The life expectancy of plastic pipes, particularly their long-term strength under constant load, is crucial for applications like pressure pipes. The Minimum Required Strength (MRS) is defined as the 50-year strength of the material under internal pressure. This value is determined through a process involving long-term burst pressure testing of pipes at different stress levels.
Test results showing rupture points at various pressure levels are plotted on a log-log graph. These points typically form a straight line, known as the regression line. This linearity is explained by the fact that the decay of polymers often follows the Arrhenius law. The MRS is statistically derived from this regression line, specifically calculated as the 97.5% Lower Confidence Limit (LCL) of the rupture line after extrapolating to 50 years (438,000 hours). Graph 3 illustrates an example of a regression line used to determine the MRS of PVCO.
For PVC, the regression line is typically straight without a "knee" in the life time graph. This indicates a consistent failure mechanism over time, provided the material is properly stabilized and processed.
Polyolefin (PO) materials like PE can show a knee in their lifetime curve (Graph 4). A knee signifies a change in the failure mode or a weakening mechanism that accelerates decay after a certain period. Potential causes for a knee in PO include loss of stabilization (polar stabilizers washing out in non-polar PO) or chemical ageing by agents such as gas condensates or disinfectants like Chloride oxide. This chemical ageing can start early brittle fracture in PE pipes.