A Method for Assessing the Long-Term Strength of Unplasticized Poly Fittings


Lifetime Analysis # 2001 Munich

Fittings used in water and gas piping systems are rarely, if ever, explicitly rated for load carrying capability relative to internal pressure. In some cases the material or resin used to manufacture the fitting is stress-rated to a period of 100,000 hours (PPI TR3) or 50 years (ISO TR 9080 and FDIS 9080). This long-term stress is sometimes used in a manufacturer’s proprietary design equations or finite element modelling to estimate the expected strength of the fittings. In the absence of extensive finite-element analysis, the strength rating is generally based on previous design practices as well as short-term hydrostatic and material property tests on prototypes with conservative design factors. In this study, a methodology is proposed and evaluated for pressure-rating fittings using general design considerations and selective product testing. The general process consists of four main steps : 1. Develop stress-rupture data on the material, 2. Evaluate the fittings series to identify a “worst-case” configuration for testing, 3. Developing creep-rupture data on actual production fittings in this worst-case configuration and 4. Conducting validation testing on remaining configurations to confirm the assumption of least common denominator (worst-case). An additional step is also proposed for fittings used in construction of piping systems that experience significant cyclic pressures, such as irrigation applications. For purposes of completing some preliminary work in a reasonable time frame, the initial study is limited to PVC-u fittings with taper socket connections, tested at 23°C. Stress-rupture testing on the PVC-u material was conducted with moulded cylindrical specimens using standard pipe testing procedures (ASTM D 2837) as well as the plane-strain specimen geometry using method ASTM F 2018. The creep-rupture testing of the production fittings is conducted following a draft ASTM method, with some modifications to ensure failure of the fitting body rather than the joint. These modifications, as well as a comparison of data from fittings vs. material are discussed. A method for determining the least common denominator (LCD) fitting configuration is proposed, and demonstrated for one series of fittings through validation testing.

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