Extrusion & Post
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Random copolymer polypropylene pipe special material (Nordic standard） PPR pipe cyclic pressure fatigue failure
Problem Phenomenon
PPR/PP-R pipes suffer fatigue failure in cyclic pressure tests, with cracks initiating on the inner wall and propagating outward.
Fatigue failure is the main failure mode of pipes in actual use.
Cause Analysis
（1）Microscopic defects on the pipe inner wall cause stress concentration\
（2）The inner wall at the welded joint is not smooth, producing stress concentration\
（3）The pipe material toughness is insufficient, fatigue cracks propagate quickly\
（4）Improper extrusion process parameters cause processing defects on the pipe inner wall\
（5）The pipe wall thickness design safety factor is too low
Solution
（1）Ensure the pipe inner wall is smooth and defect-free, inner wall Ra≤3.2μm\
（2）Improve welding quality, avoid weld beads on the inner wall of the joint\
（3）Select materials with good toughness, ensure elongation at break >500%\
（4）Optimize the extrusion process, control inner wall quality\
（5）Ensure the pipe wall thickness reaches the SDR design value, select a higher SDR value if necessary
Key Takeaway
Fatigue failure is caused by internal defects and stress concentration, and needs improvement from three aspects: pipe inner wall quality, welding quality and material toughness.
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