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Stress Test Determines Long-Term Strength of Thermoplastic Resin Pipe-Fittings

F 2018, Standard Test Method for Time-to-Failure of Plastics Using Plane Strain Tensile Specimens, gives the requirements to determine the time-to-failure of thermoplastic resins for piping applications by uniaxial loading of a grooved tensile test specimen. Published in August, it was developed by Subcommittee F17.40 on Test Methods within Committee F17 on Plastic Piping.

The long-term strength capacity for plastic-fitting applications, especially those used in the gas industry, is obtained with the test method. “The mechanical fittings standard for plastics used in gas fittings now requires that a stress-rated material be used,” said Dale Edwards, vice president, Plastics Technology, Bodycote Broutman, Inc., Chicago, Ill., and chair of the group that created F 2018. “This method can be used to obtain that stress rating.

“This standard test method allows users to develop a stress rating for plastic materials without extruding or molding the material into a pipe for testing per ASTM D 2837 [Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials 4],” Edwards continued. “F 2018 is recognized as an alternate method for developing a stress rating by the Plastics Pipe Institute (PPI).”

Created by members of ASTM Committee F17, the F 2018 was also studied by a PPI Alternate Methods Committee. Government agencies such as the Department of Transportation, universities and test laboratories, resin suppliers, pipe manufacturers, and consultants to the plastic pipe industry were involved in its approval.

As stated in F 2018, the grooved-tensile specimen achieves a multi-axial stress condition that mimics the stress condition found in pressurized solid-wall plastic pipe. The ratio of the stress in the axial direction to the transverse direction approximates that for a pressurized solid-wall pipe specimen. When the specimens are analyzed according to D 2837, the data generates a long-term strength design value for the material.

As well as its use for pipe fitting materials, the test method can be applied to plastics used in long-term load-bearing applications by applying the design principles used successfully by PPI to stress-rate piping materials, Edwards said.

Technical questions may be directed to Dale B. Edwards, Bodycote Broutman, Inc., 3424 S. State St. , Chicago, Ill. 60616 (312/842-4100, ext. 338; fax: 312/842-3583). Committee F17 meets April 2-4, 2001, in Phoenix, Ariz. For meeting or membership information, contact Staff Manager Pat Picariello, ASTM (610/832-9720). //

Copyright 2000, ASTM