Extrusion & Post
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Cyclic pressure fatigue failure of polyolefin plastomer PPR pipes
Problem Phenomenon
PPR/PP-R pipes undergo fatigue failure in cyclic pressure tests, with cracks initiating on the inner wall and expanding outward.
Fatigue failure is the main failure mode of pipes in actual use.
Cause Analysis
（1）Tiny defects on the inner wall of the pipe cause stress concentration
（2）The inner wall at the welded joint is not smooth, producing stress concentration
（3）The pipe material has insufficient toughness and the fatigue crack expands quickly
（4）Improper extrusion process parameters cause processing defects on the inner wall of the pipe
（5）The design safety factor of the pipe wall thickness is low
Solution
（1）Ensure the inner wall of the pipe is smooth and defect-free, with inner wall Ra≤3.2μm
（2）Improve the welding quality to avoid weld beads on the inner wall of the joint
（3）Select materials with good toughness, ensuring the elongation at break is >500%
（4）Optimize the extrusion process and control the inner wall quality
（5）Ensure the pipe wall thickness reaches the SDR design value, and select a higher SDR value when necessary
Key Takeaway
Fatigue failure is caused by internal defects and stress concentration, and needs to be improved from three aspects: pipe inner wall quality, welding quality, and material toughness.
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