Stress Intensity Effect on Slow Crack in Scratched PE Pipe

Session 9A
11:20 am

Jimmy Zhou, The Dow Chemical Company

Stress intensity in Polyethylene pipe is determined by applied stresses, defect sizes, and pipe geometries. Slow Crack Growth (SCG) is a major failure mode observed in field for old generations of PE pipe. SCG resistance is determined by resin material, stress intensity, and temperature. ASTM F 1473, the PENT test, is the standard test method to measure the SCG resistance. The standard testing condition is at 80°C and a stress intensity of 0.468 MPa m1/2. The minimum SCG requirement has been increased to 10h, 100h, 500h and a proposed 2000h in various codes and standard after resin material advancement is made. The stress intensity effect of intrinsic defect on the SCG resistance in PE pipe is validated by the 50-year substantiation of linearity of LTHS per ASTM D 2837 and PPI TR-3. The intrinsic material defect is essentially the same for all pipe sizes. Stress intensity is higher in smaller pipe. Hydrostatic test on smaller sized pipe is more stringent than larger sized pipe in terms of the SCG resistance caused by the intrinsic material defects. Therefore the SCG resistance of un-damaged PE pipe for all pipe is validated by hydrostatic test conducted in small diameter pipe typically 1” SDR 11.

However, defects may be introduced into pipe wall, such as rock impingement and scratching. The sizes of such introduced defect are not predictable and vary. PE industry has used 10% of scratch depth as the maximum allowable defect size for all pipe sizes as a simple inspection practice. The absolute scratch depth, though, is used to calculate stress intensity. To keep 10% scratch depth constant, the stress intensity would increase as the pipe diameter increases. The increased stress intensity will produce faster SCG failure in a larger diameter pipe than a smaller diameter pipe. We investigated the stress intensity effect in PE pipe at three design stresses: 512 psi for natural gas distribution, 800 psi for PE3608 water pipe, and 1000 psi for PE4710 water pipe. To achieve the equivalent SCG performance as measured by standard PENT test, the stress intensity should be the same 0.468 MPa m1/2 for all pipe sizes. The maximum allowable % of scratch depth as a function of pipe diameter for SDR 11 pipe thus can be obtained. If the PENT testing stress increases from 2.4 MPa to 3.0 MPa, the stress intensity would increase to 0.585 MPa m1/2. At 0.585 MPa m1/2 stress intensity, the maximum allowable % of scratch depth would increase.

Stress intensity effect is primarily a stress shift factor which varies for different PE resins. A conservative stress intensity shift factor was chosen based on published researches. The impact of stress intensity change in various pipe sizes was estimated by taking standard PENT as a reference. To predict SCG lifetime from accelerated test at elevated temperatures to application temperatures, temperature shift factor must be determined. A conservative temperature shift factor was also chosen based on published researches. To achieve 100 year SCG resistance, the minimum required PENT hours at 0.468 MPa m1/2 (2.4 MPa) and 0.585 MPa m1/2 (3.0MPa), maximum allowable % of scratch, and the pipe geometry must be balanced. The relationship among these 4 parameters is discussed in detail. Increasing the required PENT hours to compensate the increased stress intensity and decreasing the maximum allowable scratch depth to compensate the increased stress intensity are two ways to address the impact of stress intensity on the field SCG resistance of a large diameter scratched pipe.

Dr. Z. Jimmy Zhou and Dr. Dane Chang
The Dow Chemical Company