3.4. Unloading and reloading features of luffa sponge material
For any energy absorption devices, the bounce-back behaviour is critical to some of their applications. The bounce-back behaviour is dominated by the stored elastic strain during collapse (Lu and Yu, 2003). This feature can be captured by the unloading and reloading curves for a specimen at different collapse strains. Typical unload–reload characteristics at different compressive strains are presented in Fig. 9(a). The relatively high gradient of unloading curves indicates the stored elastic energy is relatively small for luffa sponge material. The unloading and reloading curves do not overlap with each other. Due to the steep gradient the dissipated energy in the hysteresis loop is very small compared to the area under the entire stress–strain curve. Thus luffa sponge can be used in those situations where a once-off, non-reversible energy absorption is required. The unloading curve is not linear and is steeper at the beginning of the unloading. The reloading curve, on the other hand, is almost linear. This phenomenon indicates the hysteresis of the luffa sponge material at a strain rate of 10−3 s−1. The hysteresis is more prominent at larger compressive strain. For a given temperature and humidity, it is observed that the unloading and reloading curves intersect at exact unloading point as shown in Fig. 9(a). The slope of reloading curve, reloading modulus, varies with the strain level at the unloading point as shown in Fig. 9(b). The reloading modulus is very similar to the Young's modulus measured in Section 3.3 at the plateau collapse region ranging from the first peak stress to the densification strain.