Cyclic stress-strain behavior of shape memory polymer based syntactic foam programmed by 2-D stress condition

Cyclic stress-strain responses of a shape memory polymer based syntactic foam and pure SMP were investigated after a two dimensional (2-D) programming process. Three types of cyclic loading patterns were used: multiple compression-tension cycles at 5% maximum strain, one compression-tension cycle at 40% maximum strain, and one coupled thermo-mechanical cycle at 40% maximum strain. The results indicate that the foams initially harden after the first cycle and then soften in subsequent cyclic loadings, while the pure SMP shows a monotonous softening. The hysteresis loops tend to shrink and approach asymptotically to a steady state before fatigue failure for both the foam and the pure SMP. The programmed foam and the programmed pure SMP are less sensitive to their strain histories, and dissipate more energy as compared with the non-programmed counterparts, showing better fatigue properties at the same strain level. Also, the programmed foam exhibits better mechanical properties under one cyclic thermo-mechanical loading, possessing a good adaptability to ambient temperature changing. Furthermore, the programmed foam demonstrates an increase in yield strength during compression and a decrease in residual tensile stress during tension. These particularly facilitate the programmed foam to be used as a self-healing sealant in expansion joints.