Stress relaxation preconditioning of porcine aortic valves.

In uniaxial tensile testing, load preconditioning is used to generate repeatable load/elongation curves and set a "reference state" for subsequent tensile tests. We have observed however, that for porcine aortic valve (PAV) tissues, preconditioning does not lead to repeatable stress relaxation curves. We thus investigated possible experimental protocols that could be used to generate repeatable load/elongation and stress relaxation curves. To quantify repeatability of stress relaxation, we compared normalized loads at the same time points from repeated stress relaxation curves and computed a repeatability ratio. We found that PAV specimens can generate repeatable stress relaxation curves (repeatability ratio >0.95) if they are subjected to at least five cycles of repeated load preconditioning and stress relaxation. We also found that a single cycle of loading/unloading prior to each stress relaxation phase is sufficient to generate repeatable stress relaxation curves. Stress relaxation preconditioning is therefore required to generate repeatable load/elongation and stress relaxation curves. It is expected that such curves will generate more accurate material constants for the characterization and modeling of PAV mechanics.