I. Material Selection and Medium Compatibility

• Metal Material Defects:
  Common materials include 304 stainless steel (general corrosion resistance for neutral media), 316L (chloride ion resistance for seawater/chemical media), and Hastelloy (resistance to strong acids and alkalis). If the material is not selected according to medium characteristics (e.g., using 304 for chloride-containing wastewater), stress corrosion cracking (SCC) may occur, shortening the service life to 1-2 years (normal service life: 5-8 years).
• Rubber Layer Aging:
  For rubber expansion joints, materials like NBR (oil resistance) and EPDM (temperature/weather resistance) will accelerate swelling and hardening if exposed to incompatible media (e.g., EPDM in contact with mineral oil), leading to sealing failure (service life reduced from 5 years to 1-2 years).

• Without zinc-nickel alloy plating or fluorocarbon spraying on the outer surface (e.g., in outdoor environments), long-term erosion by acid rain and salt spray will accelerate oxidation of the metal matrix, thinning the wall thickness by 0.1-0.2mm annually. When the thickness falls below the design safety threshold, rupture is likely.

II. Working Conditions and Displacement Overrun

• Frequent Displacement Overrun:
  If the expansion joint is designed for ±15mm axial displacement but actually undergoes long-term displacement exceeding ±20mm due to thermal expansion/contraction, the metal fatigue strength at the bellows' crests and valleys will decrease. For every 10% displacement overrun, the number of cycles drops from 10,000 to below 5,000 (halving the service life).
• Vibration Overload:
  When pipeline vibration frequency resonates with the expansion joint's natural frequency (e.g., pump vibration at 100Hz matching the joint's resonance frequency of 95-105Hz), micro-cracks form in the bellows metal, potentially leading to fracture within 1-2 years.

• Over-temperature Operation:
  304 stainless steel is designed for ≤400℃. If the medium temperature remains at 450℃ long-term, metal grain coarsening occurs, reducing tensile strength by 30% and making the bellows prone to plastic deformation.
• Sudden Pressure Fluctuation:
  Instantaneous pressure fluctuations exceeding 20% of the design value (e.g., a 1.6MPa expansion joint subjected to 2.0MPa impact) cause peak stress on the bellows wall to exceed the yield strength, leading to permanent deformation or tearing.

III. Installation Errors and Maintenance Neglect

• Coaxiality Exceedance:
  If the coaxiality deviation of the two ends exceeds 3mm (for DN100 and above), the expansion joint bears eccentric loads, causing stress concentration on one side of the bellows crest and shortening the service life from 8 years to 3-4 years.
• Uneven Bolt Tightening:
  Asymmetric tightening of flange bolts causes excessive local extrusion of the rubber sealing surface, leading to cracking and leakage (common in rubber expansion joints).

• Foreign Matter Accumulation:
  If the medium contains particulate impurities (e.g., without a filter), deposition in the bellows valleys will wear the wall thickness (0.05-0.1mm/year), eventually causing perforation and leakage.
• Unaddressed Corrosion:
  Failing to grind and repaint rust spots on the outer surface, or replacing the expansion joint when local corrosion pits exceed 10% of the wall thickness, drastically increases accident risks.

Extension: Comparison Table of Service Life Optimization Measures