quality control is a major contributor to inadequate material life. Drawing requirements and specifications, however adequate, do not guarantee that the designated material will be the material received and that it will perform as intended. Quality and product control are achieved by enf

38、orcing compliance to the receiving inspection requirements of the specification.4.9.1 In reference to packaging and storage of rubber parts, optimum protection against circulating air, sunlight, fuel, oil, water, dust, and ozone may best be accomplished through use of sealed, polyethylene bags in co

39、vered containers or sealed poly-lined Kraft bags. Inside storage of items so packaged at ambient temperatures not exceeding 120 F (49 C) 100 F (38 C) preferred will minimize deteriorating effects of adverse environmental conditions.5. SUMMARYMany of the failures experienced in components and systems

40、 containing aged rubber items can be eliminated entirely, or at least delayed by avoiding known susceptibilities, using high quality materials in a compatible design, enforcing specifications, requiring accelerated aging of components in hardware tests, and employing adequate quality and product con

41、trol programs. Assistance in meeting the objectives may be obtained from material engineers versed in the technology of elastomers, from the literature or from representatives of manufacturers of elastomer products for the aerospace industry.6. NOTES6.1 Revision IndicatorA change bar (l) located in

42、the left margin is for the convenience of the user in locating areas where technical revisions, not editorial changes, have been made to the previous issue of this document. An (R) symbol to the left of the document title indicates a complete revision of the document, including technical revisions. Change bars and (R) are not used in original publications nor in documents that contain editorial changes only.PREPARED BY AMS COMMITTEE “CE”