Lever, Palermo, Wright, Oliphant
Material Science # 2006 Washington DC
The piping industry has a need for a well-founded methodology to predict lifetime of pipeline components subject to stress concentration. Another useful tool would be a method to assess the impact of manufacturing processes on the slow crack growth (SCG) properties of a resin that can be used for process control in a manufacturing facility. In this paper we will propose an innovative methodology for predicting the time to failure of molded fittings due to the SCG failure mechanism that can also be used for optimized manufacturing process control. This method initially utilizes the well-known Rate Process Method (RPM) analysis of molded fitting assemblies for lifetime prediction. Stresses in the molded fittings are determined by finite element analysis. The method also utilizes the new Notch Ring Test (NRT) with rings cut from the molded fittings for assessing the impact of process conditions on SCG characteristics of PE fittings. Stresses in the NRT rings are also determined by finite element analysis. With these NRT data, the fitting manufacturer can develop a reliable method for process control by determining the effect of process variables on SCG performance. Correlation of the RPM data with the NRT data using the same molded fittings will provide an NRT process control method that has been validated by the RPM lifetime prediction method. By fully characterizing the SCG properties of the particular material used and fully characterizing the stress concentrations, a rational methodology for lifetime prediction is then available to the pipeline engineer, and a rational methodology for process control is available to the fitting manufacturer. This methodology can then be incorporated into national and international standards.