Sooho Pyo, Junghyun Nam, Yoonsang Kim and Sunwoong Choi
Papers # 2010 Vancouver
The Notched Ring Test (NRT) has been recently developed to measure slow crack growth (SCG) in plastics pipes and fittings. The NRT specimen in the form of a notched full ring is directly obtained from the extruded pipe or injection molded fitting and has demonstrated its effectiveness in generating slow crack growth in a time frame of order of magnitude shorter than the currently available test methods. Recently, the NRT test has been utilized to include larger pipes up to 400mm diameters and the results are presented. Also, a solution to the difficulty associated with the detection of on-set slow cracking will be demonstrated with the development of the modified displacement method. In addition, a generalized solution for the stress intensity factor calculation for NRT specimens will be described and examples presented.
The measurement of slow cracking behavior of plastics pipes and fitting has been interest to piping industries for many years as they can provide information towards making proper judgments in design against early failures and lifetime assessment. As improvements on slow crack resistance much improved over the years, the ongoing incentives for the development of accelerated test method for assessing slow cracking behavior of these resins becomes clear in terms of extending the scope of applications with increased safety and reliability as well as cost effectiveness in installation and maintenance. The notched ring test (NRT) has been recently develop for this purpose [1] and has demonstrate its effectiveness in creating slow cracking in times significantly shorter than those from other currently available methods (Figure 1). The characteristics of the NRT method are that the slow cracking test are performed on a full ring specimen directly removed from pipes or fittings so that all processing variables that can influence slow cracking are retained; specimen geometry allows optimum crack tip stress field to be attained; razor notching done in a simple and accurate manner and capital intensive pressure testing equipment not a requirement. In addition, beside the simplicity and the accelerated manner in which NRT is demonstrated to be useful, one other significant advantage involves slow crack growth testing of large diameter pipes (400mm, 600mm, etc) with similar ease as testing smaller diameter pipes (egg. 63mm, 110mm). The sample and loading geometry of the NRT specimen is illustrated in Figure 2. In general the notched ring specimen is loaded in a three point manner and the slow cracking is measured through ring deflection that would occur with time. An internal notching is done at the inner surface and to aid sample loading and in maintenance of the plane-strain condition at the crack tip, side notches were also produced as shown. This presentation provides most recent results of NRT method on larger diameter pipes up to 400mm.
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