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Regression Lines for Life Expectancy of PVC Pipes (50 Years)

Picture of Patrick van Bezouw Patrick van Bezouw Jun 10, 2025 12:20:54 PM

Mechanical Performance & Durability
Regression Lines for Life Expectancy of PVC Pipes (50 Years)
Regression Lines for Life Expectancy of PVC Pipes (50 Years)
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Overview

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.

Determining MRS and Life Expectancy

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.

Key Terms

  • MRS (Minimum Required Strength): The statistically determined 50-year strength of the material under internal pressure, derived from the 97.5% LCL of the regression line.
  • Regression Line: A straight line plotted on a log-log graph representing the relationship between stress level and time to rupture for pipe samples under constant pressure.
  • Knee: A significant change in the slope of the regression line, typically seen in some materials like PE, indicating a change in degradation or failure mechanism over time.

 

FAQ Section

How is the 50-year strength (MRS) of a plastic pipe material determined?

The MRS is determined by conducting long-term burst tests on pipe samples at different constant pressure levels until rupture. The rupture points are plotted on a log-log graph to form a regression line, and the MRS is statistically calculated as the 97.5% Lower Confidence Limit (LCL) of this line extrapolated to 50 years.

 

What does a "knee" in a regression line indicate, and why might it occur in PE but not typically in PVC?

A knee indicates a change in the material's failure mechanism or accelerated degradation over time. While PVC typically shows a straight line, PE can exhibit a knee due to factors like loss of stabilization or chemical ageing from substances such as disinfectants or gas condensates.