Why do some transportation-sector sandwich panels require PET instead of PU? Explain from fatigue fracture mechanics.

PET is a thermoplastic material. Its fatigue crack growth exhibits a “blunting-re-initiation” mechanism, with low crack growth rate da/dN and a distinct fatigue threshold (stress intensity factor range ΔKₜₕ≈3 MPa√m). PU foam, being thermoset, has no plastic deformation capability; once a crack initiates, it rapidly propagates unstably, with da/dN an order of magnitude higher. For truck floors subjected to approximately 5000 micro-vibrations per day from wheel rolling, accumulating 10⁸ cycles over 10 years, PET sandwich panels can operate safely, while PU panels may develop core fatigue cracking within six months to a year.

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Why do some transportation-sector sandwich panels require PET instead of PU? Explain from fatigue fracture mechanics.

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