Fig. 3. Effects of the (predetermined) stretch of a tensile-unloading (to zero stretch) cycle on (a) the Young's modulus (E_c) of the cycle, i.e. slope at the onset point of the tensile curve of the cycle where the stress begins to become non-zero, and on (b) the degree of hysteresis (U_hys/W_t), i.e. ratio of the irreversible viscous work (U_hys) to the total viscoelastic work (W_t), until the cycle for a poly(vinyl alcohol) (PVA)/alginate sequential, hydrogen-bonded-ionic dual-physical double-network (DN) hydrogel of maximum mechanical properties (tensile strength, elongation at break and toughness) having a mole ratio of the PVA- to Na alginate repeat units of 60/1 and Ca-dicarboxylate ionic cross-links of the minor dispersed alginate networks dramatically denser than the hydrogen bond cross-links of the major global PVA network, which, standing gripped on a universal testing machine as it is across the entire process at an initial separation of 25 mm, is subjected to sequential tensile-unloading (to zero stretch) cycles of predetermined constant-incremental stretches at a crosshead speed of 50 mm min⁻¹ without intermission between the neighbouring cycles: at its small stretches, one fresh specimen is run from an initial stretch of 0.07 mm mm⁻¹ until 0.55 mm mm⁻¹ at an increment of 0.08 mm mm⁻¹; while, at its larger stretches, another fresh, parallel specimen is run from an initial stretch of 0.70 mm mm⁻¹ until its fracture at an increment of 0.80 mm mm⁻¹.