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4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/AIR4738A AEROSPACE INFORMATION REPORT AIR4738 REV. A Issued 1992-10 Revised 1998-
5、07 Reaffirmed 2013-09 Superseding AIR4738 Hardness Testing of Elastomeric O-Rings RATIONALE AIR4738A has been reaffirmed to comply with the SAE five-year review policy. FOREWORDHardness measurements are widely used in industry as a simple way of determining the elastic modulus of elastomers. Hardnes
6、s tests are referenced in specifications for the following:a. To specify elastic modulus (stiffness)b. As a quality assurance measurec. To determine property changes due to time, heat, or fluid immersionStandard test methods have been developed which utilize a flat, parallel faced specimen of a mini
7、mum thickness, but for quality assurance and property changes it is often desirable to test production O-rings. Three standard methods for testing the hardness of O-rings have been developed and are described in ASTM D 1414, ASTM D 1415, and ASTM D 2240. Test apparatus has also been developed specif
8、ically to test O-rings but standard test methods are not available for use with the equipment. The test instruments are generally known as microhardness testers. Hardness measurements on O-rings do not necessarily give the same results as those obtained on standard flat test pieces and the reproduci
9、bility of results is inferior.1. SCOPE:This SAE Aerospace Information Report (AIR) provides a general description of methods for hardness testing of O-Rings including factors which affect precision and comparison of results with those obtained in standard tests.1.1 Purpose:To provide information on
10、methods for hardness testing of O-Rings. 2. REFERENCES:2.1 ASTM Publications:Available from ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959.ASTM D 1414 Testing Rubber O-RingsASTM D 1415 Rubber Property - International HardnessASTM D 2240 Rubber Property - Durometer Hardness2.2 Other Pu
11、blications:R. P. Brown, Physical Testing of Rubbers, Elsevier Applied ScienceRubber Manufacturers Association, 1400 K Street, NW, Washington, DC 20005RMA Bulletin OR-5 O-Ring Hardness3. DISCUSSION:3.1 Test Methods:Due to the difficulty of preparing a parallel faced flat specimen from an O-Ring, test
12、s are conducted on the curved surface of the ring. To minimize testing error, it is essential that the apparatus provide:a. A means of providing a constant load on the indentorb. Control of the rate at which the indentor contacts the surface of the O-Ringc. A jig to prevent the ring from moving late
13、rallyd. A method of aligning the indentor above the thickest cross-section of the O-RingTest apparatus is available which meets the above criteria and a suitable jig is described inASTM D 1414.3.2 Comparison of Standard and Microhardness Tests on O-Rings:When using a microhardness test instrument on
14、 O-Rings, the results obtained are not necessarily the same as those obtained on the same compound using a standard test instrument on hardness buttons. Variations in the results between the two methods are influenced by a number of factors.SAE INTERNATIONAL AIR4738A Page 2 of 4_ 3.2 (Continued):a.
15、The indentation force and depth of penetration into the specimen are much less with a microhardness test than with a standard hardness test. Because of these variations in test conditions, the difference in surface and interior hardness of the specimen becomes an important factor in the hardness det
16、ermination. The effect is particularly pronounced on O-Rings which have been post cured in an oven after press cure. The surface may be harder or softer than the interior depending upon the type of cross-linking, oxidation, or reversion which has occurred.b. Test specimen thickness has an effect on
17、test results with both standard and microhardness tests. Thinner test specimens give higher readings. Since a range of O-Ring cross sections may be tested, the results will show more variability than those obtained on a standard thickness test button. Recommendations in ASTM D 1414 are for a minimum
18、 cross sectional thickness of.25 in (6.4 mm) for a standard durometer hardness test. For cross-sectional thickness under .25 in (6.4 mm) the microhardness test of ASTM D 1415 can/should be used. ASTM D 1414 also provides a description of a holding fixture for O-Rings during testing and a modificatio
19、n for using two plied O-Rings when cross sectional thickness is under .13 in (3 mm). However, plyingO-Rings leads to additional variation in test results.c. In both standard and microhardness testing it is essential that the indentor contact the test piece at right angles to the surface and in the c
20、enter of the specimen. The microhardness testers commercially available are provided with systems to ensure correct alignment and centering. Regular maintenance and calibration of hardness test equipment is essential for retention of proper alignment.d. The hardness change in O-Rings measured after
21、fluid immersion, heat age, or functional tests is carried out to predict service life in conjunction with other tests. The results obtained may be different from standard hardness tests on buttons. Oxidation and other chemical reactions are more pronounced at the surface of the test specimen, partic
22、ularly in short-term tests. If a microhardness tester is used, where the results are greatly influenced by the surface hardness of the test piece, differences from standard tests can be significant, for the same reasons as in 3.2(a).e. For practical purposes, it is normally assumed the durometer A s
23、cale (ASTM D 2240) and the IRHD scale (ASTM D 1415) are similar. This assumption is true for highly elastic rubber (e.g., natural rubber) but for rubber with lower resilience (e.g., nitrile, fluorocarbon), differences of up to 10 points may occur. This applies for both standard and microhardness tes
24、ts and is caused by differences in application of force and time of readings.It is important to recognize this difference because most rubber specifications use the durometer A scale (ASTM D 2240) to specify hardness but many laboratories use an IRHD microhardness tester (ASTM D 1415) to test produc
25、tion O-Rings. Hardness limits on control tests may require adjustment for different test procedures.SAE INTERNATIONAL AIR4738A Page 3 of 4_ 3.3 Precision:Precision data on hardness testing are given in ASTM D 1415, ASTM D 2240, and RMA BulletinOR-5. Most of the published precision data are several y
26、ears old. Manufacturers of more recently developed test instruments show a much higher degree of precision than previously published information.3.4 Discussion:Hardness testing of O-Rings is desirable for both research and development and quality assurance testing. It is important to recognize that
27、the results obtained on O-Rings may be different from those obtained on standard hardness tests using flat specimens. Factors involving test piece geometry, surface cure effects, and indentor penetration must be taken into account when interpreting results and establishing specification limits.PREPARED BY SAE AMS COMMITTEE CESAE INTERNATIONAL AIR4738A Page 4 of 4_
