Nicholas Piazza, Bill VanHoose, P.E.
Papers # 2014 Chicago
In recent years, ASTM D6992  has been and continues to be referenced as an acceptable test method for creep phenomenon in corrugated pipes. Example product specifications include AASHTO M330 , ASTM F2418 , ASTM F2736 , ASTM F2947 , ASTM F2764 , Washington DOT Standard Practice T 925 . Traditionally, ASTM D2990  has been used for testing of creep phenomenon of viscoelastic materials. While ASTM D6992 continues to gain favor due to speed and economics of this method, additional test data showing the correlation between traditional creep testing and accelerated creep testing is warranted. This paper presents comparative test data for traditional creep testing, ASTM D2990, and accelerated creep testing, ASTM D6992, for an extrusion grade high-density polyethylene resin and an extrusion grade polypropylene resin.
In order to obtain long-term material properties (i.e. 50, 75, 100 years) from creep testing, a method of superposition is typically used to scale and shift creep test data to generate a master curve in which the time scale extends to 50 years or greater. Historically, conventional creep test methods require the testing of multiple specimens at different stress levels or temperatures, and the duration of these tests can exceed 10,000 hours to obtain the data set required to extrapolate the long-term design life. ASTM D6992 describes the Stepped Isothermal Method (SIM) in which time-temperature superposition is used to generate the long-term creep strain and creep modulus of a material under a constant stress. The test duration of this method is typically less than 24 hours and can generally provide the data required to extrapolate beyond 100 years. While SIM has been well established as a method for predicting long-term strains in geosynthetics and some thermoplastic resins including polyethylene terephthalate (PET), extensive research for high-density polyethylene (HDPE) and polypropylene (PP) has not been well established to accurately show the viability of SIM with regard to these materials.