Comparison of fatigue in fiber-backed PVDF and PFA fluoropolymer linings

Abstract

In this work comparative mechanical fatigue experiments were performed in order to quantify the mechanical stress delamination rates for both PVDF and PFA lining materials. Evidence was found for a Paris Law behavior when samples are cycled in blister test configurations. PFA liners exhibited crack growth constants C=0.0486cm/cycle and n=0.9 while PVDF liners exhibited crack growth constants C=0.0999cm/cycle and n=0.8. Linear crack growth rates were observed which ranged from 0.042 ((a/a0)/cycle) at 3.10 bar up to 1.47 ((a/a0)/cycle) at 4.48 bar for PVDF and 0.024 ((a/a0)/cycle) at 2.59 bar up to 0.262 ((a/a0)/cycle) at 3.79 bar for PFA. PFA liners were found to fail at 5.52 bar while PVDF liners failed more violently at 6.21 bar. Overall fatigue ratings of the PVDF vs PFA linings should balance the faster delamination rates of PVDF liners vs the lower strength of PFA liners. It is unlikely that vessels with PVDF or PFA liners under fatigue failure due to vacuum-induced mechanical stresses since much larger stresses were required to cause low cycle fatigue failure. Instead, sample delamination is likely due to thermal stresses arising from a mismatch in thermal expansion between liner, fiber backing, epoxy, and metal substrate.