Mechanical responses of Chrysopogon zizanioides roots under cyclic loading conditions

Background and aims: Data on cyclic root mechanical behaviour (i.e. roots subjected to repeated loading and unloading cycles at a given frequency) are lacking. In this study, cyclic root mechanical traits were measured. Methods: Chrysopogon zizanioides L. roots were sampled for monotonic and cyclic tensile tests. The effects of pre-cyclic loading on the monotonic mechanical properties (n = 92) and reversal strains on any changes in cyclic mechanical properties (n = 88) were measured. The root water content and root traits, including diameter, dry density, and specific root length (SRL), were also measured for each sample. Results: Pre-cyclic loading at 5% and 50% strain levels reduced the root tensile strength (T_r) by 18.17% and 27.10%, respectively, yet increased the breakage strain (ε_r) by 30.85% and 52.13%, respectively. Although T_r and the Young’s modulus (E) were reduced after cyclic loading, the trend of their correlations with root diameter (i.e. negative power law correlation) was unaffected. Remarkable hysteresis was found in any loading cycle. The hysteresis loop size and peak tensile stress decreased abruptly during the first 20 cycles. The peak tensile stress explained more than 86% of the variability of Young’s modulus along the unloading path at all the reversal strains. Conclusion: The root mechanical traits obtained from monotonic tests should not be used to explain the root reinforcement in cyclic events because of cyclic-induced changes in the T_r and E. Regardless of the strain range experienced in a cyclic event, determining the changes in root mechanical properties during the first 20 regular cycles of loading is crucial.