The nonlinear viscoelasticity of a thixotropic yield stress fluid (3.0 wt% Laponite hydrogel) and a non-thixotropic one (0.25 wt% Carbopol hydrogel) under large amplitude oscillatory shear (LAOS) was investigated by Fourier-transform rheology (FTR).The relative intensities of the 3rd harmonic (I3/I1) and the relative phase angle of the 3rd harmonic (3) of these two hydrogels under different stress amplitudes were compared.It is found that both the nonlinear viscoelasticity and the solid-liquid transition behavior of these two hydrogels differ greatly because of their different microscopic structures.For the Carbopol hydrogel

the nonlinear intensities change gradually with the increase of applied stress

and the transition behavior shows the correlations between the deformation

relaxation as well as relative slip of the microgel particles and the oscillatory frequency.While for the Laponite hydrogel

the nonlinear viscoelasticity exhibits a non-monotonic change

which indicates several times of shear rupture of local house-of-card structure before the full solid-liquid transition.