Résumé
To characterize the mechanical behaviour of soft biological tissue, various experimental constraints appear. Actually, physiological conditions are generally required to preserve material properties. Thus, samples must be immersed in a thermo-regulated bath during experiments which prevent from using classical strain measurement techniques. The alternative is to rely on image analysis methods which present the benefit to be non-invasive. This work focuses on annulus fibrosus tissue (AF), i.e., the external tissue of intervertebral disc located between consecutive vertebral bodies. Lumbar discs were harvested from cadaver of a domestic pig and prepared to perform a tensile test in physiological conditions. Two high resolution cameras are positioned perpendicularly to the tensile direction to visualize both transverse planes. By using Digital Image Correlation technique on natural pattern, surface displacement and strain fields are obtained. Heterogeneous fields are observed in all directions with strong strain localisations. Transverse strain in the plane of lamellae highlights the porous nature of AF tissue with a swelling in the central part and symmetric strain gradients directed towards both sample sides. Over multiple tensile loading cycles, some hysteretic loops are observed on both stress and strain evolutions emphasizing the essential dissipative function of such material. A poro-mechanical explanation associated with the biphasic and composite constitution of AF tissue is proposed allowing to quantify coupling effects between the mechanical behaviour and liquid flows. This method lead to develop a better identification procedure by fitting numerical models with 3D tissue surface behaviour. Introducing such description of the poro-mechanical behaviour of annulus tissue will have strong implications on the coupling between strains and fluid transfers.