Abstract
Uniaxial tensile and relaxation tests were carried out on annulus fibrosus samples carved out in the circumferential direction. Images were shot perpendicularly to the loading direction. Digital image correlation techniques accurately measured the evolution of full displacement fi elds in both transverse directions: plane of fibres and plane of lamellae. In the fibre plane, strains were governed by the reorientation of fi bres along the loading direction. This implies strong transverse shrinkage with quasi-linear behaviour. Conversely, a wide range of behaviour was observed in the lamella plane: from shrinkage to swelling. Strong non-linear evolutions were generally obtained. The strain field in the lamella plane generally presented a central strip section with more pronounced swelling. Our physical interpretation relies on the porous nature of annulus tissue and its anisotropic sti ness. Indeed, the liquid overpressure generated inside the sample by the strong shrinkage in the fi bre plane discharges in the perpendicular direction since rigidity is lower in the lamella plane. Regarding the strain eld measured in the lamella plane, this interpretation agrees with (a) symmetric strain distribution with respect to the longitudinal axis of samples, (b) the reversal in behaviour from shrinkage to swelling, (c) the decrease in strain during relaxation tests associated with outward ows. The variety of transverse behaviours observed experimentally could result from uncertainties regarding the initial reference state of tissue samples. Since the mechanical behaviour is highly non-linear, experimental results underline that a slight uncertainty concerning the pre-stress applied to samples can lead to wide variability in the mechanical properties identi ed.