Equation 1 Proposed Method for Evaluating Complex Molded Polyethylene Geometries

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1 Innovative Methodology for Fitting Lifetime Prediction and Process Control by Correlating Rate Process Method Analysis of Molded Fittings with Notch Ring Test Data Authors: Ernest Lever, Central Plastics, Shawnee, OK Dr. Gene Palermo, Palermo Plastics Pipe Consulting, Friendsville, TN Jeff Wright, Central Plastics, Shawnee, OK Dr. Ken Oliphant, Jana Laboratories, Aurora, ON, Canada ABSTRACT 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. Background The use of RPM analysis has been accepted as an effective tool for establishing the life expectancy of polyethylene piping systems for many years (Bragaw, (1), (2), Palermo et. al., (3)). The method has been successfully applied to pipes, fittings and assemblies. Palermo (4) has also advocated the RPM as a quality control tool and has pointed out the problems associated with using a single minimum time to failure test for quality control. Full use of

2 detailed RPM analysis is however, a time consuming and expensive process and it is not routinely used in production environments. It has also been known for many years that processing conditions at the time of manufacture can impact the SCG properties of the finished product. Chaoui et al (5) pointed this out in MDPE pipe and those experienced in the field know this to be true for all grades of resin used in the injection molding of fittings. The role of stress concentrations in initiating SCG in complex geometries has been noted by Lever (6) and Palermo et al (7). Both these authors have noted the applicability of FEA in understanding true stress distributions in complex geometries. In particular they have used FEA to arrive at true stress concentration values and apply these values to RPM analysis of test data. Over the past two decades a multitude of test methods have been proposed to expedite the evaluation of SCG properties of polyethylene. Most of these methods use either pipe or standard test sample geometries and are not readily applicable to complex molded geometries. In this paper we describe a standard molded test sample that effectively introduces a stress concentration very similar to that found in socket type fittings. This sample has applicability in establishing RPM equations for molded resin and evaluating the impact of process conditions on the RPM coefficients: Equation 1 Where t f = time to failure, σ hoop = constant hoop stress in pipe test, T = absolute temperature and C 1, C 2, C 3 and C 4 are fitting constants that are part of the RPM analysis. Choi (8) has proposed an interesting method the Notched Ring Test (NRT) that utilizes full rings cut from actual product. This method introduces high levels of stress concentration that lead to tests of short duration that can easily be utilized in production environments. In this paper we attempt to establish a correlation between the NRT method and the RPM method applied to the standard injection molded test sample. Proposed Method for Evaluating Complex Molded Polyethylene Geometries Figure I below is a graphic representation of the interplay between Material, Design Geometry, Manufacturing Process and the ultimate resistance to SCG in service for a complex molded geometry. The following sequence of steps captures the logic of the proposed material selection, design assessment and process control methodology. This method can replace current approval and process control tests. 1. FEA will be used to fully evaluate the stress concentrations of any manufactured geometry. 2. Full RPM analysis of molded specimens will establish the basic suitability of the resin used by establishing the 3 or 4 coefficients for Equation 1. This will allow an expected lifetime at the actual operating condition to be calculated utilizing the lower confidence limit provided by the RPM analysis. 3. The NRT test can be used as a process control test on all production runs to verify that the material has not been compromised in the manufacturing process.

3 Material Design Geometry Manufacturing Process Resistance to SCG in service Figure I Standard Injection Molded Test Specimen Figure II

4 Figure II shows an actual specimen with SCG failure on the left side (arrow) and a sectioned model of the test specimen showing the internal geometry with molded in stress riser on the right. Two molded specimens are butt fused together to form a pressure vessel for RPM testing. FEA of the Stress Concentration in the Standard Molded Test Specimen Figure III

5 Figure IV Figures III and IV show the detailed FEA mesh and a typical stress distribution at the point of stress concentration. The stress distribution is essentially identical to that found in a typical electrofusion coupler at the first wire as reported by Lever (6). Detailed non-linear and linear analyses were run. Material properties at 23 C, 40 C, 60 C and 80 C were input for a typical commercially available PE 100 material. The objective of these analyses was to determine the effect of loading intensity, notch radius and temperature on the value of the stress concentration factor K. Two methods of calculating the factor K were employed: 1. The ratio between the maximum notch stress and the standard hoop stress associated with a pipe of diameter ratio (DR) equal to that of the barrel of the standard test specimen. This factor is denoted K standard. 2. The ratio between the maximum notch stress and the stress at the center of the barrel of the standard test specimen. This factor is denoted K true as it is a measure of the true stress concentration in the test specimen. In all cases Von Mizes stress was used unless otherwise noted. It was found that linear FEA is of no use in determining stress concentration factors. Non linear analysis using true stress strain curves for the material shows that the internal pressure has a major effect on the level of stress concentration. At higher loads the increased displacement relieves the maximum stress and reduces the stress concentration. The notch radius has a significant effect on the stress concentration as expected. Figures V and VI below show the respective influence lines for these two factors. Temperature was found to have a second order effect on the stress concentration and is not shown here. It is interesting to note that K standard is relatively low due to the high DR of the specimen barrel. The stresses achieved at the concentration would lead to ductile yield in a pipe of similar DR. The barrel of the specimen is at much lower stress due to the short length of the barrel and the stiffening effect of the thick area of the specimen. This is an accurate simulation of a thin walled coupling fused to heavy wall pipe. This strengthening effect was reported by Lever et al (9). The FEA results from these analyses suggest a true stress concentration factor for a coupling/pipe assembly as that investigated by Lever (6) ranging from 1.7 to 2.1. The empirically arrived at factor reported by Lever (6) was 1.8 to This agreement is excellent and lends credence to detailed FEA as a method for assessing and approving complex geometries under known loading conditions.

6 Figure V Figure VI

7 Figure VII NRT Test Equipment Figure VII above shows some details of the NRT equipment. A ring is notched with a standard blade and placed under three-point loading. A pneumatic cylinder applies tensile or cyclic loading. A sensitive LVDT is able to log displacement over time. Crack initiation, propagation and failure can be detected with considerable accuracy.

8 The Correlation Between RPM Analysis of the Standard Molded Test Specimen and NRT Tests on Rings Cut from The Standard Test Specimen RPM and NRT testing is currently underway on specimens molded from three different PE 100 resins and two different PE 3408 resins. Insufficient data are available to present at the time of writing. The correlations for the various materials will be presented as a supplement at the conference as all testing is expected to be completed by that time. Conclusion A logically sound approach to ensure adequate polyethylene piping system performance has been presented. FEA has been shown to accurately predict true stress levels in complex geometries encountered in polyethylene piping systems. The advantages of rigorous RPM testing to approve materials for the fitting manufacturing process have been pointed out. The case has been made for more appropriate process control tests utilizing the NRT. The authors believe that this proposed design and testing methodology will lead to more rapid approval of polyethylene piping applications with better quality assurance and a sound basis for system lifetime prediction.

9 ACKNOWLEDGMENTS The Authors would like to thank Dr. Sunwoong Choi, Department of Polymer Science and Engineering, Hannam University, Korea for his support in generating data for correlating his NRT method with the method presented in this paper. Oren Lever, Central Plastics, Shawnee, OK deserves mention for the many hours spent crunching numbers in the detailed FEA. REFERENCES 1. C. G. Bragaw, The Forecast of Polyethylene Pipe and Fitting Burst Life Using Rate Process Theory, 5 th International Conference on Plastic Pipes, York, UK, September C. G Bragaw, Service Rating of Polyethylene Piping Systems by The Rate Process Method, American Gas Association, New Orleans, Louisiana, November Dr. E. F. Palermo, Ivan K. DeBlieu, Rate Process Concepts Applied to Hydrostatically Rating Polyethylene Pipe, The Ninth Plastic Fuel Gas symposium, New Orleans, La, November 12-15, Dr, E. F. Palermo, Rate Process Method As a Practical Approach to a Quality Control Method for Polyethylene Pipe, The American Gas Association Eighth Plastic Fuel Gas Pipe Symposium, Nov. 29 Dec. 1, K. Chaoui, A. Moet, A. Chudnovsky, Effects of Processing on the Long Term Strength of MDPE Pipes, 6. E. Lever, Design and Pressure Rating of PE Fittings: Stress Concentrations, Slow Crack Growth and Use of ISO 9080 Regression Coefficients in Material Choice, Plastics, Rubbers and Composites, 2005, VOL. 34, NO 1 7. Dr. Gene Palermo et. al., New Test Method to Determine the Effect of Recycled Materials on the Life of Corrugated HDPE Pipe as Projected by the Rate Process Method, Draft Paper for PPXIII 8. Dr. Sunwoong Choi, Private communications and interim reports on NRT testing at various ISO task group meetings 9. E. Lever, C. Allcard, Y. Gross, J. Bowman, The Pressure Rating of EF Couplers; The Role of Material Strength and Wall Thickness, 16 th International Plastic Fuel Gas Pipe Symposium, New Orleans, 1999